Chuck,
In general, mills will give you a better surface finish than pieces
produced by an RP machine which will give you a wood grain effect,
some more noticable than others. As far as an accuracy of .05mm,
you’ll get responses from equipment reps/manufacturers touting thier
accuracy down to (pick a measurement). Those measurements tend to be
a little misleading for the real world as they might be the
theoretical mechanical resolution of the machine, the smallest
movement it can make, which doesn’t take into account the smallest
movement the software will tell the machine to move, which tends to
be bigger, or that the minimum thickness of the layer does not
provide you with usable detail of that same thickness. I find that
a.05mm detail included in a CAD model would no longer exist after a
piece is cast and finished, even if you could see it in the original
prototype.
The advantages with mills are the superior surface finish, speed and
the ability to work in many materials and they tend to be
significantly less expensive. The disadvantages are that you have to
learn how to run the thing and in effect, learn how to be a
machinist to some extent, a considerable outlay in time. You can
create just about any shaped object with a 4 axis mill, but you
might have to be creative depending on the overall shape of your
scultpure and/or mill a piece in sections and assemble later.
An RP machine requires less investment in time and training and can
produce pretty much any shape your heart desires. It’s slower, which
isn’t a huge issue as you can just run it overnight and will give
you a slightly rougher model. I’m not wild about some of the
materials used in some RP machines, wax based materials are pretty
fragile, it’s tough to touch up at all, but they burn out well,
whereas the resins, which you can file and sand to smotth the
surface, are a little touchy to burn out.
That said, both mills and RP machines are used widely and with
success by many, it just depends on your needs and pocketbook. I’d
suggest sending out some models to service bureaus using a varitey
of systems, so you can compare.
I use a Minitech Minimill Pro 3, which is a great mill, capable of
producing very accurate models with great surface finishes. The
Minimill 2 is a very capable machine as well, suitable for jewelry
models and @ $10K
If you like to get feedback from users of a different mills and RP
systems, check out www.3dcadjewelry.com a free forum for jewelers
working in CAD. The best place for such info IMHO, I have no
affiliation, just a member.
Harry Hamill