Drilling and Milling Processes PDF

CHAPTER 8 DRILLING Round-Hole-Making Methods 8.1 Definition Drilling is a cutting procedure designed to generate holes predominantly with a two-flute tool, the twist drill. When drilling on the drilling mach...

CHAPTER 8 DRILLING Round-Hole-Making Methods 8.1 Definition Drilling is a cutting procedure designed to generate holes predominantly with a two-flute tool, the twist drill. When drilling on the drilling machine, the tool carries out the feed- and the cutting motions. If the hole is machined on a turning- or automatic lathe, then the workpiece performs the cutting motion. 8.2.1. Drilling The most common tool for drilling, a twist drill, is a rod with helical flutes and two or more cutting edges at the end. It is rotated about its axis and fed axially into the work. As it advances, it produces or enlarges a round hole in the workpiece. The chips are carried away from the hole by the flutes in the drill. (When drilling an axial hole with a lathe, the workpiece rotates rather than the drill.) There are other types of drills that may not have helical flutes. Others may have only one cutting edge. The drilling process is very common and is used with a wide variety of machines ranging from the most sophisticated computer-controlled or multiplespindle machines to hand-held electric or crank-driven drills. The most common diameter range for drilled holes is about 1/8 in (3 mm) to 11/2 in (38 mm) although diameters from 0.001 (0.025 mm) to 6 in (150 mm) can be drilled with commercially available special drills. Fig. 7.2 includes an illustration of drilling as performed on turning equipment. Fig. 8.1 shows various types of drill and drilling operations. Fig. 8.2 shows some typical drills. Figure 8.1. Various types of drill and drilling operations. Figure 8.2. A series of drills and, at the bottom of the group, a typical reamer. Chapter 8 1  8.2.2 Counterboring counterboring - enlarges a hole for part of its depth and usually machines a flat bottom in the enlarged portion. The operation is most often performed to provide clearance for a bolt head or multi-diameter part. The rotating cutter is guided by a pilot that fits into the existing hole, so that the counterbored surface is concentric with the original hole. A multi-diameter counterboring tool can produce stepped counterbores. Fig. 8.3 illustrates a counterboring tool in view a) and the counterbored hole it produces in view b). (The tool also can be produce spotfacing, as shown in view c). Figure 8.3. A counterboring/spotfacing Figure 8.4. Reaming is used to improve tool (view a) and a cross-sectional view the accuracy, surface finish and of the counterbored hole it produces straightness of round holes. (view b). View c) shows cross-sections of two slightly different spotfacings produced by the same tool. The purpose of counterboring is to produce a recess of prescribed depth while spotfacing is performed to provide a smooth and perpendicular flat surface for a fastener or other object. 8.2.3 Countersinking Countersinking - is an operation that adds a chamfer at the entry end of a hole. A rotating cutting tool, with the edge set to the angle of chamfer desired, is fed into the hole and removes material at the edge. The tool is centered by the hole; therefore the chamfer is concentric with the hole's axis. The operation is typically used to remove burrs or a sharp edge at the end of a hole, or to provide space for a tapered screw head or other tapered object. 8.2.4 Reaming Reaming - is a secondary machining operation for existing holes. It can provide a more accurate diameter, improved straightness, and a smoother surface finish as it slightly enlarges the hole. A rotating tool, a reamer, is used. The operation can be performed on a drill press or other Chapter 8 2 drilling machine and is sometimes done by hand. Reamers normally remove 0.005 to 0.015 in (0.13 to 0.38 mm) of diameter. Reamers normally float, that is they follow the direction and location of the existing hole, but they can also be guided by bushings to slightly improve the hole's direction or location. The operation is most common with holes from 1/8 to 11/4 in (3 to 32 mm) in diameter but both smaller and larger holes can be reamed. A typical reamer is illustrated in Fig. 8.2 and the reaming operation on a lathe is shown schematically in Chapter 7 and Fig. 8.4. Taper reamers are used for finishing tapered holes. 8.2.5 Boring Boring - is an operation that enlarges and improves the accuracy of an existing hole. Either the work or the cutting tool rotates about the center axis of the hole. The single point tool describes a circle, removing material from the surface of the existing hole as it advances, enlarging the hole, normally increasing the precision of any of a number of factors. They are: its location, diameter, direction, cylindricity, and finish. When this operation is performed on a boring machine, the workpiece is stationary and the cutting tool rotates; when performed on a lathe, the workpiece rotates. On a lathe, the operation can then be considered to be internal turning. The tool spindle and the workpiece holder must be rigid enough to provide the desired accuracy in the bored hole. The operation is performed on holes from about 1/4 in (6 mm) in diameter and larger but is more common on larger holes, especially those too large to be drilled accurately, and for the machining of cast or forged large holes. Fig. 8.5 illustrates the process, and Fig. 7.2 shows it as one of a series of lathe operations. a b Figure 8.5. Boring operation. a) Boring operations slightly enlarge and improve the precision of an existing hole. b) A horizontal boring mill. This machine can perform boring, milling, and drilling operations. Jig boring - is performed on jig boring machines, which are vertical boring machines of very high accuracy. The table movement is extremely accurate and the spindle and spindle bearings are very precisely made. The machines are mainly used for making jigs, gages, dies, and fixtures, especially where accurate layout and hole location are essential. Horizontal boring mills - are basically large horizontal milling machines capable of performing boring, milling, and other machining operations on large and often complex parts. These units are sometimes called, horizontal boring and milling machines. The table can move in x and y directions. (Some machines have a table that also swivels.) The headstock that holds the spindle can be raised or lowered. The tool-holding spindle can move inward or outward. These machines are used in the machining of large components that have horizontal holes requiring the precision that boring provides. The machines normally include an end support column, opposite the spindle, for Chapter 8 3 long boring bars. Tolerances with the machines can be as low as one or two ten thousandths of an inch (0.003 to 0.005 mm). Fig. 8.5 b illustrates a horizontal boring mill. Vertical boring mills - are machines with a horizontal table rotating on a vertical axis, and a precision tool head (often two tool heads) capable of movements up and down and side to side (in and out radially). There may be more than one cross slide with tool-holding capability. These machines can be considered to be large lathes turned on end. They are especially suited to boring and other operations on parts too large for a conventional lathe. Workpieces are typically round and heavy with large diameters and shorter lengths. The workpiece is clamped to the rotating table, which can be as large as 40 ft. (12 m) in diameter. Both boring and facing are possible. There is no spindle for milling cutters; all cutting is by single point tools. Fig. 8.6 illustrates a typical vertical boring mill. Figure 8.6. A vertical boring mill. (vertical boring and turning machine.). 8.2.6 Gun drilling Gun drilling - is shown in Fig. 8.7. A rotating single-flute drill, normally carbide-tipped, is guided by a bushing at the start of the drilled hole and is self-guided thereafter by a bearing surface opposite the cutting edge, A hole through the whole length of the drill provides a means for oil coolant to flow at high pressure to the cutting edge and to flush chips from the hole. Deep, straight, holes are possible with the process which was originally developed for manufacture of gun barrels. Hole depths of over 250 times diameter are possible. Figure 8.7. A typical gun drill, viewed from the cutting end. Chapter 8 4  8.3 Drilling tools 8.3.1. Twist drill The twist drill (Figure 8.8) consists of the body with the drill point and the shank. Figure 8.8. Elements of the twist drill. While the drill point performs the actual metal removal, the body with the flute is engineered to remove the chips. The shank has to hold the drill in the drill spindle of the drilling machine. 8.4 Drilling machines Drilling machines are used for drilling holes, tapping, reaming and other general-purpose, small-diameter boring operations. Drilling machines are generally vertical, the most common type of which is a drill press. Its major components are shown in fig. 8.9. The workpiece is placed on an adjustable table, either by clamping it directly into the slots and holes on the table or by using a vice, which in turn can be clamped to the table. The workpiece should br properly clambed, for safety and accuracy, because the drilling torque can be high enough to rotate present rates. The drill is lowered manually by hand wheel or by power feed at preset rates. Manual feeding requires some skill in judging the appropriate feed rate. In order to maintain proper cutting speeds at the cutting edges of drills, the spindle speed on drilling machines has to adjustable to accommodate different sizes of drills. Ajustment are made by means of pulleys, gear boxes, or variable-speed motors. Drill presses are usually designated by the largest worpiece diameter that can be accommodated on the table. Size typically range from 150 mm to 1250 mm. Chapter 8 5  Figure 8.9. Schematic illustration of the components of a vertical drill press. Chapter 8 6  CHAPTER 9 MILLING 9.1 Definition Milling is defined as a metal cutting technology in which a multi-edged tool (multi-toothed cutter) removes the metal. During milling, the tool performs the cutting motion, whereas the workpiece (that is, the milling machine table on which the workpiece is mounted) executes the feed motion. The milling techniques are defined according to the tool axis position relative to the workpiece and according to the tool denomination. The axis of rotation of the cutter may be either horizontal or vertical. The cutter can provide cutting action on its side or at its end (face), or both. The cutter rotates rather rapidly and its position is normally stationary; the work moves past the cutter with a suitable depth of cut at a relatively low feed rate. Milling is the most common machining operation for producing flat surfaces, but slots, and contoured or stepped surfaces and screw threads can also be produced. A variety of milling operations and the cutters used are illustrated in Fig. 9.1. peripheral milling - The milled surface, if flat, is parallel to the axis of rotation of the cutter, and is produced by cutting teeth located on the periphery of the cutter body. The operation is usually performed on horizontal-spindle machines. The milling cutter or cutters are mounted on an arbor that has outboard support. The surface may be flat or contoured, depending on the profile of the cutter. Flat and contoured surfaces, slots, and key-ways, are machined by this method. (Fig. 9.1, in views a), b), f), and g), shows peripheral milling. Views c), d), and e) show both peripheral and face milling.) face milling - produces a flat surface at a right angle to the axis of rotation of the cutter. Depending on the depth of cut, some machining also takes place on the periphery of the cutter. For flat surfaces, face milling is generally preferable to peripheral milling from the standpoints of tool economy, simplicity of set-up, and cutter rigidity. However, the operation is limited to flat surfaces. end milling - uses a cutter, commonly of smaller diameter, with teeth on both the end (face) and periphery. Fig. 9.1, in views n) and o), illustrates the operation. The approach is versatile in that slots, recesses and profiles can be machined. Machining can also be carried out in areas not accessible to other types of cutters. However, the length-to-diameter ratio of end mills is high and they can be supported only at one end, so they are less rigid than cutters for other milling methods. Lighter feeds may be required to reduce cutter deflection. Material removal rates are less than with other milling methods and accuracy may not be as great. slab milling - is peripheral milling with cutters that produce a flat surface over a wide area. The axis of rotation of the cutter is parallel to the machined surface. The cutter often removes large amounts of material. Sometimes, two or more cutters are used per arbor with opposing helixes to balance cutting forces. See view b) of Fig. 9.1. form milling - When the peripheral cutting edges of the milling cutter are ground with a form rather than in a straight line, that form is transferred to the workpiece as the milling operation proceeds. The operation is called form milling and is illustrated schematically in Fig. 9.1, views i), k), 1) and m). Milling of gear teeth is a common application of this approach. gang milling - is simply milling with more than one cutter on the arbor of the milling machine. This produces multiple surfaces on the workpiece with one pass of the cutters. Also see straddle milling, as follows. straddle milling - involves the use of two cutters on one arbor with a space between them. Two surfaces are cut in one pass, but the area between them is not machined, as illustrated in Fig. 9.1, view q). fly cutter milling - involves the use of a singlepoint cutter rather than a multiple-tooth cutter to perform a milling operation. It is face milling with only one cutting tooth. The method is useful Chapter 9 1 for producing flat surfaces in a tool room situation where the optimum multiple-toothed cutter may not be available. Obviously, cutting feed rates are much less than with face mills but may be satisfactory when flycutters are the only tools available and requirements are for only one piece or a small quantity. Figure 9.1. A collection of milling cutters and the operations that they perform. Chapter 9 2  Figure 9.1. (Continued). pin routing - involves the use of a template to guide the movement of a high speed routing cutter (small diameter end mill). Typically, the process is used to blank flat stock of sheet metal or other materials. Stacks of thin material can be cut by this method, to produce multiple parts. spotfacing - is a simple operation, shown in Chapter 8 Fig. 8.3 view c) that is normally used to provide a small flat bearing surface, perpendicular to the axis of a hole, for a bolt head or nut. An endcutting rotating tool is fed into the workpiece along the axis of the bolt hole, often with a drill press rather than a milling machine. Depth of cut is often not critical as long as the surface machined is flat and perpendicular to the axis of the bolt hole. The operation is the same as counterboring except that the depth of cut is shallow, only enough to create a flat machined surface. It is most commonly performed on castings and forgings where the surface prior to the operation has some irregularit Chapter 9 3  9.2 Milling techniques 9.2.1 Peripheral milling Peripheral milling is a milling method which functions with horizontal tool axis. The cutting edges of the plain milling cutter are located at the tool’s periphery. Peripheral milling is subdivided into up- and down milling. 9.2.1.1 Up milling During up milling (Figure 9.2), the milling cutter rotates in a direction opposite to the feed direction of the workpiece. The feed motion direction (Figure 9.3) is characterised by the feed motion angle ϕ. If, over the course of a single tooth’s contact with the material (from the moment the tooth comes into contact with the material – tool entry - up to tool exit), ϕ remains less than 90°, then it is an up milling procedure. During up milling, workpiece material is removed by the resultant force. There is the risk that the workpiece may be pulled out of the mounting or that the milling table will buckle. Specially designed clamping jigs and undercuts in the table guide-ways avoid damage to the workpiece or tool. Figure 9.2. Figure 9.3. Up milling principle, inserted force Feed motion angle ϕ during peripheral direction relates to the workpiece milling in the up milling mode ( ϕ < 90°), illustrated velocities relate to the tool, ve effective cutting speed. 9.2.1.2 Down milling During down milling (Figure 9.4), the direction of milling cutter rotation is the same as the workpiece’s feed direction. The milling cutter approaches from the thickest part position of the chip. In down milling, the feed motion angle ϕ (Figure 9.5) ranges from 90° to 180°. The resultant force presses the workpiece against the base. In cases where the cutter arbour is insufficiently stiff, the milling cutter “climbs” onto the workpiece, and cutting edges break off. Figure 9.4. Figure 9.5. Down milling principle, marked Feed motion angle ϕ during peripheral force direction is related to the milling in the down milling mode (ϕ > workpiece 90°), recorded velocities relate to the tool. Chapter 9 4  During down milling the resultant force direction coincides with the feed motion direction. Thus, if the feed screw experiences backlash, the resultant force makes the lead-bearing flank at the feed screw changes at each start of the cut. Milling machines for down milling should have a feed drive with no backlash, cutter arbours and frame components of high stiffness. 9.2.2 Face milling In face milling the tool axis is orthogonal to the surface to be generated. However, in face milling, the tool does not only cut with its face, as the name of the method indicates, but, as in peripheral milling, removes metal primarily with the peripheral cutting edges. The face cutting edges act as secondary cutting edges and smooth the milled surface (Figure 9.6). As a result, face milled surfaces have a high surface quality. During face milling, down- and up milling procedures Figure 9.6. are carried out alternately. At the beginning of the cutting Face milling principle. procedure, the direction of rotation is opposite to the feed direction of the workpiece. However, starting from the middle of the workpiece (Figure 9.7), the procedure merges into down milling. Alternate metal cutting by down- and up milling is able to compensate as much as possible for deviations of the cutting force and thus to relieve the cutting edges of load. Consequently face milling allows for high metal removal rates. Figure 9.7. If, during face milling, work is done with a feed motion Alternate down- and up milling angle ϕA > 0 , then, when starting the cut, sufficient sectional during face milling. areas of the chip are available, and the blades of the mill clutch the chip at once and cut it off without first sliding. 9.2.3 Form milling Form milling is the name for a milling procedure carried out with milling cutters whose shape corresponds to the finished contour to be generated (Figure 9.1 h, i, k, l, m). If it is impossible to generate a specific workpiece geometry with one cutter of the former type, then it is common practice to put together several milling cutters (Figure 9.8) in a set, called a gang cutter. Figure 9.8. Gang cutter (with 6 elements), 1 spacing collars, 2 peripheral milling cutter, 3 staggered- tooth side and face milling cutter, 4 peripheral milling cutter, 5 angle cutter, 6 cutter arbour. Chapter 9 5  Form milling also implies thread milling, because milling cutters corresponding to the thread profile are used. The following tools are distinguished: Long-thread milling In long-thread milling (Figure 9.9) a disk-shaped profile milling cutter (cutter of the former type) penetrates the workpiece. The long-thread milling machine with feed gear system and lead screw generates the longitudinal feed of the milling cutter. Here the workpiece may rotate either in the same direction as or in the opposite direction from the milling cutter (down- or up milling). Figure 9.9. Tool- and workpiece configuration during long-thread milling. Short-thread milling In short-thread milling, the roller-shaped milling cutter penetrates the workpiece to its full depth, whereas the workpiece rotates around 1/6 of its circumference. The thread to be milled is finished after 1¼ workpiece revolutions. 9.2.4 Groove milling Grooves are cut out with end milling cutters or side and face milling cutters. Depending on the procedure that takes place when generating a groove, these methods are classified as given below: 9.2.4.1 Plunge milling to generate grooves At the beginning of plunge milling (Figure 9.10), the end mill cutter cuts down to the full groove depth like a twist drill. Subsequently the whole length of the groove is machined in one cut. Due to the large depth of immersion of the mill, one can only set up small longitudinal feed values here. Figure 9.10. Plunge milling principle 1 mill to depth, 2 milling feed in longitudinal direction, 3 move out tool 9.2.4.2 Line milling to generate grooves With this method, the depth of a groove is machined using stepwise metal removal line by line rather than in one step. The end mill cutter penetrates the workpiece only shallowly and then mills the groove to its full length. In the final position, the milling cutter cuts slightly deeper. Then it goes on milling the groove to its full length in the opposite feed direction. This cycle is repeated until (Figure 9.11) the desired depth of groove is achieved. Due to the low downfeed in each step, in this technique, one can set up higher longitudinal feed values. Chapter 9 6  Figure 9.11. Principle of line milling to generate grooves 9.2.4.3 Groove milling with side and face milling cutter Continous or through going slots or grooves with a large exit (e.g. for multi-spline profiles) are mostly cut with a disk-shaped plain milling cutter. The chip metal removal per unit of time is greater than that achieved with the methods described under 9.2.4.1 and 9.2.4.2. Figure 9.12. Principle of groove milling with side and face milling cutter. 9.3 Application of the milling techniques 9.3.1 Peripheral milling It is impossible to achieve excellent surface qualities due to the very disadvantageous cutting conditions (uneven sectional area of chip) during peripheral milling. Consequently, peripheral milling is primarily used for cutting smaller surfaces and to shape profiles with the cutter gang (Figure 9.8). Peripheral milling in conjunction with face milling is advantageously applied as face side milling even to create shouldered surfaces (Figure 9.13). When using machines with the appropriate design, down milling generates better surface qualities than up milling. Figure 9.13. 1 contour generated with shell end mill DIN 841 2 groove generated with side and face milling cutter Chapter 9 7  9.3.2 Face milling Face milling is used to generate plane surfaces. In face milling, cutter heads tipped with inserted cemented carbide tips are used at present. A general rule of thumb holds that face milling takes priority over peripheral milling. 9.3.3 Form milling Formed surfaces with specific contours like radiuses, prisms, angles for dovetail slides etc. are created with form milling. Gang cutters are applied to produce contours with different form profiles. Thread milling, long-thread milling with profile milling cutters and short-thread milling with profile-plain milling cutters are special form milling variants. Form milling can also be applied to mill toothed gears with the single pitch technique. 9.3.4 Groove milling Groove milling is defined as a method to generate grooves limited in length; e.g. grooves for feather keys according to DIN 6885, or continuous grooves, e.g. of multi-splined profiles for splineshafts according to DIN 5461. 9.4 Accuracies achievable with milling 9.5 Milling machines Milling machines are classified as especially dangerous machines. In addition to the normal requirements of the Health and Safety at Work Act, these machines are also subject to the Horizontal Milling Machine Regulations. Copies of these Regulations are available in the form of a wall chart which is supposed to be hung up near to where such machines are being used. 9.5.1. The horizontal spindle milling machine The horizontal milling machine gets its name from the fact that the axis of the spindle of the machine, and therefore the axis of the arbor supporting the cutter, lies in a horizontal plane as shown in Fig. 9.14. The more important features and controls are also named in this figure. Basic movements and alignments of a horizontal spindle milling machine The basic alignments and movements of a horizontal milling machine are shown in Fig. 9.15. The most important alignment is that the spindle axis, and therefore the arbor axis, is parallel to the surface of the worktable. The depth of cut is controlled by raising the knee and table subassembly. The position of the cut is controlled by the cross-slide and the feed is provided by a lead screw and nut fitted to the table and separately driven to the spindle. Unlike the feed of a lathe which is directly related to the spindle speed and measured in mm/rev, the feed of a milling machine table is independent of the spindle speed and is measured in mm/min. Chapter 9 8  Figure 9.14. Horizontal spindle milling machine Figure 9.15. Horizontal spindle milling machine: movements and alignments. 9.5.2. The vertical spindle milling machine The vertical milling machine gets its name from the fact that the axis of the spindle of the machine, and therefore the axis of the cutter being used, lies in the vertical plane as shown in Fig. 9.16. The more important features and controls are also named in this figure. Basic movements and alignments of a vertical spindle milling machine The basic alignments and movements of a vertical milling machine are shown in Fig. 9.17. The most important alignment is that the spindle axis, and therefore the cutter axis, is perpendicular to the surface of the worktable. The depth of cut is controlled by raising the knee and table subassembly or, for some operations raising or lowering the spindle. For maximum rigidity, the spindle is normally raised as far as possible. The position of the cut is controlled by the cross-slide and the feed is provided by a lead screw and nut fitted to the table and separately driven to the Chapter 9 9 spindle. As for horizontal milling, the feed of a vertical milling machine table is independent of spindle and is measured in mm/min. Figure 9.16. Vertical spindle milling machine Figure 9.17. Vertical spindle milling machine: movements and alignments. Chapter 9 10  9.6 Milling cutters There is a fundamental difference between pointed tooth- and round cutting edges. The pointed tooth milling cutter edge (Figure 9.18) is generated by milling, whereas the rounded cutting edge form is made by relieving (the shape of a logarithmic helix). The standard milling cutter is pointed tooth. It is used for almost all milling tasks. Figure 9.18. Cutting edge forms on milling cutters a) Tooth form of the pointed tooth mill, b) Tooth form of the relieved mill Only cutters of the former type are relieved milling cutters. Pitch, tooth height and tooth fillet form the tooth space that collects the removed chips. 9.6.1 Horizontal milling machine cutters Figure 9.19 shows some different shapes of milling cutter and the surfaces that they produce. When choosing a milling cutter you will have to specify: Figure 9.19. Horizontal milling machine cutters and the surfaces they produce: (a) slab milling cutter (cylinder mill); (b) side and face cutter; (c) single-angle cutter; (d) double equal-angle cutter; (e) cutting a V- slot with a side and face mill; (f) double unequal-angle cutter; (g) concave cutter; (h) convex cutter; (i) single and double corner rounding cutters; (j) involute gear tooth cutter. Chapter 9 11  The bore of this must suit the arbor on which the cutter is to be mounted. In many workshops one size of arbor will be standard on all machines and all the cutters will have the appropriate bores. The diameter of the cutter. The width of the cutter to suit the work in hand. The shape of the cutter. The tooth formation. 9.6.2 Vertical milling machine cutters A selection of milling cutters suitable for a vertical milling machine is shown in Fig. 9.20 and some typical applications are shown in Fig. 9.21. Note that only slot drills can be used for making pocket cuts from the solid. All the other cutters have to be fed into the workpiece from its side as they cannot be fed vertically downwards into the work. When choosing a cutter you will need to specify: The diameter of the cutter. The length of the cutter. The type of cutter. The type of shank. Some cutters have solid shanks integral with the cutter for holding in a chuck, whilst other cutters are made for mounting on a separate stub arbor. Some large face milling cutters are designed to bolt directly onto the spindle nose of the machine. Figure 9.20. Typical milling cutters for vertical spindle milling machines Chapter 9 12  Figure 9.21. Vertical milling machine cutters and the surfaces they produce: (a) end milling cutter; (b) face milling cutter; (c) slot drill; (d) recess A would need to be cut with a slot drill because it is the only cutter that will work from the centre of a solid; recess B could be cut using a slot drill or an end mill because it occurs at the edge of the solid; (e) this blind keyway would have to be sunk with a slot drill; (f) dovetail (angle) cutter; (g) T-slot cutter; (h) Woodruff cutter Chapter 9 13  CHAPTER 10 PLANING AND SLOTTING 10.1 Definition Planing is a cutting technology in which the workpiece is machined in slices with a single blade tool, the planing tool. Planing can be understood as a turning procedure that can be performed with an infinitely large diameter; that is, the cutting motion moves along a straight line. 10.2 Planing- and slotting methods 10.2.1 Shaping During shaping, cutting and infeed motions are carried out by the tool, while the feed motion is performed by the worktable. Maximal shaping length (as a rule less than 1 m) is determined by the maximum stroke of the shaping machine (Figure 10.1). Figure 10.1. Hydraulically driven shaping machine with clamped planing tool. 10.2.2 Slotting Slotting is a variant of planing in which the single blade tool (Figure 10.2) carries out the cutting motion vertically, and the workpiece performs the infeed motion. Chapter 10 1  Figure 10.2. Slotting machine. 10.3 Application of the techniques 10.3.1 Shaping is used for planing of plates and moldings with straight-lined boundaries for tooland die- making and mechanical engineering. Copying attachments can be used to generate sculptured surfaces strip by strip. 10.3.2 Slotting (vertical planing) is applied to manufacture inner contours in tool elements and wheels, such as (Figure 10.2) keyways in holes of toothed gears. It is also possible to use the slotting technique to machine breakthroughs in blanking dies. 10.4 Accuracy values achievable with planing The accuracy values that can be achieved with planing range from IT 7 to IT 8. Figure 10.3 Cutting parameters for planing l workpiece length, Bw workpiece width, f feed, ap depth of cut, κ tool cutting-edge angle Chapter 10 2  CHAPTER 11 BROACHING 11.1 Definition Broaching is a metal cutting technique with a multi-edged tool in which the tool performs the cutting motion. This method functions without any feed motion due to the offset of the cutting teeth on the broaching tool. The material is removed in one stroke (pulling or pushing) with the broaching tool, called the broach. 11.2 Broaching methods In broaching, two working procedures – internal- and external broaching - are distinguished 11.2.1 Internal broaching During internal broaching, the broaching tool is brought into the premachined opening of the workpiece. Working motion then begins, whereby the broach, equipped with many cutting blades, is pulled or pushed through. The broach contour (square, hexagon, etc.) is generated in the workpiece opening. Figure 11.1 elucidates the configuration of workpiece and tool during broaching. Figure 11.1 Broaching principle. 11.2.2 External broaching During external broaching, the broaching tool finishes a premachined outer workpiece contour, such as the opening of a forged open-end wrench. 11.3 Application of the broaching techniques 11.3.1 Internal broaching Internal broaching is applied to generate openings of different shapes. Thus, for example, serrations, taper bushings for splines, and spline profiles for movable gears are created with this method. Some typical examples are seen in Figure 11.2. As a rule, broaching is used if high accuracy to shape and size is demanded in addition to high surface quality. For this reason, broaching is sometimes used instead of reaming to generate holes. Broaching is an economical procedure since it is possible to produce very sophisticated geometries that require no further reworking very quickly and with a single stroke. The generation of a spline profile in internal broaching is illustrated in Figure 11.3. Chapter 11 1  Figure 11.2. Broaching profiles for internal broaching. Figure 11.3. Generation of an internal spline during internal broaching a - before; b and c – during; d – at the end of the broaching procedure. Figure 11.4. Broaching profiles to be produced by external broaching. Chapter 11 2  11.3.2 External broaching External broaching is the method used for the generation of external profiles. However, this method is also used for the manufacturing of shaped grooves, such as Christmas tree-shaped grooves (Figure 11.4c), in which the turbine blades are mounted in turbine wheels. External broaching is also applied for machining of external teeth (Figure 11.4b) and guiding surfaces (Figure 11.4d), as well as guide grooves and similar items. Figure 11.4 shows some typical workpieces for external broaching. 11.4 Achievable accuracy values 11.4.1 Accuracy to size The accuracy values that can definitely be achieved with internal- and external broaching range from IT 7 to IT 8. However, with more effort, it is also possible to achieve IT 6. 11.4.2 Surface quality The final finishing tooth that cuts in the offset at a depth of h = 0,01 mm has a substantial effect on surface quality. Furthermore, reserve teeth are included in internal broaching. These teeth improve the surface by regrooving and shaving. During the generation of profiled surfaces with all the teeth of a broaching tool or of straight surfaces by broaching tools with lateral offset, the surface is influenced by the minor cutting edges of these tools. The surface roughness values Rt that can be achieved during broaching of mild steels range from Rt = 6,3 to 25 µm High surface qualities can also be achieved in the case of easily broached free cutting steels and materials for casting. Acceptable broaching results may also be expected from case hardening- and tempering steels, if a homogeneous ferrite-, perlite distribution is available for normally annealed material. 11.5 Broaching tools Figure 11.5. Broach components l1 shank, a1 pilot, a2 cutting portion, a3 pilot, l2 rear support, L total length detail X see Figure 11.6. Chapter 11 3  11.5.1 Broach – blade geometry On broaches, rake angle and tool orthogonal clearance (Figure 11.6) have the same effect as on the turning tool. Rake face chamfers reinforce the wedge and only slightly diminish the positive properties of greater rake angles. Due to complicated grinding, broaches are generally made without any rake face chamfers. Flank wear lands with a land angle of the flank from 0° to 0,5º and a land width of 0,5 mm have only finishing- and calibrating teeth. Only small land angles of the flank with small land width are selected in order to maintain accuracy to size of the broach even in the case of repeated re- sharpening. Figure 11.6. Cutting edge geometry of a broach according to Figure 11.5, at the bottom right, cutting edges of a broach with inclined blades. Figure 11.7. Formation of the tooth space. 11.5.2. Broaches - design types A variety of broach forms is given in Figure 11.8. For difficult profile types, the external broach is composed of several cutting portions. Chapter 11 4  Figure 11.8. Broach types for internal- and external broaching. Chapter 11 5  CHAPTER 12 GRINDING 12.1 Definition Grinding is a metal cutting procedure in which a multi-edged tool, whose cutting edges are geometrically undefined, removes the chips. During grinding, the tool carries out the cutting motion. The cutting speeds commonly used in grinding are approximately 20 times those used in turning (25 to 45, sometimes up to 120 m/s). The feed movement is executed as a function of the cutting technique, the tool or the workpiece. The grinding techniques are categorised according to the workpiece shape - in face- and cylindrical grinding, or according to component mounting - as grinding between centres or centreless grinding. It would also make sense to further subdivide these methods according to their ranges of application, such as grinding of slide ways or tools. At the point where the cutting takes place, grinding is very similar to other machining operations, the difference being that the workpiece is cut by the sharp edges of small pieces of abrasive material, rather than the edge of a hardened steel or carbide cutting tool. The irregularly- shaped abrasive particles may be bonded to a wheel or coated belt, or may be used loose. The particles commonly consist of aluminum oxide, silicon carbide, cubic boron nitride, diamond, or other hard materials. The individual abrasive grains are each smaller than a conventional metalworking cutting tool, and the grains on a typical wheel make a multitude of minute cuts. Fig.12.1 illustrates the grinding process schematically (Some grains, depending on their shape, do not cut but instead rub or slightly deform the surface of the workpiece.) Cutting speeds are high but the depth of cut from each grain is shallow. A water or water-oil emulsion is often sprayed on the wheel and workpiece to control the dust that otherwise arises and to overcome the heating effect of the operation. Grinding wheels are often porous, especially those designed for use with softer materials. Figure 12.1. The grinding process. Sharp edges of individual abrasive grains act as minute cutting tools, removing small amounts of material from the workpiece. Chapter 12 1  As the wheel cuts, it wears, causing some abrasive particles to become smooth but causing others to fracture, exposing new sharp edges. New wheels, and those that have become worn, are dressed with a diamond tool that removes some of the abrasive material and bonding agent, exposing sharp edges of new abrasive grains and providing a straighter, more uniform, cutting surface. Grinding is most commonly a finish-machining operation to provide a smoother surface or greater dimensional accuracy, particularly with hardened materials. When used as the primary metal removal method, the term, abrasive machining is often used. Figure 12.2. (a) Grinding chip being produced by a single abrasive grain. (A) chip, (B) workpiece, (C) abrasive grain. Note the large negative rake angle of the grain. The inscribed circle is 0.065 mm in diameter. (b) Schematic illustration of chip formation by an abrasive grain with a wear flat. Note the negative rake angle of the grain and the small shear angle. The grinding techniques are categorised according to the workpiece shape - in face- and cylindrical grinding, or according to component mounting - as grinding between centres or centreless grinding. It would also make sense to further subdivide these methods according to their ranges of application, such as grinding of slide ways or tools. 12.2 Grinding techniques 12.2.1 Flat grinding Plane or flat grinding is the grinding of plane surfaces. During flat grinding, the tool performs the cutting motion, whereas the workpiece executes the feed motion. The grinding procedure can be performed by the circumference or face of the grinding tool. Consequently, the following types are distinguished: 12.2.1.1 Circumferential grinding In circumferential grinding (Figure 12.3), the wheel spindle is in horizontal position. The machine table with the workpiece travels back and forth in a straight line. As a rule, the lateral feed per stroke is carried out by the table. Machines with a rotary table are an alternative. In these machines, the workpiece moves in a circle on a face chuck, and the lateral feed is performed by the grinding tool. Since the grinding wheel contacts the workpiece only on a small portion of its circumference during circumferential grinding, the metal removal rate is limited for these methods. Using special wheels and appropriate machines, the full-width grinding method is competitive with milling. Chapter 12 2  Figure 12.3. Face - and profile grinding Figure 12.4. Face grinding principle machine with vertical wheel spindle 12.2.1.2 Face grinding In face grinding, the grinding procedure is carried out with the front end of the grinding wheel (Figure 12.4). During face grinding, the grinding wheel (designed as segmented grinding wheel or as a ring wheel) performs the cutting motion, whereas In contrast to circumferential grinding, the contact area between workpiece and tool is much greater in face grinding. Consequently, this method makes it possible to achieve higher metal removal rates. In face grinding, the tool axis may be vertical (Figure 12.4) or horizontal (in case of larger machines, see Figure 12.5). Due to their compact design and great cutting capacity, machines with vertical wheel spindle axis are predominantly used for face grinding. Machines with horizontal wheel spindle axis are used only if the surface pattern is decisive, usually just for appearance’s sake, such as in profile grinding operations (Figure 12.5). Figure 12.5. Segmented- surface grinding machine with horizontal wheel spindle axis The face grinding procedures are distinguished according to the surface pattern generated (Figure 12.6): In cross grinding K, the grinding contours cross each other, whereas in arc grinding S, the grinding contours are allocated radially at one side. The mutually crossing grinding contours in cross grinding are generated if the wheel spindle axis is located normally to the workpiece. The radial allocation in arc grinding is created if the wheel spindle axis is inclined towards the workpiece. Chapter 12 3  Figure 12.6. Grinding patterns in face grinding a) Cross grinding K if wheel spindle axis is normal to workpiece. b) Arc grinding S if spindle axis is inclined towards the workpiece. 12.2.1.3 Profile grinding Profile grinding is a circumferential grinding method carried out with profiled grinding wheels. In this procedure, as a rule, lateral feed is inapplicable. There are two common methods used to profile grinding wheels. Simple profiles like radiuses, angles and grooves are generated with the common dressing attachments. Complicated profiles are created with the so-called diaform attachment. This attachment is used to profile the grinding wheel along a template following the copying principle. Making use of CNC equipment, dressing and profiling are more and more being implemented by means of controlled motions. Figure 12.7. Schematic illustrations of various surface grinding operations. (a) Traverse grinding with a horizontal-spindle surface grinder. (b) Plunge grinding with a horizontal-spindle surface grinder, producing a groove in the workpiece. (c) A vertical-spindle rotary-table grinder (also known as the Blanchard type). 12.2.2 Cylindrical grinding Cylindrical grinding refers to the grinding of rotary parts. In machining, a distinction is made between grinding from the outside (grinding the outer diameter of a shaft) and from the inside (grinding of a hole). Another distinctive feature is the type of workpiece mounting, for example, whether the workpiece is held with or without a centre. Centreless grinding is explained in Chapter 12.2.4. 12.2.2.1 External cylindrical grinding During external cylindrical grinding, the wheel performs both the cutting- and die infeed motion. The workpiece, which is fixed between centres or clamped in the chuck, is brought into rotation by a driving plate. Grinding wheel and workpiece have the same direction of rotation. Chapter 12 4  Figure 12.8. The types of workpieces and operations typical of grinding: (a) cylindrical surfaces, (b) conical surfaces, (c) fillets on a shaft, (d) helical profiles, (e) concave shape, (f) cutting off or slotting with thin wheels, and (g) internal grinding. 12.2.2.1.1 External cylindrical grinding with longitudinal feed In grinding with longitudinal feed (Figure 12.9), as a rule, the table of the cylindrical grinding machine, and thus the workpiece, performs the longitudinal feed. It is necessary to harmonize longitudinal feed and workpiece speed. If selecting longitudinal feed is set too high, the result is spiral-like markings on the workpiece. A clean grinding pattern is obtained if feed s per workpiece rotation is less than grinding wheel width B. Thin shafts may only be ground with small depths of cut due to the risk of deflection. For thick shafts, infeed is limited by the machine’s input power. Too high depths of cut lead to greater contact areas between workpiece and wheel. Consequently, they result in increased cutting forces. For this reason, extreme infeed values may result in grinding wheel fracture. To work with greater depths of cut, decrease longitudinal feed. Figure 12.9. External cylindrical Figure 12.10. Plunge grinding – grinding with longitudinal feed – working principle. working principle. 12.2.2.1.2 Plunge grinding In plunge grinding (also plunge-cut grinding, see Figure 12.10), there is no longitudinal feed. The grinding wheel only performs the motion for depth setting. This method is needed, for example, to grind chamfers of shafts. For the infeed amount, the same criteria as for external cylindrical grinding with longitudinal feed are valid. Chapter 12 5  12.2.2.1.3 Thread grinding Thread grinding is defined as cylindrical grinding with profiled grinding wheels. In this method as well, a distinction is made between longitudinal grinding (grinding with longitudinal feed of the workpiece) and plunge grinding. During thread grinding with longitudinal feed, the thread can be generated with a “single- edged” wheel or a “multi-edged” wheel. The narrow single-edged wheel, which has the profile of the thread to be generated (Figure 12.11), has a width of 6 to 8 mm. The width of the multi-edged wheel is about 40 mm. This wheel is dressed conically. The threads (grooves) of the grinding wheel that first come into contact with the profile rough-grind it, while the two threads at the end (Figure 12.12) finish-grind it. This way, the entire chip removal is distributed over several grooves of the grinding wheel. This reduces the load per groove. For this reason, multi-edged wheels have a longer tool life than single-edged wheels. Since the multi-edged wheel (Figure 12.12) is dressed conically, it is impossible to grind a thread directly on a shoulder with this wheel. As a result, this wheel can only be used for through threads. Single-edged wheels are preferred to generate exact threads, since with these wheels one can achieve accuracy values of ± 2 µm for the effective diameter and ± 10 angular minutes for the thread angle. Figure 12.11. Longitudinal grinding Figure 12.12. Longitudinal grinding of a thread with singleedged wheel. of a thread with multiedged wheel. During thread -plunge grinding (Figure 12.13), the thread is generated with a multiedge grinding wheel. Here, the grinding wheel is dressed in parallel. During plunge grinding, as in the milling of short threads, the workpiece performs only 11/6 rotation. On each side, the grinding wheel should be about 2 mm wider than the thread to be generated. For internal thread grinding, the same conditions as external thread grinding are valid; however, the grinding wheel diameters are correspondingly smaller in this case. Depending on workpiece size, they range from 20 to 150 mm. During thread grinding, workpiece and grinding wheel have the same rotation direction. A motion for depth setting, which is executed by the grinding wheel, only exists in plunge grinding. For thread grinding, the grinding result depends to a great extent on selecting an adequate wheel. The range of grain sizes (80 to 600) is the same for all leads, and the choice depends only on the minor thread radius. Chapter 12 6  Figure 12.13. Plunge-thread grinding with multi-edged wheel 12.2.2.2 Internal cylindrical grinding Internal cylindrical grinding (Figure 12.14) corresponds to external cylindrical grinding in terms of its main criteria. Figure 12.14. Internal cylindrical Figure 12.15. Contact length l of the grinding –principle view 1 grinding grinding wheel in workpiece d wheel, 2 workpiece, 3 three-jaw workpiece diameter in mm, D chuck. grinding wheel diameter in mm The contact area between workpiece and wheel (Figure 12.15) is greater. The contact length l depends on depth of cut a and the diameter ratio between grinding wheel and workpiece. Cutting motion, longitudinal feed and the motion for depth setting are carried out by the workpiece. In internal grinding, the cutting speeds that are optimal for grinding can generally not be reached due to the small grinding wheel diameter. Optimal conditions are obtained when the following are selected D ≈ 0,8 d D in mm wheel diameter d in mm diameter of the workpiece hole. Chapter 12 7  12.2.3 Cutting data for flat grinding and cylindrical grinding with clamped workpiece The depth of cut a (infeed e of the grinding wheel) chosen depends on the wheel’s grain size and the dimensions of the workpiece that is to be ground. Coarse-grained wheels allow greater depths of cut than fine-grained ones. Also, when fine-grained wheels are used, the pores clog more quickly. When this occurs, the wheel no longer cuts, but rather squeezes and lubricates. The general rule for common grinding is: “Depth of cut must be less than the height of the abrasive grains protruding out of the bonding.” In full-width grinding, this rule is broken. This is made possible by open-porous wheels of special design. In finishing, the following must be observed: 1. The speed of the grinding wheel must be kept high and that of the workpiece low, if excellent surface quality is required; 2. Sparking emanating from the grinding wheel means that the wheel needs to be guided over the workpiece without infeed until no sparking no longer appears; 3. Reversal of the longitudinal feed must be adjusted so that the grinding wheel travel exceeds the workpiece only by one third of its width (1/3 B); otherwise the workpiece dimensions will be smaller than specified. 12.2.3.1 Grinding wheel speed, workpiece speed Both speeds v and vw should be in a predefined mutual speed ratio q. vc q= vw q speed ratio vc in m/s cutting speed of the grinding wheel (peripheral speed) vw in m/s peripheral speed of the workpiece For the corresponding q values of different materials, see Table 12.1. Table 12.1 Speed ratio q for different materials Material q Steel 125 Grey cast iron 100 Ms and Al 60 12.2.4 Centreless grinding Centreless grinding is a grinding procedure in which the workpiece is located freely on a guide bar, in contrast to external- or internal cylindrical grinding in which the workpiece is between centres or clamped in a chuck (Figure 12.16). The workpiece rotation is generated through frictional resistance between the grinding- and regulating wheels. The axes of both wheels are located horizontally in one plane. The workpiece centre is situated above the connecting line of grinding- and regulating wheel centre. The 3 major elements for centreless grinding are: grinding wheel regulating wheel workpiece seat The workpiece seat is made of steel. It is hardened or equipped with a cemented carbide bar. Chapter 12 8  Figure 12.16. Centreless grinding principle 12.3 Application of grinding techniques 12.3.1 Flat grinding Flat grinding is applied to generate plane-parallel and profiled surfaces. Typical parts with plane-parallel surfaces are die blocks for cutting dies, die shoes for pressand draw dies, clutch lamellae, rings of different design (Figure 12.17) and many other machine elements. Grinding of external splines and punches with gear-tooth profiles, as well as grinding of profiled tools with complicated profiles from solids, are examples of uses of profile grinding. Figure 12.17. Flat grinding machine with rotary table. 12.3.2 Cylindrical grinding Both external- and internal cylindrical grinding are used to machine rotary parts of any design (Figure 12.18). Figure 12.18. Examples of external cylindrical grinding Chapter 12 9  12.4 Achievable accuracy values and allowances during grinding Table 12.2 Allowances and achievable accuracy values As a general rule: The greater the machining diameter or the machining thickness and the longer the workpiece, the higher the allowance. The allowances are valid for unhardened workpieces. For hardened workpieces, increase the values from the tables by 20–40%. Chapter 12 10  CHAPTER 13 GEAR MANUFACTURING 13.1 Definition Gear Cutting Most gear-cutting processes can be classified as either forming or generating. In a forming process, the shape of the tool is reproduced on the workpiece; in a generating process, the shape produced on the workpiece depends on both the shape of the tool and the relative motion between the tool and the workpiece during the cutting operation. In general, a generating process is more accurate than a forming process. 13.2 Form cutting In the form cutting of gears, the tool has the shape of the space between the teeth. For this reason, form cutting will produce precise tooth profiles only when the cutter is accurately made and the tooth space is of constant width, such as on spur and helical gears. A form cutter may cut or finish one of or all the spaces in one pass. Single-space cutters may be disk-type or end-mill-type milling cutters. In all singlespace operations, the gear blank must be retracted and indexed, i.e., rotated one tooth space, between each pass. Single-space form milling with disk-type cutters is particularly suitable for gears with large teeth, because, as far as metal removal is concerned, the cutting action of a milling cutter is more efficient than that of the tools used for generating. Form milling of spur gears is done on machines that retract and index the gear blank automatically. For the same tooth size (pitch), the shape (profile) of the teeth on an involute gear depends on the number of teeth on the gear. Most gears have active profiles that are wholly, partially, or approximately involute, and, consequently, accurate form cutting would require a different cutter for each number of teeth. In most cases, satisfactory results can be obtained by using the eight cutters for each pitch that are commercially available. Each cutter is designed to cut a range of tooth numbers; the no. 1 cutter, for example, cuts from 135 teeth to a rack, and the no. 8 cuts 12 and 13 teeth. Figure 13.1. Nomenclature for an involute spur gear. Chapter 13 1  Figure 13.2. (a) Producing gear teeth on a blank by from cutting. (b) Schematic illustration of gear generating with a pinionshaped gear cutter. (c) Schematic illustration of gear generating in a gear shaper using a pinionshaped cutter. Note that the cutter reciprocates vertically. (d) Gear generating with rackshaped cutter. 13.3 Gear generating In a gear generating machine, the generating tool can be considered as one of the gears in a conjugate pair and the gear blank as the other gear. The correct relative motion between the tool arbor and the blank arbor is obtained by means of a train of indexing gears within the machine. One of the most valuable properties of the involute as a gear-tooth profile is that if a cutter is made in the form of an involute gear of a given pitch and any number of teeth, it can generate all gears of all tooth numbers of the same pitch and they will all be conjugate to one another. The generating tool may be a pinion-shaped cutter, a rack-shaped (straight) cutter, or a hob, which is essentially a series of racks wrapped around a cylinder in a helical, screwlike form. On a gear shaper, the generating tool is a pinion-shaped cutter that rotates slowly at the proper speed as if in mesh with the blank; the cutting action is produced by a reciprocation of the cutter parallel to the work axis. These machines can cut spur and helical gears, both internal and external; they can also cut continuous-tooth helical (herringbone) gears and are particularly suitable for cluster gears, or gears that are close to a shoulder. On a rack shaper the generating tool is a segment of a rack that moves perpendicular to the axis of the blank while the blank rotates about a fixed axis at the speed corresponding to conjugate action between the rack and the blank; the cutting action is produced by a reciprocation of the cutter parallel to the axis of the blank. Since it is impracticable to have more than 6 to 12 teeth on a rack cutter, the cutter must be disengaged from the blank at suitable intervals and returned to the starting point, the blank meanwhile remaining fixed. These machines can cut both spur and helical external gears. A gear-cutting hob (Fig. 13.3) is basically a worm, or screw, made into a generating tool by cutting a series of longitudinal slots or “gashes” to form teeth; to form cutting edges, the teeth are “backed off,” or relieved, in a lathe equipped with a backing-off attachment. A hob may have one, two, or three threads; on involute hobs with a single thread, the generating portion of the hob-tooth Chapter 13 2 profile usually has straight sides (like an involute rack tooth) in a section taken at right angles to the thread. Figure 13.3. Schematic illustration of three views of gear cutting with a hob. In addition to the conjugate rotary motions of the hob and workpiece, the hob must be fed parallel to the workpiece axis for a distance greater than the face width of the gear. The feed, per revolution of the workpiece, is produced by the feed gears, and its magnitude depends on the material, pitch, and finish desired; the feed gears are independent of the indexing gears. The hobbing process is continuous until all the teeth are cut. The same machines and the same hobs that are used for cutting spur gears can be used for helical gears; it is only necessary to tip the hob axis so that the hob and gear pitch helices are tangent to one another and to correlate the indexing and feed gears so that the blank and the hob are advanced or retarded with respect to each other by the amount required to produce the helical teeth. Some hobbing machines have a differential gear mechanism that permits the indexing gears to be selected as for spur gears and the feed gearing to be chosen independently. The threads of worms are usually cut with a disk-type milling cutter on a thread-milling machine and finished, after hardening, by grinding. Worm gears are usually cut with a hob on the machines used for hobbing spur and helical gears. Except for the gashes, the relief on the teeth, and an allowance for grinding, the hob is a counterpart of the worm. The hob and workpiece axes are inclined to one another at the shaft angle of the worm and gear set, usually 90°. The hob may be fed in to full depth in a radial (to the blank) direction or parallel to the hob axis. Although it is possible to approximate the true shape of the teeth on a straight bevel gear by taking two or three cuts with a form cutter on a milling machine, this method, because of the taper of the teeth, is obviously unsuited for the rapid production of accurate teeth. Most straight bevel gears are roughed out in one cut with a form cutter on machines that index automatically and then finished to the proper shape on a generator. The generating method used for straight bevel gears is analogous to the rack-generating method used for spur gears. Instead of using a rack with several complete teeth, however, the cutter has only one straight cutting edge that moves, during generation, in the plane of the tooth of a basic crown gear conjugate to the gear being generated. A crown gear is the rack among bevel gears; its pitch surface is a plane, and its teeth have straight sides. Chapter 13 3  The generating cutter moves back and forth across the face of the bevel gear like the tool on a shaper; the “generating roll” is obtained by rotating the gear slowly relative to the tool. In practice two tools are used, one for each side of a tooth; after each tooth has been generated, the gear must be retracted and indexed to the next tooth. Figure 13.4. (a) Cutting a straight bevel-gear blank with two cutters. (b) Cutting a spiral bevel gear with a single cutter. The machines used for cutting spiral bevel gears operate on essentially the same principle as those used for straight bevel gears; only the cutter is different. The spiral cutter is basically a disk that has a number of straight-sided cutting blades protruding from its periphery on one side to form the rim of a cup. The machines have means for indexing, retracting, and producing a generating roll; by disconnecting the roll gears, spiral bevel gears can be form cut. Gear Shaving For improving the surface finish and profile accuracy of cut spur and helical gears (internal and external), gear shaving, a free-cutting gear finishing operation that removes small amounts of metal from the working surfaces of the teeth, is employed. The teeth on the shaving cutter, which may be in the form of a pinion (spur or helical) or a rack, have a series of sharp-edged rectangular grooves running from tip to root. The intersection of the grooves with the tooth profiles creates cutting edges; when the cutter and the workpiece, in tight mesh, are caused to move relative to one another along the teeth, the cutting edges remove metal from the teeth of the work gear. Usually the cutter drives the workpiece, which is free to rotate and is traversed past the cutter parallel to the workpiece axis. Shaving requires less time than grinding, but ordinarily it cannot be used on gears harder than approximately 400 HB (42 HRC). 13.4 Gear Grinding Gear Grinding Machines for the grinding of spur and helical gears utilize either a forming or a generating process. For form grinding, a disk-type grinding wheel is dressed to the proper shape by a diamond held on a special dressing attachment; for each number of teeth a special index plate, with V-type notches on its periphery, is required. When grinding helical gears, means for producing a helical motion of the blank must be provided. For grinding-generating, the grinding wheel may be a disk-type, doubleconical wheel with an axial section equivalent to the basic rack of the gear system. A master gear, similar to the gear being ground, is attached to the workpiece arbor and meshes with a master rack; the generating roll is created by rolling the master gear in the stationary rack. Spiral bevel and hypoid gears can be ground on the machines on which they are generated. The grinding wheel has the shape of a flaring cup with a double-conical rim having a cross section equivalent to the surface that is the envelope of the rotary cutter blades. Chapter 13 4  Figure 13.5. Finishing gears by grinding: (a) form grinding with shaped grinding wheels; (b) grinding by generating with two wheels. 13.5 Gear Rolling The cold-rolling process is used for the finishing of spur and helical gears for automatic transmissions and power tools; in some cases it has replaced gear shaving. It differs from cutting in that the metal is not removed in the form of chips but is displaced under heavy pressure. There are two main types of cold-rolling machines, namely, those employing dies in the form of racks or gears that operate in a parallel axis relationship with the blank and those employing worm-type dies that operate on axes at approximately 90° to the workpiece axis. The dies, under pressure, create the tooth profiles by the plastic deformation of the blank. When racks are used, the process resembles thread rolling; with geartype dies the blank can turn freely on a shaft between two dies, one mounted on a fixed head and the other on a movable head. The dies have the same number of teeth and are connected by gears to run in the same direction at the same speed. In operation, the movable die presses the blank into contact with the fixed die, and a conjugate profile is generated on the blank. On some of these machines the blank can be fed axially, and gears can be rolled in bar form to any convenient length. On machines employing worm-type dies, the two dies are diametrically opposed on the blank and rotate in opposite directions. The speeds of the blank and the dies are synchronized by change gears, like the blank and the hob on a hobbing machine; the blank is fed axially between the dies. Chapter 13 5  CHAPTER 14 FINISH MACHINING OPERATIONS 14.1 Honing Honing - is a low-velocity abrasive machining process. One or more bonded abrasive stones or “sticks” are put in area contact with the surface to be machined (in contrast with grinding processes where the abrasive and the work are in contact essentially on a line). Slow movement of the sticks in several directions abrades the high spots on the workpiece surface and makes the surface smoother and more true (i.e., with greater dimensional and geometric accuracy). Because of the slow movement, there are no heat effects to the metal surface. The process, illustrated in Fig. 14.1, removes only a small amount of material - less than 0.005 in (0.13 mm)2 - but improves the dimensional accuracy and surface finish of the surface being honed. The operation can be performed by hand but, for production situations, machines that impart reciprocating motion in several directions, or a combined reciprocating and rotary motion, are used. Cutting fluids are normally employed. The most common application is the finish machining of bored holes to remove tool marks, waviness or taper. The stones are allowed to “float” and follow the direction of the hole's axis. The cylinder walls of internal combustion engines are typically finished by honing. Gear teeth and bearing races are also honed. Figure 14.1. Honing. a) end section view of typical single-stone abrasive honing tool in a hole. Views Figure 14.2. Honing tool equipped with 6 b) through e) show, greatly exaggerated, honing stones hole discrepancies that are improved by honing: b) waviness, c) taper, d) out-of-roundness, e) bowed shape. 14.1.1. Application of the honing procedure Honing as a finishing operation can be used for practically all materials, such as grey cast iron, hardened and unhardened steels, hard metal, non-ferrous metals and aluminium. Honing is applied as a finishing operation after drilling or grinding of cylinder sliding surfaces, housing holes, holes in toothed gears and connecting rods, tubes and bushings. Chapter 14 1  14.1.2. Achievable accuracies and allowances Table. 14.1. 14.2 Superfinishing (shortstroke honing) The superfinishing method, also called superhoning or shortstroke honing, is a precision finishing process in which a workpiece rotates and an abrasive wheel, which is pressed against the workpiece, simultaneously performs a rapid longitudinal vibration of only few millimetres (Figure 14.3). Figure 14.3. Superfinishing 1 workpiece, 2 abrasive wheel, 3 vibrating head, a) machine, b) principle. The abrasive wheel is similar to the honing stone. Sometimes the wheel or wheels is/ are fixed on a tool carrier, which carries out the oscillating motion. The slide on which the tool carrier is based generates the feed motion. The overlapping of the two motions (the rotary motion of the workpiece and the oscillating – and feed motions of the tool) causes the grinding grains to pass over the workpiece surface on always different trajectories that are never the same. This results in particularly high surface qualities. Due to the short longitudinal motion of the abrasive wheel, which is similar to honing, the method is also called “shortstroke honing“. Since the abrasive wheel carries out a vibrational motion (back and forth motion), the technique is also called “superhoning“. In lieu of the term “superfinishing“, the terms “precision honing“ or “superfine honing” are also used. The German equivalent of these terms is derived from honing. 14.2.1. Application of superfinishing This method is used when, in addition to the best possible surface quality, the structure of the machined workpiece, up to the outermost load-bearing layer, needs to be totally heterogeneous. If the part’s microstructure has to fulfil high requirements, then a superfinishing technique is Chapter 14 2 indicated. Requirements like these occur, for example, in the case of bearing yokes, heavily loaded bearing pins on shafts and heavily loaded anti-friction bearings. As a result of superfinishing, in these elements, the surface structure is refined by removing the surface layer, which is dispersed by the machining procedure to such an extent that, for instance, breaking in of rotating machine parts is unnecessary. Moreover, the elements machined this way have good wear characteristics. With superfinishing, peak-to-valley heights from 0,1 to 0,4 µm can be achieved, the measuring accuracy ranges from IT 3 to IT 4. Allowances from 0,002 to 0,003 mm are sufficient. 14.3 Lapping Lapping is a precision grinding technique with loose grains, in which the workpiece and the tool slide over each other in constantly changing directions. In combination with oil, fine grinding grains form a lapping paste, or, with petroleum, a lapping fluid. This paste is deposited on the lapping tools, the lapping plates (Figure 14.4). Figure 14.4. Surface lapping principle a) upper lapping plate, b) bottom lapping plate, c) workpiece The lapping grain moves due to the irregular sliding of lapping plate and workpiece on each other. During sliding, both tool and lapping plate are abraded. The rate at which each part is machined depends on its materials. Lapping plates are predominantly manufactured from grey cast iron (special cast material) with a strength of 2000 N/cm2. The workpiece motion results from frictional coupling or a forced guidance as shown in Figure 14.5. In the configuration shown in Figure 14.5, the workpiece holders are in contact with a fixed rim of the gear (lying outside) and a driving ring gear (inside). Figure 14.5. Hydraulic two plate-lapping- and precision grinding machine Chapter 14 3  In the configuration of workpiece holders, it is necessary to look for an optimal solution for each individual application. Internal- (hole lapping) and external cylindrical lapping are executed on lapping machines with a vertical spindle, whose rpm can be controlled. The circumferential speed of the lapping arbour should be between v = 10 to 20 m/min. In most cases, the oscillating stroke motion is generated hydraulically. Lapping tools for holes (Figure 14.6) consist of a hardened tapered arbour, (cone ratio 1 : 40) made of steel, which bears the intrinsic lapping sleeve made of cast iron. The sleeve is slotted in order to readjust the lapping sleeve diameter, which becomes smaller as a result of the drive. During lapping, the grinding grains break as a result of pressure between lapping tool and workpiece. Consequently, new, smaller lapping grains are formed, continuously improving the surface during the lapping procedure. Figure 14.6. Internal cylindrical lapping tool 1 lapping arbour, 2 lapping sleeve. 14.3.1. Application of the lapping technique Plane lapping is used for face machining of piston rings, stampings, coupling rings, toothed segments and components for measuring devices. Holes of bushings, sleeves, pump cylinders are finished with internal cylindrical lapping. Lapping is applied if surface roughness values less than Rt = 0,5 µm and maximum accuracy to shape are required simultaneously. Accuracy to size is from IT 4 to IT 5, allowances range from 0,02 to 0,04 mm. 14.4. Burnishing Burnishing - is a means of smoothing machined or other surfaces by rubbing a smooth hard object against the surface with considerable pressure. Burnishing is not a machining operation - no workpiece material is removed - but instead deforms and presses down localized high spots from cutting tools, and thereby smooths rough areas. Figure 14.7. Schematic illustration of the roller burnishing process. Chapter 14 4  Roller burnishing - refines the surface of a workpiece by pressure rolling rather than by removing metal. The burnishing tool incorporates one or more hardened, finely polished rollers, which bear against the workpiece at high pressure. Each roller achieves the desired effect by deforming the surface material of the workpiece as it rolls against it, compressing the minute peaks of surface roughness into the valleys. The force applied to the burnishing tool depends on the amount of pressure required to exceed the yield point of the workpiece material. Multiple rollers may be used, depending on the shape of the surface being rollerburnished. Gear-tooth finishing is one common burnishing operation, performed by rolling the gear workpiece against three smooth, hardened, burnishing gears. Fig. 14.7 shows the principles of the operation and Fig. 14.8 a, b, and c illustrates typical configurations that are roller burnished and the burnishing tool that is used. Compressive stresses, left in the surface after the operation, and surface work hardening improve resistance to wear and fatigue failure. The process is limited to workpieces up to about Rockwell 40C in hardness and with walls thick enough to withstand the forces involved. Roller burnishing of holes can be a substitute for, or a supplement to, reaming and boring. Typical parts that undergo the operation are cylinder bores, valve stems, piston rods, turbine shafts, pump plungers, and rolls for the plastics and paper industries. Figure 14.8. Typical surfaces that can be roller burnished and the burnishing tools used. Chapter 14 5  CHAPTER 15 NONCONVENTIONAL MACHINING PROCESSES 15.1 Introduction The recent in the use of hard, high-strength, and temperature resistant materials in engineering has made in necessary to develop many new machining techniques. With the exception of grinding, conventional methods of removing material from a worpiece are not readily applicable to these new materials. Even when such machining is possible, it is usually slow and highly inefficient. Although most of the new machining processes have been developed specifically for materials that are difficult to machine, some of them have found use in the production of the complex shapes and cavities in softer, more readily machined materials. Descriptions of the main nonconventional machining processes are given in this chapter. Nonconventional machining process may be used as alternatives to the more traditional processes for a number of reasons, including the following: 1. Machinability of the workpiece material: Many workpiece materials are dificulut to machine by conventional methods because of high hardness, high thermal resistance, or high abrasive wear. For these materials many of the nonconventional machining processes are suitable. 2. Shape complexity of the workpiece: Many shape features are difficult or impossible to produce by conventional machining methods. For example, it is usually relatively simple to produce a cylindrical hole in a workpiece, but to produce a square, sharp-cornered hole of similar size is much more difficult. Nonconventional machining process are often capable of machining complex shape features with comparative ease-even in materials that are normally difficult to machine. 3. Surface integrity: Conventional machining processes can result in surface cracks and residual stresses in the workpiece. Many nonconventional machining processes can remove material without causing these effects. 4. Precision: Many nonconventional processes are capable of high levels of precision, which cannot be achieved by conventional machining. 5. Miniaturization: Some nonconventional processes are capable of producing very small shape features and fine detail not possible by conventional machining (e.d., the machining of deep holes with very small diameters). 6. Automatic data communication and computer integration: Many nonconventional processes can be computer-controlled and linked directly CAD/CAM systems in an integrated manufacturing environment. 15.2. Electrical Machining Processes Electrical discharge machining or EDM - is sometimes called, spark erosion machining. It uses a series of fine, electrical discharges or sparks to erode the workpiece material. The discharges pass from the tool (cathode) to the workpiece (anode) at a rate greater than 20,000 times per second. The workpiece material, which must be electrically conductive, melts or vaporizes at the point where it is touched by the spark. A dielectric fluid, usually kerosene, circulates between the electrode and the work. The fluid confines the spark, cools and solidifies molten material, and carries away the residue. The process is advantageous for hard, conductive material including hardened steel and carbide. There is no significant cutting force, so delicate shapes can be produced. The gap between the anode and the work is only about 0.002 in (0.05 mm) and is servo controlled. There is an overcut equivalent to the length of the spark from the electrode to the work. The cutting rate is slow compared with conventional machining but is still advantageous for Chapter 15 1 materials too hard or otherwise not easily machined. The rate of cutting and surface finish are controllable by varying the frequency, voltage, and current in the electrical pulses. Higher energy levels in the pulses produce faster erosion of the workpiece but a rougher surface finish. Typically, the rate of cutting is reduced toward the end of the operation, in order to provide smoother surfaces. Spark erosion produces small craters in the workpiece and these craters are manifested as a matte finish on the workpiece. The tool also is eroded since some spark action is in the reverse direction. Additionally, some secondary operations may be required to remove a hard, thin, re-cast surface layer, or fine surface cracks caused by thermal stresses, depending on the material used and the function of the part produced. 15.2.1. Ram EDM Ram EDM - is sometimes called “diesinker” EDM, and the principle is illustrated in Fig. 15.1. The electrode is shaped to fit the desired cavity and is fed into the workpiece, which is eroded to match the electrode shape. The electrode is made undersize to allow for the expected overcut, commonly 0.0005 to 0.020 in (0.013 to 0.5 mm). Feed is downward into the work, but CNC controls can also provide transverse movement of the electrode to produce special shapes in the cavity. Since the tool also erodes and becomes tapered, extra tools are often made. Graphite is a common electrode material, but copper, brass, aluminum, copper-tungsten, zinc-tin, and other alloys are also used. The process is mainly used to machine cavities in hardened steel dies and molds. It is also used in machining carbide, and in salvaging hardened parts or tools such as broken taps. Slots, nonround holes, small deep holes, and the machining of honeycomb and other fragile parts, are additional applications. Figure 15.1. Ram EDM. A series of fine, rapidly repeating electrical sparks from the electrode to the workpiece erode a cavity in the workpiece. The cavity matches the shape of the electrode. 15.2.2. Wire EDM Wire EDM - uses a constantly-moving wire instead of a shaped electrode. The wire, of 0.001 to 0.013 in (0.025 to 0.33 mm) diameter, passes through the work, with a vertical axis, (though it may be set at an angle when required when cutting apertures for stamping tools.) Tungsten, copper, and brass are common wire materials. The wire or the work, is fed horizontally as the cut progresses, to cut a slit or shaped through-hole in the workpiece. Chapter 15 2  Different wire material is constantly exposed to the spark, so wear of the wire is widely distributed and is not a problem. The process, shown in Fig. 15.2, is often used to cut die openings in hardened stock to produce dies and die components. A high level of accuracy and fine detail can be achieved. Figure 15.2. Wire EDM. Electrical sparks from the wire to the workpiece cut a contoured slit in the workpiece as the wire advances. Figure 15.3. Electrical discharge grinding (EDG) - is EDM with a rotating wheel electrode. Electric sparks from the electrode erode the workpiece. The shape of the periphery of the wheel is transferred to the ground surface of the workpiece. 15.2.3. Electrical discharge grinding (EDG) Electrical discharge grinding (EDG) - is similar to electrical discharge machining, except that the electrode is a rotating wheel instead of an electrode that is stationary except for its downfeed. The workpiece and wheel are immersed in dielectric fluid, and metal is removed by the same kind of spark erosion as that which occurs with EDM. The wiping action of the wheel produces better surface finishes than with ram-type EDM. The graphite wheel is dressed as necessary to Chapter 15 3 compensate for its wear in the process. The volume of material lost by the wheel from wear averages about 1/3 of that removed from the workpiece but is considerably less in many instances. Since the wheel wear is spread over its circumference, the amount of reduction of wheel diameter from dressing is normally small. The process is used in shaping carbide form tools and in grinding fragile or brittle materials. Material to be ground by EDG must be electrically conductive. There is a thin heat-affected layer from the process. The layer varies from 0.0001 to 0.0015 in (0.0025 to 0.038 mm) in depth. Fig. 15.3 illustrates the process. 15.3. Electrochemical machining (ECM) Electrochemical machining (ECM) - removes metal by a reverse electroplating process. The workpiece becomes the anode and the tool is the cathode of the electrolytic process. A highly conductive electrolytic fluid is pumped into the space between the workpiece and the tool, and highamperage current removes workpiece material by anodic dissolution. The workpiece takes a mirrorimage shape of the tool. The gap between the electrode (tool) and workpiece is a small as 0.001 in (0.025 mm) and pressure of the electrolyte must be high to insure adequate flow. Temperature control of the electrolyte, control of the gap and feed rate of the tool and maintenance of electrolyte cleanliness are important elements in the process. The electrolyte is circulated through equipment that removes the operational debris from the fluid. The process is particularly suited to hard materials and others that are difficult to machine by conventional methods. Workpiece hardness does not affect its machinability by the process, but the workpiece material must be electrically conductive. The tool does not wear and no stresses are induced in the workpiece by the operation. Common tool materials are copper, brass, bronze, stainless steel, and copper-tungsten. Metal removal rates are good compared to a number of other non-traditional machining processes, and average 1 in3 (16 cm3) per minute per 10,000 amperes of current. The process is used for die sinking, manufacture of jet engine parts, cam profiling, and the machining of small, deep holes. ECM is most advantageous for materials that are difficult to machine by conventional methods. A typical electrochemical machining set up is illustrated in Fig. 15.4. Figure 15.4. Electrochemical machining (ECM). Heavy electrical current passing through the electrolyte between the electrode and the workpiece causes anodic dissolution of the workpiece material. Chapter 15 4  15.3.1. Electrochemical grinding (ECG) Electrochemical grinding (ECG) - is similar to electrochemical machining but replaces the relatively stationary tool with a rotating conductive grinding wheel. The wheel normally consists of aluminum oxide abrasive bonded to a metal wheel, which acts as the cathode of the electrolytic circuit. Electrolytic fluid circulates in the area where the abrasive contacts the work. A combination of electrolytic and mechanical action removes material from the workpiece but the electrolytic action predominates, accounting for about 90 percent of metal removal. Anodic dissolution of the workpiece metal leaves surface metal oxides. In conventional ECM, the flushing action of the electrolyte removes these oxides. In electrochemical grinding, the abrasive mainly functions to remove the oxide film, exposing a new metal surface to the electrolyte. The abrasive also separates the metal wheel from the work, preserving a fine (0.001 in or 0.025 mm) gap between the two. It also carries the electrolyte solution to the gap. The process has the advantage of relatively high metal removal rates for hard metals, freedom from heat damage to the workpiece, and the ability to grind fragile parts. Plunge, surface, cylindrical, and internal grinding are all feasible with the process. However, capital costs a