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Question on lathe accuracy
(Martin Perry) wrote in message . com...
I have an old Boxford lathe. The bed shows signs of wear but otherwise
it is in excellent working order. Everything works smoothly without
any undue slack.
I checked the accuracy the other day by turning an eight inch length
of two inch diameter bar. There was a slight taper on the finished
work. The bar was 0.001 inches smaller nearest to the chuck.
I then bored a two inch diameter hole in a four inch diameter bar and
found that the bore was 0.001 inches larger nearest to the chuck.
These two errors seem to be in opposite directions. Can anybody
suggest an explanation please.
Regards, Martin Perry
Was the bar supported at the outboard end? If not, you'll get
deflection when cutting stuff with that much overhang. If you did,
you might want to check your tailstock centering with a couple of dead
centers.
On your boring project, a lot depends on how rigid your setup is, if
you're using a boring bar a lot smaller than your 2" hole diameter,
you'll get deflection that way, too. I usually make a number of
finish passes at the last cross slide setting to compensate for bar
deflection. I also try to make the boring bar a fairly close match to
the hole size if I'm trying for a good, parallel hole. A lot depends
on how deep you're boring your hole and the amount of boring bar that
sticks out past the holder.
You might also want to check to see that your lathe bed isn't twisted.
If your Boxford is the copy of the 9-10" Southbend that I think it
is, they aren't that rigid. They're nice small lathes, I've done a
lot of work on a 9" Southbend, but they do flex and if your shims have
slipped and the bed has a little wind in it, you'll end up with some
untrue results.
Stan
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