Re: [ATM] hogging out

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

My preference is to do the hogging of the hole in the glass with
a iron flange or pipe end cap to get the approximate radius of
curvature desired.  To this end, you will also want to make up a
cheap spherometer which can be made with a piece of gfflat metal
and a few screws.  You're not looking so much to measure the ROC
so much as to make sure that the ROIC is about the same over the
whole surface.   As such, three screws on a diameter of about
three or four inches diameter and another fourth screw in the
approximate center will do just fine.  Peter has even built some
spherometers with a hard plastic that will do the job just fine.
You're looking ofr differences so just see how much rocking of
the devicce there will be and that will be good enough.  A
flashlight test, sun test or other such test will determine the
ROC to a very nice value and then you can start workign with the
subdiameter tool.
A 40" tool on a 13" mirror isn't very subdiameter so your strokes
are not that particular.  Do put the larger surface on the bottom
and go at it.  There will be a tendency for the ROC to decrease
even with the mirror on the bottom so the storkes will be a bit
llonger than otherwise for the smaller tool.  Adjust the stroke
size as needed to keep the ROC from changing and things will be
fine!
If you're not shooting for an exact ROC, just keep on going and
remember that where you put the pressure is where the glass gets
ground off of the surface.
fFor real numbers, I'd robably start going at it with a stroke
that leaves about three inches of the tool off of the edge of the
mirror.
Biggest thing here is that you just need to look at things
occasionally to verify that you are still grinding where you want
to.
Bob May
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