Being able to print out custom gears is one area where 3D printing can really shine, and [Karl Lew] has been busy doing exactly that with pinion gears printed in PLA and mounted to stepper motor shafts, but there are tradeoffs. Pinion gears need to grip a motor shaft tightly – normally done with a screw through the gear and onto the motor shaft. But a motor and its shaft can get quite warm when doing a lot of work, and a tight screw on a hot motor’s shaft will transmit that heat into the PLA, which can then deform.
[Karl Lew] managed to improve things in an unusual way: using a hot water bath to anneal the gear while it is attached to the stepper shaft. Annealing PLA has the effect of increasing the crystallinity of the material, which – according to an article going into some detail about the process of annealing PLA – increases stiffness, strength, and heat deflection. The annealing process also shrinks the part slightly, which happens to result in a very tight joint made between the gear and the slotted stepper shaft if the gear is annealed while connected to the motor.
[Karl Lew]’s wiki page on PLA annealing sums up the important parts of his test:
Attach your pinion gear to the stepper shaft with a loose set screw and anneal it for 50 minutes at 160°F/71°C water using double air-evacuated plastic bags (e.g., Ziploc). In this case, we are making use of the shrinkage associated with annealing. Part shrinkage is normally a bad thing, but here we are using it to good effect to obtain a solid and tight joint with a slotted stepper shaft. Be sure to use high-infill (80% or greater) for annealed pinion gears. Shaft-annealed pinion gears require little or no set screw tension to stay in place on a slotted drive shaft.
If using hot water to improve the gear side of things is too low-tech, there’s always converting the motor itself into a closed-loop stepper servo to complete the upgrade circle.