Tooling by Design
Increasing Press Speed and Efficiency
What you don't know can hurt your bottom line.
Your pressroom likely faces increased pressure to reduce the cost of the stampings you produce. One to reduce stamping cost is to increase stamping-press efficiency, i.e. produce more parts per hour. But before you turn up the speed dial on your presses, be sure to understand the effect it can have on the overall process.
Forming, piercing and cutting steel at higher speeds requires greater force, and this increased force can create stock-feeding and scrap-removal problems; excessive tool defections; damaging tipping moments; and increased snapthrough forces that can shock and break tools.
Cutting punches, if not properly engineered, can fail prematurely in higher-speed stamping operations. A common practice has been to use a relatively tight cutting clearance of 5 to 8 percent of stock thickness per side. This clearance produces an acceptable burr height and provides reliable slug control, but when stamping speeds are increased, these tight cutting clearances can cause increased downtime and tool maintenance due to wear, galling and breakage.
Higher press speeds increase impact forces acting on the punch face and snapthrough forces (reverse tonnage) acting on the punch head. Impact occurs when the punch first contacts the part material. The punch’s travel stops briefly as backlash and deflections in the ram and press are absorbed. Compressive loads build rapidly, sending a shock wave through the punch. Then the part material begins to deform. With conventional clearance, the part material bulges outward under the punch face as the slug presses into the die matrix. Once the tensile load exceeds the shear strength of the part material, the slug suddenly separates from the part. This sudden unloading of compressive stress generates a reverse (tensile) shock that can break punch heads.
Punches sometimes are staggered in length to minimize impact and snapthrough shock. Splitting the punch lengths into two or three lengths can reduce impact and snapthrough shock by as much as 30 to 50 percent. Staggering the punch lengths to be equal to the shear band width in the hole being pierced—approximately one-third of stock thickness—will greatly reduce impact snapthrough shock.
Staggering the punch lengths allows the next group of punches to contact the part material prior to the first group snapping through. The snapthrough energy from the first group of punches is absorbed and used to drive the next set of punches through the part material.
Because a punched hole can close as much as 0.002 in. smaller than the punch point, each hit creates a press-fit condition between the punch and the workpiece. The tight fit results in high frictional forces during stripping, which generates significant heat. At higher speeds the punch has less time between hits to cool. Heat builds up rapidly in the punch point, causing galling and heat damage. Even with adequate cooling, the abrasive stripping wear on the punch created by tight cutting clearances will result in increased tooling maintenance or repair.
Use Engineered Clearance, High-Speed Steels
Significant improvements in life expectancy can be realized when punch to die clearances are increased. These engineered clearances—typically 10 percent per side and greater—keep burr height to a minimum while significantly increasing tool life. When using engineered clearances, the part material across the punch face stretches, placing the material in tension. When the slug fractures, the hole becomes slightly larger than the punch-point diameter. This eliminates as much as two-thirds of the abrasive wear incurred using traditional clearances.
A negative side effect of using engineered clearances: the slug sits loose in the die matrix, which can lead to slug pulling. To avoid this effect, stampers should ensure they’re using a properly designed shear angle, or use a spring-loaded ejector pin extending through the center of the punch face.
Punching small holes at higher speeds may require special attention to tool-steel selection. Higher operating speeds generate greater heat and also decreases the tooling contact time by the same factor. This reduces any cooling 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 instability. 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.
Imagine two identical mechanical presses running at the same average slide velocity. For simplicity, we’ll assign an arbitrary 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.
KE for die no. 1 = 1/2 (500 x 0.22) = 20 J
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 second die runs with a velocity of 80 ft./min.
KEm = 1/2 (500 x 0.22) = 20 J
KEM = 1/2 (500 x 0.42) = 80 J
Here, kinetic energy increases proportionally with the square of slide velocity. So, when press speed doubles, kinetic 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 material—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 present a comprehensive High Strength Steel seminar at the Holiday Inn Detroit Metro Airport. Complete information can be found on www.pma.org.
Related Enterprise Zones: Tool & Die
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