Precautions for sheet metal processing design
Sheet metal processing is a non-additive and non-subtractive process. It starts with flat material and maintains a constant material thickness throughout the part. In some cases, it is possible to selectively thicken part areas by welding multiple metal sheets together, but this is an expensive and uncommon practice.
The design of sheet metal processing has its own standards, which are different from those of other production processes. Early in the design phase, the more you know about the features and functionality of a part, the sooner a manufacturable design can be completed. Here are some design considerations:
l Sheet metal fabrication is most economical when using "generic" tooling rather than part-specific tooling configurations. If individual parts become too complex and welding or riveting together is considered, parts made with general purpose tools can be used.
l Because the bend stretches the metal, the feature must be kept away from the bend to avoid deformation. A useful convention is 4T - four times the thickness of the material.
• Stamping brakes create bends by pressing a sheet metal into a die using a linear punch, so the design does not allow for closed geometry.
l Sheet metal tolerances are much larger than machining or 3D printing tolerances. Factors that affect tolerance include material thickness, the machine used and the number of steps in the part's production.
l A uniform bend radius such as 0.030 inches (industry standard) should be used on individual parts to reduce the number of machine setups and speed up production.
l Where possible, the standard distance from the elbow to the edge should be maintained at four times the thickness of the material. This eliminates the need to remove excess material required for bending.
l Welding of thin materials may cause cracking or warping. Other assembly methods are preferred.
l When using PEM hardware, always consider the manufacturer's minimum requirements for mounting location and material thickness.
Sheet metal can be finished in many ways and for different reasons. There are finishes that protect fragile materials from rust or corrosion, as well as other finishes used for aesthetic reasons. In some cases, finishing serves two purposes: some treatments are just changes to the metal surface itself; others consist of another material or process applied to the metal. Surface treatments include:
Sandblasting, which involves blasting abrasives on metal to roughen and clean surfaces. Sandblasting is commonly used on stainless and carbon steel as a preliminary step before painting to remove impurities and improve adhesion.
Brushing is similar in function to sandblasting, but uses an abrasive brush to clean and scratch the metal surface to form lines on the surface, reflecting the texture of the metal material. It can be used as a final finish for materials such as aluminum and stainless steel, and is often used as an appliance finish.
Polishing produces a smooth surface that can be used on metals such as stainless steel, aluminum and copper. It can be used as a final finish or in preparation for other finishing processes such as electroplating. It is not suitable for metals to be painted as it will not enhance adhesion.