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1、7.1 INTRODUCTIONAfter lathes, milli ng mach ines are the most widely used for manu facturi ng app licati ons. In milling, the work piece is fed into a rotat ing milli ng cutter, which is a multi-point tool as show n in Fig. 7.1, un like a lathe, which uses a sin gle pointcuttingtool. The tool used i

2、n millingis called the millingFig. 7.1 Schematic diagram of a milli ng op erati onThe milli ng p rocess is characterised by:(i) Interr up ted cutting Each of the cutt ing edges removes materialfor on ly a part of the rotatio n of the milli ng cutter. As a result, the cutting edge has time to cool be

3、fore it again removes material.Thus the milling operation is much more cooler compared to the turni ng op erati on. This allows for a much larger material rates.(ii)(iii)Small size of chips Though the size of the chips is small, in viewof the multiple cutting edges in contact a large amount of mater

4、ialis removed and as a result the component is generally completed ina single pass unlike the turning process which requires a largenumber of cuts for finishing.Variation in chip thickness This contributes to the non-steadystate cyclic conditions of varying cutting forces during the contact of the c

5、utting edge with the chip thickness varying from zero to maximum size or vice versa. This cyclic variation of the force can excite any of the natural frequencies of the machine tool system and is harmful to the tool life and surface finish generatedA milling machine is one of the most versatile mach

6、ine tools. It is adaptable for quantity production as well as in job shopsand tool rooms. The versatility of milling is becauseof the large variety of accessoriesand tools available with milling machines. The typical tolerance expected from the process is about 0.050 mm.7.2 TYPES OF MILLING MACHINES

7、To satisfy various requirements milling machines come in a number of sizes and varieties. In view of the large material removal ratesmilling machines come with a very rigid spindle and large power.The varieties of milling machines available are:(i) Knee and Column type(a) horizontal(b) vertical(c) u

8、niversal(d) turret typeThese are the general purpose milling machines, which have a high degree of flexibility and are employed for all types of works including batch manufacturing. A large variety of attachments to improve the flexibility are available for this class of milling machines.(ii) Produc

9、tion (Bed) type(a) simplex(b) duplex(c) triplexThese machines are generally meant for regular production involving large batch sizes.The flexibility is relatively less in these machines which is suitable for productivity enhancement.(iii) Plano millersThese machines are used only for very large work

10、pieces involving table travels in meters.(iv) Special type(a) Rotary table(b) Drum type(c) Copy milling (Die sinking machines)(d) Key way milling machines(e) Spline shaft milling machinesThese machines provide special facilities to suit specific applications that are not catered to by the other clas

11、ses of milling machines.7.2.1 Knee and Column Milling MachinesThe knee(升降臺(tái))and column type is the most commonly used machine in view of its flexibility and easier setup. A typical machine construction is shown in Fig. 7.2 for the horizontal axis. The knee housesthe feed mechanism and mounts the sadd

12、le and table. The table basically has the T-slots running along the X-axis for the purpose of work holding. The table moves along the X-axis on the saddle while the saddle moves along the Y-axis on the guide ways provided on the knee.The feed is provided either manually with a hand wheel or connecte

13、d for automatic by the lead screw, which in turn is coupled to the main spindle drive. The knee can move up and down (Z-axis) on a dovetail provided on the column.Fig. 7.2 Horizontal knee and column type milling machine4LThe massive column at the back of the machine houses all the power train includ

14、ing the motor and the sp indie gearbox. The power for feeding the table lead screw is taken from the main motor through a sep arate feed gearbox. Sometimes a sep arate feed motor is p rovided for the feed gearbox as well.While the longitudinal and traverse motions are provided with automatic motion,

15、 the raising of the knee is generally made manually.The sp indle is located at the top end of the column. The arbour used to mount the milling cutters is mounted in the spindle and is p rovided with a support on the other end to take care of the heavy cutting forces by means of an overarm with beari

16、ng. As shown in Fig.7.2 the overarm extends from the column with a rigid design. The spindle nose has the standard Morse taper of the suitable size dep ending upon the machine size.The milling cutters are mounted on the arbour at any desired position, the rest of the length being filled by standard

17、hardened collars of varying widths to fix the position of the cutter. The arbour is clamped in the spindle with the help of a draw bar and then fixed with nuts.Milling machines are generally specified on the following basis:(i) Size of the table, which specifies the actual working area on the table

18、and relates to the maximum size of the workpiece that can be accommodated.(ii) Amount of table travel, which gives the maximum axis movement that is possible.(iii) Horse power of the spindle, which actually specifies the power of the spindle motor used. Smaller machines may come with 1 to 3 hp while

19、 the production machines may go from 10 to 50 hp.Another type of knee and column milling machine is the vertical axis type. Its construction is very similar to the horizontal axis type, except for the spindle type and location.The vertical axis milling machine is relatively more flexible (Fig. 7.4)

20、and suitable for machining complex cavities such as die cavities in tool rooms. The vertical head is provided with a swiveling facility in horizontal direction whereby the cutter axis can be swivelled. This isuseful for tool rooms where more com plex milling op erations are carried out.The sp indle

21、is located in the vertical direction and is suitable for using the shank mounted milling cutters such as end mills, In view of the location of the tool, the setting up of the work pi ece and observing the machining op eration is more convenient.Fig, 7.3 Vertical knee and column type milling machineI

22、L_Fig.7.4 Some of the milling op erations normally carriedout onvertical axis machinesThe universal machine has the table which can be swivelled in a horizontal pl ane at about 45o to either the left or right. This makes the universal machine suitable for milling spur and helical gears as well as wo

23、rm gears and cams.7.2.2 Bed Type Milling MachineIn p roduction milling machines it is desirable to increasethe metal removal rates. If it is done on conventionalmachines by increasing the depth of cut, there is po ssibility of chatter. Hence another varietyof milling machines named as bed type machi

24、nes are used which are made more rugged and are cap able of removing more material. The ruggedness is obtained as a consequenceof the reduction in versatility.The table in the case of bed type machines is directly mounted on the bed and is p rovided with only longitudinal motion.The sp indle moves a

25、long with the column to p rovide the cutting action. Sim plex machines (Fig. 7.5) are the ones with only one sp indle head while duplex machines have two sp indles (Fig. 7.6). The two sp indles are located on either side of a heavy work pi ece and removematerial from both sides simultaneously.Fig. 7

26、.5 Simplex bed type milling machine-ZWorkpiece,Face mitling cunerQuillX ColumnMeadbj Side viewFig. 7.6 Duplex bed type milling machineFace millFntjColumnlead7.3 MILLING CUTTERSThere are a large variety of milling cutters available to suit sp ecific requirements. The versatility of the milling machin

27、e is contributed to a great extent by the variety of milling cutters that are available.7.3.1 Types of Milling CuttersMilling cutters are classified into various types based on a variety of methods.(i) Based on construction:(a) Solid(b) Inserted tooth typeBased on mounting:(a) Arbor mounted (b) Shan

28、k mounted(c) Nose mountedBase on rotation:(a) Right hand rotation (counter clockwise) (b) Left hand rotation (clockwise)Based on helix:(a) Right hand helix (b) Left hand helixMilling cutters are generally made of high speed steel or cemented carbides. The cemented carbide cutters can be of a brazed

29、tip variety or with indexable tips. The indexable variety is more common since it is normally less expensive to replace the worn out cutting edges than to regrind them.Plain milling cutters These are also called slab milling cutters and are basically cylindrical with the cutting teeth on the periphe

30、ry as shown in Fig. 7.7. These are generally used for machining flat surfaces.(a) Stab mining cuttar(b) Slab mining cutter(c) Slitting SAWFig. 7.7 Arbor mounted milling cutters for general purpose(d) Sicie and 怙幅 cutterTuvo side and face cutter 怡 Staggered tooth cutter (g) Side and lace cutterLight

31、duty slab milli ng cutters gen erally have a face width, which is small of the order of 25 mm. They gen erally have straight teeth and large nu mber of teeth.Heavy duty slab milli ng cutters come with a smaller nu mber of teeth to allow for more chi p sp ace. This allows tak ing dee per cuts and con

32、 seque ntly high material removal rates.Helical milli ng cutters have a very small nu mber of teeth but a large helix an gle. This type of cutter cuts with a sheari ng acti on, which can p roduce a very fine fini sh. The large helix an gle allows the cutter to absorb most of the end load and therefo

33、re the cutter en ters and leaves the work piece very smoothly.Side and face milling cutters These have the cutting edges not only on the face like the slab milling cutters, but also on both the sides. As aresult, these cutters become more versatile since they can be used for side milli ng as well as

34、 for slot milling.Staggered tooth side milli ng cutters are a variatio n where the teeth are arran ged in an alternate helix p atter n. This type is gen erally used for milli ng dee p slots, since the staggeri ng of teeth pro vides for greater chi p sp ace.Ano ther variati on of the side and face cu

35、tter is the half side milli ng cutter, which has cutt ing edges only on one side. This arra ngeme nt pro vides a po sitive rake an gle and is useful for machi ning on only one side. These have a much smoother cutt ing acti on and a long tool life. The powercon sumed is also less for these cutters. A

36、ngle milling cutter也）Argle milling cuner罔 Form relieved circular cutterFig. 7.8 Special forms of arbor mounted milling cuttersSlitting saws The other com mon form of milli ng cutters in the arbor moun ted category is the slitti ng saw. This is very similar to a saw blade in appearanceas well as func

37、tion. Most of these have teeth around the circumference while some have side teeth as well. The thickness of these cutters is generally very small and is used for cutting off operations or for deep slots.Special form cutters In addition to the general type of milling cutters described above, there a

38、re a large number of special form milling cutters available which are used for machining specific profiles.Angular milling cutters are made in single or double angle cutters for milling any angle such as 30, 45 or 60o Form relieved cutters are made of various shapes such as circular, corner rounding

39、, convex or concave shapes.T-slot milling cutters are used for milling T-slots such as those in the milling machine table. The central slot is to be milled first using an end mill before using the T-slot milling cutter. Woodruff key seat milling cutters are used for milling as the name suggests, woo

40、druff key seatsSome other special form cutters are dovetail milling cutters and gear milling cutters.End mills These are shank mounted as shown in Fig. 7.9 and are generally used in vertical axis milling machines. They are used for milling slots, key ways and pockets where other type of milling cutt

41、ers cannot be used. A depth of cut of almost half the diameter can be taken with the end mills.The end mills have the cutting edge running through the length of thecutt ing p orti on as well as on the face radially up to a certai n len gth. Thehelix an gle of the cutt ing edge pro motes smooth and efficie nt cutt ingeve n at high cutt ing sp eeds and feed rates. High cutt ing sp ee轉(zhuǎn)速?) aregen erally recomme nded for this type of milli ng cutters.Lerg(th overall Length otcutOia of 鈕-DiaDia of cur閭 Four flute a