On 2007-12-17, wrote:
Cant get my head around this one, so, to the group........

Beware that our various locales disagree on the spelling of
"center/centre", so ignore that as you read.

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel 20mm in diameter. (at least, thats what
its called here - its shiny steel rod, ground (I think) so its
straight and parallel. Set it up in a 3 jaw chuck, centre drilled each
end.

Probably centerless ground -- which may have lobes which won't
show up with a standard micrometer, but which will often show up when
measured with a V-anvil micrometer.

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe. Will the centre
drills be in the CENTRE of the bar, so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage,

Given the typical 3-jaw chuck, the center holes which you
drilled will *not* be in the true centers of the end of the stock.

Either put them in a collet (and verify that the collet is a
good one by indicating on the 10mm or so which are projecting out the
end of the collet), or chuck up close in a 4-jaw and take the time to
center that with a tenths-reading dial gauge.

with the dial indicator on it, down the bars
length..). It does show deviation at each end if I rotate it...

Yep -- typical 3-jaw chuck. You really *can't* depend on those
being true -- they get work close to center quickly, but not on center,
even brand new -- and if they've been in use some time things are likely
to be pretty bad.

Its probably simple, its not a search for world peace, but I cant werk
it out....

OR - do I need to set up the bar in a 4 jaw, centre drill, then put it
in a cylindrical grinder to make sure it is 100% accurate?......

That depends on just how accurate you want it to be. 100% is
not achievable -- unless you define it to be "errors not detectable with
the equipment which *I* have". :-)

If you have a 0.001" reading dial indicator at home, and use a
0.0001" reading dial indicator to center it in the lathe at school, it
is probably good enough. (If you're working in metric, make that a
0.001mm indicator at school, and a 0.01mm indicator at home.

And if the tailstock ram at school has any play, you'll probably
need to face off the previous center holes totally before starting over,
or the center drill will follow the previous center as much as the slop
in the ram allows it to do. And snug the lock on the ram as much as you
can while still being able to feed the ram out for drilling so the ram
does not wobble around while you're drilling.

Drilling the centers and then grinding it *between centers* will
probably give you the best results -- on a between centers grinder, not
on the lathe, because you don't know how much you can trust the
tailstock of the school lathe.

Good luck,
DoN.

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