Disclaimer! Currently, the design is in revision DRAFT. As such, it is not complete and will be updated as I complete the build myself

Hey Folks!

I'm a big supporter of the Open Source movement. So, I'd like to share for you all a simple G0704 Belt Drive Conversion. The plans are free to use and share, but not for commercial purposes, meaning they fall under Attribution-NonCommercial 4.0 International of the Creative Commons Standard.

This design is made to be elegant but simple. I could have gone with a two bolt pivoting method for tensioning of the belt, but that would require shoulder bolts and other more difficult to find fasteners. The key plan here is to make everything reasonably easy to do, with easy to find hardware. The gearing chosen is a slight bump in speed, but not enough to exceed the stock spindle bearings. For me, this is mostly a jumping-off point for me to play with gearing and motor sizing until I'm happy. And, of course, to quiet down that damn gear drive! I will update the hardware list once I have the full build speced out, but for anyone trying this early you can figure out which bolts need to be longer I'm sure. To reduce complexity its entirely possible to leave some of the more decorative features out (counterbores) to make this easier to manufacture.

Stock Required

Steel or Aluminum
2x 1.25 x (0.5 or 0.625) x 7.85
1x 4.375 x 3.75" x .5

Models and Drawings for Standoff and Motor Mount Plate

Link to Google Drive

Belt Drive Components
42 tooth "L" Series Belt, 1/2" Wide - PN# A 6R 4-042050
.375" (L) Pitch,21 Teeth, 0.625" Bore, 2 Flanges / With Hub, Aluminum Alloy Timing Pulley for 0.5" Wide Belt - PN# A 6A 4-21DF05020
.375" (L) Pitch,14 Teeth, 0.375" Bore, 2 Flanges / With Hub, Aluminum Alloy Timing Pulley for 0.5" Wide Belt - PN# A 6A 4-14DF05012

Quick Math
The max motor RPM I can see on my Spindle motor is around 4000. The above gearing choice gives a stepdown of 0.6667, equating to a max RPM of 2666. This is just slightly higher over stock, but take care if you wanted to gear your spindle up, you will have to select different pulleys. This system was designed with a 4.574" center to center distance. SDP/SI's center distance calculator should be able to provide you with some options. I chose to not bump my spindle speed up much as I plan to install a larger motor and bearings that can handle a higher speed soon.

Pulley Prep
Finish ream small pulley to 10mm shaft diameter (or 14mm, depending on your motor). This part is now done

Attachment 312542

Bore large pulley to 0.8515". Measure your quill shaft diameter to find your actual measurement, the preceding one is what worked for me.
Bore a step down 0.15" and to an ID of 1.000". This step will be where the threaded cap that retains your drawbar will sit.
Part protruding hub of large pulley. (Note: I knocked off one of the retaining side plates in order to get a good clamping location for my lathe, then pressed it back on with an arbor press after turning)
Drill and tap 4x 6-32 screws through the root of 4 equally spaced grooves in the center of your pulley. These are used to hold set screws to secure the pulley to your quill shaft

Attachment 312544
Attachment 312546
Attachment 312548

Installation: Will flesh this out with photos when I have completed my work.

Additional Hardware: Will flesh this out with photos when I have completed my work.

Sample G Code: Will flesh this out with photos when I have completed my work. <--- Would anyone actually want this? Conservative toolpaths with standard tooling, or is everyone like me and wouldn't trust a random .nc from the internet?