Help

I have a Sherline 5400 CNC mill, and I have a problem. The problem started as: The manual that Sherline hands out with their CNC mill emphasizes that you should know exactly what each command will do before running the program.

I had wanted to improve the finish on the completed parts, because there is a series of stripes in the direction of milling. It is only appearance; there is no detectable edge, so .001" or less difference. But I thought I would smooth it perpendicular to the direction of the milling. So I modified the excavate program to mill in the X direction, then Y, instead of X then Y. The excavateyx program would be called with parameters to remove .0001" of material.

Fine. I wrote the program, and failed to notice that when computing the end of the X movement, I needed to subtract the mill radius from the destination. The right test strategy would have been to start the program, single step each line, then override the feed rate for the first line. Nope! The mill banged into the edge of the workpiece at 6 in/min, the rotation then yanked the Y axis towards the table hard enough to strip the teeth from two star gears that control the Y axis feed. Fortunately, these are $5 for both parts, but I ordered two, in case I screw up again.

The problem is that to replace the antibacklash nut on the Y axis lead screw, you have to remove the old nut, screw the new one on, and then reinstall the lead screw. Easier said than done. I moved the X axis far enough left to unscrew the set screw that prevents the lead screw from coming out, unscrewed the stepper motor mount from the base, and unscrewed the lead screw. (It is a left hand screw thread, just to humble me.) Okay, the new antibacklash screw did not want to go on the lead screw easily, but eventually, I managed to grip it with a channel lock, and get it on the lead screw. It turned freely for most of the screw's length, but it would not slide easily into hole for the backlash nut, probably because the channel lock deformed it. The same thing with the other backlash nut. I have a solution to this--sand the outside of the nut.

The bigger problem is that the lead screw does not want to screw back into the X table. If I use the 3/8" nut just in from the motor mount housing, turning CCW (left hand thread on lead screw), it just moves up the lead screw instead of turning the lead screw in. How to solve this?

UPDATE: I see evidence the nut was held by some LocTite-like adhesive in position in the past, so I applied LocTite to keep it from moving up  the lead screw.  I am also buying left hand 1/4"-20 die and tap tomorrow to clean up the lead screw and the brass backlash nut.