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.