Re: [ATM] Grind More...

["Jason Hissong" <jhissong@gmail.com>]

Thanks Bob,

Doing the chordal strokes did the trick.  I went from a sagitta of .113 to .106.  I think I am going to go ahead and start COC strokes now to get me back to a sphere.  I can live with a focal length of 38" instead of 40".  I will continue to keep the tool on the bottom.

On my 14" I will follow your advice.

Best Regards,
Jason

------- Original Message -------
From: "Bob May" <bobmay@nethere.com>
To: atm@atmlist.net
Sent: 8/17/05, 2:28:10 PM
Subject: Re: [ATM] Grind More...

When trying to change the ROC, chordal strokes (grind with the
tool overhanging the side) is the best way.  That way, you grind
where you need to grind.  You do need to also do some center over
center strokes to bring the inside part to mate with the center
but that is more later on when you start getting close.  Don't go
all the way to the sag indication with the chordal strokes or you
will have then overshot the desired ROC the other way but rather
get most of the way there and then go back to the strokes that
you're doing.
You may also want to sharpie the mirror to see where you are
grinding and how much of a hole you will end up with.
I'll note that I tend to take pregenerated blanks with a fairly
smooth surface and start at the 320 grit as the start.  This
keeps the ROC from changing so much as well as using the shorter
strokes needed for going down through the grits.  You've already
done the hogging with the grinder and all you need to do is to
smooth the surface in preparation for polishing.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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