Re: ATM Sub-diameter lap

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

>If you short stroke the mirror, you will be on a path to a TDE

By not bringing the lap up over the edge, the center digs out much faster 
than the edge.  This "tilts in" the zone just inside the edge, which 
increases the relative slope between the edge and the zone just inside of 
it.  This seemingly strange and unprovoked increase in edge correction often 
confounds beginners trying to "protect" the edge by not working on it too 
much -- the edge just seems to get long for no reason other than pure spite! 
  It's so unfair!

For my "W" based, low RPM polishing setup, I get a fairly edge-neutral 
stroke by bringing the lap about 10% of lap diameter over the edge of the 
mirror -- this is for laps > about 60%.  Less overhang will lengthen the 
edge, and *up to a point* more overhang will shorten it.  Of course, if 
you're looking for a nice, wide TDE, consider trying to shorten an over 
corrected edge using normal figuring techniques.

It's interesting to think about the relative percentage that each of the 
outer zones is covered by the lap at the lap turn-around point.  When the 
overhang is small, a slight change in turn-around position greatly changes 
the "work ratio" between the outer zones.  It might be possible to develop a 
way of estimating the figuring consequences of various amounts of overhang 
for a particular machine or technique.

Bill T.




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