The first thing I checked was the motor mount and the anti-backlash nut. To do this I removed the spoil board and the floor. I found a large amount of brass filings everywhere. The previous owner was a shop that used this CNC as a proof-of-concept machine. They were ready to move to a larger CNC and sell this one. It’s clear that they cut a lot of brass.
The motor is mounted at the back of the lead screw and the anti-backlash nut is mounted on the tie plate of the gantry. Both of these looked to be in good shape.
I did notice that the gantry sides had some slop. The right side had the most motion. I could see a gap between the tie plate and the gantry sides. I brought this up with ShopBot to confirm. I had some issues with communication. They wanted me to check the bearings next. I had been suspicious of the bearings for a while and put the slop on the back burner for a bit.
You can see the two linear bearings on the right side in the photo above.
The process is to remove one bearing, check it, reinstall it and then repeat for the second one. That way the gantry stays supported.
I started with the front bearing on the right side. This is the side that was not holding position well. I got the 4 mounting bolts removed and slid the bearing forward. There was a LOT of drag! I managed to pull the bearing forward and off the rail. This is what I found.
There were over a half inch of balls missing. Also, the plastic cage to hold the balls in position was shredded.
You can see another ball that had fallen out. I checked the back bearing on the right side. It felt much better. The left side bearings felt great. I suspect that the previous owners did not clean out the filings very often. It looks like the gantry was dragging through a pile of brass filings.
After checking with ShopBot, I had to verify the type of rails on my machine.
I had the older PMI rails. Since Shopbot no longer supplies those they sent me two new rails, 4 bearings and a new limit switch.
With the bearing problem identified, I returned my attention to the slop in the gantry. I took a look under the gantry and could see problems.
This is the left side. The front bolt is missing, the center bolt is halfway out and the back bolt is tight. On the right side the front bolt is missing, the center bolt is halfway out and the back bolt is loose. So there is ONE bolt holding the tie plate to the gantry!!
The running gear on a a CNC is most important to accuracy. That one has not seen any maintenance for quite a while. Those balls missing must have made the rolling very sloppy! I'm sure you will have it like new condition with the replacement of all those worn parts. Thanks for the information!
Cheers, Jim ........................ Variety is the spice of life...............Learn something new every day
Everybody knows ball bearing slides run better with a few missing balls Steve 🤣 The good news is that you found the culprit pretty quickly and you don't have to spend months trying to run down an unknown problem. FWIW, I'd leave the enclosure. Dust is the arch enemy of machines with ways/slides and bearings.
It looks like the carrier is bolted into wood? I can't tell from the pictures but I assume there are threaded inserts in the wood? In either case, I think I'd swap the wood out for MDF. It's a lot flatter and a lot more stable.
