Rob McDonald wrote:

-- snip --

The lead screw looked OK, so I disassembled the apron and voila! The half
nuts were badly damaged. The threads were mostly torn away, and the entire
casting (they look like a die cast part) had many small cracks. I believe
this was allowing the nuts to move after engagement, occasionally changing
the position of the cutting tool relative to the previous pass.

-- snip --

Thanks for the advice guys, you convinced me that there was more at play
than my technique.

Rob

I hope the pun was intentional.

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Tim Wescott
Wescott Design Services
http://www.wescottdesign.com