Test Lamp Types
DAVE MCCARTER (50VE3GSO@qstar.fanshawec.on.ca)
Tue, 18 Apr 1995 22:56:39 -0400 (EDT)
I built my own Foucault tester out of an old tandon floppy disk drive chassis.
The trick was to take almost everything off the discarded drive, except the
head carrage and rails. The disk head moves smoothly on a pair of stainless
rails, so seemed good for use as the knife support, as the knife must be
nodded back and forth quite a bit, and only a very small amount while looking
for the equal nulls in the coude mask cut outs.
I used the disk clamp spring, a wide steel spring band, as the knife. It was
simple to cut with a Dremmel cutting disk, and took a very sharp edge.
I cut off the actual magnetic head, leaving most of the carrage, and epoxied
the knife to the carrage.
The disk center bearing and drive motor was taken apart to yeald the mount ring
which was cut into a C and I mounted two knife blades with epoxy on the one,
and small 4-40 bolts on the other to allow adjustment of the slit.
The mount was modified and the mount holes were slitted to allow the mount ring
to rotate plus and minus 10 degrees, so that the slit could be made parallel
to the knife edge.
The slit wa at first illuminated through a difussion plastic by a super bright
LED, running from a five volt wall type supply at a current of twenty milliamp.
I now use a small "grain of wheat" lamp, which gives a white light. I control
the current with a 100 ohm 3 watt potentiometer, so the lamp runs cool, and a
bit yellow. Hopefully it will last a long time, as it is held in the metal
lamp bracket by, you guessed it, epoxy glue. Great stuff.
The entire cut down disk drive chassis, now a mirror tester, is mounted on a
piece of excruded aluminum, sitting on small cut tubes of nylon, which provide
a smooth slide. A threaded brass rod is used between an end plate and the
disk drive to slowly move the slit and knife back and forward, at 1/32 inch
per turn. I attached a dial micrometer to the stage to measure the knife
position change to an accuracy of 1/100 millimeter or so.
Total cost? It was straight out of the junque box, which is the best way.
Any questions?, I would be glad to attempt a reply.
Dave.
Clear Skies or Grind Glass