How to go from 2.5 D to real 3 D

Message: 8
Date: Wed, 21 Nov 2001 20:29:06 -0000
From: tom.kay@...
Subject: How to go from 2.5 D to real 3 D ?? [Snipped for brevity]

Hi All

I think I get the concept about what's 2.5 D and what's 3D. In 2.5 D,
you do some x-y movements, then stop, lift the tool in the z axis,
then stop, and do more x-y cutting. In 3D, all three axes can move at
once. Or 4, if you have a rotary device.

[It's not really about how the tool moves, but what it's doing. There are
various strategies for cutting 3d parts, and for some, you only need to move
2 axes at a time. Even with a 4th axis, it's not necessary to move more than
2 axes at a time, although some programs will use all 4 at once. 2.5 D
refers to parts that are defined in 2 dimensions, but are cut at different
levels, like for instance a shape with a pocket removed from it.]

But how do you make the jump up to 3D? Right now, I can draw
something in Corel Draw, save as a DXF file, convert to G-code, and
away I go. My drawings are 2-D. I would REALLY prefer to stay with
Corel Draw if I can (instead of AutoCad) but I'd like to try 3D. Do I
have to draw differently, essentially a 3D drawing? Does anyone know
if this can be done in Corel Draw?

[No, I don't think Corel Draw produces anything but lines. For 3d, you need
something that makes surfaces. They don't necessarily have to be solids-
that's a whole 'nother level of CAD software. DXF files are confusing,
because the definition encompasses both linear output, like you get from
Corel, and "3d faces" output, which AutoCad and other 3d programs can use to
define surfaces as sets of adjacent triangles. STL files are similar, but
this format isn't used for anything but surfaces and solids.]

Also, how does the G-code get generated for a real 3D operation? Are
the files typically huge compared to 2D work?

[They certainly can get huge. At least with the less expensive programs I
know about, the G-code is actually fairly simple, consisting mostly of
directional commands like G0 and G1, but there tend to be a lot of them if
you're describing a curvy surface. A typical strategy will make a toolpaths
that run down the X axis varying in Z with lots of small incremental moves,
then move over a fraction of the tool diameter in Y and come back down X,
etc. With enough of these parallel toolpaths stacked next to one another, a
complex 3d surface relief can be carved.]

Andrew Werby
www.computersculpture.com
3d Scanning, Modeling, and CNC Products at a Discount