By Gerald Davis, Contributing Writer
Areader recently asked for guidance in using 3-D CAD for sheet metal parts. This guid- ance, continued here in the second part of
a four-part series, is partially related to design and
largely related to communication. Once the “trade
secrets” are revealed, the modeling choices become less mysterious.
As described in Part I of this series, a job shop is
essentially a production line that is time-shared
by many similar products. The same tooling and
machinery are simply reprogrammed to make a
variation rather than something entirely di;erent.
Setup of that production line and the methods
used to bend sheet metal are important to sheet
CAD details have an impact on fab shop setup. As
an example of bad design for manufacturing, consider Figure 1a. The corner treatment shows a tear
in all four corners of the part. This ripped design is
bad for at least two reasons: It is di;icult to manufacture with precision, and it leaves a sharp and
The torn feature is removed from the design in
Figure 1b. It is a good practice to always have a
cutout meet a bend at 90 degrees. Figure 1b also
features an open corner. Either bend may be over-bent to allow for material springback. This flexibility
in fabrication may translate into faster and more reliable production.
When a closed corner is required, design an overlap to extend the short sides as shown in Figure
1c. The idea is that one long section of tooling can
be used to form both the short and long flange
lengths. The first two bends use the long tooling to
form the short sides. The next two bends require
carefully tucking the tooling in place before starting the bend. A couple of pretty good illustrations
of this can be found in last month’s edition (“What
sheet metal shops wish you knew: Part I,” Precision
Matters, The FABRICATOR, May 2017, p. 40).
Whether or not you are overlapping, a corner relief cutout removes the material that is likely to rip
or tear (see Figure 1d). The filleted corners in the
bend relief cutout reduce the propagation of microfractures in the material. This is good practice if the
part is going to be subject to vibration. The design
in Figure 1d is better than Figure 1a because it has
fewer sharp corners to slow down the laser, fewer
burrs a;er forming, and is structurally more robust.
Read more from Gerald Davis at www.thefabricator.com/author/gerald-davis
What sheet metal shops
wish you knew: Part II
A CAD jockey who understands metal fabrication makes
better decisions in applying CAD tools. Let’s focus on the
accuracy of things made from rolled sheet stock.
This is a bad corner treatment. The torn corners specified in this design leave sharp edges as a result of ripping and cracking.
This is an example of a rectangular cutout corner treatment. The material that will rip and tear is cut away.
In this overlap corner treatment, an overlap is made to
make the short sides longer. This might allow one tooling setup to form four flanges instead of requiring two
setups, each bending two flanges. The problem with
ripped corners remains, however.
Bend relief cutouts in the overlapped corner remove
the material that will rip or tear. The use of radius instead of sharp corner cutouts reduces the propagation
of microfractures in the material. This is good practice
for parts subject to vibration.
Practical manufacturing tolerances are limited by
the physics of rolling hot metal to create sheet stock.
The accuracy of the machinery that processes that
raw material also contributes to what is practical in
terms of precision in sheet metal.
Here’s a design for manufacturability (DFM) tip:
Include a default callout on drawings for precision
sheet metal destined for job lot production. For example, “Tolerance unless otherwise specified: ±0.006
in. hole-to-hole, ±0.012 in. hole-to-edge, ±0.015 in.
fold-to-fold, angles ±;;; degree.”
Greater precision, which translates into smaller
manufacturing variation, might be economical,
especially if you are working with die sets in a progressive stamping line. However, more precision
requires greater consistency in the tooling setup for
blanking and bending the sheet metal parts.
As raw material, sheet metal has behavioral weirdness. Some of that weirdness, stress in particular, is
relieved during manufacturing. The resulting bend
angle variation, and sometimes the required adjustment to the flat layout, contributes to the reason
that machined parts (typical tolerance of ±0.002 in.)
can routinely have tighter tolerances than precision
sheet metal parts (typical tolerance of ±0.010 in.).
The raw sheet stock is manufactured by rolling ingot between rollers to squeeze the billet into a ribbon. An early stage of the process is shown in Figure
2a. As the hot ribbon cools, it is fed through finishing
rollers to achieve the final gauge thickness. Those
finishing rollers deflect somewhat in the process. An
exaggerated view of this deflection is shown in
Figure 2b. The emerging sheet is o;en thicker in the
middle than it is at the extreme edges of the ribbon.
The rollers used to finish the sheet emboss the
surface finish onto the sheet. Typically, the rollers
are precision-ground with a smooth finish. The re-
sulting mill-rolled finish in the sheet stock is termed
“2B” as a finish designation for stainless. An exam-
ple drawing note might read, “Material: Stainless
Steel Sheet, 16 gauge 304 2B.”
The rolling process also introduces a grain di-
rection in the microstructure of the sheet. This mi-
crograin runs parallel to the edge of the ribbon of
sheet metal as it comes out of the roller.
As a third source of “grain” in the material, the
rolling mill can sand the surface of the ribbon to give
it a cosmetically appealing finish. “Material: Stainless Steel Sheet, Pregrained, 16 Gauge 304 #4 with
PVC” is a typical designation for pregrained stainless steel sheet stock. Of course, sanding the workpiece for deburring and cosmetic grain might occur
at any stage of fabrication.
Whether prefinished or not, the ribbon of sheet
metal is subsequently rolled into coils for rail transport (see Figure 2c). A 25-ton coil requires dedicated machinery for handling and storage. This coiling of the ribbon winds nonuniform stress into the