[FoRK] The other CMM -- personal coordinate measuring machines nowavailable.

Ken Meltsner meltsner at gmail.com
Sun May 22 03:23:37 PDT 2005

The state of the art about 10 years ago was hitting exactly that
point.  With a CNC mill, you could carve out a wood or foam block
pretty well, and that block could be used as the master for a mold. 
Biggest problem, from my point of view, is that casting isn't the
greatest process for most parts.  There are problems with strength,
toughness, etc.  Not insurmountable.

Brief casting side note:  the most economical short-run castings start
with *patterns* -- wooden replicas of the part to be made.  The
pattern is placed in the bottom of a box and sand (mixed with various
binders and goops) is rammed around it.  The box is flipped and that's
your mold, or at least half of it.  We're definitely at the point
where CNC can make patterns, or you could even use a fancy solid
lithography system to make the pattern.  Patterns do need to be sized
a bit bigger than the final product, both to give a bit of material to
machine away to get a good surface (if that's important) and because
castings shrink when they solidify.

10 years ago, the big problem was data exchange and conversion -- your
typical 3D model (which includes both solids and surfaces-based
models) would have to be converted into the correct tool paths, tool
sizes (hogging out lots of material with a big mill, then switching to
a little one for the final cuts), etc.   That was the real bottleneck.
 I've been out of this business for a while, but things must have
improved ever since ACIS came out with a solid modeling kernel that
was available under reasonable licensing terms.  In the bad old days,
as I've mentioned, it was often easier to make a part with one set of
CAD/CAM tools, measure it, and then create a new model than it was to
convert models from one representation to another.
A skilled foundryman can make moderately complex castings this way; in
the old tools world, people are constantly finding tools that were
copied from originals this way -- frequently, it's a bronze or
gunmetal variant of a plane  only manufactured in cast iron; they're
often close to perfect except that the dimensions are off by a few

On 5/21/05, Russell Turpin <deafbox at hotmail.com> wrote:
> Yeah, that's pretty cool. But the biggie in this
> area is going the other way: creating a mold
> from a 3D CAD model. When that can be
> automated, inexpensively, you'll be able to
> create custom components for a wide range
> of applications where only a small number of
> copies are needed.
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Absolute power corrupts absolutely, but model train sets do a pretty
good job as well

-- 2/28/05, in a odd dream

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