[ATM] Dog Biscuit

[wa4guu at verizon.net (Jerry)]

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Ray@J-Engineering.com


What causes dog-biscuit?  Could an etching action during pressing have 
something to do with it?  How about the slight pause at the end of each 
stroke when figuring by hand?
-- 
Ray
******************




Stroke speed. Pitch hardness plays a part in what a good stroke speed is.

With the stroke too fast any small area of contact between glass and lap
that has increased friction compared to the rest of the lap will heat those
areas and swell them on the lap and the areas on the mirror that the swelled
area of the lap passes over. The areas on the lap will heat and flow in time
except the fast stroke allows the warmer swollen pitch to unevenly heat the
mirror surface before the pitch has time to flow. This causes small areas of
the glass to swell which in turn become areas of excess drag that heats
other small areas on the lap. Also the small locals on the lap that are
warmed by excess friction also polish more efficiently. And around and
around it goes. The surface of the glass is heating and swelling in a number
of small areas across the surface and become bumps that get worn on top and
when the work is over and the glass cools down where the bumps were becomes
low areas.

So with too fast a stroke you have a multitude of bumps, dimples and valleys
rising and falling on the lap and the glass and the faster you work the
larger the magnitude of the surface deviations. Slower stroke causes less
localized heating and also allows the pitch more time to flow to shape.

The thing about "stopping at the end of the stroke" does not seem to be a
problem for me. Now of course you don't want to stop and then linger at the
end of a stroke because if you do you allow the lap to press while off
center and not in full contact across the lap, at least with full size laps
and almost always the same with small laps. 
But just stopping for an instant to reverse direction has not caused me a
problem. I have done stokes that curve back to reverse direction and see no
improvement in Foucault test. I find it easier to stroke straight and stop
and reverse than to hook at the end so that is how I do it.

I am not saying that elliptical, circular or sine wave like strokes don't
have a place in figuring but much of the worry over the momentary stop to
reverse direction is unnecessary.


Jerry