By Gerald Davis
When designing an enclosure, it is useful to have accurate models for sizing and planning. Sometimes “
reverse engineering” is required.
For a recent project I needed a bottle opener.
Figure 1a is a photo of an example item. Note
that the lanced hooks cut into adjacent bends
because of the sequence of hits in a progressive
A 3D CAD version of this item is shown in
Figure 1b. It is a reasonably good visual copy. Dimensionally speaking, it is what I measured with
hand-held calipers—probably within 0.004 inch,
but no guarantees.
Selecting CAD Techniques
The following review of the CAD techniques used
mentions software-specific terms such as sheet
metal, forming tool, unfolding, and mirroring.
For planning how to model this, the design intent, or guiding light, is to create a model suitable
for appearance in a photorealistic rendering. As
always, speed matters. We are on the clock. This
item is merely a prop for visualization.
Given my CAD skill set, I frequently use sheet
metal modeling tools in CAD. The speedy technique is sheet metal-oriented. An artifact of this
technique is that the model can be unfolded. In
other words, it is nearly suitable for designing a
progressive stamping die. However, manufacturing bottle openers is not the intent.
A CAD Sheet Metal Tutorial
This model starts with a sketch, shown in Figure
2a. The sketch line drawn is merely an imaginary
edge. That sketch is subsequently used to create
a sheet metal base flange. This is only half of the
model. It will be mirrored later to complete the
The Mirror feature was used in the spirit of
speedy CAD. Why model two identical edges if
they are symmetrical? Model one and mirror it.
That “goof-proofs” the symmetry and sort of
speeds the CAD.
According to the rules of CAD, the sequence
of modeling matters. A Base Flange > Unfold >
Lance > Fold sequence allows the lances to exist
before the base bends. If the forming tool is applied to the opener in the bent state, the lance
will not cut into the adjacent bend.
The Forming Tool is a versatile modeling resource. In this example, a Forming Tool feature
is used to add lances. As shown in Figure 2b,
the lances mimic the appearance of the actual
part. By the way, according to the rules of CAD,
the Forming Tool feature must create normal
edges in the sheet metal model. In Figure 2c
the lanced tabs extend past the midpoint of
the model. These unneeded artifacts from the
Forming Tool will disappear when the part is
mirrored (see Figure 2d).
Read more from Gerald Davis at
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A few tricks can help a CAD jockey
produce a model for show, not necessarily for dough
EXPERTISE » PRECISION MATTERS
» Figure 1a
We have a need for a 3D CAD model of this bottle opener.
Note that the lanced hooks cut through adjacent bends.
» Figure 1b
With 3D CAD, we have reverse-engineered the opener
from Figure 1a. Only the name has changed to protect
» Figure 2a
Sheet metal base flange is based on a single line sketch.
The dimensions are as measured by the author.
» Figure 2b
The bends in the base flange do not exist. They have been
unfolded at the time that the Forming Tool is applied to
create lanced hooks.
» Figure 2c
After folding the unfolded, this opener model starts to
look like half of itself.
» Figure 2d
The mirroring operation ensures symmetry and
potentially reduces labor by nearly 49 percent, resulting
in a brighter smile and mightier model.