[ATM] Grinding tips (was Pencil Test/Sphere)

[Mike Lockwood]

Francis,

It's great that your videos are very popular, and it is good that you 
are spreading interest about mirror making.  For that, I thank you.

However, as you are basically taking on the role as an instructor, you 
need to check to make sure that what you advise is carefully thought 
out.  In some of your posts I'm seeing a pattern emerging, and I want 
to raise a few points.

I don't want to sound like I'm picking on you - I am most certainly 
not.  This is simply friendly advice.  I want to make sure you 
understand the issues and why things have been done a certain way for 
a long time.  I would have sent this as a private message, but I want 
those grinding mirrors for the first time to read the points I have to 
make here.

On Sept 22, 2007, Francis J. O'Reilly wrote:
> I did not do any surface tests between grades of abrasives, none 
> whatsoever. I relied solely on the clock. 45 minutes to an hour of 
> each grade then clean up and move on.

"Grind more, worry less" will only get one so far.  It can't be taken 
to the extreme where it becomes "grind more think less".  At some 
point one has to stop and evaluate progress, rather than just 
proceeding with more grinding.  The only way to tell if the grind is 
complete is to LOOK at the surfaces being ground by holding them up to 
a strong light and using a loupe to check for pits.  45 minutes might 
not be enough time to ensure a complete grind, especially if the 
stroke is not quite right, or if the radii don't match or are changing.

I'm not saying you are using bad grinding strokes, just pointing out 
that others might not be grinding with the proper stroke, and thus it 
will almost certainly take them longer to finish a particular grit.  I 
have had students grind for 1.5 hrs on their own time with the same 
grit and not grind the huge pits out of the center.  They never saw 
the pits, but I spotted them as soon as I glanced at the surface. 
With light behind them, they looked like craters.

On March 4, 2008, Francis J. O'Reilly wrote:
> My advice at this point is to ignore the pencil test. You probably
> are fine as you are, even strokes 1/3 center over center. Walk
> around the stand and rotate the mirror the opposite direction
> regulary, every three strokes should work well. 1 stroke per
> second. Grind with 120 grit for about two hours, alternating tool
> and mirror on top. It's too early to worry about the figure and you
> don't have the ability to accurately discern it in any event. GRIND
> MORE, WORRY LESS!

I agree with Bob and Jeff who asked about the condition of the outer 
1/4" of the mirror.  It needs to be looked at, not ignored.

It is never too early to check tool/mirror contact or to learn about 
how to do this.  This is NOT figuring, it is how the two pieces mate, 
and can reveal what is going on.  In this case, if the outer 1/4" is 
not in contact, there might be some pits there.  That should be 
checked with a loupe and a strong light.  It is a clue that the center 
is being ground faster than the edge of the mirror.  This can lead to 
"frosty edge syndrome" and when the pitch meets the Pyrex.

Also, with carbo, one can grind about as fast as they care to, and it 
will speed things up.  In my experience, slow strokes can be reserved 
for the very end of a coarse grit to make the pits smaller, and for 
fine grinding and polishing.

	Mike Lockwood