For many parts there are physical reasons for finished parts to be off specification, which cannot be avoided by good workmanship.
Springback After Bending
Springback is a natural property of bending metal. It is the amount by which the metal will return to its original shape after it has been bent. This can affect the finished metal part in several ways. For example, the amount of springback can vary depending on the grade and thickness of the metal. Therefore, the designer must allow for some springback when designing the bend. He can allow for this by making the bend angle a few degrees tighter, or he can ask the person who is bending the metal to confirm that his press brake is set to allow for the correct amount of springback.
Kerf Width and Cut Dimensions
Every cut method will remove material from the part being cut. Typically, the amount of material that is removed by laser cutting is very narrow (roughly 0.1-0.3 mm) compared to plasma cutting. However, if your drawing has been completed to include dimensions on the cut line (as opposed to the edge of the part), then the amount of material that is removed as part of the cut will affect the overall size of the finished part.
Holes and Slots Near Bends
The distance between holes and bends is very critical. Holes close to bends will distort when the part is bent. A good rule is to keep the distance between a hole and a bend at least 1.5 times the material thickness.
Tooling Wear on Repeat Runs
On long production runs accuracy can change as punch tips wear out and die clearances increase. If required, please ask your metal fabrication service for a confirmation of the tooling in use for your repeat order.
A useful reference for Sheet Metal Fabrication is https://dkmsheetmetal.co.uk/.
Coating Adds Measurable Thickness
Powder coating will typically add around 60 to 100 microns per surface area to your part. This must be accounted for in the clearance of any clearance holes or in the width of tight fitting slots, to allow for assembly.
Getting the dimensions right on a part made from metal is a complex business, not least because the process of making the part will affect its dimensions.
