Foucault Tester


  There are nearly as many designs for Foucault testers as there are makers of testers.  I admit that my design is rather bulky but it has a couple of features that make it user friendly.  First of all, the stages roll on steel balls which run in " aluminum angle tracks.  Second, I've always disliked having to work the "Z" axis adjusting screw in my chest as I lean over to the knife edge.  To overcome this I use a string attached to the Z stage and the other end winds around a " threaded rod which is turned by a knob on the side opposite the X axis adjusting knob.  One bearing for this rod is a "  nut, the other is a " teflon plug, cut with a plug cutter from a slab of teflon and drilled and taped for " thread.  This also allows a dial gauge  to be installed in the front side.  The steel balls need to be about 3/8" diameter or larger and can be obtained by cutting  apart discarded 1"  or 1" ID bearings (from electric motor repair shop) with a small angle grinder and cut off wheel.  Hardwood blocks glued to the lower rail keep the balls in their desired location. Two balls are used in one track and one ball in the other and a flat metal surface is recessed in the under side of the moving part of the stage for this ball to roll against.  To keep the stages from accidentally falling apart if bumped, a 3/8" wide slot is routed in the center of the moving part of a stage and a "  bolt with a wide head screwed into the non moving part directly below so that the bolt does not touch the upper part in normal operation.  Ordinary elastic bands are sufficient to return the rolling stages.  They are looped over the heads of small screws between the two parts of a stage.



In the top picture the base has been extended and V rails added as guides for a Ross null lens platform.  I have no way that I can think of to test the accuracy of this tester but I think it should be accurate enough for the caustic test.  I'm starting to build another tester, much the same but this time I'm using  "  polished aluminum angle from the decorative trim section at the lumber store; thinner and cheaper.  I also plan to extend the bottom rails and add a separate  "Z" stage for the Ross null lens that will be indexed to the main Z stage.  The lens can then be set accurately with respect to the knife edge and then moved with it.

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10 May 2010