It is important to work with a design-for-manufacturability (DFM) engineer who has expertise in metal engineering for sheet metal fabrication during the prototype phase before manufacturing begins.
Here are a few helpful design tips.
Excessive Forming - Incorporating cuts or bends that do not have a functional purpose can create added costs. Excessive forming can also make the part impossible to bend.
Critical Dimensions – call out information not available on models—datum planes, tolerances (block and critical), material type, finish requirements, hardware specifications, hole tapping, welding requirements, surface requirements, and edge requirements, just to name a few.
Standard Tolerances - Although the machinery and tooling will repeat within .004", it is a mistake to simply engineer all mating parts, to be within +/-.005". This excess forces additional labor in sorting and inspection.
Tolerances that are too tight result in higher costs and lower productivity. Correct tolerance will still produce excellent fit and function, with the added benefit of manufacturing efficiency.
Hole Sizes +/- .003” - The size and shape of the punch and die tooling determine the size and shape of the hole. A minimum hole or relief size is determined by the thickness of stock to be used.
For best results, the punched feature can be no less than the material being punched. The die tool is slightly larger than the punch to minimize tooling wear and to reduce the pressure required to punch the hole. Generally speaking, 10% of the material thickness is used for most applications.
For example, if the material is .100” aluminum and the punch diameter is 1.000”, the die diameter would be 1.010”. The size of the hole on the punch side will be the same size as the punch tool. The size of the hole on the die side will be the same size as the die tool. Except for tooling wear, there is very little variation from one hole to the next. Generally speaking, +/-.003” is a reasonable and functional tolerance.
Hole to Hole +/- .005” - Accuracy of the distance from one hole to another is dependent primarily upon the machinery used to process the sheet. Some equipment will hold better than +/-.005” with little difficulty.
However, all holes and features punched through the sheet can introduce stress into the sheet metal. If the part has a closely spaced perforated pattern or formed features such as dimples or counter sinks, the result can cause the sheet to warp and distort. This can cause unwanted variation between holes or features. If this condition exists, a greater tolerance should be applied to certain areas surrounding this characteristic.