1. Cam, Rack, Worm, and Clutch Milling
Unit 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. 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. Positive-Type Cams Control follower at all times
Follower remains engaged in groove on face or periphery of cam
Cylindrical
Grooved plate
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5. Nonpositive-Type Cams
Cam pushes follower in given direction and depends on some external force to keep follower bearing against cam surface
Plate
Toe and wiper
Crown
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6. Types of Followers
Roller: least frictional drag, little or no lubrication
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)
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7. Cam Motions
Three standard types of motions imparted by cams to followers and machine parts:
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. Uniformly accelerated and decelerated motion
Harmonic-motion
Provides smooth start and stop to cycle
Used when uniformity of motion not essential and high speeds required
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. Radial Cam Terms Lobe Rise
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. 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. 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. 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. Calculations Required
If circumference is divided into 100 equal parts, it will necessary to calculate the lead as follows:

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. L = lead to which machine geared
Feed
Table
Dividing head
Cutter
Cam
a= 360º
L = lead to which machine geared
It will be necessary to swing work and vertical head to definite angle.
i = angle of inclination of dividing head spindle in degrees
H = lead of cam
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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:

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

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

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

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

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

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

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

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. Straight-Tooth Clutch
Permits rotation in either direction
More difficult to engage
Mating teeth and grooves must be in perfect alignment
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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
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31. Sawtooth Clutch Permits drive in only one direction
More easily engaged than other clutches
Angle of teeth generally 60º
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32. To Machine a Straight-Tooth Clutch Having Three Teeth
Note: method applies to all clutches having odd number of teeth
Mount dividing head on milling machine table
Mount three-jaw chuck in dividing head
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
Mount side milling cutter on milling machine arbor
Set proper spindle speed and table feed
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
Move table laterally half diameter of work plus about .001 in. for clearance, and lock saddle in position
Set depth of cut, and lock knee clamps
Take cut across full width of work
Return table to starting position

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

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. Procedure to Machine Straight-Tooth Clutch Having Four Teeth
Mount work as in previous example and set proper indexing
Start cutter and adjust workpiece until edge nearest front of machine just touches the inner edge of cutter
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
Set depth and lock knee clamp
Take first cut
Index and cut remaining teeth on one side
Revolve work one-eighth of turn
Touch opposite side of work to other side of cutter
Repeat operations 3, 4, 5, 6, and 7 until all teeth have been cut