>How about laying the flat upper surface on a flat table and holding a straight steel ruler from thick to thick across the area >to be thinned, and observing the gap below the ruler?
>John

You would need a table flatter than a pool table, if you think about it any deviation from straight line in any direction would have to be less than 1/10 of a millimeter because that is the level accuracy normally aimed for. Yes steel rulers are not straight edges aluminium straight edges are quite good, at least for checking the heads of engines. What might work would be 2 perfectly matched straight edges placed each side of the board you could check the deviation from the maximum thickness with feeler gauges in fact the straight edges don't need to be straight as long as they fit against each other with no measurable gap.

Richard

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