On 09/25/2018 03:02 AM, andy pugh wrote:
I can't help thinking that the way to determine the amount of
correction needed is to actually machine a test bar and measure it.
The problem here is that deflection of the bar, and also the
lathe, will affect the accuracy of the result.
You can't turn between centers, or at least that adds a
whole new dimension to possible errors.
If you know that you turn the same diameter at each end,
then maybe turning the whole length will give a profile of
the bed. But, then, diameter errors on the test bar could
be pure shift in the X axis, or some combination of that
plus a rolling of the carriage due to height differences on
the two ways.
Having (painstakingly) redone a lathe bed, it is more
complicated than at first glance.
My preference is to use a hardened and ground bar of known
straightness, held just in the spindle, and a sensitive
indicator to ride up and down the length of it.
Jon
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