RE: ATM Machine Grinding Advice Needed

[Alfredo Neves <arainho_cl@yahoo.com>]

Hi Shawn,

Glad to see I am not the only one with this "sweet
spot" problem. My post on this subject is still not
answered! 

Using the spinning method help me hoggout and fine
grind my mirror, but when it came to polishing it was
a disaster! It seems that this method gives a flat
(high) center and caused me severe problems since the
mirror was not truly spherical. Maybe at moderate F
ratios that is not a problem.

Anyway, I reconfigured my machine back to the more
traditional method of a slow turntable and a faster
stroke (7rpm turntable and 22rpm stroke), went back to
#800 grit and ground for an hour. Got a beutifull
sphere and started again polishing. Tonight I should
be polished out and will examin the figure again.

Regards,

Alfredo

--- "McHatten, Shawn" <Shawn.McHatten@aliant.ca>
wrote:
> Alfredo.
> 	Have you received an answer to the question below
> on how to calculate the sweet spot. I have been
> wondering the same thing for some time. If there is
> no calculation that can be done then has anyone ever
> tried to put together a table comparing mirror dia,
> tool dia and f-ratio to find the sweet spot.
> 
> Thanks
> Shawn
> 
> > -----Original Message-----
> > From:	Alfredo Neves [SMTP:arainho_cl@yahoo.com]
> > Sent:	Friday, June 14, 2002 11:25 AM
> > To:	atm@shore.net
> > Subject:	Re: ATM Machine Grinding Advice Needed
> > 
> > 
> > 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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