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Q: What is precision sheet metal?
A: "Precision" has to do with the dimensional accuracy of the parts. The thickness of the raw material is generally within 5% of nominal. The equipment used to fabricate the parts will typically hold within .005" (0.12mm). The significant variables in accuracy are typically the condition of the machinery and tooling, the size of the parts, the skill of the craftsmen, and the type of finish applied to the parts.

 

 

In general, "sheet metal" ranges in thickness from .020" (0.5mm) up to .250" (6.5mm). Thinner sheets are often called "shims" or "foils". Thicker material is referred to as "plate" or "ingot".

Q: How do I specify cosmetic or appearance standards?
A: Cosmetic inspection at DSM uses a time and distance inspection procedure and relies on the judgment of trained inspectors and suppliers. We recommend using the procedure defined in the DSM Cosmetic Reference Specification (down load it at the bottom of this page) for training inspection personnel and may be used to assist in making an accept or reject decision.
Q: What tolerances are appropriate for sheet metal parts?
A: Although the machinery and tooling will repeat within .005" (.13mm), it is a mistake to simply engineer all mating parts expecting /-.005" (.13mm) accuracy. Such over kill forces additional labor in sorting and inspection. The result of tolerances that are too tight is simply higher cost and lower productivity. Correctly toleranced parts still have excellent fit and function, with the added benefit of efficiency. In the following, you can read more about tolerances for hole size, hole-to-hole, hole-to-edge, hole-to-fold, and fold-to-fold.

 

 

Hole Size

 

Generally speaking, /-.003" (.08mm) is a reasonable hole size tolerance. Unless the hole is produced with a metal cutting laser, the hole size should not be smaller than the material thickness.

 

Hole-to-hole

 

The accuracy of the distance from one hole to another hole is largely dependent upon the machinery. DSM's equipment will hold better than /-.010" (.25mm) with little difficulty. However, each hole that is punched introduces stress into the sheet metal. If the part has many holes closely spaced, the result can resemble a baker rolling cookie dough -- the sheet metal can be expanded causing unwanted variation between holes. Whenever possible, engineers should allow /-.010" (.25mm) hole-to-hole. Resort to /-.005 (.12mm) only when absolutely necessary.

 

Hole-to-Edge

 

Because the profile (or edges) of the part are generally punched just like any hole, the same considerations for hole-to-hole apply. When punching very near to an edge (less than 2X material thickness) the edge will be pushed out by the stress of punching the metal. This edge migration introduces variables in the accuracy of the hole location. Whenever possible, engineers should allow /-.010" (.25mm) hole-to-edge. Resort to /-.005 (.12mm) only when absolutely necessary.

 

Hole-to-Fold

 

Engineers must consider the natural variation in material thickness (5% of nominal thickness), the /-.005" (.12mm) from cutting the metal, the effects of cosmetic graining, and the variation introduced by the press brake. Whenever possible, engineers should allow /-.015" (.38mm) hole-to-fold. Resort to /-.010 (.25mm) only when absolutely necessary.

 

Fold to Fold

 

All of the considerations of "hole-to-fold" apply, compounded by the fact that multiple material surfaces and thicknesses are involved. Whenever possible, engineers should allow /-.020" (.50mm) fold-to-fold. Resort to /-.010 (.25mm) only when absolutely necessary.

Q: What are the design limitations on holes?
A: Keep the hole size greater than or equal to the material thickness (unless you're willing to pay for laser cutting).  Keep the edge of the hole at least 2 material thicknesses away from the start of the inside bend radius.
 
Q: What are the design limitations on bends?
A: The distance from the edge of the part to the start of the bend radius should be at least 3 material thicknesses.  Less than this will increase the cost of manufacture.
 
Q: Which metal alloy should I choose?
A: Use aluminum for light weight.  However, aluminum reacts with water, so it is recommended to coat it for corrosion protection.  Steel is generally less costly, but will rust unless carefully coated.  Stainless steel can be used without any extra coating; it is durable, attractive, and the most expensive.
 
Q: What kind of paint is best?
A: To minimize air borne pollutants, "powder coat" paints are widely used. Solvent based paints use a liquid to suspend the paint solids during application. The carrier solvent evaporates, leaving the paint film behind. Powder coat paints, on the other hand, take advantage of static electricity to cause the fine, dry powder of paint solids to cling to the part being coated. Heat is used to liquefy the power to form a film. As the part cools, the paint returns to a solid.
Q: Which kind of plating is best?
A: That's not an easy answer, but here's a quick summary: for aluminum, chromate or anodize;  for steel, zinc or nickel; for stainless, bare is fine but passivate will remove iron from the surface to minimize the appearance of rust.