8. Cam Generation

The packing size derivation and calculation to obtain the correct offset for generation of the eccentric circular cam to generate the required stroke using a 3-jaw lathe chuck is given below.

To accommodate both Standard 8 mm and Super 8 mm film sprocket hole spacing a stroke of 5.4 mm was chosen thus the offset d = 2.7 mm. Using a 18 mm diameter rod the radius r = 9 mm. Substituting the values for d and r in the above formula the packing P required is 3.74094 mm. For quickness and accuracy I use the shank of the nearest drill size I have to provide the required packing. So from my drill selection this is a No.26 or No.25 series drill with a diameter of 3.734 mm or 3.797 mm. The cam diameter must have a clearance in the cam rider of about 0.1 mm to ensure free rotation of the claw arm about the cam. The free rotation of the claw arm facilitates low drag disengagement of the claw from the film sprocket hole during normal operation and allows lifting of the claw during film threading or splice repair operations. The cam claw aperture is 12 mm high by 18.1 mm wide and the cam diameter is 11.9 mm. The cam rider running slots position the rider so the cam only ever touches the top and bottom of the cam claw aperture and never exerts side thrust on the cam rider.

Photograph 22 shows my lathe chuck arrangement for making the cam. I coated the end of the cam with black marker pen to aid clarity. Figure 10 shows a drawing of the cam. For safety always wear protective goggles, secure the drill to the jaw using soft copper or iron wire and make sure the drill ends are within the outside diameter sweep of the chuck.

  • 1. Face off and then turn down rod to 18 mm diameter.
  • 2. Centre drill and then bore 5 mm hole to fit NEMA 17 motor shaft.
  • 3. Reverse rod so that only the 18 mm diameter section is within the jaws.
  • 4. Insert packing piece at jaw 1 midway along the section within the jaws to get the required offset, ensure the unpacked jaws 2 and 3 are in full contact and then fully tighten chuck. Secure packing to jaw 1 with copper wire.
  • 5. Face off the end of the rod. Then turn down the eccentric circular cam to the required diameter of 11.9 mm. The cut is intermittent so many light cuts are safer. For extra safety I have left the 5 mm hole blind so I could centre drill the cam end of the rod when offset and fit a revolving centre in the tailstock. Before removing the piece from the chuck scribe a top dead centre (TDC) line along the 18 mm section of the cam using the lathe tool. Use an engineers square on the lathe bed and rotate the chuck by hand until jaws 2 and 3 are furthest from you and both jaws touch the vertical of the square. Adjust the lathe tool to scribe a line along the exposed parts of the surface of the 18 mm section as it is traversed. Remove and then reverse the cam in the chuck without any packing, with the jaws on the 18 mm section only, rotate the cam so the TDC line is midway between jaws and tighten the chuck, rotate the chuck by hand so the lathe tool and TDC mark align and then scribe the line the full length of the 18 mm section and across the face of the cam.
  • 6. Mark the TDC scribed line with black marker pen.
  • 7. Drill 2.55 mm tapping hole for grub screw. Then tap 3 mm.



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