Page 14 - MetalForming November 2011

Die Timing
to Control
Snapthrough
A loud “boom” heard when sheetmetal is punched or sheared indicates excessive snapthrough and likely an out-of-control process. Here we present die-timing adjustments that stampers can make to control snapthrough.
BY DAVID ALKIRE SMITH
The loud “boom” heard when sheetmetal is punched or results from the release of snapthrough energy. This can cause broken crankshafts and the failure of other press components. This article covers simple die-timing adjust- ments that stampers can make to control snapthrough; severe cases may require the installation of hydraulic dampers.
Another technique to control snapthrough is to reduce the peak pressure required to cut through material, by grinding one or more shear angle(s) on the punch or
die. If the cutout, such as a slug, is discarded, the shear is placed on the punch. Punch length adjust- ments also can be made.
Waveform Signature—a Case Study
The task here (Fig. 1) involved press-damage con- trol on a severe punching operation. Shown is a large chain sidebar fabricated of 0.625-in.AISI-SAE 1039 fine-grained carbon steel (a 6-in. scale is shown for size comparison). The punch has a pointed angular shear optimized for the task.
Waveform signature analysis (Fig. 2) provided a way to measure die-timing opportunities to reduce the
David Alkire Smith, a tool engineer with more than 50 years of experience, is the editor of the “Die Design Handbook” and other engineering reference works; dsmith@smithassoc.com, www.smithassoc.com.
Fig. 1—A chain sidebar of 0.625-in. SAE-ASTM 1039 fine- grained carbon steel.
destructive negative reverse load on the press. Shown is the waveform signature of the stress-strain relationship when cut- ting off and piercing two holes in the part. The data was taken with a chart-recorder speed of 8 in./sec.; the vertical axis indi- cates strain or force, the horizontal axis represents time.
A
200 tons 150 tons 100 tons
50 tons
Zero
–25 tons
–50 tons – 75 tons –100 tons –150 tons
B
12 MetalForming/November 2011
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Fig. 2—This chart recording displays the waveform signature of a com- bined punching and cutoff operation, and points to excessive snapthrough (or reverse load).