By Gerald Davis
Areader recently asked for guidance in using 3-D CAD for sheet metal parts. We continue the theme from last month: Skilled designers know how the sheet metal parts get bent.
The recommended CAD work flow is to design
with size and function of the end product as the
modeling intent. A concluding step in the work flow
is to derive the flat workpiece from the finished
product’s manufacturing information.
As an alternative CAD work flow, model the flat
workpiece first and then add bends to create the
final shape. While that would resemble the fabrication process, such a CAD work flow—where the
conclusion is presented and an argument is to follow—is not recommended.
Working on the Hem
A hem is a single, 180-degree bend. Some hems are
closed—basically a zero inside bend radius. If the
hem has a radius greater than zero, it resembles a U
in cross section. Unlike the U-channels discussed in
Part III, the hem has only one bend instead of two. If
no flat section to the hem is evident, it is simply an
In 3-D CAD a hem is very easy to create. As shown
in Figure 1a, pick an edge or edges, set a depth,
and decide on the gap. The CAD hem’s angle can be
adjusted past 180 degrees to create a closed tear-
drop or underbent to create an open hook. To bead
an edge, simply select that hem type in the hem
feature’s setup. The model with hemmed edges is
shown in Figure 1b.
As a design element, the hem creates the appearance of greater thickness without adding much
weight. It creates a very smooth edge and can hide
the raw edge of the part. This is sometimes a way to
add durability to painted parts. Hems can be useful
as a way to stiffen a sheet metal panel.
Hems have drawbacks, however. With plated
steel parts, the interior of the hem can be very difficult to cover for corrosion control. This can be a serious problem because the inside of the hem is easy
to wet but challenging to dry.
In the fab shop, the hem is created with multiple
bending strokes or machine cycles. Machine cycles
take time, and as a result, hems add expense to the
design. As additional expense, the hem requires a
unique tooling setup separate from that for 90-de-
Figure 2a shows the flat part positioned, ready
for the first bend. The first bend in creating a hem
is an acute angle bend, something greater than 120
degrees to define where the center of the hem bend
As the top punch descends to contact the workpiece, the spring-loaded die set is compressed (see
The initial acute bend in a hemming operation
is often completed rather slowly as the workpiece
flops and whips during the initial forming cycle.
Even a moderately sized workpiece may require a
pair of brake operators to support the whip of the
workpiece. Extra labor is another reason that hems
are such expensive features when compared to simple 90-degree bends.
As the press brake completes this first stroke, the
top punch is raised, and the die set springs lift to
their normal position. At that moment, the workpiece is released.
The workpiece is then positioned for the final
bend (see Figure 2c). This is a spanking operation
made with flat faces in the die set. The top punch is
lowered into the empty V die and continues down
to compress the springs in the die set. The spanking operation deforms the acute starting bend into
the final 180-degree angle (see Figure 2d). Note
that the radius on an open hem bend is usually deformed because of this spanking operation.
Once again, as the press brake completes this final stroke, the top punch is raised, and the die set
springs lift to their normal position. The hemmed
workpiece is then released (see Figure 2e).
Read more from Gerald Davis at
What sheet metal shops
wish you knew: Part IV
Hems and jogs are easy in 3-D CAD, but they require
planning, tooling, and setup in the fab shop
This is a CAD setup for an open hem on two edges.
Note that closed hems, teardrop, and edge bead are all
available from this control panel.
The completed hem is shown. Note that the perfect
arc for the inside radius may be difficult to fabricate.
It probably will be more parabolic than a true radius.
This model represents
the function of a “Dutch
bend” hemming die set.
The brake operator positions the workpiece
over the V die. The upper tool descends under
press brake power to
contact the workpiece.
An acute bend is formed in
the workpiece as the upper
tool is lowered. The pressure
from the upper tool com-presses springs in the lower
After the upper tool is
returned to the top of
stroke, the workpiece is
repositioned in the spanking section of the die set.
The upper tool is lowered
into the empty V die to compress the springs once again.
This time, the acute bend in
the workpiece is spanked to
form a 180-degree bend.
The upper tool returns to top of
stroke to release the finished
workpiece. This example shows a
closed hem. Open hems are created using a shorter stroke distance for the upper tool, sometimes with a manually inserted
spacer to help with accuracy.