Troubleshooting Sheet Metal Forming Problems, Part 1: The Sheet Metal

After a part has been in routine production for some time, we can trace most subsequent forming problems back to some process variable different from what had been used in the past. Dozens of possible parameters may have changed, but the more common ones include the sheet metal composition and mechanical properties, the die, the lubricant, and the press.  

Sheet metal characteristics and properties are relatively easy to check. The shipping tags should include the composition (levels of carbon, manganese, etc.) and mechanical properties (yield and tensile strength, elongation, n-value, etc.). If not, any number of laboratories can perform the testing with just a 12-in.-square material sample; testing only costs a few hundred dollars. While stampers may be reluctant to spend for testing, consider the cost of scrap pieces, lost opportunities while remaking the scrapped parts, and the additional manpower required for this additional work and problem solving.  
With test results in hand, you then can compare them to the historical values received, and the allowable range shown in the material specification. Of course, this can be a challenge without historical information; periodic testing of incoming material can help establish a baseline.  

You probably can skip the chemistry check. Quite likely, the tags of every shipment list the composition of at least five important elements and will be the same for all coils with the same heat number. Each heat—a single batch of molten steel or aluminum (about 300 to 400 tons at the high end)—will have uniform distribution of all elements throughout. The production mill typically conducts a chemical analysis from the heat and then applies the results to all coils produced from that heat.  

Casting converts the molten metal into a sold slab, which for steel measures as much as 10 in. thick, 20 ft. long and 72 in. wide. Slab width does not significantly change during processing, so reducing the thickness from the as-cast thickness to satisfy order requirements will extend the coil length, sometimes to more than a mile. Production of sheet aluminum occurs with similar thickness reduction and sampling.

We can expect that the five to 20 coils typically produced from the same heat will have different mechanical properties, even if all processing followed the same recipe. This occurs because strength, elongation and other mechanical properties are output variables of the sheet metal processing operation. Key input variables include incoming composition, processing temperature profiles and the degree of thickness reduction at numerous rolling stations. Each of these parameters has a tolerance of acceptable and achievable values, which results in normal and inherent variability in the product that leaves the mill.

The properties tested should reflect the portion of the coil received at the stamping facility. Although great strides have been made in terms of edge-to-edge and tip-to-tail uniformity, variations will occur in thickness and mechanical properties throughout a coil. When a mill generates certified sheet properties, it is easier to sample at the quarter or middle portion across the width at one end of the coil, and allows for shipment of the full-weight coil. Therefore, sheet metal-related problems may result from in-coil variability, where your specific lift does not sufficiently reflect the properties obtained at the coil ends.