Re: [ATM] Rough generating blanks

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

> I'm ordering a 19" or so pyrex blank and want the curve rough generated 
> into it, if I
> want to end up with a finished f/5 mirror, should I order the blank with 
> an f/5 ROC

If you want an f/5, order the generation to f/5.  Don't be second guessing 
how the focal ratio might drift, because there's as much chance that it will 
drift in the opposite direction!  Use alternating tool on top and mirror on 
top.  If subdiameter tool, vary from short/narrower to longer/wider to 
control.  Depending on the type of generation (ie, marks left in the glass), 
one typically starts at 220 grit.  The biggest issue is fitting a tool to 
the pre-gen. surface.  Cast the plaster against the surface using plastic 
barrier, then glue tiles form fitted same way.  Otherwise 220 will take 
longer than necessary as you grind the tool into shape

Mel Bartels 

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