When I worked for a medical device company at the turn of the century, I was tasked with coming up with a method for measuring the thickness of slabs of epoxy composite that ranged from 2mm to 50mm in thickness and sizes up to 50cm square. I needed to measure thickness to .01mm precision at random locations on the face of the slabs.
One of the concerns is that the slabs were possibly not flat so I needed to account for that. My solution was to make a platform with five ball bearings arrange in a pattern as is found on the face of a die. The central ball was a few mm higher than the other four, ensuring that the bottom of the slab would make contact at that point. The remaining balls were there simply to limit the amount of tilting of slab resting on them could experience.
Directly opposing the central ball was a Mitutoyo digital indicator on an adjustable arm much like a traditional height gage. To account for different slab thicknesses, gage blocks were used to substitute for the slab thicknesses. To use, a thin stainless steel sheet was placed on the balls and the digital indicator was zeroed at the location the central ball, using a gage block if necessary. The gage block was removed and the slab inserted in its place and positioned at a location where a thickness measurement was desired. The SS sheet was thin enough that it would conform to the surface of the slab, ensuring that intimate contact with the central ball was achieved. The slab thickness at that point was equal to the indicator reading plus the gage block length. This was a simple and quick way to make multiple thickness measurements that could not be achieved with a micrometer.
