Clearance for valve body

[Harold_V]

Bill,
Roundness in centerless grinding depends on two things. One of them is to NOT be on center. Either above or below center will function properly, with running below center somewhat more desirable for long objects, as wheel pressure tends to keep less than straight objects in contact with the blade (which supports the part while being ground).

Given the proper numbers of passes through a centerless, assuming the amount to be removed does not exceed the amount of error, less than straight parts can be brought true. Heavy passes eliminate the possibility, however. Here's where the skill of the operator spells all the difference in the world.

If a long object is not straight, it tends to ride up as the high spot contacts the blade. With light passes, only the highs well be removed. You can see the part develop a grind pattern, uniform on each end, and in the center, 180° opposite from the grind on the ends. If the item is long and slender enough, wheel pressure will keep it in contact with the blade (by flexing), so that the center of your stock is round makes sense. That's a good indication that the piece was not ground on center.

However, in such a case, it's common for the ends to be ground with an irregular configuration, which is often observable if the material has made more than one trip through a centerless, which is often the case. The first pass will remove material in excess on the high side, leaving nothing for cleanup. That results in a not round condition, but also not three lobed. It's hard to say which condition you have with your piece of material, but I suspect it's not three lobed. The error is so small that it may be difficult to determine. A sure sign it's three lobed is if you get a repeatable pattern (three times per revolution) as the piece is rotated. Otherwise, you may find there's a general low spot, with the balance being round.

Thanks for confirming my hunch.

Harold

[seal killer]

All--

I reamed the hole in question to 0.3760" x 3 1/2" and the 3 1/2" length of 0.375" drill rod slips in with very low friction and no detectable side play. I am sure it will handle low pressure (<=25 psi) air.

Thank you for the help.

--Bill