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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine.

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Presentation on theme: "Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine."— Presentation transcript:

1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. PowerPoint to accompany Krar Gill Smid Technology of Machine Tools 6 th Edition Cam, Rack, Worm, and Clutch Milling Unit 72

2 72-2 Objectives Calculate and cut a uniform-motion cam Set up the machine and cut a rack Understand how a worm is cut Set up the machine and cut a clutch

3 72-3 Cams Used to change rotary motion into straight- line or reciprocating motion and to transmit motion to other parts of machine through follower –Plate or bar cams (templates) –Often used on tracer-type milling machines May also be used as locking devices –Application in jig and fixture design

4 72-4 Positive-Type Cams Control follower at all times Follower remains engaged in groove on face or periphery of cam Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CylindricalGrooved plate

5 72-5 Nonpositive-Type Cams Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cam pushes follower in given direction and depends on some external force to keep follower bearing against cam surface Plate Toe and wiperCrown

6 72-6 Types of Followers Roller: least frictional drag, little or no lubrication Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Tapered roller: Used with grooved plate or cylindrical cams Flat or plunger: Used to transmit large forces and requires lubrication Knife-edge, pointed: Used on intricate cams (follows sharp contours)

7 72-7 Cam Motions Three standard types of motions imparted by cams to followers and machine parts: 1.Uniform motion –Moves follower at same rate from beginning to end of stroke –Goes from zero to full speed instantaneously and ends in same abrupt way

8 Harmonic-motion Provides smooth start and stop to cycle Used when uniformity of motion not essential and high speeds required 3.Uniformly accelerated and decelerated motion Moves follower slowly at first, then accelerates or decelerates at uniform rate Gradually decreases in speed permitting follower to come to slow stop Smoothest of three motions and used on high- speed machines

9 72-9 Radial Cam Terms Lobe –Projecting part of cam imparts reciprocal motion to follower –May have one or several lobes depending on application Rise –Distance one lobe will raise or lower follower as cam revolves

10 72-10 Lead –Total travel imparted to follower in one revolution of uniform-rise cam, having only one lobe in 360º Lead for double-lobe cam twice lead of single-lobe –Lead of cam NOT rise that controls gear selection Uniform rise –Rise generated on cam that moves inward at even rate around cam, assuming shape of Archimedes spiral –Caused by uniform feed and rotation of work when cam machined

11 72-11 Cam Milling Plate cams without uniform rise –Laid out and machined by incremental cuts –Black rotated through angular increment and cut taken to layout line (then repeated) –Ridges left between each successive cut removed by filing and polishing

12 72-12 Cam Milling Uniform-rise cam produced in milling machine with vertical head –Combined uniform rotation of cam blank, held in spindle of dividing head and uniform feed of table –Work and vertical head swung at angle so axis of work and axis of mill parallel Required lead to be cut on cam always less than forward feed of table

13 72-13 Calculations Required If circumference is divided into 100 equal parts, it will necessary to calculate the lead as follows:

14 72-14 Example: A uniform-rise cam having a rise of.375 in. in 360º is to be cut. Calculate the required lead, the inclination of the work, and the vertical head. Solution: Lead of cam =.375 in. This is impossible, since shortest lead that can be cut with regular change gears is.670 in. By consulting a handbook, note that gears required for.670 in. lead are 24, 86, 24, 100

15 72-15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Feed Table Dividing head Cutter Cam a= 360º It will be necessary to swing work and vertical head to definite angle. L = lead to which machine geared H = lead of cam i = angle of inclination of dividing head spindle in degrees

16 72-16 Example: Specify steps required to cut a uniform-rise cam having three lobes, each lobe occupying 120º and each having a rise of.150 in. Procedure: 1.

17 Smallest lead over.450 to which machine can be geared is.670 in. (handbook) 3.Change gears required to produce lead of.670 in. 4.Mount dividing head, and connect worm shaft and lead screw with above gears. Disengage index plate locking device 5.Scribe three marks 120º apart on periphery of cam

18 Mount work in dividing head chuck 7.Mount end mill of sufficient length in vertical attachment 8.Calculate offset of work and vertical head 9.Swivel dividing head to 42º12´ 10.Swivel vertical milling attachment to 90º - 42º12´ = 47º48´

19 Centralize work and cutter 12.Rotate work with index crank until one scribed mark on cam blank is exactly at bottom dead center 13.Adjust table until cutter touching lower side of work and center of cutter in line with mark 14.Set vertical feed collar to zero 15.Start machine

20 Using index head crank, rotate work one- third turn or until the second scribed line is in line with forward edge of cutter 17.Lower table slightly and disengage gear train, or disengage dividing head worm 18.Return table to starting position 19.Rotate work until next line on circumference is in line with center of cutter

21 Re-engage gear train or dividing head worm 17.Cut second lobe 18.Repeat steps 17, 18, 19, and 20, and cut third lobe

22 72-22 Rack Milling Rack, with gear (pinion) used to convert rotary motion into longitudinal motion Pitch line of rack distance of one addendum (1/DP) below top of tooth Pitch of rack measured in linear pitch, obtained by dividing by diametral pitch

23 72-23 Rack Indexing Attachment Moves table for each tooth when cutting rack using rack milling attachment Consists of indexing plate with two diametrically opposed notches and locking pin Two change gears (set of 14) mounted –Different combinations permit accurate increments corresponding to linear pitch of rack –All diametral pitches from 4 to 32 as well as circular pitches from 1/8 in to 3/4 in.

24 72-24 Worms and Worm Gears Used when great ratio reduction required between driving and driven shafts Worm – cylinder on which is cut single- or multiple-start Acme-type thread –Angle ranges from 14.5º to 30º pressure angle Worm gear machined on peripheral groove –Radius equal to half root diameter of worm Drive ratio between worm and worm gear depends on number of teeth in worm gear and whether worm has single or double-start thread

25 72-25 To Mill a Worm Worms often cut on milling machine with rack milling attachment and thread milling cutter –Setup of cutter similar to rack milling –Work held between index centers and rotated by gears between worm shaft and lead screw Short lead (<1in.) attachment used when worm milled because thread has short lead

26 72-26 Procedure to Mill a Worm 1.Calculate all dimensions of thread: lead, pitch, depth, and angle of thread 2.Mount worm blank between dividing head centers located at end of milling machine 3.Determine proper gears for lead and mount them so they connect worm shaft and lead screw 4.Disengage index plate locking device

27 Mount proper thread milling cutter on rack milling attachment 6.Swing rack milling attachment to required helix angle of worm thread and proper direction for lead of worm 7.Center work under cutter 8.Raise work up to cutter 9.Move work clear of cutter, raise table to required depth of thread 10.Cut thread using automatic feed or by manually turning index crank handle

28 72-28 Positive-Drive Clutches Used extensively to drive or disconnect gears and shafts in machine gearboxes –Headstocks on most lathes use clutches to engage or disengage gears to provide different spindle speeds Positive drive produced by means of interlocking teeth Three forms: straight-tooth, inclined-tooth, and sawtooth

29 72-29 Straight-Tooth Clutch Permits rotation in either direction More difficult to engage –Mating teeth and grooves must be in perfect alignment Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

30 72-30 Inclined-Tooth Clutch Provides easier means of engaging or disengaging driving and driven members because of an 8º or 9º angle on faces of teeth –Must be provided with positive means of locking it in engagement Permit shafts to run in either direction without backlash Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

31 72-31 Sawtooth Clutch Permits drive in only one direction More easily engaged than other clutches Angle of teeth generally 60º Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

32 72-32 To Machine a Straight-Tooth Clutch Having Three Teeth Note: method applies to all clutches having odd number of teeth 1.Mount dividing head on milling machine table 2.Mount three-jaw chuck in dividing head 3.Mount workpiece in chuck (vertically)

33 Set sector arms to proper indexing, that is 40/3 = 13 1/3 turns or 13 turns and 13 holes on 39-hole circle 5.Mount side milling cutter on milling machine arbor 6.Set proper spindle speed and table feed 7.Start cutter and adjust work until edge nearest front just touches the inner side of cutter. Set crossfeed graduated feed collar to zero

34 Move table longitudinally until work clear of cutter 9.Move table laterally half diameter of work plus about.001 in. for clearance, and lock saddle in position 10.Set depth of cut, and lock knee clamps 11.Take cut across full width of work 12.Return table to starting position

35 Index for next tooth and take second cut 14.Return table to starting position 15.Index next tooth 16.Take third cut

36 72-36 To Machine a Straight-Tooth Clutch Having Four Teeth Even number of teeth require that machining one side of each tooth first then machining second side –Procedure requires more time Clutches should be designed with odd number of teeth –Reduce machining time –Reduce chance of error

37 72-37 Procedure to Machine Straight- Tooth Clutch Having Four Teeth 1.Mount work as in previous example and set proper indexing 2.Start cutter and adjust workpiece until edge nearest front of machine just touches the inner edge of cutter 3.With work clear of cutter, move saddle over half diameter of work plus thickness of cutter minus.001 in. for clearance

38 Adjust work so cutter over center hole of clutch 5.Set depth and lock knee clamp 6.Take first cut 7.Index and cut remaining teeth on one side 8.Revolve work one-eighth of turn 9.Touch opposite side of work to other side of cutter 10.Repeat operations 3, 4, 5, 6, and 7 until all teeth have been cut


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