Bill of Materials

This is where I will embed the BOM google sheet as well as walk through the different design choices.

Because of my recent change to the joints, I must also re-evaluate the arms capabilites. I am looking for 16in of range and ~1kg of payload, which would need ~10-12nm of torque. As an impossible figure for a Nema 17 without ridiculous gear ratios, I will switch to a NEMA 23 stepper with a planetary gearbox for the shoulder joint. The other joints will still be NEMA 17s with planetary gearboxes.

But, if I wanted tight tolerances and really good repeatability, I would need to use a cycloidal or strain-wave (harmonic) gearbox. I have found cheap versions of both (relatively, at ~$80 for the harmonic and ~$60 for the cycloidal on Ali Express; amazon's results were minimum $150). The harmonic/cycloidal drives also offer superior mounting, with a flat circular plane with 6 screw holes, as opposed to the keyhole shaft for the planetary drives. As reputable sellers consistantly offer prices 3x that, coupled with bad reviews, I can only assume it is a scam. I will then simply use planetary gearboxes from StepperOnline or Amazon, with a metal coupler to connect the key-ed shaft to the 3d printed arm. In the future, if the load proves to be too great, I will switch to using extruded aluminum as the load-bearer for the arm.

After more research, I found this gearbox. It uses a NEMA 23 with and has a 10:1 reduction with a max torque of 20 NM, perfect for my shoulder joint. While it may be overkill, it gets the job done for less than a NEMA 17 of the same size. For the motor itself, I will be using this motor/driver combo.It has a 2NM NEMA 23 closed loop stepper and a CL57t closed loop stepper driver. The reason I am using this driver over the basic TB6600s is because this driver recieves the built-in encoder's data and automatically adjusts the stepper, which is extremely convenient and takes out a large chunk of software work.

For the base joint, I would like to use an open loop NEMA 23 with a belt drive connecting it to the rotating base assembly, or I will have the motor mounted to the base which will rotate with the base assembly and whose driver pulley will mesh with the static base teeth. I am currently researching different base designs, and I will continue to document ideas.

After this, I have determined that the belt drive is the best option for the base, and I will also use a belt to power the elbow joint too, in order to keep the bulk of the arm's mass closer to the center, reducing load on the shoulder.