Lapping-to-thickness alert device ?

I use a Lapmaster to lap titanium "pucks" flat from 0.4000" (machined height) to no less than 0.3970". The process which uses these pucks is
optimum when the pucks are 0.3970" - 0.3980". Currently we analyzed the time to lap these things and have it dialed accordingly, but sometimes we get something under the limit (0.3970").
Any ideas on devising something that will signal an alert when the accept thickness "window" as been reached? The fixtures used to hold the pucks (and provide weight) are cylindrical and freely spin as the puck is riding on the lapping plate.
Thanks to all responses. Marcel
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Lapmaster sells two fixtures that essentially do just that. They prevent the workpiece from becoming too thin. http://www.lapmaster.com/accessories/lapping-polishing-machine-tooling/default.html
In the first, the back of the workpiece is mounted to a plate. The outside of the plate has three diamond-tipped adjustable screws that determine the stop distance. The second has an outer metal ring that replaces one of the 3 conditioning rings. Inside is a vacuum chuck on a spring loaded piston with an end-stop setting. I am not sure what they cost new, I got mine (the second type) used. They would not be too hard to duplicate if you were inclined. Both can accomodate indicators to set the thickness. In theory they could monitor progress too, but I do not think I would want to keep indicators there with abrasive dripping around.
I am not sure, but the diamond-tipped screw fixture might not work so well on a lap with grooves. Pressure is momentarily relieved on one screw at a time as it passes over the groove.
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On Fri, 9 Oct 2009 11:38:22 -0700 (PDT), " snipped-for-privacy@comcast.net"

Can they not be surface ground?
You would need a fixture to hold them, but a 0.001" tolerance band is hardly precision. It should be very quick work.
Mark Rand RTFM
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Mark Rand wrote:

Not flat enough Mark.
--
John R. Carroll



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On Fri, 9 Oct 2009 11:38:22 -0700 (PDT), " snipped-for-privacy@comcast.net"

Depending on the surfaces and edges that are accessible, an optical sensor could work.
This type of sensor will detect the position of an edge quite accurately. http://www.bannerengineering.com/en-US/products/8/Sensors/38/Laser-Sensors/67/PicoDot-PD-Series /
Some of the measurement sensors on this page may be appropriate. http://www.bannerengineering.com/en-US/products/8/Sensors/38/Laser-Sensors
Omron and Keyence also make this sort of thing.
--
Ned Simmons

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