"Tim Wescott" wrote in message
...
I was going to say "acrylic", but then I realized that maybe I would be
narrowing it down too much.

How hard is it to machine a part out of hard clear plastic and then make
it clear again? Any gotchas? What plastic should one start with? What
question am I failing to ask?

The part will be cylindrically symmetric (i.e., turned on a lathe), about
1.5" diameter, 0.5" tall, and accuracy can be as sloppy as 0.01 in all
directions. But it would need to be "pretty".

Some plastic that is a bit more rugged in impact than acrylic would be
nice (do they make crystal clear Delrin?) but I can't think of anything
like that which would actually work in this case.

--

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


From what I have read the biggest thing is to get the feed and speed right.
I don't think any method of cutting is going to leave you a perfect finish,
but cutting with feeds to slow for the RPM is going to leave the hard white
stringy melted surface that you hate. I've not done any "clear" plastic,
but I occasionally cut stencils out of PETG photo mounted to a sacrificial
backer. On my little CNC router I turn the spindle speed down to about
18,000 RPM, and scream along with a fresh sharp down cut carbide router bit
at a very high feed. Usually around 100 IPM with my smallest bits and even
faster with larger ones. Since these are usually one offs and not frequent
I have not played with getting it exactly right, but I've found in general
the faster I feed (and the fresher the cutter) the better my finish.

I think my approach would be to define the types of cuts first. Cutting a
stencil is certainly going to be different than cutting a mold, and both
would be different from cutting a part with all outside profile machining.
Then I would wander over to CNCzone and ask some of the sign makers for some
help with the speed/feed for your chosen plastic material.

After you have figured out how to get the best possible cut to begin with
then I would look at ways to improve the surface if desired.

Tangent Mode On

.... And after I had it mostly figured out but before I set it up I would
also consider other approaches for atleast some types of cuts. For instance
if I was making a 3D clear bust I would be more likely to consider making a
mold and selecting a nice casting resin rather than machining the bust
itself out of clear acrylic. I know some folks will freak at that last one
because they are mostly familiar with the thermal issues of thick pours of
epoxy resins (usually not good because most yellow when cured) and most more
common poly resins, but there are resins designed to be poured thicker.
There are also techniques like filling a mold and then emptying it leaving a
thin layer of resin to form a thin shell in the mold.

There are so many other things that can be done when it comes to plastics
and resins that it really depends on what you are making, how it will be
used, and how many of them you need to make. I bet there are even clear
plastics that can be blow molded economically.

Tangent Mode Off

Bob La Londe






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