Tooling by Design



Stamping Thin Materials

By: Peter Ulintz

Tuesday, January 1, 2019

A reader contacted me recently regarding a problem he’s having with a part shaped like a pie pan and produced from very thin material. It reminded me of a column that Larry Crainich wrote back in 2000; much of that column is presented here.

When a process or tooling engineer considers the robustness of a production stamping process, prime considerations include sheetmetal thickness and strength. In general, the thicker the material, the more robust the die construction. At least that seems to be the consensus when designing stamping dies.

Often overlooked when processing parts from very thin material: the need for strength in the die. Components must remain rigid and resist flexing during stamping. Too much flexing in parts made from thin-gauge material fall short of the desired level of quality.

Of course, all things metal flex: a 6-in.-thick bolster flexes, a 3-in. die shoe flexes and a 11⁄2-in. hardened die section flexes. How much they flex depends on the load placed on them, but they all flex. If forming a 0.060-in.-thick part causes flexing, so does the formation of a 0.006-in.-thick part. Although the magnitude of deflection may be far less when stamping 0.006-in.-thick material, the detrimental results are far more pronounced.

Addressing Oil-Canning

A reader e-mailed me regarding a problem with a part shaped like a small pie pan, produced from very thin material. The problem encountered: severe oil-canning after forming and a lack of flatness. Oil-canning occurs due to loose material—material that has not been subjected to any appreciable stretch—in the center of the part.

Oil-canning also causes problems with large, flat panels, especially if the outer ends have been stretched or strained in a manner other than pure bending. Often, to overcome oil-canning in this type of stamping, stampers will emboss the material to provide rigidity after the forming step that resulted in oil canning. Stippling (coining a waffle pattern into the material) may work in thicker materials, but that’s difficult to do in thin material.

Look at a rectangular air-handling duct in a heating and cooling system and you may find an “X” pattern creased into large panels. This geometry stabilizes the panel by consuming the loose sheetmetal into the X pattern and prevents it from resonating when air flow starts and stops through the duct. Additional deformation removes the oil-can effect. The same logic applies to the problem part originally discussed.

The second problem with the subject part: lack of flatness. When exiting the die, the part exhibited severe bowing. The bowing on the rim of the pan did not result from oil-canning. The bowing resulted due to the form tools flexing.

If a form tool flexes from the ends to the center as little as 0.001 in., with 0.005-in.-thick material, the forming forces can vary by 20 percent or more. That’s considerable and creates distortion.

The problem can be resolved in a couple of ways. One method: Rebuild the die and make it stronger and more rigid, minimizing the significance of any flexing. Obviously, cost and time to rebuild the die make this an impractical solution.

Another solution: Compensate for the flexing. But how much? That is, how do we measure the deflection to compensate for the flexing?

This old trick may work: Cut notches into the blank and place it into the die. Next, place strips of solder into the notches. Cycle the press and measure the thickness of the solder strips. Measure carefully. Accuracy of the measurement is very important; you’re not looking for much difference.

With that in mind, say that you discover an area of the die that flexes 0.002 in. To verify your results, conduct this experiment: Place a part to be formed into the die, add 0.002-in. shims where the solder was thicker by 0.002 in., and cycle the press. Part flatness should improve if proceeding correctly. To achieve compensation, shape the form tools inversely to their flexing, possibly a painstaking and laborious process. Another method: Place a shim under the center of a flat form tool. During part processing, the section ends flex to compensate for the original flex and a straight section results, producing a flat part.

Is flexing the form steel with a shim an ideal solution? No. Rather, using robust die designs when forming thin-gauge materials is always best. MF


Related Enterprise Zones: Tool & Die

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