Re: ATM:Oversized Lap Not Removing TDE

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

Bill,

I've done quite a bit of experimentation with oversized, channeled laps.  
Based on my personal experience, it would surprise me greatly if TOT with an 
oversized lap did not plow the edge!  I've done all my practical work MOT.  
One of the biggest issues in my experience is that the amount of mirror 
overhang is critical.  Too little overhang, and the edge of the pitch 
doesn't stay dressed adequately, plowing the edge.  Too much overhang and an 
inside zone develops at about the border of the overhang. For an f5 20" on a 
23" lap, a 3" overhang with about +- 1" random variation works well for me.

http://www.kupercontrols.com/misc/20top23.jpg

Oversized laps are single minded spherizing machines.  Polishing times are 
considerably faster than for smaller laps.  They can require less pressing 
-- that's good because pressing can be, uh, challenging.  It takes a little 
longer to get good contact established.  Once you're "in the groove" you 
want to stay there as long as you possibly can, to minimize pressing.  When 
contact gets funky a good option is to just slow way, way down for a while 
which amounts to a kind of slow motion, on-the-fly pressing.  And yes, once 
you've got the parameters worked out the edge is beautiful.

Oversized laps are not much use at all for figuring, other than for majestic 
marches back to the sphere.  Even the briefest spells with a partially 
figured mirror will create wide spherical bands somewhere in the outer 
zones.  I still like to have an approximately full sized lap available for 
figuring.

Based on a limited evaluation of your setup, I'd recommend a lot of MOT 
only.   Never fail to overhang.  Monitor for edge zones until you get a feel 
for the right amount of overhang and variation.  Every so often pull the 
mirror straight up without pausing and check for any imprint of the mirror's 
edge in the pitch -- that means you're plowing the edge, and should probably 
lengthen the stroke for a while to equalize the pitch.  Right now your only 
real choice with the oversized lap is to lower the whole interior of the 
mirror to match the edge, a long process.  It might be time-efficient to 
make a new same-sized lap and apply "normal" anti turned edge interventions.

Bottom line is, not having to swap mirror and tool positions is beyond 
wonderful, especially with big discs.  But it's necessary to keep going for 
very long work spells to avoid re-pressing as much as possible.  "15 minutes 
here, half an hour there" work schedules are not feasible.  You need to 
think in terms of 3 or 4 hours at a shot (which may be enough to polish some 
mirrors!).  This may not be a good method for one-off mirrors.  But it's a 
revelation for production.

I hope this helps.  My oversized lap experiences are still very isolated -- 
I'm sure others will offer valuable insights.

Bill Tondreau

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