Page 51 - MetalForming August 2012
P. 51
Tooling by Design
using a properly designed shear angle, or use a spring- loaded ejector pin extending through the center of the punch face.
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Punching small holes at higher speeds may require spe-
cial attention to tool-steel selection. Higher operating
speeds generate greater heat and also decreases the tool- ing contact time by the same factor. This reduces any cool-
ing afforded by contact with the tooling or workpiece.
Because small punches have less ability to dissipate heat,
they are prone to overheating. This can result in a loss of
hardness, reduced wear resistance and dimensional insta-
bility. High-speed or high-alloy tools steels such as M2
and 10V are tempered at temperatures above 1000 F, giving
them increased tempering resistance compared to A2 and
D2 tool steels.
Press in Danger
The stamping press can be adversely affected by increased stamping speeds, too.
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Imagine two identical mechanical presses running at the same average slide velocity. For simplicity, we’ll assign an arbi- trary speed value of 0.20 m/sec. (approximately 40 ft./min.).
Let’s further assume that the mass of the upper half of the die in press no. 1 is 500 kg (1100 lb.) and the mass of the upper half of the die in press no. 2 is twice that (2200 lb.)
We are (or should be) interested in understanding the kinetic energy (KE)—the energy that an object possesses due to its motion—for each of the press slide tools. KE = 1⁄2 (mv2), where m is the object’s mass, v its velocity.
KEfordie no.1=1⁄2(500x0.22)=20J
KE for die no. 2 = 1⁄2 (1000 x 0.22) = 40 J
Kinetic energy increases linearly with mass. Now let’s
assume the upper die weights are identical for both presses— 500 kg—and both presses are identical, except that the sec- ond die runs with a velocity of 80 ft./min.
KEm =1⁄2(500x0.22)=20J KEM =1⁄2(500x0.42)=80J
Here, kinetic energy increases proportionally with the
square of slide velocity. So, when press speed doubles, kinet- ic energy increases by a factor of four. Some of the kinetic energy from the press will be transferred and expended in the form of mechanical energy to deform the workpiece mate- rial—a desirable outcome. However, some of the energy will transform into less-desirable energy states such as heat, dynamic deflection, impact energy and vibration energy that transfers into the press frame. MF
On August 21-22, Stuart Keeler and Peter Ulintz will pres- ent a comprehensive High Strength Steel seminar at the Hol- iday Inn Detroit Metro Airport. Complete information can be found on www.pma.org.
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MetalForming/August 2012 49
