I had a manual Sherline Rotary Table (#3700) that I wanted to convert to CNC. This design is for a frame to secure a Stepper Motor to the Rotary Table's Worm gear's shaft, as well as a spider coupler to transmit the rotary forces from the stepper motor to the shaft.

You will need PLA material (or ABS) and TPU for the spider coupler. Apart from the 3D printed parts, you will also need:

1) A stepper motor. I used a 17HS4401 NEMA 17 Stepper Motor
2) 3 x M3x40mm screws - to secure the mount bracket to the mount holdfast, and then to motor frame
3) 4 x M3x5mm screws - to mount the stepper motor to the motor frame.
4) 2 x M3x10 screws - to secure the mount and shaft couplers to the stepper shaft and worm gear shaft
5) 7 (or possibly less) M3 nuts

Printrbot

Printrbot Simple Metal

No

Yes

All the printed parts can be of PLA or ABS, but the Spider should be printed with TPU or other compliant material.

For the mount bracket and mount holdfast, I chose to print them with their long shafts pointing straight up. You will need supports under them as that surface is not critical,

All the holes will probably need to be widened up as they were designed to be a tight, snug, or even to be press fit. You will need drills of the correct sizes. The ones I needed were 5mm, 6.3mm, and 2.5mm for the stepper motor shaft, the worm gear shaft, and the M3 screws holes respectively.

The handwheel of the Rotary Table should be removed from the Worm Gear Shaft by removing the 2 screws holding it on.

The Mount Bracket and Mount Holdfast should be assembled on the Worm Gear housing, and secured with the M3x40 screw and nut. Check for wobble and shim with cardstock if necessary so that the wobble is minimized.

The Motor Frame should be mounted to the stepper motor with the 4 M3x5 screws.

The Motor Coupler should be mounted on the Stepper motor shaft, and secured with an M3x10 screw. The shaft should be flush, and should not protrude from the coupler.

The Shaft Coupler should be mounted on the Worm Gear shaft and secured with an M3x10 screw. The shaft should be flush, and should not protrude from the coupler.

The Spider should be placed on the Shaft Coupler.

The Motor Frame with Stepper Motor and Motor Coupler should then all be mounted to the Motor Bracket and Holdfast, making sure that the Spider Coupler is properly meshed with the Motor and Shaft Couplers. Secure together with 2 M3x40 screws and nuts. You may need to place 2 nuts per screw as spacers depending on the length of the stepper motors shaft.

Test by turning the motor coupler by hand and make sure that the worm gear moves. Any wobble or non-centricity of the motor shaft and worm gear shaft should be absorbed by the spider. Some wobbling is to be expected in the housing due to the lack of precision in the parts.

I hope it works as well for you as it does for me!

I did study some of the other designs to couple the stepper motor to the handwheel of a Sherline lathe. I did design a coupler to turn the handwheel, but I found the outcome to be unsatisfactory as the wobble was quite large. The turning of the handwheel via the handle is ultimately eccentric, so transmiting the forces that way was not ideal. I found that removing the handwheel was very simple (just don't lose the parts!), and coupling the shafts using a spider coupler was the best I could come up with.

I also wanted to have the Mount Bracket grip the Worm Gear housing tightly on 3 sides, so you'll notice that the Mount Holdfast has some sloped edges to transmit the forces correctly (it should be pulled IN to the housing as the screw is tightened). This didn't work as well for me as the tolerances were not good enough, and I didn't want to reprint the parts, so I ended up shimming the gaps.