Page 87 - MetalForming October 2011

Fig. 2
concern. All hole extrusions start as hole expansions (Fig. 2), where a punch is forced into a blanked hole, causing a circumferential elongation or stretching of the cut edge. Absolute values of expansion limits depend on the mate- rial, tool design, lubrication and edge quality of the punched hole. Fig. 3, illustrates the negative impact that cutting-edge damage can have on hole expansion and hole extruding.
If the amount of stretching required to form the hole extrusion exceeds the residual stretchability of the cut edge after punching, consider of one several options available for restoring edge stretchability:
• Improve the quality of the original cutting operation; • An additional cut of higher quality; or
Fig. 3—Damage done by various hole-punching operations tested by the traditional hole-expansion test. (Research by R.R. Hilson, Inland Steel, and presented at Microalloying 75 in Washington, D.C.)
• A shaved cut and deburred break edge.
Next month I’ll present some proven hole-punching methods and extrusion-punch designs for forming extrud- ed holes in sheetmetal stampings. MF
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Hole expansion, %