Milling Machine and Operation PDF

Milling Machine and Operation M About Milling Milling is a process of producing flat and complex shapes with the use of multi- tooth cutting tool, which is called a milling cutter and the cutting edges are called teeth. Types of Milling Operation Milling operations are broadly classified as periph...

Milling Machine and Operation M About Milling Milling is a process of producing flat and complex shapes with the use of multi- tooth cutting tool, which is called a milling cutter and the cutting edges are called teeth. Types of Milling Operation Milling operations are broadly classified as peripheral milling and face milling: Horizontal Milling Machine Milling operations are broadly classified as peripheral milling and face milling: Peripheral Milling. This operation is also called plain milling operation. In this operation axis of rotating tool is always kept parallel to the surface being machined. This operation is done by the cutting edges on outside periphery of the milling cutter. Different types of peripheral milling operations are possible as described below. Slab Milling. In this milling operation the cutter width extends beyond the workpiece on both sides. Slotting. It is also a type of milling operation, also called as slot milling operation. In this case width of the cutter is less than the width of workpiece. It is used to make slot in the workpiece. Thin slots can be made by using very thin milling cutters. The workpiece can be cut into two pieces by making a very thin slot throughout the depth of workpiece. Cutting the workpiece this way be slot milling is called saw milling. Side Milling. The cutter is used for milling of sides of a workpiece. Straddle Milling. It is just like side milling with difference that cutting (milling operation) takes place simultaneously on both the sides of workpiece. Up Milling. It is also called conventional milling in this case movement of cutter teeth is opposite to the direction of feed motion. Down Milling. It is also called climb milling. In this case direction of cutter motion is the same so that of direction of feed motion. Face Milling. In the operation of face milling, axis of the milling cutter remains perpendicular to the surface being milled. In this case cutting action is done by cutting edges of both sides (end and out side) periphery of the milling cutter. Depending upon the relative geometry of workpiece and milling cutter face milling is different types as described below. Conventional Face Milling. In this case diameter of milling cutter is greater than the width of workpiece. The milling cutter remains over hanging on both sides of workpiece. Partial Face. Milling In this case the milling cutter overhangs on the workpiece on one side only. End Milling. In case of end milling thin (low diameter) cutter are used as compared to workpiece width. It is used to make slot in the workpiece. Profile Milling. This is just like end milling in which the outer side periphery of a flat part is machined (milled). Pocket Milling. This is a selective portion milling on the flat surface of workpiece used to make shallow packets there. Surface Contouring. In this operation a ball nose cutter if feedback and forth across the workpiece along a curvilinear path at short intervals. This creates the required contours on the surface of workpiece. This operation is used to make contours of molds and dies and this time the operation is named as die sinking. Two basic types of column-and-knee milling machines, (Left) horizontal, and (Right) vertical.  Types of Milling Machines Milling machines can be classified into different categories depending upon their construction, specification and operations. The choice of any particular machine is primarily determined by nature of the work to be done, its size, geometry and operations to be performed. The broader classification has threecategories and each category has its sub-classifications given below: Column and Knee Type Milling Machine a. Head milling machine b. Plain milling machine c. Universal milling machine d. Omniversal milling machine e. Vertical milling machine Fixed Bed Type Milling Machine a. Simplex milling b. Duplex milling c. Triplex milling  Special Type Milling Machine a. Rotary table milling b. Drum milling c. Planetary milling d. Tracer controlled milling In addition to above three types, there is one more type of milling machine, the planner type milling machine, which is rarely used. A machining center is a highly automated machine tool capable of performing multiple machining operations under CNC control. The features that make a machining center unique include the following: -Tool storage unit called tool magazine that can hold up to 120 different cutting tools. -Automatic tool changer, which is used to exchange cutting tools between the tool magazine and machining center spindle when required. The tool changer is controlled by the CNC program. -Automatic work part positioning. Many of machining centers are equipped with a rotary worktable, which precisely position the part at some angle relative to the spindle. It permits the cutter to perform machining on four sides of the part. Column and Knee Type Milling Machine. Main shape of column knee type of milling machine consists of a base having different control mechanisms housed there in. The base consists of a vertical column at one of its end. There is one more base above the main base and attached to the column that serves as worktable equipped with different attachments to hold the workpiece. This base having worktable is identified as “ knee ” of the milling machine. At the top of the column and knee type milling machines are classified according to the various methods of supplying power to the table, different movements of the table and different axis of rotation of the main spindle. Head Milling Machine. In the case of head milling machine feed motion is given by hand and movements of the machine are provided by motor. This is a simple and light duty milling machine intended for basic operations. Plain Milling Machine. The Plain milling machine is similar to the hand milling machine, but feed movement can be power-controlled in addition to manual control. Plain Milling, also called Surface Milling or Slab Milling, is the milling of flat surfaces with the milling cutter axis parallel to the surface being milled. Generally, plain milling is done with the workpiece surface mounted parallel to the surface of the milling machine table and the milling cutter mounted on a standard milling machine arbor. The arbor is well supported in a horizontal plane between the milling machine spindle and one or more arbor supports. Mounting the Workpiece. The workpiece is generally clamped directly to the table or supported in a vise for plain milling. The milling machine table should be checked for alignment before starting to cut. If the workpiece surface to be milled is at an angle to the base plane of the piece, the workpiece should be mounted in a universal vise or on an adjustable angle plate. The holding device should be adjusted so that the workpiece surface is parallel to the table of the milling machine. Selecting the Cutter. A careful study of the drawing must be made to determine what cutter is best suited for the job. Flat surfaces may be milled with a plain milling cutter mounted on an arbor. Deeper cuts may generally be taken when using narrow cutters than with wide cutters. The choice of milling cutters should be based on the size and shape of the workpiece. If a wide area is to be milled, fewer traverses will be required using a wide cutter. If large quantities of metal are to be removed, a coarse tooth cutter should be used for roughing and a finer tooth cutter should be used for finishing. A relatively slow cutting speed and fast table feed should be used for roughing, and a relatively fast cutting speed and slow table feed used for finishing. The surface should be checked for accuracy after each completed cut.  The milling cutter is positioned on the arbor with sleeves so that it is as close as practical to the milling machine spindle while maintaining sufficient clearance between the vise and the milling machine column. This practice reduces torque in the arbor and permits more rigid support for the cutter. A typical setup for plain milling. Column & Knee type Milling Machines. Used for general purpose milling operations, Column and knee type milling machines are the most common milling machines. The spindle to which the milling cutter is may be horizontal (left; slab milling) or vertical (right; face and end milling). The basic components are: Work table, on which the workpiece is clamped using the T-slots. The table moves longitudinally with respect to the saddle. Saddle, which supports the table and can move transversely. Bridgeport systems. Another important development came in the 1930s when Rudolph Bannow and Magnus Wahlstrom brought out the Bridgeport-style vertical milling machine. This design offers versatility and economy in place of the higher metal removal rates of traditional horizontal milling machines. Because of this versatility, there are more Bridgeport-style mills in existence today than any other milling machine design. Horizontal mills are now usually reserved for production applications where high metal removal rates on identical parts are needed, not prototyping and short runs. Bridgeport style machines are also called knee-and- column machines and turret mills.. The key features of these machines are a: Knee-and-column support for the milling table, which provides vertical motion of the work with respect to the tool. Saddle which supports the table to provide in-and-out motion from the vertical column. One-piece tool head which holds the motor, drive pulleys, and spindle. Sliding overarms or rams were eventually added to allow the tool head to be moved in or out with respect to the vertical column. Some machines have provisions for the tool head to be tilted side to side or back to front. The Bridgeport-style machine offers many advantages over the older horizontal milling machine design: The biggest advantage is the quill ’ s ability to advance and retract the cutter easily without cranking to raise and lower the milling table. This speeds production and reduces operator fatigue. The retractable quill lets the operator quickly withdraw the tool to clear chips from a hole or check its progress. Tactile feedback through the quill feed handle or handwheel also tells the machinist how the tool is cutting and lets him optimize feed with less danger of tool breakage. Vertical table movement is still available for high accuracy depth adjustment or when more force on the tool is required.  The second largest advantage is the Bridgeport-style machine ’ s ability to make angle cuts. With the horizontal milling machine, either the milling cutter is made on an angle, or the work must be positioned at an angle to the spindle axis. With the Bridgeport-style machine the operator merely needs to tilt the spindle to make an angle cut. Of course, the Bridgeport can also use an angled cutter or mount the work on an angle. Vertical milling machines must use smaller cutting tools than horizontal mills because they have less rigid, less massive castings, and lower horsepower motors. Still, they can accomplish the same end results as the horizontal mill, just more slowly. Vertical milling machines are less complex than horizontal machines because the onepiece tool head eliminates the need for complicated gearing inside the vertical column. Bridgeport-type milling machines usually have 1 to 5-horsepower motors, and smaller castings than most horizontal mills. Because of this they generally cost less. Knee-and-column mills offer versatility and economy in place of the high metal removal rates of traditional horizontal milling machines. The Milling Process  Modern milling is a very universal machining method. Milling is a strong contender for producing holes, cavities, surfaces that used to be turned, threads, etc. Milling is principally metal cutting performed with a rotating, multi-edge cutting tool which performs programmed feed movements against a workpiece in almost any direction. Classification of Milling Cutters. Milling cutters are usually made of high-speed steel and are available in a great variety of shapes and sizes for various purposes. You should know the names of the most common classifications of cutters, their uses, and, in a general way, the sizes best suited to the work at hand. Two views of a common milling cutter with its parts and angles identified. These parts and angles in some form are common to all cutter types. The pitch refers to the angular distance between like or adjacent teeth. The teeth of milling cutters may be made for right-hand or left-hand rotation, and with either right-hand or left-hand helix. Determine the hand of the cutter by looking at the face of the cutter when mounted on the spindle. A right-hand cutter must rotate counterclockwise; a left- hand cutter must rotate clockwise. The right-hand helix is shown by the flutes leading to the right; a left-hand helix is shown by the flutes leading to the left. The direction of the helix does not affect the cutting ability of the cutter but take care to see that the direction of rotation is correct for the hand of the cutter.. Saw Teeth. Saw teeth are either straight or helical in the smaller sizes of plain milling cutters, metal slitting saw milling cutters, and end milling cutters. The cutting edge is usually given about 5 degrees primary clearance. Sometimes the teeth are provided with off-set nicks which break up chips and make coarser feeds possible. Helical Milling Cutters. The helical milling cutter is similar, to the plain milling cutter, but the teeth have a helix angle of 45 ° to 60 °. The steep helix produces a shearing action that results in smooth, vibration-free cuts. They are available for arbor mounting, or with an integral shank with or without a pilot. This type of helical cutter is particularly useful for milling elongated slots and for light cuts on soft metal. Metal Slitting Saw Milling Cutter. The metal slitting saw milling cutter is essentially a very thin plain milling cutter. It is ground slightly thinner toward the center to provide side clearance. These cutters are used for cutoff operations and for milling deep, narrow slots, and are made in widths from 1/32 to 3/16 inch. Side Milling Cutters. Side milling cutters are essentially plain milling cutters with the addition of teeth on one or both sides. A plain side milling cutter has teeth on both sides and on the periphery. When teeth are added to one side only, the cutter is called a half-side milling cutter and is identified as being either a right-hand or left-hand cutter. Side milling cutters are generally used for slotting and straddle milling. Interlocking tooth side milling cutters and staggered tooth side milling cutters are used for cutting relatively wide slots with accuracy. Interlocking tooth side milling cutters can be repeatedly sharpened without changing the width of the slot they will machine. After sharpening, a washer is placed between the two cutters to compensate for the ground off metal. End Milling Cutters. The end milling cutter, also called an end mill, has teeth on the end as well as the periphery. The smaller end milling cutters have shanks for chuck mounting or direct spindle mounting. End milling cutters may have straight or spiral flutes. Spiral flute end milling cutters are classified as lefthand or right-hand cutters depending on the direction of rotation of the flutes. If they are small cutters, they may have either a straight or tapered shank. The most common end milling cutter is the spiral flute cutter containing four flutes. Two-flute end milling cutters, sometimes referred to as two-lip end mill cutters, are used for milling slots and keyways where no drilled hole is provided for starting the cut. These cutters drill their own starting holes. Straight flute end milling cutters are generally used for milling both soft or tough materials, while spiral flute cutters are used mostly for cutting steel. Large end milling cutters (normally over 2 inches in diameter) are called shell end mills and are recessed on the face to receive a screw or nut for mounting on a separate shank or mounting on an arbor, like plain milling cutters. The teeth are usually helical and the cutter is used particularly for face milling operations requiring the facing of two surfaces at right angles to each other. T-Slot Milling Cutter. The T-slot milling cutter is used to machine T-slot grooves in worktables, fixtures, and other holding devices. The cutter has a plain or side milling cutter mounted to the end of a narrow shank. The throat of the T-slot is first milled with a side or end milling cutter and the headspace is then milled with the T-slot milling cutter. Woodruff Keyslot Milling Cutters. The Woodruff keyslot milling cutter is made in straight, tapered-shank, and arbor-mounted types. The most common cutters of this type, under 1 1/2 inches in diameter, are provided with a shank. They have teeth on the periphery and slightly concave sides to provide clearance. These cutters are used for milling semicylindrical keyways in shafts. Angle Milling Cutters. The angle milling cutter has peripheral teeth which are neither parallel nor perpendicular to the cutter axis. See Figure 8-8. Common operations performed with angle cutters are cutting V-notches and serration ’ s. Angle cutters may be single-angle milling cutters or double-angle milling cutters. The single-angle cutter contains side-cutting teeth on the flat side of the cutter. The angle of the cutter edge is usually 30 ° , 45 ° , or 60 ° , both right and left. Double-angle cutters have included angles of 45, 60, and 90 degrees. Gear Hob. The gear hob is a formed tooth milling cutter with helical teeth arranged like the thread on a screw. These teeth- are fluted to produce the required cutting edges. Hobs are generally used for such work as finishing spur gears, spiral gears, and worm gears. They may also be used to cut ratchets and spline shafts. Concave and Convex Milling Cutters. Concave and convex milling cutters are formedtooth cutters shaped to produce concave and convex contours of 1/2 circle or less. The size of the cutter is specified by the diameter of the circular form the cutter produces. Helical milling tool. (Datron) Finger joint milling cutter. (Weiss) Roughing milling cutter. (Dormer Pramet)  Drilling milling tool. (Starcut) Burr milling cutter. (Fiudi) Gear cutting milling tool. (Knuth)  Slot milling cutter. (Mikron) Roughing milling cutter (Dormer Pramet) Thread milling cutter. (OSG) Corner Rounding Milling Cutter. The corner-rounding milling cutter is a formed tooth cutter used for milling rounded corners on workplaces up to and including one-quarter of a circle. The size of the cutter is specified by the radius of the circular form the cutter produces, such as concave and convex cutters generally used for such work as finishing spur gears, spiral gears, and worm wheels. They may also be used to cut ratchets and spline shafts. Special Shaped-Formed Milling Cutter. Formed milling cutters have the advantage of being adaptable to any specific shape for special operations. The cutter is made specially for each specific job. In the field, a fly cutter is formed by grinding a single point lathe cutter bit for mounting in a bar, holder, or fly cutter arbor. The cutter can be sharpened many times without destroying its shape. Selection of Milling Cutters. Consider the following when choosing milling cutters: Highspeed steel, stellite, and cemented carbide cutters have a distinct advantage of being capable of rapid production when used on a machine that can reach the proper speed. The milling cutter should be small enough in diameter so that the pressure of the cut will not cause the workpiece to be sprung or displaced while being milled. Size of Milling Cutter. In selecting a milling cutter for a particular job, choose one large enough to span the entire work surface so the job can be done with a single pass. If this cannot be done, remember that a small diameter cutter will pass over a surface in a shorter time than a large diameter cutter which is fed at the same speed. Care and Maintenance of Milling Cutters. The life of a milling cutter can be greatly prolonged by intelligent use and proper storage. New cutters received from stock are usually wrapped in oil paper which should not be removed until the cutter is used. Take care to operate the machine at the proper speed for the cutter being used, as excessive speed will cause the cutter to wear rapidly from overheating. Arbors. Milling machine arbors are made in various lengths and in standard diameters of 7/8,1,1 1/4, and 1 1/2 inch. The shank is made to fit the taper hole in the spindle while the other end is threaded. NOTE: The threaded end may have left or right-handed threads. The milling machine spindle may be self-holding or selfreleasing. The self-holding taper is held in the spindle by the high wedging force. The spindle taper in most milling machines is self- releasing; tooling must be held in place by a draw bolt extending through the center of the spindle. Arbors are supplied with one of three tapers to fit the milling machine spindle: the Standard Milling Machine taper, the Brown and Sharpe taper, and the Brown and Sharpe taper with tang. The Standard Milling Machine Taper is used on most machines of recent manufacture. These tapers are identified by the number 30, 40, 50, or 60. Number 50 is the most commonly used size on all modern machines. Machine tapers. The Brown and Sharpe taper is found mostly on older machines. Adapters or collets are used to adapt these tapers to fit machines whose spindles have Standard Milling. The Brown and Sharpe taper with tang is used on some older machines. The tang engages a slot in the spindle to assist in driving the arbor. Standard Milling Machine Arbor. The standard milling machine arbor has a tapered, cylindrical shaft with a standard milling taper on the driving end and a threaded portion on the opposite end to receive the arbor nut. One or more milling cutters may be placed on the straight cylindrical portion of the arbor and held in position by sleeves and the arbor nut. Screw Arbor. Screw arbors are used to hold small cutters that have threaded holes. These arbors have a taper next to the threaded portion to provide alignment and support for tools that require a nut to hold them against a taper surface. A right-hand threaded arbor must be used for righthand cutters while a left-hand threaded arbor is used to mount left-hand cutters. The slitting saw milling cutter arbor is a short arbor having two flanges between which the milling cutter is secured by tightening a clamping nut. This arbor is used to hold metal slitting saw milling cutters used for slotting, slitting, and sawing operations. The shell end milling cutter arbor has a bore in the end in which shell end milling cutters fit and are locked in place by means of a cap screw. The fly cutter arbor is used to support a single edge lathe, shaper, or planer cutter bit for boring and gear cutting operations on the milling machine. Collets, Spindle Adapters, and Quick-Change Tooling Milling cutters that contain their own straight or tapered shanks are mounted to the milling machine spindle with collets, spindle adapters, and quick-change tooling which adapts the cutter shank to the spindle. Collets A collet is a form of a sleeve bushing for reducing the size of the hole in the milling machine spindle so that small shank tools can be fitted into large spindle recesses. They are made in several forms, similar to drilling machine sockets and sleeves, except that their tapers are not alike. Spindle Adapters A spindle adapter is a form of a collet having a standardized spindle end. They are available in a wide variety of sizes to accept cutters that cannot be mounted on arbors. They are made with either the Morse taper shank or the Brown and Sharpe taper with tang having a standard spindle end. Chuck Adapter. A chuck adapter is used to attach chucks to milling machines having a standard spindle end. The collet holder is sometimes referred to as a collet chuck. Various forms of chucks can be fitted to milling machines spindles for holding drills, reamers, and small cutters for special operations. Quick-Change Tooling The quick-change adapter mounted on the spindle nose is used to speed up tool changing. Tool changing with this system allows you to set up several milling operations such as drilling, end milling, and boring without changing the setup of the part being machined. The tool holders are mounted and removed from a master holder mounted to the machine spindle by means of a clamping ring. Vises Either a plain or swivel-type vise is furnished with each milling machine. The plain vise, similar to the machine table vise, is used for milling straight workplaces and is bolted to the milling machine table either at right angles or parallel to the machine arbor. The swivel vise can be rotated and contains a scale graduated in degrees at its base to facilitate milling workplaces at any angle on a horizontal plane. The universal vise, which may be obtained as extra equipment, is designed so that it can be set at both horizontal and vertical angles. This type of vise may be used for flat and angular milling. The allsteel vise is the strongest setup because the workpiece is clamped closer to the table. The vise can securely fasten castings, forgings, and rough-surfaced workplaces. The jaw can be positioned in any notch on the two bars to accommodate different shapes and sizes. The air or hydraulically operated vise is used more often in production work. This type of vise eliminates tightening by striking the crank with a lead hammer or other soft face hammer. Adjustable Angle Plate. The adjustable angle plate is a workpiece holding device, similar to the universal vise in operation. Workpieces are mounted to the angle plate with T-bolts and clamps in the same manner used to fasten workplaces to the worktable of the milling machine. The angle plate can be adjusted to any angle so that bevels and tapers can be cut without using a special milling cutter or an adjustable cutter head. Indexing Fixture. The index fixture consists of an index head, also called a dividing head, and footstock which is similar to the tailstock of a lathe. The index head and footstock attach to the work table of the milling machine by T-slot bolts. An index plate containing graduations is used to control the rotation of the index head spindle. The plate is fixed to the index head, and an index crank is connected to the index head spindle by a worm gear and shaft. *Workpieces are held between centers by the index head spindle and footstock. Workpieces may also be held in a chuck mounted to the index head spindle or maybe fitted directly into the taper spindle recess of some indexing fixtures. High-Speed Miling Attachment The rate of spindle speed of the milling machine may be increased from 1 1/2 to 6 times by using the high-speed milling attachment. This attachment is essential when using cutters and twist drills which must be driven at a high rate of speed in order to obtain an efficient surface speed. The attachment is clamped to the column of the machine and is driven by a set of gears from the milling machine spindle. Vertical Spindle Attachment. This attachment converts the horizontal spindle of a horizontal milling machine to a vertical spindle. Universal Milling Attachment. This device is similar to the vertical spindle attachment but is more versatile. The butterhead can be swiveled to any angle in any plane, whereas the vertical spindle attachment only rotates in one place from horizontal to vertical. Rotary Table or Circular Milling Attachment. This attachment consists of a circular worktable containing T-slots for mounting workplaces. Offset Boring Head. Boring, an operation that is too often restricted to a lathe, can be done easily on a milling machine. The offset boring head is an attachment that fits to the milling machine spindle and permits most drilled holes to have a better surface finish and greater diameter accuracy. Offset Boring Head and Tools. The boring bar can be adjusted at right angle to the spindle axis. This feature makes it possible to position the boring cutter accurately to bore holes of varying diameters. This adjustment is more convenient than adjusting the cutter in the boring bar holder or changing the boring bar. NOTE: On some boring heads, the reading on the tool slide is a direct reading. On other boring heads, the tool slide advances twice the amount shown on the micrometer dial. Mounting and Indexing Work An efficient and positive method of holding workplaces to the milling machine table is important if the machine tool is to be used to its fullest advantage. The most common methods of holding are clamping a workpiece to the table, clamping a workpiece to the angle plate, clamping the workpiece in fixtures, holding a workpiece between centers, holding the workpiece in a chuck, and holding the workpiece in a vise. Indexing. Indexing is the process of evenly dividing the circumference of a circular workpiece into equally spaced divisions, such as in cutting gear teeth, cutting splines, milling grooves in reamers and taps, and spacing holes on a circle. The index head of the indexing fixture is used for this purpose. Index Head. The index head of the indexing fixture contains an indexing mechanism which is used to control the rotation of the index head spindle to space or divide a workpiece accurately. Mounting and Indexing Work A Index Plate. The indexing plate is a round plate with a series of six or more circles of equally spaced holes; the index pin on the crank can be inserted in any hole in any circle. With the interchangeable plates regularly furnished with most index heads, the spacing necessary for most gears, boltheads, milling cutters, splines, and so forth can be obtained. Plain Indexing. The following principles apply to basic indexing of workpieces: Suppose it is desired to mill a project with eight equally spaced teeth. Since 40 turns of the index crank will turn the spindle one full turn, l/8th of 40 or 5 turns of the crank after each cut will space the gear for 8 teeth, If it is desired to space equally for 10 teeth, 1/10 of 40 or 4 turns would produce the correct spacing. Direct indexing is accomplished by an additional index plate fastened to the index head spindle. A stationary plunger in the index head fits the holes in this index plate. By moving this plate by hand to index directly, the spindle and the workpiece rotate an equal distance. Direct index plates usually have 24 holes and offer a quick means of milling squares, hexagons, taps, and so forth. Mounting and Indexing Work Differential Indexing. Sometimes, a number of divisions is required which cannot be obtained by simple indexing with the index plates regularly supplied. To obtain these divisions, a differential index head is used. The index crank is connected to the wormshaft by a train of gears instead of a direct coupling as with simple indexing. Indexing in Degrees. Workpieces can be indexed in degrees as well as fractions of a turn with the usual index head. Indexing Operations. The following examples show how the index plate is used to obtain any desired part of a whole spindle turn by plain indexing, Milling a hexagon.