Why not reverse and put the narrow edges on the cylinder.  The depressed
surface could possibly be ground accurately using the method telescope
makers use to grind mirrors to very fine tolerance.

Randy
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> Charles,
>
> You could create the same "rolling" effect by having 2 narrow plates a =
> short distance apart on the upper edge of each a shallow curve of about =
> 1000' radius. On these you sit a ground and polished rod of, say, 2" =
> (~50mm). I'm sure this would give the same effect as your cylinder, the =
> only problem being of course - the creation of the curves. The =
> measurement of any movement of the rod could be done by some sort of =
> optical sensor looking either up or down passed a narrower extension of =
> the rod, or even a flat reflective surface attached to one end of the =
> rod...
>
> Regards
>
> Ted
>
> 
..>

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