Datasheet

tolerance—at least, they’re supposed to. The question is, where in that range should you
create your accurate-to-15-decimal-places geometry when using a CAD system? (You
know it’s impossible to make anything an exact size. If you think you can, you’re not using
a precise enough measuring tool.)
There are several possibilities. Some people draw objects in the middle of their size tol-
erance range. Others use what they consider the design size or nominal size—the base size
given before the plus/minus sign. After all, isn’t that the ideal size? Well, it may be the
ideal size, but neither of these approaches is good practice, whether you’re creating a
2D drawing or a 3D model. The fact is, a mechanical part has no ideal size. If a part is
designed properly and given a functional tolerance, as required, it will work fine as long as
its dimensions fall within that tolerance. No size in that tolerance will make the part per-
form better than any other size. As a machinist, you may aim for the middle of a range,
but not because that makes a better part. You do that to reduce the chances of making the
part too small or too large and having to throw it out.
Draw all parts and create all solid models at MMC unless it’s one of those rare features
that are controlled at Least Material Condition (LMC) in a tolerance frame. Features with
outside dimensions, like a shaft or a pin, should be drawn at their largest acceptable size.
Features that have inside dimensions, like a hole or a slot, should be drawn at their small-
est possible size. In both cases, the result is a part with the maximum amount of material
in it. That’s usually when parts are least likely to go together; this approach allows you to
draw parts in the situation where they’re most likely to fail and to check for interferences
more readily. It’s also easier to be consistent this way, because the rules for Geometric
Dimensioning and Tolerancing assume MMC for many of the specific situations where
the rules apply. I recommend this approach for both 2D and 3D modeling, including
when you’re using Mechanical Desktop, Inventor, or any other CAD package.
Use Logical Increments
When designing something, you can use any size increment you wish. If you choose incre-
ments that are easy to work with, or that result in less waste during fabrication, you save
both time and materials. During the early design stages, you can set a SNAP to that incre-
ment in AutoCAD, with a GRID set to twice the increment. Doing so can speed up the ini-
tial layout.
For architectural design, I recommend using the largest increment possible, such as
whole inches, one foot, two feet, or four feet. Doing so makes it much easier to use stan-
dard-sized sheets of material during construction. For mechanical parts, use increments
of 2mm or 0.1 inches if possible.
4 ■ chapter 1: AutoCAD Productivity
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