Some beliefs are so widely accepted in press shops that they’ve achieved legendary status. But stop the presses, these statements are actually completely off the mark:
The statements below are so ingrained in many shops that they’re “press-shop legends,” says Stuart Keeler, who’s acclaimed worldwide for developing press shop analysis tools. But that doesn’t make them true. In his March 2004 and December 2004 columns in Metalforming magazine, Keeler tackles these common misconceptions:
MYTH #1: You can get rid of springback by permanently deforming sheetmetal beyond the elastic region.
FACT: The amount of springback depends on the level of flow stress at the punch stroke’s end. For example, if the initial yield stress is 20,000 psi and the material is work-hardened to a flow stress of 40,000 psi (a 100% jump), then the amount of springback will also increase by 100%. In short, they’re proportional.
MYTH #2: You can significantly decrease the amount of springback by quickly pausing the punch at the bottom of the stroke.
FACT: As you reduce the punch force during the retraction stroke, the total elastic stress in the stamping at the bottom of the stroke must either be spread out by springback or kept as a residual stress. Cutting the offal will increase springback as the residual stress tries to reach 0.
MYTH #3: You can only work-harden a material during tensile (stretch) deformation and not compressive deformation. That’s the reason why stampings can only fracture in stretch areas.
FACT: Work-hardening occurs almost equally in tensile and compressive deformation. The main distinction is that when using tensile elongation, sheet thinning and necking resulting in failure can happen. In contrast, compressive deformation thickens the material.
MYTH #4: The hardness test accurately gauges sheetmetal’s forming capacity.
FACT: The hardness test measures the resistance of the sheetmetal to being pushed out from under an indenter, which is forced into it during this test. Fractures, on the other hand, are caused by too much thinning and necking due to tensile deformation in the plane of the sheet.
MYTH #5: By measuring the uniform elongation during a tensile test, you get the maximum stretch limit for sheetmetal forming.
FACT: The Forming Limit Curve, and not the uniform elongation, determines the maximum allowable sheetmetal stretch. Thus, a uniform elongation of say, only 25%, will not preclude the sheetmetal from being stretched by 35, 50, 100 or even 200%.
MYTH #6: You should avoid stretching sheetmetal beyond its uniform elongation. That’s because the softening of the sheetmetal at the end of uniform elongation causes the load drop in the tensile test.
FACT: Work-hardening, and not softening, of the sheetmetal is occurring in the deforming neck. What’s more, a uniaxial tensile test does not simulate sheetmetal forming as well as biaxially stretching domes. By forming these domes, you can observe that the maximum load is reached at the end of punch stroke or when the dome tears–not at a force equal to uniform elongation.
MYTH #7: Applying a force exceeding the UTS (ultimate tensile strength) as measured in a tensile test will cause the sheetmetal to fail.
FACT: UTS and load maximum occur in tensile tests, not actual sheetmetal stampings. True stress and strain curves indicate a steady rise in deformation stress and no load maximum.
MYTH #8: Cold-working causes sheetmetal to become hard and brittle.
FACT: Cold-working or deforming sheetmetal makes it stronger. Because increased strength has implications on both tensile deformation (stretching) and compressive deformation (reaction to a hardness indenter), the material does get harder as it gets stronger. However, if you examine the fracture via a microscope, the surface will reveal a ductile failure, not a brittle fracture.
MYTH #9: Sheetmetal would be easier to manage if it did not work-harden because when it does it can jam the press.
FACT: High work-hardening capability is a good thing to have for sheetmetal stretchability. Without that capacity, the deformation would accumulate at its origin–typically at the point of initial punch contact. Failure would then result without much stretch. In contrast, work-hardening capacity enables the sheetmetal to gain strength at the early deformation location and forces the strain to spread more uniformly to other parts of the material.
MYTH #10: The left side of forming limit diagram (FLD) applies to only compressive forming in cup drawing.
FACT: The whole FLD charts the limits of tensile stretching, delineating the start of a local neck generated by different combinations of stretching. The left side of the curve depicts the strain path for the tensile test.
MYTH #11: To increase steel density, you should cold-reduce the sheetmetal.
FACT: Cold-rolling steel to make it thinner can be compared to flattening out a piece of putty. While you are increasing the surface area as you make it thinner, you are not affecting the volume at all. Variances in calculated density often result from inaccurate measurements.
MYTH #12: You can use the press load monitor only to avert press overloads.
FACT: While preventing press overloads is a good way to use press load monitors, an even better application is providing early warning to the press shop if the tonnage veers outside of normal production ranges. This could prevent problems such as different metal flow in the die, divergent distributions of stress, and out-of-specification variances in stamping dimensions due to altered springback.
The Science of Forming: Myth or Truth in Metalforming
Metalforming, March 2004
The Science of Forming: Myth or Truth in Metalforming
Metalforming, December 2004