Re: ATM Machine Grinding Advice Needed

[Alfredo Neves <arainho_cl@yahoo.com>]

Hi Denis,

Thanks for responsing my questions. It was the actual
mirror-o-matic operations manual that got me started
in the spinning method. I had originaly built my
machine on the convention that I would have the mirror
on top stroking over a slowly rotating tool which
emulates hand grinding. After reading the operation
manual I changed my machine so that I could use the
spinning method and the more traditional method for
figuring as the manual recommends.

Last night I finished fine gringing and tonight I will
start polishing. I am not too sure I found the so
called "sweet spot", but spherometer readings seem to
be constant throughout the surface to +- 2 microns. 

Denis, is there a way to calculate where the sweet
spot should be? My mirror is 8" and my tool is 6". The
overhang at maximun is about an inch and the eccentric
is set at 1/2 inch!

Alfredo

--- Dennis Rech <dennisrech@mirror-o-matic.com> wrote:
> At 07:08 AM 6/13/02 -0700, Alfredo Neves wrote:
> Hello Alfredo,
> 
> >After completing my grinding machine, I started
> work
> >on an 8" mirror. Rough grinding to F2.8 took about
> 16
> >hours.
> >My questions are:
> >
> >1. Should I use a coarser grit next time to hog
> out,
> >Or it is not worth the effort afterwards to get rid
> of
> >the huge pits?
> 
> 60 grit is what most people use and it works just
> fine. I would guess that 
> you would have taken half the time at 88 rpms and
> about half of that time 
> if you would have doubled up on the weight. Your
> mirror has a pretty steep 
> curve so it should take longer than average. I
> usually figure about an hour 
> per cubic inch so 4 to 5 hours would be normal. I
> usually start out with a 
> side to side stroke when hogging. It gets the curve
> established faster. 
> Then go to the spinning method. Make sure that you
> have almost no overhang 
> when hogging. Even a little adds a great amount of
> time to the rough 
> grinding process.
> 
> 
> >2. When polishing I intent to do it, like my
> griding,
> >using the spinning method (fixed over arm,
> undersized
> >tool). I read that my pitch lap should not have off
> >center facets of the lap will not spin. Is this
> >correct?
> 
> If the pitch lap is not symmetrical, it may not
> spin. Use about an 80% 
> diameter tool and a little side to side eccentric.
> 
> 
> >3. Can I use the same symetrical lap afterwards for
> >figuring by stroking and not spinning? Or will that
> >cause zonening ?
> 
> You will probably want a smaller figuring lap than
> 80%. More like 60 to 
> start and 35 % at the end to pick up small zones.
> Bigger if you have to 
> blend areas for smoothness. Again, if the lap is too
> far off center, it 
> will just drag and mess up the surface. When
> figuring, you still want 
> the  lap to spin slowly.
> Good luck,
> Dennis
> 
> 


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