Chapter 7 - Designing for Stamping Production
Fundamental Considerations
When part drawings are submitted to metalformers, they can only be
accurately interpreted when they follow accepted engineering drawing practices for
dimensioning and tolerancing. In addition, the co-engineering concept, which emphasizes
early supplier involvement during the design stage, is essential for optimum cost and
performance in component designs. Other fundamental considerations include:
Tooling Amortization. Because tooling costs must
usually be absorbed over the length of production, careful consideration must be given to
part quantities when tooling options are evaluated. Larger quantities usually justify more
sophisticated tooling. However, optimum cost effectiveness can sometimes be achieved for
all production quantities by using standard components for blanking, piercing, forming and
drawing.
Typically, standard fixtures with quick-change inserts can be used to
hold tool segments, thereby reducing costs of conventional die sets. In hole punching, for
example, the use of "stock" hardened-steel punches helps keep tooling costs to a
minimum. Similarly, hundreds of standard tool holders and fixtures are available to
simplify tooling set-ups for forming and drawing.
For large quantities, an additional investment in tooling may be
warranted. Although prototype or universal tooling may do the job for smaller runs, more
efficient production using upgraded tooling can significantly reduce the unit cost. That's
why it's important to discuss potential quantities and the expected quality level
(including cosmetic requirements) with the metalformer prior to establishing the tooling
method.
Numerous factors can drive up tooling costs: larger part size, increased
complexity, restrictive tolerances, workpiece material characteristics, and very thin or
thick stock. To keep the costs of tooling down, restrictive tolerances should be specified
only where needed, and all alternatives should be considered when special material
characteristics are needed. If tight tolerances and material of a specific temper are not
required, less expensive sheet steel can sometimes be substituted for strip stock.
Material Utilization Affects Costs. For-tunately, there
are ways to conserve material, and most metalformers are familiar with all of them.
Nesting parts in sheet or strip stock can significantly reduce the
amount of scrap generated in blanking operations, and is particularly successful with
large L- and C-shaped parts. The amount of material that can be saved by slightly changing
the design without changing its function is often worthwhile. An example is shown in
Figure 8.
Figure 8. Nesting of blanks, as evidenced by this redesign
of a V-shaped part, can dramatically increase material utilization thereby cutting
material cost.
The slide forming process allows the fabrication of many parts using the
net strip width and little or no scrap between blanks, see Figure 9.
Figure 9. Examples of parts fabricated using net material width.
Material Characteristics. Variations in material affect
precision in the metalformed part. Such effects are more evident in forming operations
than in piercing. Dimensions and properties vary more with sheet stock than with strip.
A pertinent example: dimensional variations in forming operations that
result directly from thickness variations in the workpiece material. Because thickness
tolerances on standard sheet vary, (see Material Selection chapter), the designer should
be aware that the dimension from a hole to the edge of the bend can vary by the thickness
tolerance. If that tolerance is not acceptable, the designer should specifiy
controlled-thickness material at extra cost.
Engineering changes are very costly, but are often
unavoidable. If a product is in the developmental stage, the designer should consider use
of CNC fabrication or cost-effective hard tooling. Low-cost hard tooling, which uses
standard components for separate operations to blank, pierce and then form parts, responds
to design changes more readily than progressive, transfer, or compound dies. The latter
should only be considered when part designs have been firmly established.
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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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