Re: [ATM] Pressing was Re: Oversize tools questions

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> You said something about pressing that I have never
> seen before.  You talked about moving the lap while
> pressing.  I have never heard of that.  Most of the
> time I see people talk about warm or cold pressing for
> x amount of time with x amount of weight. The time in
> the range of 10 to 30 minutes. But they never mention
> moving the lap during the press.  You may be doing
> something that most of the rest of us don't know
> about.


I noticed that by accident early on (at least for my style of mirror making)
that my mirrors turned out smoother if I moved the top piece every minute or
two while pressing.  I theorize that I spread out the heat transfer and
attack off the glass by the hydrated polishing compound.

So that's my technique - same as I teach in my mirror making classes.  Twirl
the top piece around a bit every 30 sec to minute, offset it a tad here and
there.  As soon as desired contact obtained (1-5 minutes), freeze the pitch
by dunking in cold water for 2-3 minutes and go for it!

Within a short period of time the pitch and glass will become warm, even hot
to the touch.  If not, then I go slower and heavier.  According to the
chemical-mechanical theory of polishing, it's the pressure and hydration
that does the trick, with speed important in the sense of preventing the
silicon from re-forming back into the mirror face.

So it was a natural that I (again accidentally, then afterwards purposefully
as I investigated the literature) discovered that oversized laps controlled
the edge.  This is easy to stumble into if the tool is a bit oversized
(happens because one grabs a tool for an earlier larger mirror, pours pitch
at the smaller current mirror diameter, then allows the pitch to squeeze
outward as polishing progresses).  

Same thing when I discovered that channeling is unneeded for polishing and
figuring.  Let the channels squeeze out and keep on going - don't stop!  But
do keep the micro-faceting up - I scratch the pitch with something sharp
every so often under cold water.  I scratch with 1/4 inch separation in 3
directions at 120 degree angles - only takes a minute to refreshen.

Mel Bartels


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