In a typical supply chain, a regional depot receives
the coils of sheet stock from a rolling mill. This distributing depot uncoils and carefully restresses the
raw stock to flatten it. An example leveling line is
shown in Figure 2d. A leveling line can be set up
to flatten a variety of sheet materials while simultaneously cutting rectangular sheets. Examples are
shown in Figure 2e. Such pallets of sheet stock
are delivered to the fab shop.
Typical sheet sizes are 48 in. by 120 in. for steel
and 48 in. by 144 in. for aluminum. Other sheet sizes
are available. It is best to check with the fab shop to
see what is available from the raw material depot.
Here’s another DFM tip: Proceed with caution. Flat
part tolerances cut by well-maintained lasers or
turrets usually repeat within ±0.004 in. However, that
is not a practical tolerance callout on drawings for
job lot production of bent sheet metal.
To minimize the quality control cost of job shop
fabrication, we o;er the guideline for tolerances.
For bent part fabrication tolerances, start with
±0.006 in. to allow for accuracy of the fabrication
equipment and add 5 percent of the thickness for
each bend. In the example of a 16-ga. L-bracket with
a single bend, that’s ±0.009 in. (0.006 + 0.003). For
a U-bracket, where two bends or more are needed,
the tolerance for at least one of the legs should be
±0.012 in. with ±0.009 in. as the overall default.
Sensitive Air Benders
The fab shop sta; is sensitive to details about tool-
ing and tooling access, flat layout accuracy, grain
direction, alloy, finish, and tolerance. These caring
experts may carry a title of journeyman sheet met-
al mechanic. Well-designed 3-D CAD models and
thoughtfully prepared 2-D drawings soothe their
sensitivities. “Soothing” is roughly equivalent to
“suitable for fabrication with existing tooling.”
As an alternative to single-purpose stamping
tooling, a more versatile process is air bending, a
bending method that sometimes approaches the
technique of bottom bending with a press brake.
Many precision sheet metal job shops o;er reliable and predictable air bending as a service. When
set up for air bending, the tooling in the press brake
does not coin or thin the workpiece as illustrated
in Figure 3. As one of life’s miracles, entry-level
mainstream 3-D CAD tools have been perfected to
predict sheet metal behavior as long as the workpiece doesn’t get coined. That means it works with
folding brakes as well as it does with air bending
The rolled and then coiled and decoiled raw sheet
stress-relieves as it is subsequently cut and bent
into a useful item. While some stress runs parallel
to the sheet length, not all stress in the flat material
Here is a DFM tip: ±;;; degree on bend angles is
“precision” because of the variation in raw material
thickness, built-in stress in the workpiece, and
variation in the brake’s operation.
The raw sheet has a natural micrograin direction
that runs parallel to the 144-in. sheet length. This
micrograin impacts the design in terms of strength
Bends that run perpendicular to the material’s
natural grain require slightly more pressure than
those running parallel to the grain. The raw stock
tends to crack and fatigue parallel to the micrograin,
just as a sheet of wood would.
When sheet stock is bent at close to the fatigue
point of the workpiece—where it permanently retains shape without cracking—two closely related
characteristics of the material come into play: the
micrograin direction and the surface condition of
Surface scratches in the workpiece make it more
susceptible to cracking. As mentioned earlier,
bends that run parallel to the micrograin will fracture more easily than those that run perpendicular
to the grain.
Here’s a DFM tip: When specifying a grain direction,
perpendicular to the bend is structurally the best
When wide-belt sanding the flat aluminum workpiece for deburring and a beautiful finish prior to
forming, the fab shop makes an e;ort to avoid
cross-graining. In other words, this means running
the sandpaper perpendicular to the micrograin inside the sheet stock.
The surface of cross-grained aluminum may look
beautiful before plating, as no trace of cross-grain
can be seen, but this kind of beauty is only skin deep.
Chemical etching that takes place during the anodizing process reveals a warp-and-woof pattern in the
surface that is usually described as unattractive.
Here’s a DFM tip: The grain direction, particularly for
anodized parts, can contribute significantly to the
raw material utilization and cost for the workpiece.
Base Flange Versus Sheet Stock
In the CAD shop, the Base Flange (as designated in
most 3-D CAD modeling packages) makes the automatic flat layout possible. The equivalent in the
fab shop is a sheet of raw material with relatively
Raw material sheet metal starts out as hot ingot rolled
into a ribbon.
Air bending in a press brake is a reliable process. Many
combinations of upper and lower tooling can achieve
the same bend, making air bending a versatile process
The ribbon is thinned to final thickness with finishing
rollers. This image exaggerates the variation. Material
thickness variation of ± 5 percent is normal. Sheet is
typically rolled to the thin side of the gauge tolerance.
The cold ribbon is thinned and rolled into coils for distribution to local inventory depots.
The coil is uncoiled and flattened with a rolling leveling
line as the ribbon is cut with a shear.
The leveling line produces sheets of raw material.
These pallets of raw stock are the inventory that the
fab shop starts with. Stress is waiting to be relieved in