"Jim Wilkins" wrote in message
...
On Oct 1, 7:29 pm, "Michael Koblic" wrote:
"DoN. Nichols" wrote in message
On 2008-10-01, Michael Koblic wrote:
big snip

But you probably don't really need to do this. Just learn to
live with your 3-jaw chuck, or get a 4-jaw and learn to set it properly
when (and only when) you need the accuracy.

Thank you Don and Jim.

In my case it is more about understanding the concepts than about
immediate
practical application. I am getting a better grasp.

One final question: Grinding - does it damage a chuck or is it mainly the
ways and the bed you have to be careful about? I was thinking it was
almost
inevitable for the grinding dust to get inside the chuck and causing
mischief there. I am talking about grinding things clamped in the chuck
rather than doing the chuck itself as Don suggested.
Michael Koblic,

Opinions vary on lathe grinding. The dealer I bought mine from warned
never to grind on it because it has hardened ways. I haven't need to
anyway because I bought new chucks for it and have another small lathe
that is easily disassembled for thorough cleaning, plus a surface
grinder with a tool grinding swivel table. The small lathe and the
surface grinder each cost less than a new chuck.

Hard or soft, grinding grit is hell on the ways of a lathe, and anything
else it gets into. Old timers avoided grinding on a good lathe if at all
possible, because even the best precautions sometimes let you down. That
grit wants to get everywhere.

If you need to improve on a lathe-turned finish you can lap it with
fine loose abrasive like Clover compound in a home-made cast lead lap.
This doesn't spread abrasive grit all over like dry grinding with a
high speed wheel.

Hand lapping is becoming a lost art. There are lots of lapping tools and
tips available in the really old books and the MAP booklets from the UK.
They really ought to be re-promoted for hobby machinists, because lapping is
easy and versatile, and it can save a lathe if the alternative is grinding.

However, it's important to keep in mind what lapping will and will not do.
It can produce better roundness than grinding. But it does not improve
concentricity. In other words, a lapped shaft may measure excellent
roundness but the concentricity at one spot may not agree with that 1/2 inch
further along the shaft. Another way to look at it is that the center of the
lapped shaft at any point may not be in line with the center at another
point. Lapping tends to follow the mean center of the shaft, at any given
point. If you start with a badly turned shaft you'll wind up with a badly
lapped shaft, even if you have a mirror finish and the roundness at any
point is near perfect.

In practice, this hardly matters. The differences are on the order of a few
millionths of an inch, if you use good technique throughout. But it could
matter in extreme circumstances.

--
Ed Huntress