Archive for February 28th, 2017

Mini-Lathe Tailstock Way Repair

After the faceplant caused by the crappy compound way finishing, I decided to try repairing the tailstock ways as a means of gaining experience before tackling the real problem. The general idea is to see whether filling the gouges with epoxy will suffice.

I’m using good ol’ JB Weld steel-filled epoxy, rather than graphite / molybdenum disulfide loaded epoxy, mostly because:

  • I have it on the shelf
  • This is a non-sliding joint
  • My technique needs polishing, too

The key point: the tailstock is (astonishingly) well aligned and, if I can manage to not change how it sits on the lathe bed, this should be a zero-impact operation. Scraping / filing / fiddling with the high spots will change the alignment; I expect I must eventually do such things; this represents a first pass at the problem.

Applying a fat blue Sharpie to the tailstock ways:

Tailstock way repair - blue Sharpie

Tailstock way repair – blue Sharpie

After sliding the tailstock back and forth a few times, the remaining blue shows where the ways did not make contact. Those shiny and silvery spots rubbed against the lathe bed ways.

The flat way looked like this:

Tailstock way repair - flat contacts

Tailstock way repair – flat contacts

The patch along the upper-left edge and the small dot near the upper-right corner are the only contact points across the entire flat.

The outside of the V groove:

Tailstock way repair - outer V contacts

Tailstock way repair – outer V contacts

As nearly as I can tell, that’s actually a reasonably flat and well-aligned surface, with small contact points scattered all over. Granted, there’s a larger contact patch to the left and less to the right.

The inside of the V groove:

Tailstock way repair - inner V contacts

Tailstock way repair – inner V contacts

There’s a single point near the top left, another over on the right, and that’s about it.

I cleaned the tailstock ways with acetone to get rid of the Sharpie / grease / oil / whatever. Under normal circumstances you’d roughen the surface to give the epoxy something to grip, which definitely seemed akin to perfuming a lily.

To prevent permanently affixing the tailstock to the lathe, some folks put a generous layer of oil / graphite / soot / release agent on the lathe bed ways. I used some 3 mil = 0.08 mm Kapton tape, figuring an impervious layer would pretty much guarantee I could get the tailstock off again, no matter what.

So, we begin.

Butter up the tailstock ways with epoxy and smoosh into place atop the Kapton:

Tailstock way repair - V groove on tape

Tailstock way repair – V groove on tape

Make sure the tailstock remains well-seated where it should be:

Tailstock way repair - weights

Tailstock way repair – weights

Do other things for 24 hours while the epoxy cures, pry the tailstock loose by hammering The Giant Prying Screwdriver between the lathe bed and the underside of the tailstock (just right of the V-groove, where nothing slides on the bed, but I did use a bit of plastic as a shield), chip off excess epoxy, clean things up, etc, etc.

This time, I applied Sharpie to the lathe bed, then slid the tailstock back & forth a few times. As a result, the blue areas now show the contact patches and the gray areas just slid by without touching.

The flat way looks pretty good:

Tailstock way repair - flat epoxy blued

Tailstock way repair – flat epoxy blued

That round dot over on the right seems to be a steel protrusion; I think it’s part of the same lump appearing in the “before” picture above. That rather sharp point seems to have indented the tape and produced a low area in the epoxy around it, which may not matter much: it was the only contact point before I did this.

The V groove isn’t anywhere near perfect:

Tailstock way repair - V groove epoxy blued

Tailstock way repair – V groove epoxy blued

On the upside, the ways have much, much larger contact patches spread across nearly their entire lengths, which isn’t to be sniffed at.

While reassembling the tailstock, I added a pair of M6 washers above the clamp plate so it cleared the bed with the screw tightened into the cam-lock post:

Tailstock clamp plate - washers

Tailstock clamp plate – washers

Which definitely calls for a small bushing, of course. If you put a lockwasher under the screw head, it won’t clear the end of the bed casting. So it goes.

Another washer under the ram lock screw changed the phase enough to keep the knob out of the way in both the fully locked and unlocked positions:

Tailstock ram lock - added washer

Tailstock ram lock – added washer

I slobbered some Mobil Vactra #2 Sticky Way Oil (thanks, Eks!) on the bed ways, snuggled the tailstock in place, and wow does that thing move! Verily, it slides smoothly and clamps solidly in place: a tremendous improvement over the status quo ante.

Some observations…

  • The tape (perhaps the adhesive layer) produces a slightly textured epoxy surface
  • The tailstock way’s small contact points indented the tape, even though it’s only 3 mil thick
  • Filling the low areas in the way works well
  • The high areas may not have enough epoxy for good durability
  • I expect the epoxy will wear faster than steel, so contact should improve with time
  • This is not a permanent fix

What I’ll do differently next time…

  • Apply more epoxy to avoid those small gaps along the edges
  • Use a real release agent: smoothed in place, it might provide a better finish. Might not matter
  • Verify a good prying spot before epoxying, say, the compound

All in all, though, this worked much better than I expected!