Re: [ATM] Oversize tools questions - wasPitchSubstituteforveryfastmirrors

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

> The mirror is as smooth a silk. No visible roughness what so ever. I
> mostly
> press TOT. I warm the pitch slightly, put on about 40 pounds, after 2
> minutes take off 10 pounds, let 30 sit on there for 3 minutes, then ;et it
> sit for 15-30 minutes with 5 pounds.


Well good for you and your mirror.  The key seems to be your warming the
pitch SLIGHTLY.  Another vantage point is that all mirrors have roughness,
and it's good to be able to detect it, no matter how smooth the mirror.  One
test that I use is to heat the mirror then test immediately.  You'll see
through all the horrible heat waves and oscillations particular zones and
roughness that is real and stays put, gradually subsiding to casual
non-detection state when fully cooled.


> To be honest moving it around would concern me, if your mirror isn't a
> perfect sphere, aren't u constantly repressing a slightly different shape
> into your pitch.


Not really.  The shape difference is of no consequence in that the pitch
will flow to compensate on a second by second basis (that is, the sinking of
the pitch overwhelms the change in profile).  Besides, no matter where you
press, you are creating a profile in the pitch that will be altered as soon
as you start to move the tool, no matter where you start and no matter where
you end up.  Luckily for us the sinking rate overwhelms the change in
profile.  At least by moving it around a bit (a light stroke or two every
couple of minutes) you have a chance of detecting by feel if the pitch
should not be able to accommodate changing profile by sinking.


> Awhile back my mirror was always rough because my stroke came back about 2
> inches and forward 1/2 inch instead of 1" and 1".


Oh, that's not the kind of roughness I'm talking about - I'm talking about a
finer scaled roughness.


> I also keep my tool micro-faceted with fiberglass window screen.


Ah, an important step to smoothness and pitch's ability to conform to
differing profiles while figuring.  I don't bother with window screening as
a few seconds of scratching with an X-Acto knife does the trick.

Mel Bartels


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