Saturday, October 10, 2015


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.


Will said...

Is the nut supposed to be a jam/lock nut? Is it intended to lock the lead screw into the stepper motor? Is there some way to lock up (hold the shaft from turning) the motor? Cross pin, or maybe a hex socket in the end of the motor shaft?

Are you sure the parts are correct?
If they did not want to start threading easily, it's possible that the shaft threads got distorted when the crash occurred. This may effect how accurately the cnc system can move the stages.

If you distorted the gear teeth by gripping them, this will cause problems. If it's just burrs on the outer edge, a file or grinder may fix it. What are the parts made of? Have you measured the diameter of the new and original parts, and the clearance hole/port?

I'm not familiar with the Sherline equipment, which is why I'm asking for more detail. Are there any exploded views of the setup?

Clayton Cramer said...

I see what looks like LocTite where the nut was at the near of the lead screw. I will apply some LocTite there.

Will said...

I would be very surprised if they used a threadlocking compound as the only means to hold a nut from shifting. However, there are some versions that would make it pretty much a permanent installation, especially if there is no mechanical way to hold that shaft.

One option is to install two nuts and tighten them together, and then use them to hold the shaft. That works only if you can remove them later, as they would be in the way of machine function. Be careful that you do not distort the shaft by applying too much force. Depending on size and thread, you can stretch or break it.