Chapter 7 - Designing for Stamping Production
Basic Operations
Because dedicated production tooling is designed for a specific part
configuration, it is capable of producing very complicated shapes, such as multiple bends,
tabs, protrusions and extrusions, with great precision, on each press stroke.
Simple, two-dimensional parts become more economical to produce in
dedicated tooling through multiple punch and pierce operations per stroke. Complex curved
and shaped parts can be produced using specific-purpose design unachievable by
multi-purpose tooling.
The following operations are commonly performed in dedicated
single-purpose dies of the type used by Small Lot Stamping Division members of PMA:
Blanking is the cutting of flat metal sheet or strip
stock into the required size and shape. Here, tooling consists of a die, a punch, a set of
guides and a stripper. Typically, the entire periphery, which may include rounded corners
and irregular shapes, is cut in one stroke of the press.
Designers who are in the testing stage of products that will ultimately
run at high volume in dedeicated tooling might consider the use of "pancake"
(simple push-through) dies for blanking of prototype parts. In contrast, quantities up in
the thousands or more merit production units that feed strip or coil to limit handling and
keep blanking operations as economical as possible.
In practice, different types of blanks are used, depending on the next
operation in the manufacturing sequence, such as drawing or forming. Tool development for
a drawn part may involve the building of the draw die before the blanking die to establish
the final blank size.
Piercing generally refers to cutting openings such as
holes and slots in sheet stock, strip material, or a part. This operation is similar to
blanking, but here the slug produced by piercing is scrap.
Where possible, all holes and openings in a part are pierced in one
stroke. This results in a much more consistent part than punching features in separate
strokes of the press. Since all punches are permanently mounted in the same master die and
enter the workpiece at the same time, once they are checked and located correctly,
dimensional repeatability of all features is high.
Other types of piercing operations require special punches, among them:
pierce-and-extrude (for an extruded hole), lance and form (for a small feature formed from
the body of the part), slotting, countersinking, and cutting of tabs (see Figure 3).
Additionally, just about any operation--blanking, drawing, forming--can be combined with
piercing.
Figure 3. Basic piercing operations used to
create holes, slots, etc, and corresponding tooling.
Forming, in practically all instances, can be
considered as a bending opeation. Unlike drawing, which can significantly stretch and
deform the metal, forming generates shapes with virtually no change in thickness. Part
configurations include those with simple flanges, V-bends, U-shaped channels, and more
complex contours (see Figure 4).
Typically, basic tools like V-punches and wipe dies are built into
standard holders. As with blanking and piercing operations, repeatability and high
precision are readily achieved.
Drawing is broadly defined as stretching sheet metal
stock, commonly a blank that is restrained around its periphery, into a cuplike,
cylindrical or box shape. In practice, there's virtually no limit to the shapes that can
be drawn. (See Figure 5 for typical drawn shapes).
Figure 4. Example of various part
configurations achieved through forming.
Figure 5. Typical drawn shapes.
Drawing usually takes place in either a mechanical or a hydraulic press,
where a tool (punch) forces the metal to flow into a cavity (die) creating the basic
shape. Deep drawing and shallow drawing are the two main variations, classified by the
amount of deformation. In most instances, a trimming operation is required--either a pinch
trim or a cam trim--to remove the scalloped edge (see Figure 6).
As with most other dedicated-tool operations, drawing can be done with
single-action, double-action and compound dies (most commonly for blanking and drawing),
as well as progressive dies and transfer die systems.
Figure 6. Drawn cups are usually trimmed by cutting the
wall (left) or the flange (right). Resulting scrap is shown above.
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Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association
Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!
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