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SCALING, OFFSETTING, AND PARAMETERS
SCALING, OFFSETTING, AND
PARAMETERS
Scaling the CAD file by a small percentage, S,
makes up for any discrepancies between TMAX
and the nominal ¾″. This scaling method is the
simplest and most direct way to match the vir-
tual CAD design with the material, which came
after much trial and error. It is worth explaining
scaling in the context of other methods of
adjusting a CAD file for material.
WHY SCALING WORKS
Scaling all parts by a multiplier, S, has proven to
be the most efficient and accurate way to
adjust a cut file to match TMAX. As shown in
Figure 5-5, scaling resizes every part propor-
tionately, resulting in flush connections
between slots and tabs. The only shortcoming
of this technique is that it works best with a
nominal amount of scaling.
A scale differential less than 5%, which is
within the nominal thickness variation of most
standard material stock, won’t be perceptible in
a free-standing object.
A 5%–10% scale adjustment to furniture
might be perceptible, but typically won’t affect
function.
A scale adjustment greater than 10% will
affect the function of a furniture piece, yielding
a tabletop that is 10% lower, a chair that is 10%
smaller, or having 10% less height between
shelves.
Using this function in any CAD or CAM program
inconsistently affects the X and Y dispropor-
tionately, impacting the ratio of height and
width.
Offsetting creates tabs deeper than the mate-
rial thickness, and it changes sniglet radii to
form uneven and possibly uncuttable tool-
paths. Parts and joinery don’t fit when you off-
set toolpaths.
THE PARAMETRIC SOLUTION
A third alternative is to adjust the file by pre-
cisely redrawing all 2D vectors to match TMAX,
while still preserving the proportions of each
part, shown in Figure 5-7.
However, by laboriously redrawing each part,
there is a high probability of missing a small
detail in the process. A powerful CAD program
with parametric capabilities can automate this
adjustment, while sparing you the tedium and
possibility for mistakes.
Parametric software, explained in Chapter 12,
can have a steep learning curve but has tre-
mendous potential when coupled with digital
fabrication. Chapter 13 and Chapter 14 are
accompanied by parametric applets, which
allow you to transform every furniture part, so it
precisely fits TMAX.
WHY OFFSETS DON’T WORK
At first glance, it may seem that offsetting vec-
tors is the simplest and most obvious way to
adjust a cut file to fit TMAX. However, take a
look at Figure 5-6, and examine what happens
with the example slot and tab as vectors are
offset.
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DESIGN FOR CNC
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