Friday, October 16, 2015

Sherline Repair

I doubt most of you care about this, but I am putting it as a resource for others who might need help.

I followed instructions at for installing new Y axis backlash nut  Problem is that the nut does not go on the leadscrew easily.  It requires grabbing it with channel locks to turn on to lead screw, which chews up the brass barrel of the nut, preventing it from sliding into the recess where it goes.  Trying to turn the lead screw back into the X-axis carrier base, requires turning the nut at base of motor mount, but that rapidly unscrews from the base because it is threaded the opposite direction of the lead screw.   Solution, Loc-Tite to prevent turning.

I ordered two more backlash nuts and a new lead screw (which was $15.50--much cheaper than I expected).  Why the new lead screw?  Because I suspected the problem wasw damage to the lead screw, and Sherline warned me that they thread their lead screws about .100" oversize, and just using a LH 1/4"-20 die would cause backlash problems.

The new backlash nut turned onto the new lead screw without difficulty, but the nut would not turn into the  hole, so I assumed the problem was debris in the hole and I removed the lead screw.  But removing the nut was harder than going on, and after verifying the nut would slide into the hole just fine by itself, it would not go easily back onto the lead screw.  It looks like the place where the set screw through the table hits the lead screw is the problem.  (Don't screw it down so tightly next time.)  I used a file and sandpaper on that section without success.

Because the problem was apparently damaged threads on the Y lead screw(s), and Sherline makes them .100" oversize, I used an adjustable die open as far as possible.  As I had hoped, the die met no resistance except in the sections where backlash nuts had stuck. As a result, I can now turn the backlash nuts down the lead screw(s) by hand without difficulty.  So I reassembled, and even though the star washer is grabbing the projections on the backlash nut, the backlash nut still reverses out of the hole, failing to pull the table back on the Y axis.  I tried stacking two star washers to increase the range of spaces to grab the backlash nut, without success.  I am rapidly developing gobs of experience on repair of this mill, but I still don't have a working CNC mill.

Some pictures.


Jim Dunmyer said...

Not to be a nit picker, but Sherline can't possibly be making their leadscrews .100" oversize. An oversize of .001" makes more sense.

Will said...

It appears that the large button head screw to the side of the leadscrew, on the saddle block, is intended to lock that brass washer that is under it.

That engages the brass nut the leadscrew runs through. Make sure the "anti-rotating" washer (actual name: backlash lock) properly engages the brass "backlash screw", before tightening it. Back off the button head enough to be able to lift the brash washer to find a good engagement fit to the backlash screw (nut, or insert, might be a better term). I'm assuming that the brass piece that the leadscrew runs through is threaded on the outer diameter, and threads into that saddle block.

Clayton Cramer said...

Will: you are correct. It appears the star washer is not engaging the backlash nut adequately. I am suspecting one of these parts is out of the proper tolerances.

Will said...

That's why I suggested that you back out the button head so you can lift and turn the locking washer to look for the best engagement in those radial splines. If they have gotten shredded by forced rotation of the brass insert/nut, you may have to replace one or both parts for adequate engagement.

It would seem that the way they are locking the insert, it is intended to be able to override it under severe conditions. Otherwise, it could be done better.

If the spot facing the locking setup is bad, but the rest of it is ok, you could chuck it in a lathe, and remove a little from the back side, where it seats against the block. This should allow the part to rotate a bit more into the block, bringing a fresh engagement surface into alignment with the locking washer setup.

Trying to put a washer under it, to shift the engagement spot, would not be a good thing to attempt, as it will cause interference with the locking setup, unless you get very creative with a notched washer. I doubt there is enough space to try it. And, it may cause other clearance problems.

Clayton Cramer said...

Tried that. The next step is to ship back to Sherline. The splines can't engage. I suspect there is a tolerance issue with the replacement parts.

Will said...

Does it act like the parts are too far apart, or too close? It would not surprise me to find that the holes in the block are not correctly spaced. If one of those seems to be true, you might try to measure the center to center of them, and call customer support to check what the spacing should be.

Of course, they are not called out like that, typically. There would be a 0x0y data point, but they should be able to figure it out.

If the spacing seems off, and you have compared new and old parts, getting this question sorted out before you start shipping parts back and forth may save you a fair amount of time. As part of this investigation, look closely at the original parts, and see if it looks like they have been modified to make them fit.

Clayton Cramer said...

Will: too far apart. My guess is that some handfitting is required to deal with manufacturing tolerances. A few thousandths of an inch difference on hole locations and the same for star washer and backlash nut would be a problem I have shipped the base back to Sherline. They will have it Wednesday, and I suspect I will have it back next week.