Re: ATM Grinding problem?

["Bill Kelley" <flex@commspeed.net>]

----- Original Message ----- 
From: <mbartels@efn.org>
Cc: <atm@shore.net>
Sent: Thursday, October 23, 2003 7:05 PM
Subject: Re: ATM Grinding problem?


>
> > I'm grinding the initial curve on my first mirror (6")
> > and I think I might be doing something wrong. I am
> > about 4/5 of the way to my goal now. At about 2/3 of
> > the way there I started using a center over center 1/3
> > stroke. At that point progress slowed to a crawl and I
> > expected that since I know the curve needs to extend
> > to the edge of the mirror after the chordal stroke.
> >
> > Now despite repeated wets, it isn't getting any
> > deeper. The curve is all the way out to the edge and
>
> Likely that your strokes are longer than 1/3W and/or that you are placing
> extra pressure at the ends of the strokes.  Flip the work over and
continue as
> you are doing - that will deepen the center without touching the edge very
> much.
>
> Mel Bartels
>

Hey Mel,

You've been working TOT too long on big mirrors  (G) hee hee

He's doing MOT. It's SHORT strokes that shorten the radius.
If he'll lengthen the stroke , he'll start to deepen the center again.
I have my students over-run the desired sagitta a bit, then with
shorter strokes they bring down the edge and get back to a
sphere somewhere near the F ratio they are shooting for.

Yes -to the rest of you old timers- I know there are faster ways
to get rid of the hole in the center from aggressive hogging
mirror over edge.
These are students and it's easy to explain this conservative
approach. Anyway- it builds character.

Bill