Y Axis Cross feed problem
I had not used the mill all that much since it was purchased, and it was still very much at the "running in" stage. However I have been doing the odd job with it here and there. It's been working as expected, but there was a bit of a disaster - well it seemed that way at first.
The Y-Axis (Cross feed, or as I prefer to call it, the forward and back drive) seized solid on me during a milling operation. At first I thought that some swarf might have jammed the dovetail, but I could not see anything obvious. After a bit of head scratching, I realized that the problem was the feed screw had seized in its bearing.
The bearing is just a hole drilled through the alloy block shown above. It's nothing much and to be honest it really does not need much more than a simple sleeve type bearing, but for some reason this one had jammed solid. I removed the chip guard rubber (that's something that needs a mod of some sort, really awkward to remove.) Then I removed the two socket head screws that attach the bearing block to the base of the mill. With these undone I was able to remove the lead screw assembly. Remember it's a left hand thread, so you unscrew it CLOCKWISE.
Once removed there was nothing obvious to see what would indicate what the problem was, so The next thing was to try and free up the lead screw. a few brute force attempts were made, but it would not budge. So the next stage was to remove the handle, two nuts, a washer and the handle comes off. There is a small key that locks the handle to the shaft, be careful not to let it fall out.
What I really needed was some form of press to force the lead screw out of the bearing, but I have nothing available that would do the job, so once again, brute force was applied. The nuts were replaced onto the end of the lead screw to protect the thread, and supporting the alloy bearing block in the vice, a few thumps with a hammer was applied to the nuts on the end of the lead screw, using the nuts as an anvil. I must have been a bit lucky, and after the third thump, the lead screw started to shift. A few more taps and the shaft was free of the bearing, and the problem was then very easy to see. For some reason a small flake of metal had separated from the lead screw shaft and had dug itself into the inside of the bearing jamming it. The metal flake was not swarf from the machining of the lead screw, but it looked more like a small defect in the metal that eventually broke away. As a temp. repair, I smoothed out the lead screw bearing surface with a quick skim in the lathe, and a slightly deeper skim was done on the block bearing as that had been rather badly scored. This got the mill back into operation, but having seen the problem I thought that an improved bearing would be a good idea. All I did was address the sleeve bearing, and what I did was add two ball races one at either end of the alloy bearing block.
While the lead screw was out I measured the lead screw and I found that it was 12mm (don't forget that I had skimmed it slightly, it may have been 1/2 inch orig). I used a couple of 12x18x4 bearings (S61701 ZZ from Arc Euro Trade Ltd).
I bored out the bearing block to provide clearance around the lead screw, in fact I made this slightly larger than the inner section of the ball races. I then bored a 4.1mm x 18 mm recess in each end to take the bearing. The recess is slightly deeper than the bearing, because you do not want to push against the bearings, or they will be damaged. There looks to be enough space to add thrust bearings should the need arise.
The bearing block, lead screw and handle was reassembled, and the two locknuts adjusted to provide free movement of the lead screw within the bearing, and I must admit that it turned very smoothly indeed. Installing the lead screw assembly back into the mill base, and just making sure that the lead screw did not contact the base casting before the two socket head screws were tightened up. The result is a very smooth Y axis feed many times better than what it was when the mill was first purchased.
