Film advance mechanism for 3D printed film cameras

prusaprinters

I have arthritis so I wanted a lever action film advance rather than a knob, and I wanted the lever to automatically return to the at rest position after each stroke just like the commercial roll film backs that I have.  The design goal meant I needed a torsion type return spring to return the lever to the original position each time the lever was pulled. For proper roll tension and winding I wanted to drive the film advance in the counter-clockwise direction. This meant the spring had to be a right hand wound spring so that it compressed and store energy when the lever is pulled out.  The next criteria I had a lever travel of between 110 - 120 degrees with each pull, which necessitates a spring with a compression angle of at least 110 - 120 degrees.The hardest part was finding a readily available spring that has sufficient torque rating, a suitable compression angle and suitable ends that would require the least amount of fuss to affix to the assembly. I found a right hand spirally wound spring of strip steel on amazon that had an 11mm long end that could be fixed in a slot in the lever handle, and a hooked end that could be abutted against a boss to prevent the spring rotating when the lever is activated. The boss design leaves enough compression space to allow 120 degrees of rotation.  The lever return spring is a replacement spring for a door handle. A package of 3 costs about $12 Canadian. The product name is “SeroNi 3Pcs Replacement Spring for Door Knob Handle Lever Latch Internal Coil Repair Spindle Lock Torsion Spring Flat Section Wire.”https://www.amazon.ca/Replacement-Internal-Spindle-Torsion-Section/dp/B0CDSJR2G9In order to make the design more accessible, I also needed to design a 3D printed pawl and ratchet gear to transfer the lever rotation to a central inner spindle comprised of a hex coupling. Unlike Javier's fixed single pawl design, my pawl had to freely rotate with the lever/knob assembly in both a clockwise and anticlockwise direction.  Since I was using 3D printed parts I decided to use opposing twin pawls for greater strength and to evenly distribute the forces acting on the ratchet gear. I designed a more robust ratchet gear than those proposed by others for this design, it is deep and wide with matched notches.The pawls are now in the knob rather than the body, but the recessed housing design remains closely similar. I did nest the pawl bodies in a circular bearing socket to guide the rotation of the pawls and help retain them in their position. I also added a cap with protruding races that keep the pawls in place as well. For pawl springs I plan to use an open cell 2mm thick elastomeric foamed polyurethane product called poron. But people can also easily source small springs. Poron foam can be found online in many thicknesses, I got mine from a supplier of orthotics supplies. AAA Battery springs can be cut down and used too.The design does require a couple of other purchased parts but they too are readily available on amazon at a low cost.  The hex coupling nut is M3 x 30mm coupling nut. The external dimension is 6mm. They can be found anywhere. You can buy 50 of them for under $20 on amazon. You can also search “Brass Pillar Female Threaded Spacer Nuts Standoff Nuts for Circuit Board in size M3 x30mm to find them.And two M3 screws, I like cap screws. one needs to be 12mm and the other 8mm I think. The heads are usually 5.5mm diameter and will likely need to be sanded/ground to 5mm diameter to more easily fit into film spool ends.Here are the non-printable parts:The principle is that there is a center spindle comprised of a 30mm long hexagon coupling nut with a ratchet gear on one end and a film spool drive spur on the other end. The coupling nut passes thru a hole in the lid body and the two pieces at each end of the shaft keep the assembly trapped in the lid. There is a drive lever, return spring, drive pawl assembly and camera lid through which the spindle passes. The lever assembly is held in position by the lid, the film spool advance nut and the spindle's ratchet gear. The lever is free to rotate in both directions around the central spindle due to the orientation of the ratchet gear, drive pawls and pawl springs, however the drive pawls only permit the spindle to rotate in the counter clockwise direction. When the lever pulled counterclockwise the drive pawls engage the ratchet gear and drive the spindle in the counterclockwise direction.When the lever is released the torsion return spring acts to force the lever in the clockwise direction to return the lever to the origin. As the lever assembly rotates clockwise the pawls ride up the sloped gear teeth and the pawl springs compress, after the pawl slips off a tooth the spring pushes the pawl into the tooth base and then the pawl rides up the tooth face again. The process continues until the spring has released all the stored tension.The return spring is kept in position by a slot in the lever handle and by a boss on the camera lid that prevents the spring from freely rotating around the hew coupling. There is an additional 30mm round boss on the lid that is 2mm high, and a corresponding recess in the underside of the lever. The boss acts as a pivot for the lever and the recess on the underside of the lever acts as a secondary light trap because redundancies are a good thing. Install the spring in the handle. Install film spool feed nut onto one end of the hex coupling.  Push hex nut thru lid from the underside. Place lever over the hex nut and onto the boss on the topside of the lid. Use a small screw driver to ensure the hooked end of the spring is seated against the upper boss as shown in the images.Install the ratchet gear onto the hex nut. Install the pawls in their recesses (use tweezers if necessary) and install the pawl springs. Install the snap fitting cap. It should fit snugly, but can be glued if you use loctite on the screws in the hex nut.