PM3520B quill galling from drilling

Hello all--

I have a PM3520B and recently took the tailstock assembly apart for cleaning. On that tailstock, there is a groove milled into the quill, and a threaded grub screw that rides in the groove to keep the quill from spinning. I noticed while I had it apart that there was substantial galling on the wall of that groove. I believe this is from drilling, where you're advancing the quill with a rotational load, pushing the side of the groove into the threads of the grub screw. PM has obviously recognized this problem because on the 3520C, the grub screw threads into a small steel plate that rides in the groove, which puts smooth metal on smooth metal and also increases the surface area of contact greatly. See part #28 in the diagram below of the 3520C.

Attachment 473882

Anyone have a patch or workaround to deal with this? I thought about using a brass grub screw, but the threads that extend into the groove would get smashed and I'm worried I wouldn't be able to get it back out.

Best,

Dave

Good question. Your diagnosis is probably right on. But I wonder what is the problem with having the groove galled?

That's been a known issue for maybe 15 years or more. It has gotten so bad for some owners they claim the quill won't extend. If you had a buddy with metal working machinery, he could make a custom brass screw for you.

you could try using a brass tipped set screw (McMaster-Carr sells them) the threads are not brass, just a brass tip sitting just beyond the threads. Might work. I had heard the recommendation to file a chamfer along the top and bottom of the groove to help.

My 3520B did the same thing. I filed the rough edge (burrs) smooth and keep the setscrew as tight as I can without binding. It's been OK since.

Will the 3520C setscrew and plate fit a 3520B?

Based on the description, I assume it's due to using a drill chuck in the tail stock. I've done drilling several times with the various lathes I've had, there's always some bouncing if not spinning at times which can burr the inside of the MT. Personally I've seen both at times so it was time for a solution, which I came up with one that works for me. First off, if there's burrs in the tailstock you can get a reem that can be used to clean up the inside, but that still leaves the potential for the chuck attempting to spin in the tailstock. I made a simple handle that fits into one of the holes in the chuck which I can hold onto and keep the chuck from spinning. May not be elegant but definitely works for me.
Attachment 475105

Hey Paul--

Good ideas to keep a chuck from spinning in the morse taper, but I think you're misunderstanding the issue. The issue I described is not from anything spinning, and the galling is not in the Morse taper. The galling is on the sidewall of the keyway on the outside of the quill (on a PM3520, it's at 3 o'clock when looking down the quill toward the headstock). The galling is not because something is spinning, but because all the rotational force from drilling is pushing the threads of the gib screw against the sidewall of the keyway. As you advance the quill, the threads of the gib screw, under all that rotational force, are pushed along the keyway and they cut into it.

This discussion has made me realize that a workaround, perhaps not a great one, is to drill with the quill locked, but the tailstock not locked, and advance the entire tailstock with a bar clamp, or just by hand if the hole is small diameter.

Best,

Dave

Dave
Got it. I still think something on that style might be helpful if the initial cause is where the drill bit is catching and causing the quill assembly to be hammered against the stud. As far as repair, that's another story. You might be able to have a machine shop spray weld the damaged area first to build it back up then remachine the groove and cylinder. I don't know how accessible it would be but something like cryo treating it once the repairs are done should make it far stronger than it currently is IMO. If there's a hardened replacement, that could be a cheaper option.
Just some thoughts.

Paul

I removed the screw and found the threads were wearing because the quill was harder. I put the screw in the metal lathe and turned the threads off the end a little more than the keyway depth so it is smooth metal riding on the quill.

Quote:

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Originally Posted by Dan Hunkele

I put the screw in the metal lathe and turned the threads off the end a little more than the keyway depth so it is smooth metal riding on the quill.

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I thought about this approach, wasn't sure if the additional slop would create any issues, I take it it does not?

Earlier someone asked about retrofitting a 3520B with the sliding key like the 3520C has. I measured the keyway on both machines and on the 3520B the keyway is roughly 3/16" wide, but closer to 5/16" wide on the 3520C. So the parts would not interchange, though one might be able to fabricate something (or have a machine shop widen the keyway on the 3520B quill).

Best,

Dave

Quote:

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Originally Posted by Dave Mount

I thought about this approach, wasn't sure if the additional slop would create any issues, I take it it does not?

Earlier someone asked about retrofitting a 3520B with the sliding key like the 3520C has. I measured the keyway on both machines and on the 3520B the keyway is roughly 3/16" wide, but closer to 5/16" wide on the 3520C. So the parts would not interchange, though one might be able to fabricate something (or have a machine shop widen the keyway on the 3520B quill).

Best,

Dave

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I don't believe it does. I don't often tighten mine but if you do it may mushroom on the end easier than it does now. On the upside it will come out of the hole easier if it is mushroomed.

Could you not make a steel 'slider' like the C? This sounds like the best fix to me? Or get the part from the B and file or cut it to fit?

Hummm - maybe time to do some maintenance on my 3520B. I found this video that might be helpful:

Quill Maintenance

It happens on just about every lathe with the same quill design.
IMO, it's just a poor design. You're asking one set screw act as a locking device for forward and backward advancement of the quill as well as be a positive stop against rotation. This two functions should be performed by two different systems.
I had the same thing on my Jet and like others, I just had to do a little R&R to the quill and it should be good for another few thousand miles.