Summary

Simple 2D SCARA like robot arm using inexpensive 28BYJ-48 motors, 6807-2RS bearings and inexpensive contact switches.

Videos:
https://youtu.be/o8eF27kjyfY (homing and line drawing)
https://youtu.be/S-C0HrY7dIo (homing)

Software (on github in mcp folder: https://github.com/beckdac/SCARA ) to control the arm from the command line includes threaded server with homing

Instructions

The full up-to-date source for the hardware and software are available here: https://github.com/beckdac/SCARA

Please use the github sources. The files uploaded here may not be up to date. I definitely recommend using the Makefile which is setup for running openscad from the command line on a Mac. Definitely check the adjustable parameters in SCARA.scad. The arm lengths can be adjusted in this file but need to be matched in the mcp program's kinematics.h that is part of the github repo. The current inverse kinetics code is complete, but the command line drawing program doesn't support much else from drawing simple lines. Run with mcp
core run

A montage of assembly photographs including which M3 screw lengths to use is available in the assembly.pdf file: https://thingiverse-production-new.s3.amazonaws.com/assets/f1/b3/03/a4/0d/assembly.pdf

The hardware is M3.

Two 6807-2RS bearings per joint (4 total).

Two 28BYJ-48 steppers are required. You can mount them on the top or bottom of each joint. If you change the arm lengths and find the steppers lack power, you can mount two on each joint (total = 4) and run the top and bottom for a joint in opposite wiring patterns so that they run in opposite direction from the same pulse train.

Two cheap contact switches for limit detection per joint.

The laser used in the demos: https://www.amazon.com/gp/product/B00R73MC1S/ref=cm_cr_ryp_prd_ttl_sol_2

The arm can easily support double the version I printed here (which was for testing) and will realize quite a large envelope.

I printed the shoulder to elbow lengths upside down from the orientation of the STL uploaded to thingiverse, with very light support (~ 7%). Then, I cut the support out with an exactoknife and patience to make a clean fitting for the bearing.

Inexpensive SCARA arm (2D) w/ 28BYJ-48 steppers