Page 38 - MetalForming November 2014

Tooling by Design By Peter Ulintz
High-Speed Metal Stamping
What may be considered a high- speed stamping operation at your company may not be considered high speed at other com- panies or in other industries. In the electronics, fastener and lamination industries, 500 to 1200 strokes/min. (SPM) are not uncommon. High-speed progressive dies for automotive or gen- eral-purpose stampings may run at 50 to 120 SPM, and high-speed transfer dies producing large, complex deep- drawn automotive parts may run at 15 to 40 SPM.
Although it can be difficult to find agreement based on strokes-per- minute, we can generally agree that high-speed stamping operations will require special attention. For example, forming and cutting more quickly requires greater stress and increased forces that can pose significant chal- lenges for progressive and transfer tool- ing. These challenges include stock feeding and scrap removal, excessive tool defections, damaging tipping- moments and increased snapthrough forces that can shock and break stamp- ing tools and damage press equipment.
The Effects of Higher Speed
As the upper die half increases in velocity, its kinetic energy also increas-
Peter Ulintz has worked in the metal stamping and tool and die indus- tries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His back- ground includes tool and die making, tool engi- neering, process engi-
neering, engineering management and product development. Peter speaks regularly at PMA semi- nars and conferences. He is also vice president of the North American Deep Drawing Research Group. Peter Ulintz
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
es. Recalling some high-school physics, we know kinetic energy can be expressed as:
KE = (mv2)/2
Where:
KE = kinetic energy m = mass
v2 = velocity squared
This makes it clear that the kinetic energy of an object can be influenced by its mass and speed. For example, a car traveling at 20 miles/hr. will deliv- er a greater force and do more damage as it hits some solid object than a bicy- cle moving at the same speed.
A car traveling at 40 miles/hr. has four times the kinetic energy as it does when traveling at 20 miles/hr., since the velocity term in the equation contains an exponent (velocity squared).
Now let’s apply this principle to a high-speed stamping die. Kinetic ener- gy increases linearly with mass. So, when mass (the upper die weight) is doubled, kinetic energy also doubles.
Now assume that the upper die weight remains constant and we run the stamping process twice as fast as the previous production run. The mechanical press is identical except that the slide velocity will be twice as great as the previous run. Here, kinet- ic energy increases proportionally with the square of its speed. When speed doubles, kinetic energy increases by a factor of four. If speed were tripled, kinetic energy would increase by a fac- tor of nine.
Some of the kinetic energy will be transferred and expended in the form of mechanical energy to deform the workpiece into the actual stamping. However, much of this energy (approx- imately 95 percent) will transform into less-desirable forms, such as heat, dynamic deflections and damaging vibrations.
Clearly, kinetic energy and impact vibrations associated with high-speed stamping operations must be under- stood and controlled in order to pre- vent detrimental damage to the stamp- ing tools and press equipment.
Preventing Damage
After a mechanical press slide reach- es peak velocity—at approximately 90 deg. of crankshaft rotation—the slide begins to decelerate until it eventually reaches zero velocity at bottom dead center. The slide immediately acceler- ates again in the opposite direction on the upstroke. Because the ram motion generates inertia—the greater the press speed the greater the inertial forces— the ram wants to continue its down- ward travel as the press crank tries to accelerate it in the opposite direction on the upstroke. These inertial forces place tremendous stress on press com- ponents, especially the pitman con- nections. Depending on the magni- tude of inertia forces and the rigidity of the machine design, the pitman will elongate, effectively reducing shut height. The shut-height reduction caus- es die damage and introduces addi- tional press stresses, such as impacting the large end of the pitman on the crank journal and introducing addi- tional frame deflections.
Slug breaking in high-speed punch- ing operations generates higher snapthrough forces, transferring addi- tional kinetic impact energy into the frame structure in the form of vibra- tions. As press speeds increase, there is less time to dissipate these vibrations and they eventually can reach critical levels where magnified stresses are pro- duced. These stresses can create a range of nuisance problems, from loos- ening of nuts and bolts to catastroph- ic problems such as a broken crank- shaft or tie rods.
36 MetalForming/November 2014
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