# RE: artist wants carved styrene shapes

From: Derek Smith-EDS014 (Derek_Smith-EDS014@email.mot.com)
Date: Mon Jul 06 1998 - 23:09:08 EEST

Gautham,

I don't quite understand your question. You seem to make the assumption that
you should change the foam slab thickness to compensate for scaling. I don't
agree. The reason is that in practice, you probably have a limited selection
of foam thicknesses you can buy. The foam thickness is not infinitely
variable unless you are producing it, and still there are limits.

What you want to consider first is what thickness of foam you WANT to use.
Your decision should be based upon how easy it is to cut, how long it will
take to build (thin foam means many cuts), how much step error you can
accept (assuming you don't take the surface angle into account), etc.

Once you decide which foam thickness is best to work with, you should also
decide how big your final "projection" will be. Actually, this decision and
the last one are somewhat related, but I think this order will work for you.

Think about a 1 inch or 1 centimeter cube to make things easier.

An example might make it easier as well.
Let's assume your final size for the 1 inch cube is 2.5 feet in X&Y.
Let's assume that you get a really good deal (per unit of volume is how you
need to measure cheap!) on 0.75 inch foam, and this is easy to cut. So you
have a CAD model that is 1 inch cubed, and a final size in real life that is
30 inches (2.5 feet). Divide the CAD size by the final size, and you now
have a ratio of 0.033, or (1/30). Be careful to keep the units consistent.
Next, you need to compensate for the foam slab thickness. Multiply
the ratio above by your foam thickness, and this gives you the amount by
which to slice in Quickslice. 0.033 * 0.75 = 0.025 inches.
If you want to check this calculation, divide the height of your
final model by the thickness of your foam, and this tells you how many
layers you will need. The result is 30/0.75 = 40 layers of foam. You now
need to calculate what layer thickness you need to use to slice the CAD file
in Quickslice. You know that you want 40 layers, and you know that the
height in CAD is 1 inch. Divide the CAD height by the number of layers, and
you get: 1/40 = 0.025 inches, so it checks out.

So, simplifying this into an equation, it looks like this:
CST = (CAD / FOAM) * FST
Where:
FOAM = Foam Model Size
FST = Foam Slab Thickness

Here it is with the numbers above:
CST = (1/30)*0.75
CST = 0.025 inches

To help you plan how many sheets (layers) of foam you will need before you
start, just divide the foam model size (FOAM) in the Z-axis by the foam
thickness (FT).

A little planning up front makes things much more practical. You may need to
do a little "setup" with your projector by moving it in and out to get the
final size to match what you planned. Make sure you include a couple of
holes, or something similar, in the CAD model so you can align the slices,
using broom sticks, or other suitable rods.

Good luck,

...Derek

----------
From: gautham@asu.edu[SMTP:gautham@asu.edu]
Sent: Monday, July 06, 1998 2:09 PM
To: Smith Derek-EDS014
Subject: RE: artist wants carved styrene shapes

Derek,
What you said is right. The z relative to scaled up x & y is
important. doesnt the thickness of the individual slabs of styrofoam
decide
the scaling factor then- bigger the scale of the prototype, for the
slices
corresponding to the orginal part size, you would need to use
thicker slabs to
compensate for the scaling?

gautham

------------------------------------------------------------------------
Gautham Kattethota Home : 950, South Terrace Road,
#C349
Dept. of MAE,Mail Code: 6106 Res Ph# : (602)967-4362
Arizona State University, Off Ph# : (602)965-7830
Tempe, AZ 85287 Email : gautham@asu.edu

------------------------------------------------------------------------

On Mon, 6 Jul 1998, Derek Smith-EDS014 wrote:

> Gautham,
>
(perpendicular to the
> slices) to be the proper scale relative to the X & Y, then the
size of your
> projection relative to the original, the foam thickness, and the
slice
> thickness chosen in QuickSlice must all be considered. I think you
said you
> were doing a big tooth, and you would end up with one that is too
tall or
> short otherwise.
>
> Regards,
>
> ...Derek
> ____________________________________
>
> E. Derek Smith
> 3DP Program Manager
> Technology Scout
>
> Motorola Land Mobile Products Sector
> 8000 West Sunrise Blvd., Room 2329
> Ft. Lauderdale, Florida 33322
>
> 954-723-4790 (Phone)
> 954-723-5584 (Fax)
> eds014@email.mot.com
> ____________________________________
>
>
> ----------
> From: gautham@asu.edu[SMTP:gautham@asu.edu]
> Sent: Saturday, July 04, 1998 5:12 PM
> To: Marshall Burns
> Cc: rapid tooling bulletin board
> Subject: Re: artist wants carved styrene shapes
>
> marshall,
>
> Ok- thermocoal is no magic material. Its called
styrofoam
> here.
> Thermocoal is what we call it in the country where I am
from.
>
> Here's a brief review of the process of using styrofoam
sheets to
> build scaled
> prototypes:
>
> The process is a simple one, but time consuming .
All it
> would neeed
> is a slicing software, a overhead projector, slabs of
styrofoam &
> some
> patientce. Once the part is sliced, these students used a
projector
> to project
> each slice on a wall. THey adjusted the distance of the
projecttor
> to the wall
> to scale the slices up to the scale they wanted. Then they
placed a
> slab of
> styrofoam on the projection of each slice , outlined the
contours
> with a pen ,
> cut against the outline with a hot knife & then glued the
individual
> slices
> together to get the prototype. The thickness of the
styrofoam slabs
> or sheets
> determine the layer thickness.
>
>
> Regards,
> gautham
>
>
>
------------------------------------------------------------------------
> Gautham Kattethota Home : 950, South
> #C349
> Grad. student, Tempe AZ 85281
> Dept. of MAE,Mail Code: 6106 Res Ph# : (602)967-4362
> Arizona State University, Off Ph# : (602)965-7830
> Tempe, AZ 85287 Email : gautham@asu.edu
>
>
------------------------------------------------------------------------
>
> On Sat, 4 Jul 1998, Marshall Burns wrote:
>
> > Dear Gautham,
> >
> > What is thermocoal, please?
> >
> > Marshall Burns
> >
> >
> >
> >
> >
> > gautham@asu.edu wrote:
> >
> > > Sharon,
> > > THis may not be what you are exactly looking
for, but
> never the
> > > less....
> > > A couple of arts students here from a
Visualization &
> PRototyping
> > > class used an ingenious way to build big prototyppes out
of
> thermocoal from a
> > > smaller part on the computer. What they did was they
used
> QuickSlice , a
> > > software that forms the interface for A FDM machine, to
slice up
> the stl file
> > > & then projected each layer on to a thermocoal slab
pressed
> against the wall (
> > > the distance of projection decides the size of the
projected
> slices). They
> > > then cut along the outlines & stuck the individual
layers of cut
> thermocoal to
> > > make a scaled up prototype of a tooth.
> > > It is a simple ingenious way to build scaled
prototypes
> yourself.
> > >
> > > gautham
> > >
> > >
>
------------------------------------------------------------------------
> > > Gautham Kattethota Home : 950, South
Terrace
> > > Grad. student, Tempe AZ
85281
> > > Dept. of MAE,Mail Code: 6106 Res Ph# : (602)967-4362
> > > Arizona State University, Off Ph# : (602)965-7830
> > > Tempe, AZ 85287 Email :
gautham@asu.edu
> > >
>
------------------------------------------------------------------------
> > >
> >
> > --
> > Marshall Burns
> > Marshall@Ennex.com
> >
> >
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