Milling PDF

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...

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