Re: ATM unsupported edge effect on polishing

["Bill T." <twentiethwave@hotmail.com>]

>Bubble wrap shifts
>way to much... I cannot see figuring a piece of glass with any size of 
>bubble wrap...

The thickness of the wrap has to be correct.  For mirrors around 20" I use 
1/4" wrap, which seems to give just the right amount of flex for my 
polishing techniques, which is to polish-out overarm-style at the 50% zone, 
and then figure using stroke based techniques.  That's about an 80:1 ratio 
of mirror diameter / bubble height.

"1/4 inch" refers not to the bubble diameter, but to the spacing that 
results if the wrap is sandwiched between two planes.

Standard 3/4" wrap definitely rocks too much even for a 20" mirror.

On my setup, an interesting effect of figuring a bubble supported mirror is 
that it is not only hard to turn the edge, but when working with laps around 
50% diameter it is difficult to get enough "delta" in the slope between the 
zones outside the 70% radius.  At the start of figuring I have to bring in 
some edge correction with an ~80% lap, and then go over to deepening the 
center with a 50 or 60% lap.  This is a great way to wind up with a crisp 
edge.  This effect may or may not be completely the effect of supporting 
with bubble wrap, but I first noticed it when I started using wrap.

Another nice feature of bubble wrap is that it compensates for uneven table 
surfaces better than other supports.

Bill T.

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