30-01-2013, 12:54 PM
Sheet Metal Fabrication
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Introduction
Sheet metal fabrication plays an important role in the manufacturing world. Sheet
metal is used to make everything from hinges to automobiles. There are many types of
sheet metal fabrication for engineers to choose from. Deep drawing, stamping, rubber
forming, hydroforming, and high-energy-rate (HERF) forming are all different methods
used for creating desired shapes out of a sheet metal. It is the responsibility of design
engineer to determine the mechanical properties of the material, select the material that
meets these requirements, and select the best fabrication process for the job.
There are other considerations a designer must decide on besides fabrication
process when creating a new part or assembly. Cosmetic appearance, cost, engineering,
manufacturing method, and assembly are just some of the factors to be taken into account
when a new design is being developed.
Material Properties
Before the material is selected for a design project, engineers have to determine
the mechanical properties of the material. Designing for strength, material class and mode
of loading are important considerations. Several factors have to be considered when
selecting a material such as, yield strength, ultimate tensile strength, ductility, thermal
conductivity, wear, and corrosion resistance. When designing, all these factors are
weighed against the properties of the available materials and it is the job of the design
engineer to select the material that best fits the application.
The most common measure of strength is the yield strength. Yield strength is
equal to the minimum load which produces permanent deformation (see Figure 1).
The ultimate tensile strength is calculated by dividing the maximum load on the
material by the initial cross section area.
Material Selection
After all engineering parameters have been calculated and established, the next
step is to select the type and size of sheet metal. Typically sheet metal can be purchased
as sheet, plate, or coil stock ranging from .0187 inches to 12.0 inches in thickness. Sheet
stock is a flat rectangular sheet of metal measured in thickness between .0187 inches (28
GA.) and .179 inches (7 GA.). Plate stock is a flat rectangular sheet of metal with a
thickness between .187 inches to 18.0 inches. Coil stock is made from sheet metal slit or
cut to a certain width then rolled up into a coil. Coil is generally cut to a width between
.312 inches to 3.0 inch. When ordering coil or sheet stock it is usually ordered by gauge
thickness (GA) and width. If coil is ordered the coil diameter (ranging from 36.0 inches
to 72.0 inches) or total weight (lbs) is required. Different surface finishes or protective
coatings may be required for different applications. For example, an application exposed
to water requires some form of protection. Galvanized finish is a corrosion inhibiter used
for an application exposed to water. Another type of surface finish is mirror or polished.
Mirror or polished finishes are used for cosmetic applications. All of these factors must
be considered when selecting and ordering sheet metal from a vendor.
Rubber – Pad Forming
There are other alternatives to forming complex shapes besides deep drawing.
Rubber-pad forming, hydroforming, explosive forming, and electromagnetic forming are
alternative sheet metal fabrication processes. Rubber-pad forming, also know as flexibledie
forming, uses a rubber diaphragm as one tool half. This method only requires one
solid tool half, which is usually the punch. The rubber-pad is used to distribute equal
pressure on all workpiece surfaces as it is pressed around the form block. Rubber-pad
forming is designed for the use of fabricating parts with relatively complex shapes and
configurations. The form block height is usually less than four inches.
The following figures were taken while touring the facilities of Southwest Fabricators.
The rubber forming process was used to prototype a button panel used on a slot machine.
Figure 4 is a picture of the pre-punched flat pattern for a button panel prior to forming.
The part is a semi-flat blank .040" thick piece of sheet metal. The blank was punched
using a numerical controlled punch press and shear. The blank was checked against the
blueprints and inspected prior to the next phase in the fabrication process. Next, the part
is set in the hydraulic press ready to be formed. The operator places two pieces of rubber
(urethane) between the part and the die. Two pieces of rubber are used to create the
required force distribution to form the part (see Figure 5). The urethane forces the sheet
metal to form around the aluminum punch which creates the flanges on the part. After the
operator applies the required force, the punch is backed away from the part and the
rubber pads are removed. Although it is difficult to see in the picture, Figure 6 shows
that rubber removed and the front and side flanges formed. Because of the complex shape
of the part, the operator had to come back and rework the front radius with a smaller
piece of urethane. This was done to finish the front radius of the button panel (see Figure
7). This was done until the finished part met the required dimensions specified by the
drawing. Once the forming process was complete, the part was inspected for surface
defects from the forming process and dimensional requirements. Figure 8 shows the
finished part (left) after the forming process and the sheet metal blank (right) prior to the
forming process.