"Tim Wescott" wrote in message
...
On Tue, 16 Jun 2015 21:46:30 -0400, Ned Simmons wrote:

On Tue, 16 Jun 2015 19:25:47 -0500, Tim Wescott
wrote:

I'm working on a project that may well end up having a case made of
fiberglass. As I'm working on a 3D model that will use mostly to make
pretty PDFs for my fiberglass guy and my customer, It occurs to me that
I could just make a 3D model of a mold and send it to a machine shop.

A 1.25 x 12 x 48 inch piece of 6061 costs $430 from Online Metals. Is
that pretty much what I would expect to pay for materials from a machine
shop? Is there any material that's significantly cheaper, that can be
tossed into a CNC mill and made into something mold like, and then hand-
polished to a high shine?

That's almost twice what I'd expect the shop to pay for that piece.

If you're cutting a large cavity in one side of a piece of 6061 plate
it's not going to stay flat. Cast tooling plate (e.g. Alcoa MIC6) is
stable, but is relatively gummy, so will be more difficult to machine
and finish to a polish. Also more expensive.

I knew about the issue with 6061 -- that's part of the reason that I'm
asking questions here!

I wonder if one could just pour a couple of gallons of epoxy casting
resin into a box, then send that off to the machine shop -- if you can do
it without bubbles you certainly have something that you can polish to a
high shine.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com


One trick for pouring resins with few bubbles is to pour a long thin stream
from a long distance above the forms. However I am not sure if epoxy would
work well for what you are doing. I've found epoxy pours thicker than about
1/2" tend to scorch from their own reaction heat. Even 1/2" is pushing it.
I've had the small amount left in a mixing cup scorch once or twice if it
was still fairly thick. I've read that you can get past the issue with
epoxy by making multiple pours, but I seem to recall that you either need to
have your timing just right, or you need to let it fully cure and then prep
the surface before making the next pour. Sounds like a long tedious
process. There are other casting resins that might work better for thick
pours, but I really don't have any first hand experience with them.