Re: ATM mirror grinding mechanics ...?

["Bob May" <bobmay@nethere.com>]

With your statement about: "Someone here pointed out when this thread first
started that the upper becomes concave because of the distribution of work
toward the edge of the lower and the center of the upper" you missed the
specific thing that makes this happen is that both of the pieces are about
the same size.  It is the pressure on the grinding grits that makes the
difference.  Light pressure does cause some chipping but adding more
pressure causes larger divots when they happen thus the top piece having
more pressure at the center will cause larger divots in the center than at
the edges.
When you start using a small tool to do grinding, the thing that happens is
that the grinding happens where the pressure is and since that isn't all
over both of the surfaces, the large piece gets carved where the grinding
is - if you grind in the middle of the large piece then the large piece gets
a hole in it where you grind.  If you grind around the edges of the large
piece then it goes convex.  This is again, the idea that where you press,
you lose material.
I might note that when you hang way off the side of the tool with long
strokes you grind the curves you want a lot faster than grinding with very
short strokes COC.
As to grinding the two vertically, you have to look at the way that the
pressure is applied in order to determine which is going to go which way.
One of the surfaces needs to be able to change it's angle of repose in order
to be able to get the two surfaces to start forming a curve.  If the both
are unable to tilt any then you will get a flat surface.

Bob May
http://nav.to/bobmay
bobmay@nethere.com