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Nick Mueller

wrote:

Made up a test bar at school for my home lathe tailstock alignment.
Took a piece of silver steel Â*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.

And what if the 3 jaw chuck is excentric? It certainly is! Put a dial
indicator to the round when it is in the chuck and rotate it. You'll
see ...

Question - if I then set it up between centers of tailstock and
headstock, will it be square to the bed of the lathe.

No, see above.

Will the centre drills be in the CENTRE of the bar,

No, see above

so any rotational error as show on
a dial gauge indicates the lathe bed being out of true (as shown by
running the carriage,

The dial will only show how much off-center you drilled.

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

You need to make a dead cylindrical bar between centers OR ascertain that
your center drilling is dead on drumroll center. You can do the later by
clamping the round in the chuck, center drill it AND make a small recess.
Flip it over and do the same on the other end. But then, measuring in
between is useless.

BUT, you can calculate out the excentricity by measuring the min and max
when you rotate the bar and use the average.

There's a **MUCH** better way to align your tailstock dead to the spindle
(what doesn't mean that your spindle is dead parallel to the bed):
Some bar in the chuck, a dial indicator at the end of the bar. The
indicator's tip touches a dead center in the tailstock. Rotate the spindle
and watch the indicator. Now you see how the tailstock is misaligned back
and forth and up and down. You'll be astonished!

This method can be used to align the spindle to the bed too. I'm assigning
that to you as homework. :-))

Nick
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