> I don't know that I have either, sure would be nice to see a first here
FWIW, I did some experiments in that area. Somewhat tedious and some
home-made tools involved, but I was able to achieve a pretty good
calibration, within about 2% from 25 mph to 65 mph (according to my
reference, a "digital" tachometer from HF). (Also cured a severe waver in
the reading.) Above 70 mph it still creeps up though, to about 5% high @
100 mph.
To adjust the calibration, I changed the strength of the flying magnets. To
weaken them, I used a simple degaussing coil made from some 14 AWG insulated
wire wrapped around a 5/16 bolt and connected to my soldering gun (which
supplies low voltage, high current AC).
http://goo.gl/fqPE4
(The chart in the background is my table of spindle speeds for the lathe,
along with what the speedo should read at each speed.)
To strengthen, I wrapped some more wire around a muffler hangar U-bolt and
used some big capacitors from the junk box to supply a big spike of DC
current.
http://goo.gl/IbfMB
I originally used the small 12v battery visible; but it didn't deliver
enough current through my crude electromagnet. My bench power supply would
charge the capacitors to about 60 volts, giving a much larger jolt.
After each adjustment of the magnet of course, I had to move the needle to
the best position and recheck the results.
Now all I have to do is put the 3.45 gears in the rear axle (to match my
short tires) and I should be at least closer than I've ever been before. My
current project is breaking down the hubs so I can replace the seals &
bearings (hopefully without ruining the hubs). In spite of the
frustrations, I enjoy learning how to do these things myself. (Which is why
I don't just take them to someone else with the right tools.)
-- Randall
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