Page 13 - Metallforming Magazine December 2020

steel with adiabatic cutting.
MF
Cutting Edge
AB
Fig. 3—Shear zones: (A) thermal image during adiabatic cutting, (B) optical-microscope image of interrupted adiabatic cutting to show the width of the shear zone.
formers. An automotive-OEM engineer once told me that if the draw-tool temperature of a door-inner panel increases by 7 C, the binder tonnage must be reduced to avoid cracks. The heating effect also limits the maximum parts produced per minute, since press speed must be reduced in order to control the tool temperature.
During blanking, the plastic deformation—and so the heating—is very localized to just the periphery of the punch (Fig. 2a). A typical blanking press would operate at a maxi- mum speed of 80 strokes/min., depending on the feed length. If the press has a 300-mm stroke length and is cutting a 1.0-mm-thick blank, punch speed at contact would be approximately 25 mm/s. With this punch speed, the sheet first deforms and then is sheared (Fig. 2a). Heat would be generated in the deformation area (to around 150 C) but would quickly dissipate.
If punch speed exceeds 3 m/sec., then the time required to cut the sheet decreases to a few milliseconds (Fig. 2b). There would be no time to dissipate the heat in the shear zone, thus the name adiabatic. This would generate a very narrow adiabatic shear band (10 to 50 mm) and reach 700 C (Fig. 3).
When to Use Adiabatic Cutting
We can gauge the edge quality of a blank by its surface roughness (burr vs. shear), perpendicularity and the presence of microcracks. To improve surface roughness, stampers use smaller blanking clearances, which will increase tool wear. Blanking higher-strength materials using relatively tight clearances also increases the risk of microcracking. Microc- racks can shorten the life of a fatigue-limited component or reduce the impact/crash resistance of a formed part.
We recommend adiabatic cutting when blanking high-
strength steels in excess of 1300-MPa tensile strength, includ- ing hot-stamped steels. I know of at least one Tier One sup- plier planning to replace laser trimming of hot-stamped
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