Re: [ATM] The 15.5 Ultra Thin Lap Discussion

[vorblesnak@peak.org]

>>Right now I am
>>leaning towards a stacked wood surface
>
> ... Can you please post them for discussion
> purpose.
>
> Regards
> Vladimir.


Well, for me the main reason is, it is cheap, I have the materials, I have
the technology and experience to shape it.

The lap is almost a one shot item in doing a mirror.  Usually the curve
and size are unique to the glass.  I have a collection of smaller
"universal" laps that I use.  Everything from a one inch up to an 8 inch. 
About half of these are on a wooden base.  I like the wood because it
floats.  I fill a pan with warm water, float the lap pitch side down for a
few minutes, then press new facets in with a piece of scrap wood, and
press the mirror on top to mate.  The whole idea for the pitch being to
allow perfect contact on a changing surface.  I will sometimes warm a lap
a bit to get better contact.

Glass or concrete sinks and by the time the pitch has warmed so has the
substrate, and the pitch stays softer longer.  One of the 8 inch laps is
on a nearly worn out glass tool and I use it for just that reason, when I
want the pitch to stay softer longer.

When I did the lap for the 16 inch, I cast a thin tool using rapid set
concrete poured into the glass.  I then mounted that on a piece of 3/4
inch MDPB for added stiffness.  I poured a hard pitch lap on that tool,
but it was a lot of work to melt and control that much pitch.  I routinely
make broken glass grinding tools and so wondered if I could use the same
process to make a broken pitch lap.  Seems reasonable.  The shape of the
pitch facets does nothing to speed or retard the polishing.  It is the
density of the pitch in an area that affects the polishing speed or
quality of a lap, density meaning the area covered by the pitch on the
lap.  To figure a mirror for instance, we reduce the area of the pitch at
the edge by widening the channels.  Pressing a shape into the pitch
reduces contact at that point, or said another way, reduces the density of
pitch in that area of the lap.  At least that is my experience.

Given a Sagitta of 6/32nds of an inch.  That curve runs from zero at the
edge to 6/32nds at the middle of the mirror.  Half way, at the 50% zone,
the Sagitta would also be 50% or 3/32nds.  Divided into thirds, at 33% and
66% of radius the depth would be 4/32nds and 2/32nds respectively,
(working from the middle out).  Diameter of the disks would be about 5
inches and 10 inches, the bottom disk being 15.5 inches in diameter.  Now,
pour a disk of pitch  and allow it to cool. Break into pieces and stick
them on the wood.  Press and polish.

The curve should be perfect since the pitch will form to the glass,
deforming to allow exact mating of the surfaces.  Reforming the facets may
be a problem, but it should work the same way, use a stick and reform the
pitch 'mounds' press for contact and polish.

David Davis
Toledo, OR 97391

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