Reversal Methods - Absolute Measurement

One of the simplest places to see reversing is in calibrating a level.

if I = indication - Ia first measure Ib second measure with level reversed.
and el = error in level
and et = error in table..

Ia = et + el
Ib = et - el

The rest is just simple math..

Dan Gelbart briefly mentions it..

A related process happens in the 3-plate method to get a perfectly flat plate..

I found this..

Generational delinquency - degredation of straightness, flatness and measure

There is a problem in machining - the error in the tool transfers to the error in the work - so if you use a tool to create another tool - things just get worse - reversal methods are the cure..

Using 2 plates instead of 3

This trick - lets one use 2- plates... But I would make it nicer.. <grin>