The advance of the film frame by frame is made by 360° rotation of the cam for each frame advance. The cam start point should be set at 0° or slightly later as the film is then stationary in the film gate for at least the next 180° of the cam rotation. The video image is captured when the film is stationary.
The cam is mounted directly on the motor shaft of a standard NEMA 17 size stepper motor. The chosen motor has ball race bearings, a short stiff shaft that does not flex when a load is exerted on the cam and excellent step position repeatability. The motor is rigidly mounted on the chassis.
Film take up winding has to be managed to prevent movement or vibration when the image is captured. I used the same NEMA 17 stepper motor to drive the winding reel. Drive pulses were only sent to the wind stepper motor after the cam passed 180º. A spring loaded vane and slotted optical vane switch are used to determine if there is sufficient slack film to need the wind motor to start winding. See Slotted optical switch for more detail.
The wind motor shaft was fitted with a suitable sized boss made to be a press fit friction drive in the film reel used. The press fit friction drive worked well with the large hole used in super 8 mm film format reels made of plastic up to 9 inch. Steel reels with the smaller drive hole used in standard 8 mm film format reels required an extended drive boss threaded on its outer end so a nut could then be used to clamp the reel on its side faces to ensure reliable drive.
