Re: [ATM] Oversize tools questions - was Pitch Substitute for very fast mirrors

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

> How do you form and press the oversize tools?
> 
> How much bigger are they?
> 
> What are the advantages/disadvantages of oversize tools?


Check the archives for past posts from me.  

Oversized tools are formed and pressed as all other tools. You do move the
tool around periodically as you press, yes?  For oversized tools, simply do
the same, ensuring that the pressing eventually covers all of the oversized
area.  I practice the technique of 'pitch freezing' (<- my name for it)
where as soon as the pitch is in perfect contact, I dunk the tool in cold
water for a couple of minutes to freeze the shape, then begin polishing
immediately.  I never heat the pitch deeply, only heating the surface of the
pitch with a quick dunk in hot water or sitting in the sun for a few minutes
or even a torch passed over the surface.

The optimum ratio (from Brashear at the end of the 1800's and from others
back into the 1700's) is 6:5 which auto-maintains a spherical shape.  I
don't go quite this big on the ratio on the larger sizes (eg, I used a 22
inch tool for my last 20 inch).

The big advantage besides above is the tendency to avoid edge problems - I
just don't have them.  The edge of the mirror seems to avoid the evaporative
problems that often plague same sized tools in uncontrolled environments.
Also important is that the edge of the mirror avoids the nasty problem of
receiving significantly less polish time that occurs when using same sized
tools.

For parabolization (even on big fast mirrors like 20 inch f/5) shape the
pitch on the oversized tool into a starred parabolization pattern and add
differential pressure and alter strokes to accumulate differential polish
time.  The oversized tool keeps the edge zones behaving nicely - like
training a dog!

Mel Bartels


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