Tuesday, October 13, 2015

Sherline Repair

So I needed to replace the backlash nut on the Y axis lead screw.  The replacement nut did not go on easily at all.  I ended up grabbing it with channel locks to turn it on the lead screw.  The nut is brass, so the lead screw effectively acted as a tap.  But it still wasn't turning smoothly, so I went hunting for a left hand 1/4"-20 tap and die.  The tap came from Industrial Hardware in Garden City; they were surprised to find they had one.  No left hand die; but I found that at Idaho Machinery & Supply in Meridian.   This is a dangerous store for an aspiring machinist to enter.  Next door is even more dangerous: the Haas CNC factory outlet store.

Anyway, I was warned by Sherline that using my own 1/4"-20 tap on the nut and die on the lead screw would cause backlash problems.  Why?  Isn't 1/4"-20 a standard?  But I settled for tapping the replacement backlash nuts, which now turn onto the lead screw pretty easily (which I had also given a quick rubbing with sandpaper, which might have been what it needed to get the nuts on smoothly anyway)..  The end of the nut that slides into the X table base was a bit chewed up by the channel lock, so I put the nut on the tap, then put the tap into the lathe, and smoothed down the section that inserts into the table base.  Trying to hold the nut in the 3-jaw chuck by itself was pretty much impossible because of its size.

Okay, the nut now moves smoothly on the lead screw, but the remaining problem is that the backlash nut has a series of projections which lock into a star washer (also made of brass) immediately adjacent on the table base.  This prevents rotation of the backlash nut on the lead screw.  It also somehow forces rotation of the lead screw to advance the X table forward, although I don't see how.  In this case, the nut advances without dragging along the start nut and therefore the base, so I am still unable to work.  I think the projections on the nut were damaged while trying to put this on the lead screw the first time, so I ordered some replacement nuts ($2.75 each) and in case I need it, a replacement lead screw ($15.75).  I hope to be operational tomorrow.  In the meantime I am learning an enormous amount about repair and maintenance of the Sherline mill.  I really just want to get these six pieces made that I need for customers, and then consider moving up to a more serious CNC mill.


Will said...

did you remove the motor mount to access the screw that captures the end of the leadscrew? I'm looking at this:



I'm also seeing some setscrews in the stages, and suspect that they may capture the nuts that run on the leadscrews. I'll look some more later this evening.

Clayton Cramer said...

Yes, I remove the motor mount.