I have just completed the belt drive conversion to my PM-45 mill following the notes from this blog. Thanks to all who have contributed and made the conversion stress free.

A couple of notes that I did not see mentioned in this blog. The head does come off after you remove the motor (for weight reduction), the 2 locating pins (removed with 5mm screws), and the 6 socket head bolts. I had to use a hammer to break the seal (my gasket was simply some silicone sealant) and a couple of screw drivers and spacers to slowly work the head up, twisting slightly. What you are doing is actually pulling the shafts from the bearings or the bearings from the housings.

One modification I had to make was machining a new drive shaft instead of using the existing quill drive, removing the gears, and inverting it for the new pulleys as mentioned in this blog. For up and down motion, I am still using the quill and driving the front handle with a stepper motor (through a 1:4.5 reduction belt drive using a NEMA 23 motor) as shown below. Because the spline in the existing drive is on the lower half, when it is inverted the spline will be on top of the top plate. When the quill is lowered, it will disengage from the spline drive when close to the full down position. Thus I had to fabricate a new drive.

The new drive is fabricated from a piece of 1 5/8" x 1 1/8" mechanical tubing. A drawing is shown below. I used a Hub City Spline coupling (Part # 0332-00344) machined as shown in the drawing. This spline had a little more backlash that the original spline (0.04 vs 0.02 clearance on the spline) but I thought I would give it a try. I could always go back and use the original spline, or my other thought was splitting the Hub City Spline into 2 sections, and then inserting each section into the new drive and twisting one of the sections to remove the clearance and then welding the unit together.

However, I found that this Hub City spline works fine. One note is to make sure you measure the existing bearing spacer and snap ring. The new inner bear spacer needs to be greater than this width to make sure the bearings are not preloaded when you put it all together. I found a 1 3/8” x 18 nut on Ebay that I used for holding everything together.

When I assembled and ran the system for the first time there was a slapping noise coming from the driven pulley. I found that the initial Chinese alignment pins were not correctly placed and there was 0.030 TIR between the quill and the new drive spline. I removed the pins and realigned the quill/drive with a dial indicator and the slapping noise when away.

Here is a picture of the completed belt drive on the machine and a drawing of the new drive. One note, by fabricating a new drive as shown, you can always go back to the gear drive. I don’t think making a new drive is that much more complicated than machining off the gears and threading the drive for a nut. And, a new drive/spline is required if you want to use the full quill up and down motion.