Variables make part tolerancing a pipe dream
March 11, 2011
The forms and positional tolerances called out on product designs often defy the physical abilities of the product's material. Proper product and process design, as well as pressure clamping, can help maintain strict tolerances.
Trying to form a very small part or locate a pierced hole to a precise tolerance can be difficult. Achieving consistency in these tasks is even more daunting.
In my career as a consultant, I have been exposed to part tolerances so small that a change of even a few degrees in the part or inspection temperature can put the part out of specification. The forms and positional tolerances called out on product designs often defy the physical abilities of the product's material.
Let me make two facts crystal clear:
That doesn't mean the material is not within a given specification—statistically speaking, it most likely is. However, the metal has tolerance limits within which it must fall to be qualified as a given grade, and it can vary substantially within those limits.
Called-out part tolerances often are more stringent than the tolerances on the incoming material. In these cases, unless you somehow are requalifying and re-establishing a tolerance, such as the metal's thickness in the die, you already have lost the battle. For example, you might have a form tolerance of ±0.002 in. on both sides of the part, but the incoming metal has a thickness tolerance of ±0.003 in.
Sound crazy? It is. In simple terms, the stamped part must have more tolerance than the incoming metal. And you can't expect the die to correct the incoming material's inconsistency.
For the shape of a stamped part to be consistent, the amount and distribution of strain also must be consistent. In other words, the amount of stretch and compression that occurs during forming must be consistent. Many factors control the amount of strain that occurs during metal forming; lubricant type, die geometry, forming temperature, and forming speed are just a few.
Certain grades of advanced high-strength material, such as dual phase, continue to act on the effects of strain, stress, and heat after the forming process has been completed. The part can change its shape as it is sitting idle. I have seen shaped parts twist and deform as much as 0.138 in. over a period of two hours as a result of cold working. Attempting to hold a very small surface tolerance defies the natural behavior of this material type.
One way to prevent this problem is to pressure-clamp the part into its fundamental shape using a fixture before inspection. Attempting to qualify a tight-tolerance, deeply contoured formed part made from dual-phase 600, for instance, in its free, unclamped state is a waste of time. Even if the part does check within the given tolerances, chances are it won't be consistent in shape. Higher-strength materials not only react to strain and stress more adversely, but they often have a much larger tolerance range than low-carbon, high-formability materials.
In certain part shapes, you can expect a geometry change as the die warms up and cools down. Of course, all parts made from high-strength materials will not exhibit these behaviors. It all depends on the material type and thickness, the part geometry, and the process used to create the geometry.
There's a big push in the automotive industry to use thinner, lighter-weight materials. Thin, high-strength materials formed to deep contoured shapes have a higher tendency for geometry change after forming.
I was taught that the process used to manufacture a part—the dies, the press, the lubricant, and the sheet metal—must be engineered and built to 10 times the stamped part's tolerance.
However, all of these process factors also must have tolerance. Clearances that allow a die to function properly, such as the clearance around pilot pins, often result in the inability to achieve certain tolerances. Although reducing the clearance between the pilot pins and the hole they enter into may help to achieve the desired tolerance, doing so may cause the strip to stick to the upper die, resulting in a miss-hit and possible die damage.
Items such as cams that perform piercing and cutting also must have a certain amount of clearance or slope to function. Take, for example, a cam slide engineered and built with 0.001 in. of working clearance. This is a high-precision cam by today's standards, but the positional tolerance of a hole made using this cam would need to be 10 times greater, or no less than ±0.005 in. (0.010 in. overall).
Even though dies are built to very small tolerances, they are not perfect.
This might sound like gloom and doom, especially if you're a product designer or quality control inspector. However, it doesn't mean certain given tolerances or part geometries cannot be obtained. Success is possible, but it relies heavily on understanding and controlling as many factors as you can. Many problems with achieving part tolerances can be resolved by changing the product and process design.
Until next time... Best of luck!