Design for Manufacturing: Sheet Metal Cutouts and Projections

This rule provides information on how to handle narrow projections and cutouts in sheet metal design.

Both cutouts and projections require removal of material. Removal of precise amount of material must be carefully controlled. Cutouts are normally removed in a single operation; however, particularly narrow cutouts must be removed in two or more operations. Similarly, narrow projections must be treated in the same way. This is due to the particularly delicate nature of small features in sheet metal designs.




Stamped Part with Narrow Projection

Narrow features are defined as those features whose width is less than three times the material thickness. These projections tend to be at high risk of deformation or other failure modes. In the case of narrow projections, a blanking punch is required to separate the product from the strip due the delicate nature of the workpiece. In addition to a blanking punch, a notching operation is also generally required for products of this nature. This may involve two separate notching stations in order to accomplish the notching operation.

If
(a) While designing a product which will utilize sheet metal…
(b) If narrow projections are present in the design as defined by width of less than three material thicknesses
(c) Or if narrow cutouts are present in the design as defined by width of less than three material thicknesses

Then
(a) Narrow projections must be separated from the main strip carefully.
(b) Separation of narrow projections from the main strip is accomplished by creating a notch on one side of the projection
(c) Afterwards, the part is punched again to create the projection
(d) Finally, the separation step occurs at a third station.
(e) Narrow cutouts must be similarly separated from the main strip carefully
(f) Notching punches must be used in two separate stages to create narrow cutouts.

Stamped Part with Narrow Cutout

By following this rule, product designers will produce higher quality parts when stamping is used. Fewer product defects will be observed which will drive down overall product costs.

Further reading:
Poli, Corrado. (2001). Design for Manufacturing – A Structured Approach. Elsevier.

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