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Chapter 7 - Designing for Stamping Production
Tooling/Process Options
For increased producitivity, two or more basic metalforming
operations can often be combined in dedicated tooling. Beyond the single operation per
press-stroke, other options with dedicated tooling include compound dies, progressive
tooling, transfer die systems, and slide forming.
Compound dies are hard-tooling options
that allow two or more operations to be combined into one. A compound blank and pierce,
for example, permits the entire perimeter and all holes to be cut in one stroke of the
press.
This produces the closest feature-to-feature tolerances,
flattest part, and easiest to control burr height. In addition, all burrs are formed in
the same direction.
Metalformers who specialize in smaller production runs (up
to thousands of pieces) often utilize a compound system that is built into a master die
set. In production, parts typically run slightly slower than normal blanking, but a lot
faster than separate blanking and piercing.
In effect, an additional operation is gained in the
process. Similarly, compound dies can combine other operations. Among them: blanking and
forming or blanking, piercing and forming.
Progressive tooling, as typically used by
members of PMA's Metal Stamping Division, makes use of one die in which multiple
operations are performed in consecutive stations (see Figure 7). Normally, stock is
automatically fed by a feed mechanism. The workpieces remain attached to a strip or ribbon
of the material, and are thereby advanced from station to station.
Unlike single-operation dies, progressive dies do only a
portion of the work at each station. With blanking, for example, single-operation dies cut
an entire periphery at once, while progressive dies may cut the periphery in sections.
Good progressive tool design allows the incorporation of
multiple forming and piercing operations for economy and reproducibility.
Other operations such as tapping, staking, riveting and
insertion of clinch hardware may be accomplished at separate stations. Piloting holes or
tooling holes are sometimes necessary to register the strip for sequential operations,
depending on a part's geometry. Typically, progressive dies produce burrs in both
directions and may exhibit parting lines.
- Transfer-die systems differ from
progressive tooling in that individual workpieces are moved from one die station to the
next between press strokes by mechanical fingers, levers or cams which are built into or
mounted on the press. Transfer operations often require additional equipment, such as
strippers, eject pins, and die cushions. High production volumes are a prerequisite for
transfer-die systems, which often require specially designed transfer presses.
- Slide forming, as performed by members of
PMA's Slide Forming Division, is another option for high-volume production of formed
stampings and wire forms. Various operations such as piercing, trimming, and extruding, as
well as assembling, tapping and welding are carried out on coil stock in multiple slide
forming machines. Like progressive and transfer systems, all operations take place
sequentially, but here, a machine with several cam driven slides as well as a press
section does the work instead of a complex progressive die.
See a description of the slide forming process.
Economic Advantages and Limitations
Compared to CNC fabrication using single-purpose or universal tools,
dedicated tools generally provide superior dimensional precision and uniformity of product
throughout a production run, and good reproducibility from one lot to another. Being
generally more complex and costlier to design and build, they also require longer lead
times.
In selecting the appropriate tooling method, the designer weighs the
minimal tooling cost, short lead times and relatively low production rates of CNC
fabrication against the higher tooling cost, greater precision, longer lead times and
higher operating efficiencies of dedicated tooling. Ultimately, the total production
quantity is often (but not always) the deciding factor.
Many products begin as CNC fabrication prototypes, then evolve into
hard-tooled operations when production quantities reach higher levels. Early consultation
with the metalforming vendor regarding total anticipated quantities is essential in
planning the appropriate tooling approach.
Size of the part and the degree of dimensional precision may also affect
selection of the tooling method, regardless of the quantities involved. Parts with
extremely large plan-view areas (greater than 30 x 30 in. (762 mm) for example) may be too
large for all but the very largest specialized tooling and press equipment.
Likewise, some parts inherently require a degree of dimensional
precision unachievable with CNC fabrication. However, certain classes of
parts--particularly those with selectively perforated areas within the body of the
part--are often more economically run in numerically controlled presses.
Go to the Design Guidelines Overview
Go to the Glossary
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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