Introduction: Belt Drive Conversion for the Mini Mill.

About: general bloke type of tinkering

I wanted to upgrade my mill to belt drive for quieter running and a greater speed range, but the only kit available was an expensive import from California ( and it was designed with the 30mm spindle in mind.

My machine is the generic Chinese import as sold in South Africa with the 28mm OD spindle.

Another big advantage using a belt, is that in the event of a tool crash, you dont strip plastic gears as the belt just slips.


We need to build, 2 plates and 2 pulleys and 2 standoffs. The pic shows the belt drive conversion kit.

  • Aluminium plate 138mm x 135mm x 12mm thick, 10mm thick is also adequate.
  • Aluminium plate 138mm x 70mm x 6mm thick, 8mm probably a better choice.
  • 2 x 27.4mm x 25mm OD standoffs, used the riser from casting the pulleys.
  • Step pulley, 75mm x 55mm OD, 28mm ID.
  • Step pulley, 45mm x 28mm OD, 9mm ID.
  • Sewing machine belt MB 320.
  • Various M6 CSK allen screws.

Main tools were lathe to do the pulleys and standoffs, mill to machine the plates and furnace to cast the pulleys.

Step 1: Plates

I bought the 2 plates and the supplier gave me an extra 4mm, alas I thought they were cut to the size I requested and this caused a minor glitch later on as the motor mount plate didnt match the holes for the standoff plate.

It wasn't a train smash as I just offset the top M6 hole on one of the standoffs, but be aware that the A and B dimensions must be equal for everything to screw together smoothly.

All dimensions are in the attached dxf file, I basically took apart the mill head and measured everything with a vernier.

Step 2: Pulleys

I originally went with a 80:20 as the low speed and a 35:30 as the high speed, but couldn't find a single belt solution.

The belt manufacturer doesn't provide center to center belt/pulley measurements, so after some research online, I settled on the MB320 single cog belt with a 75/55 large pulley and a 45/28 small pulley. This gives a ratio of 75:28 for low and 55:45 for high.

Given my motor runs at 5300 rpm, this makes my low speed 1730rpm, which is higher than the original 1100rpm, and 4160rpm for high, which is a lot higher than the original 2500rpm.

The difference between the actual and theoretical speeds is because stock was removed when machining, so instead of a 28mm OD its more like an actual 24.5mm OD : 74.9mm OD.

The main thing is to get the grooves running parallel so that the belt doesn't bend when changing from bottom to top grooves or vice versa.

I've included the 3D printed pulley blanks that I used to make the casts, you'd need to print 2 of each and use 5mm dowels to join them. The larger pulley has a 25mm ID core so I'd have some material to machine, so you'd need to make a 25mm dia sand core for that one.

I've also attached the stl files to print a plastic replica of my aluminium ones in case you want to experiment first before committing to making an aluminium version.

Step 3: Keyways

I used the mill to cut the keyways, making sure it wasn't plugged in to power as I would be clamping the spindle to prevent any rotation. Failure to do so results in a slight spiral as the bit cuts down.

I welded a simple clamp which attaches to the bracket on the front for the chip shield. Once the M6 bolt is tightened, the spindle is rigid.

The smaller 3mm keyway cutter had a bit of flex and I used a small needle file to clean up the way afterwards.

Step 4: Sewing Machine Belt

I've included some info from Mitsuboshi Belting for the cogged sewing machine belt, its 6mm wide and 4mm deep with a 20 degree wall angle, so that's the profile one needs to cut the pulleys.

For some strange reason, the belts have different pricing, so the MB330 which was the suggested belt was more expensive than the 320. Needless to say I went the cheaper route and had to do more juggling with pulley diameters as a result.

Step 5: Left Overs

First pic shows all the pieces which will be left over as being not needed.

I also didn't design a hole in the top plate to accommodate the intermediate shaft because I was going to open the gearbox up and remove all the loose bits inside... peace of mind etc.

I left the steel gears on the spindle to preserve the spacing between the top and bottom spindle bearings.

Step 6: Final Bits

Both the motor gear and spindle top nut have grub screws, dont forget them otherwise you will damage threads/shafts.

The original motor adapter plate has very shallow holes, so I opted to retain the M6 x 9mm screws.