Well, it turns out there is an alignment issue. The tailstock/spindle are at slightly different heights. Not sure how or when this happened. I'll have to figure this out and then try and see what happens.
They may also shift alignment slightly when under a load.
Is the tailstock lower or higher? And by how much?
I understand that some good machines are made with tailstocks about a half-mil (0.0005") higher than the spindle, so as the tailstock wears over the years, it gets better before it gets as bad as when it was new. And that much offset is normally not a problem.
If the tailstock is lower, and if it is a split tailstock (ram holding part is separate from the base which slides on the bed), then you can take it apart and add the proper thickness of shim stock between them before re-assembling -- and then taking care to get the tailstock horizontal offset correct again.
In a turret, there are reamer holders which will offset slightly in all directions, so you can get it truly on center.
Enjoy, DoN.
The tailstock was lower by maybe something between 5-10 thousandths. I removed the headstock and found a small chip of metal that was keeping it from laying flat on the bed. It's a sherline lathe so it attaches with a peg and setscrew. Anways, after that everything lined up correctly. I have removed the headstock in the past to set it on a riser block to work with stuff over 3" in diameter. This must have been when it got botched up.
In the sherline the tailstock is just an aluminum extrusion. There are no adjustments for wear, but people do chop them up and add the ability to make adjustments.
I tried again with this one weird reamer, and again it left rifling marks. I'm going to play with larger and smaller holes before reaming to see what happens. I did cut a fairly deep counersink to allow it smooth entry, but it did get a bit grabby at first, which may be the problem. It doesn't seem to chatter at all when cutting.
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