RF-45 Repair – Making a new Y-Axis Nut

The most recent success in my ongoing RF-45 mill repair project is the fabrication of a new Y-Axis lead nut. The stock nut was apparently lost somewhere along the way during the attempted CNC conversion that I’m partially reversing, so I had to come up with a replacement.

My first thought at replacing the nut was to make a new one out of acetal plastic based on the method described at the Home Shop Machinist Forum. On second thought, I decided it would be much more practical to see if I could find a replacement part online. Like many of the chinese machine tools out there, this mill is one of many clones of an original design. In this case, the Rong-Fu 45 is the original, and my copy was produced by Penn Tools. Unfortunately, I was unable to find replacement parts through them, but Grizzly sells a similar mill along with replacement parts. I dug through the manual to find the part number, contacted Grizzly, and ordered the nut. Easy enough right?

Unfortunately, the importer of my mill chose to use 8 TPI leadscrews, while the Grizzly version uses 10 TPI leadscrews, so the nut didn’t work. I returned it to Grizzly, and set off on my original plan of making the nut from scratch.

I started by boring and splitting the acetal to fit the lead screw:

Then I heated the leadscrew using a heat gun while squeezing the nut pieces together in the vise:

My first attempt didn’t work out so well…

The problem appeared to be that the acetal rod I’d purchased was too close in size to the leadscrew, making the walls of the acetal pieces too thin to properly clamp around the threads. The finished piece ended up oval, and poorly fit to the threads. In an effort to make the plastic form easier, I overheated it, resulting in the goopy mess seen above. I put the leadscrew in the lathe and turned the plastic off, ready to try again.

My next attempt went much better, since I decided to make an aluminum reinforcing sleeve to support the acetal and keep it round during the forming process.

This probably wouldn’t have been necessary if the acetal rod were larger, but this made it possible to use what I already had on hand.

The heating/forming process was repeated, paying closer attention to the temperature and evenness of heating:

The plastic started to flow right around 230 deg F. I was much more careful about heating the screw evenly this time, resulting in a nice even flow across the length of the thread:

A little more time and clamping resulted in a fully formed and fused nut. The halves didn’t join together in the kerf quite as well as I’d hoped, but they stuck well enough to become one piece. I put the leadscrew back in the lathe to clean up the OD of the nut:

After a bit of a struggle, I was able to get the nut loose from the screw. I clamped the nut in the chuck of the lathe, locked the spindle, and cranked on the screw until it finally came loose. I then proceeded to face the ends of the nut square:

If you look closely, you can see that the kerf is fused a the ID, the flow just didn’t make it all the way to the OD. Good enough for me I guess :).

I then turned the leadscrew into a makeshift tap to loosen up the fit between the nut and the screw. I ran the nut in and out several time until the fit felt right, then blunted the edge on the screw so it wouldn’t cut in the future. I ended up making the notch bigger than what’s shown in the picture to provide more clearance for the cut material.

The threaded part of the nut is now finished, next I’ll make a mount for it.

Using my mini-mill and lathe, I made up a mounting block out of aluminum for attaching the plastic nut to the mill’s saddle. I’ll touch on a few highlights of the process below.

Drilling a pilot hole for the press fit bore:

Locating the hole in the four jaw chuck in the lathe (I don’t have a boring head for the mill yet, so this is the only way for me to machine accurate large diameter holes):

Boring the hole to size:

After a few more features and a retainer plate, the mount was finished:

After installing the nut on the mill, I’ve now got a working Y-Axis. All that’s left now is to repair the leadscrew mount on the end of the table, reassemble everything, and I’ll be ready to test the mill!