In article , Martin Perry
says...

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.

The errors you see are in the correct direction for workpiece
spring. For the OD cut, the part moves away from the tool so it
will have the largest diameter far away from the headstock. Here
you can imagine the part moving away from the tool, springing
towards the back of the machine.

For the ID cut, the part would be springing towards the front
of the machine, because that is how the force is being applied.
So it will bore small at the very end of the part.

You could re-try this test by cutting some aluminum, using
a very sharp tool, and taking only a thou off on the last
cut.

Also note (as others have mentioned) the chuck may be
flexing and allowing the workpiece to move, or the spindle
bearings may have enough slack in them to cause a similar
effect. You can check that with a good tenths reading dial
indicator.

Jim

==================================================
please reply to:
JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com
==================================================