RE: [CAD_CAM_EDM_DRO] home brew rapid prototype machines

I've literally had hundreds of the parts I've designed modeled with the SLA
process (stereo lith. - liquid polymer cured with laser), and I have to say
that I think I've taken the process to its current limits. They key is in
being familiar with the process and it's limitations. The guys who run the
parts for me at Dynacept in NJ (www.dynacept.com) are constantly amazed at
the parts I give them and how they come out and come together as an
assembly. The product line I've been working on for the past couple years
is basically a very elaborate, pocket sized, manually operated fluid
metering device, with about 10-15 parts when assembled, all in the space of
a fat ball point pen. I can very often take all the SLA parts and assemble
them, and have a working assembly (for a few cycles). The parts include
details such as ratchet teeth (.018" high, 60 on a .290" diameter), mating
spring beam pawls, helical guideways, leadscrews and nuts (.200" major
dia.), snaps, and whatever else you might find on a an injection molded
part.

The SLA process is much like a HIGH powered laser printer that's been
directed at a vat of liquid resin. The software takes 2D slices of your CAD
model and writes that onto the surface of the liquid to harden it and fuse
it to layers under it. Then the layer is sunk into the vat, usually .004"
for each pass, and a squeegee comes across to flow a new layer of liquid
resin across the previous. The machine I observed maybe laid down a layer
every 8-15 seconds. Most of the parts I have had made are said to take
several hours just for this part of the process. The resolution of each 2D
section of the model is EXCELLENT. With a good SLA operator that knows
his/her machine, they can get 2D layer accuracies to within less than a
thou. at the fine 2D resolutions. The thing you have to watch out for is
the Z-axis stair-stepping. Each .004" layer leaves a step between each 2D
profile. Vertical constant dia. cylinders pose no problem, but anything
else will show the stairsteps. If you're willing to pay the premium for
extra machine time, some SLA machines are good for .002" per layer, and this
will obviously take twice as long. The machines cost $150,000+, so you'll
definitely pay the price for extra time on the machine. I HAVE to use the
.002"/ layer res. when I send out small parts that have threads, and have to
fit with other SLA threaded parts. There is ALOT of post-processing on most
SLA parts. At a minimum the operator must remove the SLA material supports
that were added so the part wouldn't collpase during building, and then
oven-cure the parts to harden the resin. The supports must be added by the
operator using the SLA house's software and are actually grown as part of
the SLA process. They are just thin columns of SLA material (maybe around
.010" dia), but they must be cleaned off by the operator after "building"
using an abrasive slurry jet. Then the rest of the part is run through the
abrasive slurry jet (think: wet sandblast) or a "vapor honing" process that
gives the parts a nice matte finish, and levels off much of the stair
stepping.

So there's an overview of what I've learned.

By the way, I agree with whoever said the machinery is very much "homebrew".
The quantities these machines are seeing doesn't justify much refinement. I
saw the guts of a really high-end machine ($250k+, super fast) and it is
really just a bunch of chopped up AL plate, stepper motors, a tank, linear
guideways, BERG couplings, gears, and shafts, a 2-axis laser system (major
bucks), a run-of-the-mill PC, and some pretty pressure-formed plastic
exterior panels. The software must be pretty pricey, though, even though
the machines I observed were still DOS-based! I wouldn't be surprised if
the machine mfr. had a software dept. of under 5 people, anyway.

So back to my original thoughts:
I DO use my SLA parts for FUNCTIONAL testing to a point. I have to
extrapolate ALOT, of course, since the SLA process currently can only run
various formulations of epoxy. They DO have resins that will approximate
soft plastics such as Polypropylene, but the original resins are alot
stiffer and less compliant, and approximate plastics such as polystyrene,
Delrin, ABS, ... as far as stiffness goes. The SLA operators I work with
KNOW that when they see gears and such, to run the part features standing
vertically so as to get REALLY GOOD 2D resolution for the important
features. Of course, I have to let them know what the important features
are many times.

This is just the laser/resin (SLA) process. I'm sure many of the other RP
processes started off very home-brew also. Lasers, hot-glue, etc. But they
all need some form of CNC to move the mechanicals...

By the way, I've decided to go with the AVS step/dir input servo system that
Bearing Engineers (http://www.bearingengineers.com/) sells for $800 per
axis, including motors, for my groundup 8"X8"X12" benchtop. It's gonna be
sweet.


Carlos Guillermo
VERVE Engineering and Design

> -----Original Message-----
> From: lawrence jackman [mailto:ljack70117@...]
> Sent: Monday, March 20, 2000 4:28 PM
> To: CAD_CAM_EDM_DRO@onelist.com
> Subject: Re: [CAD_CAM_EDM_DRO] home brew rapid prototype machines
>
>
> From: lawrence jackman <ljack70117@...>
>
> If it looked like an injection molded part it has been hand
> worked. When these things come off the machine they looked like a
> plowed field. Each pass of the laser leaves a row that has rounded edges.
> I have been looking at the process for over four years to make a
> part to use with the 3M mold making process. The finished part
> comes out of the machine so rough that it can not be used with
> out smoothing up and once you do that you do not have the part
> that as made.
> It ia also my opinion that the part is only good for looking at
> and test fitting
> Larry
>
> Jon Elson wrote:
>
> > From: Jon Elson <jmelson@...>
> >
> > Joe Vicars wrote:
> >
> > > From: Joe Vicars <jvicars@...>
> > >
> > > I have a little experience with the rapid prototyping machines.
> > > The
> > > problem with these machines is that the parts they create aren't good
> > > for anything but looks and fit testing. You would have to seriously
> > > be
> > > in to product development to justify the cost.
> >
> > I have to take issue with this. A friend of mine just had an RP part
> > made for a project he
> > is working on. It is essentially a kitchen cutting board, with an
> > opening for the thing he
> > is designing. (I'm being vague here to not divulge too much.) A
> > cutting board has to
> > be pretty tough to withstand normal use, and his definitely is. I'm not
> > sure which process
> > was used for this, but I believe it was the liquid polymer & laser
> > system. I thought that
> > this was an injection molded part when I first saw it, it was so solid.
> >
> > Jon