Re: [ATM] Ain't got no shperometer

["Jerry" <wa4guu@verizon.net>]

Norm has it quite right.

It is learn by doing. All the advice is to help you recognize the experience
when you encounter it. 

Look at it, feel it. Whether it works out good or bad remember it. Recognize
the differences between what worked and what didn't.

The Sharpie test is not the only way to know the surface shape. I don't use
the Sharpie test, but I think it is a good method for someone who has not
yet learned how to see and feel the proper shape and maybe hasn't settled
into a normal stroke that will produce the correct shape.

With a full size tool, if you do what is widely considered the "normal
stroke", a W 1/3D high and 1/4D wide you will end up with a sufficiently
spherical shape.

If that is true, when would you use the Sharpie test? That would be after
you have used a large overhang stroke to change the radius of the mirror,
and you have used the "normal stroke" for a while to spread the curve
smoothly from center to edge.

After hogging in rough grinding would be one time you would work to make
them spherical again. And then if you shoot for a target ROC you might at
any or all stages of fine grinding try to force the ROC to the target with a
large overhang, either MOT or TOT. So again you will want to smooth the
curve from center to edge after altering the sagitta with the large
overhang. It will always be a hole in the middle that you are trying to
spread evenly to the edge. So you might use the Sharpie test to see if you
have done a good job of smoothing the curve.

If you had it spherical with the previous grit and you have only used
"normal" strokes, by the time you have the pits worked out with the present
grade of grit, there is no reason that it would not be a sufficiently
spherical surface.

Well I see Pete has posted again.... And that brings me to another time you
might do the Sharpie test. That would be after you have put the wrong tool
on the mirror and you have been trying to mate the curves of the proper tool
and mirror. In this case the Sharpie test may be superior to seeing and
feeling if you did not change grit grades during or after your mistake. The
surface could have the same pit sizes from center to edge and it could feel
smooth while working if the central area is large enough.

Do it like Ken says. He invented it, he can't be wrong about how to do it.

Color code the mirrors and laps so that mistake doesn't happen again.

Jerry



-----Original Message-----
From: Norm Prince
 
Look at the glass...are the pits even?...is the outer portion getting even 
pits before the inner or vice versa?....If you are alternating tot and mot 
with more or less randomly even intervals then it should be as close as you 
are likely to get if you don't have someone who has done it to give you a 
direct second opinion...Does it feel like the action of surface on surface 
is smooth?...

you seem to know enough about what you are trying to do to have actually 
tried it so keep going and you will soon find out what you have and haven't 
quite done...of course take whatever bits of advise you find on list they 
are groovey dudes and know infinitely more then me...

but from one inexperienced cat to another...without that hands on "here you 
look at it"...kind of back and forth with someone who has tried it too, we 
are working on a learn by doing basis...so go with your gut and see what 
happens...

-Norm Prince



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