Sheet Metal Do's & Dont's

DFM (Design for Manufacturability)

Designing for Manufacturability DOES NOT necessarily translate into a part design of the lowest manufactured cost.  Each part design should be reviewed for its “cost performance” as part of the overall assembly.
However, for purposes of understanding process cost implications the following design suggestions are offered.

• Minimize Processes and Feature Complexity
• Use no tapped holes
• Use no extruded holes
• Use clinch nuts and clinch studs
• Use self-locating techniques such as tabs and slots, Pop rivets and “Cleco Clamp” holes for aligning mating parts.
• Use slots for alignment of mating parts
• Length tolerances for rectangular slots, both round and square end, should be ± .02, or greater whenever possible.  Width tolerance should be ± .01.
• Eliminate need for secondary operations, such as reaming, by using standard hole sizes and tolerances.
• Specify welds on inside of corners, instead of outside, whenever possible.
• All bend angle tolerances to be specified as ± 1°.
• Do not specify locations of spot welds unless it is critical to the design.
• Specify all paint masking dimensions with a Ref. callout.
• Recommended bend radii for materials with a thickness under .120 should be .07R MAX.
• Use pre-plated such as “EZC” electro-zinc CRS, eliminate plating
• Eliminate masking for paint requirements
• Eliminate painting requirements

TOLERANCES

The following sheet metal tolerance information is based upon standards for soft tooled sheet metal to meet high yields from the process.  Tolerances outside these standard can possibly be achieved with special tooling or process.

Fabrication Tolerances

Webbing/Punching

Assembly Tolerances

Critical to function (CTF) is the characteristic whose variation has a significant on the fit, performance, or reliability of the component or system

Dimensions that are NOT CTF should not have tolerances tighter than the default tolerance rules

TYPICAL DESIGN/DOCUMENTATION PROBLEMS
Poor Modeling/Design Practices

• Parts are modeled in the flat and do not allow bend adjustments without time consuming changes to the models/data
• Hardware modeled as “Extruded Features” instead of as an “Assembly”
• Suppression of hardware feature in assembly eliminates mounting hole features
• Suppression of hardware feature in assembly eliminates mounting hole features
• Part files will NOT generate a “Flat”
• Unnecessary multiple bend radii
• No Bend Relief
• Countersinks not designed to the sheet metal minimum thickness tolerance.  Countersink designs should have a 10% material thickness “barrel”.
• Material thickness modeled to theoretical maximum
• PEM holes sizes not modeled correctly (should be -.000, +.003)
• Flange widths are less than 3 ½ material thicknesses
• No systems of numbering for data files.  Creates problems with traceability.

Data Submitted for Fabrication

• Part Assemblies (sheet metal with “PEMS” or other hardware) are missing “part” files (all references)

Drawing Issues

• Tolerances are not within the process capability for soft tooled sheet metal
• Missing dimensions
• Material specification is missing or incomplete
• Finish specifications are incomplete
• Drawing does not show bend relief and/or note for allowable use missing
• Ordinate dimensioning is used over multiple bends (tolerance accumulation issue)
• Use of tooling holes for assembly purposes is not shown or noted
• Drawing views are not properly oriented and/or rotation noted
• Hardware call outs are not specific (vendor name and part number should be referenced)
• Critical appearance of surfaces in not indicated
• 3D data does not match drawing (typical when parametric features of modeler are abandoned)

PLATING AND COATINGS

Platings & Coatings are available and typically add 2-5 business days to the fabrication process.
Electroplating is a process whereby an object, usually metallic, is coated with one or more relatively thin, tightly adherent layers of some other metal.  Electroplating is specified when there is need for surface characteristics that the basis metal selected does not possess.  Whatever the purpose (improvement of appearance, corrosion protection, and the like), the plating operation is an important and necessary part of the manufacturing process and should be planned for with the same care accorded to the fabrication operations.
It sometimes happens, however, that designers, intent upon achieving a fresh, new look with sales appeal for their product, and production engineers striving for low fabrication cost for component parts, inadvertently create configurations and situations that make it difficult and expensive to accomplish the plating objective.
These limitations are basic and to a degree, self-evident;

• Surfaces to be plated must be wetted by all solutions and rinses in the plating sequence
• One must be able to make electrical contact without resulting defects
• The amount of metal deposited on a given portion of a surface will be proportional to the current that flows to that surface portion.

It is not intended nor desirable, that designers be unduly influenced by the above or that the plater be relieved of his need for ingenuity in the process.  On the other hand, in this practical world it is necessary to be aware of the great influence that part configuration can have on the cost of plating and on the quality of the finished product.

LIQUID AND POWDER COATING

The application of a coating either as wet paint or powder coating is usually either for decorative purposes such as adding color, gloss and texture for functional value for protection from corrosion.  The preparation and cleaning of surfaces to be coated is the key factor in the adhesion of the coating.

Powder coating is an advanced method of applying a decorative and protective finish to a wide range of materials and products that are used by both industries and consumers. The powder used for the process is a mixture of finely ground particles of pigment and resin, which is sprayed onto a surface to be coated. The charged powder particles adhere to the electrically grounded surfaces until heated and fused into a smooth coating in a curing oven. The result is a uniform, durable, high-quality, and attractive finish.