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Casting a detailed CAD/CAM piece


#1

A friend of mine has asked me to help him prepare a piece for
production that is outside my area of expertise; naturally I thought
of you all. He has supplied me with several milled waxes that are
essentially flat backed hemispheres 2 centimeters in diameter with
lettering on the domed side. Here’s the problem; some the fins of
investment that form the letters have sheared off in casting. I
wasn’t particularly surprised, as the lettering has a line weight of
only 0.4mm to 0.25mm and they are nearly 1mm deep. He likes this
look and would sooner not make the lettering shallower, and I don’t
think the font could be made bolder (thicker line weight) and still
fit. Here’s the question; is there an investment strong enough to
withstand this situation and still reproduce the detail? I have used
Kerr Lab’s Satin Cast 20 for years, and it has always done what I
have asked of it beautifully (at least after I began to strictly
follow the formulated instructions as to ratio of investment to
water, and the water temperature / working time). I want to deliver
to my friend a master model that he can have cast without extreme
measures, or a high rejection rate. Any suggestions?


#2

David,

If you can’t cast it I suspect that the design is going to be even
more of a problem production casting from a mold.

Maybe the designer will have to re-think his model. Model making is
more complex than just what appears on the screen. Every
manufacturing step (limitation) has to be taken into account, good
model making is technically more complex than even really fancy
custom work.

No real evidence but I suspect that your friend is using a straight
end mill, a tapered bit (15 included with a .2mm tip) will yield
almost the same visual results but be much stronger. And hopefully
the back is hollow, thick castings are always much more difficult.
Thick and fine details just don’t mix well.

Jeff
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#3

David,

With lettering at only .4 to.25 mm wide in the master, production
pieces will have areas that are only 0.2mm wide at best. At the
specified lettering width, the CAD model letter depth should be no
more than 0.5 or 0.6 mm for cross sections that thin. Even that is
asking for a low ratio of good to bad castings/ injections. Cost wise
it is overkill, but most of the platinum investments will hold up
better that Satin Cast for sections that thin.

One other suggestion is: make your letters taper in width. That is,
if they are raised letters, wider at the inner surface tapering to
the designated width at the top of the letter; if they are inscribed
letters, make them the designated thickness at the deepest point and
taper them out to at least 0.5mm at the attachment point to the
outer surface.

If you need to see what I mean, email me off line and I will send you
examples of the two scenarios with pictures of what I’ve done in the
past for CAD/CAM clients.

Paul


#4

Hi David,

In response to your question, We cast carved /cnc models and RP
models daily. Very fine thin lettering and lines that are deep can
problematic. We sometimes have this problem with lines/ lettering
that are in the.03 mm category and not at all in the.4mm category.

What we do to reduce any possibility of error is to not use any kind
of wax wash /soap/alcohol on the machined waxes… further more, we
use high pressure air blasting ( air gun with a reduced nozzle) to
clean out and dry any small grooves/ lettering from the wax before
attaching the wax to the tree.

Our investment and water is weighed on digital scales for accuracy
and sometimes we use a 38/ 100 mix to have stronger investment on
carved models. That is also why we cast models for our customers
individually in their own flasks at the correct temperature for the
model…

I suspect that if you are having this problem at.4mm that there may
be some water/ soap in the grooves before inversting which is
weakening your mixture just enough in that area.

I would suggest cutting the depth to 1/2 mm or less as deeper is
totally useless and may cause issues in those same areas if a rubber
mold is made. I hope this helps.

Daniel Grandi
Racecar Jewelry Co. Inc.
We do casting /finishing from small to large pieces for people in
the trade.
http://www.racecarjewelry.com