Re: [ATM] Focal Length Accuracy

["Bob May" <rmay@nethere.com>]

You'rede going the right way about things.  Use longer strokes to
hog out the edges more and the ROC will change faster.  Shorten
the storkes to insure that you get more back to a sphere after
the ling strokes.  At that radius, you can get to within a tenth
of an inch if you so desire by correcting as you go down through
the grits.
For a pulley, take any old zinc cast pulley of the right size and
grind for a few moments against a piece of glass.  This will even
out the working edge and you can refine that by filing the edge
to a sharper point.  You don't have to ha the dial indicator
exactly in the middle for a larger pulley although the closer,
the more accurate.  Check the math for how much of an error you
get when you're a little bit off.  I've found that mounting the
dial indicator rigidly to the rest of the tool is more imporant
than getting it exactly in the middle of the disk as any tilt or
wobble of the dial indicator will make for larger errors.
Bob May
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