Re: [ATM] artificial convex curve for pitch lap

[Mark Holm <mdholm@telerama.com>]

Ken Hunter wrote:

>   Once you get the curve made in the glass, pour a cement, dental plaster or similar tool (upon which you will pour the pitch for the tool) using the curve in the glass as a mold for the tool. Directions are all over the I'net for doing this.
>    
>   Really simple and stable PLUS... less expensive and time consuming than any other method.

There are pictures and a pretty good description of how to pour a dental
stone tool at www.gotgrit.com  If you buy dental stone from Tom (who
runs gotgrit in what passes for his free time) he sends along a very
good written set of directions (at least he did a couple of years ago.)
  I think Richard Schwartz originally wrote the directions Tom includes.
The directions are for a tile in stone tool.  To make a lap base, you
just omit the tile.  Having done it, I have to agree with Ken.  This is
the easy way.  Once you have done it this way, you won't ever want to do
it any other way.  All the other ways are harder.  Now, one little
problem.  If you want to make a sub diameter tool, I don't know how you
easily mold in the curve.  If you are going full size, then this is dead
easy, till you get so big you can't lift the thing.

Mel Bartels (I think) has used plywood lap bases with thin plywood disks
to approximate the curve.  I don't think he used more than one or two
layers of curve approximation.  With plywood, you have a major
waterproofing problem.  Warping will be deadly.  Cured dental stone
appears to be quite dimensionally stable in the presence of water.  That
is a big advantage.  No waterproofing necessary and apparently little or
no risk of warping.

As for sizing the disks, if you decide to go that way, you should be
able to do it with the sagitta equation.

if your radius of curvature is R and the radius of your tool is r, then
the sagitta is r^2 / 2R  (approximate for a sphere, but close enough).

Now, lets say that your sagitta turns out to be 0.1 inch (for example)
and you have a 0.05 inch thick piece of plywood to approximate the curve
with in one step.  How big should the diameter of that plywood circle be?

The sagitta of the inner circle (bounded by the edge of the 0.05 inch
thick plywood disk) will be 0.1 - 0.05(ply thickness) = 0.05 inch (inner
circle sagitta).  So, the radius of the inner circle is sqrt(0.05 * 2R)
 That is, radius of inner circle = sqrt(sagitta of inner circle * 2R),
where sagitta of inner circle = sagitta of whole tool minus thickness of
layer(s).
-- 
Mark Holm
mdholm@telerama.com

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