Peter Ulintz Peter Ulintz
PMA Technical Consultant

Increasing Press Speed and (In)Efficiency

March 30, 2023
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Press shops face constant pressure from customers to reduce manufacturing costs. A common method: Increase strokes/min. on the stamping press. But, before turning up the speed dial, understand the effect it can have on the overall stamping process.

Increasing press speed can amplify issues related to coil feeding, scrap removal and tool deflections. The forces needed to punch and trim at higher speeds can increase unloading forces—snapthrough—that shock and break tools, and sometimes presses. 

Hole punches may fail prematurely at higher speeds if not properly engineered. Common practice has been to use 5-8 percent of stock thickness per side for cutting clearance. This clearance produces an acceptable burr height and provides reliable slug control. But as stamping speeds increase, tight clearances can be a source of increased downtime and tool maintenance due to wear, galling and breakage. 

Staggering cutting punches at heights equal to or slightly less than the shear-band depth in the holes that they produce greatly reduces impact and unloading shock. This method allows the succeeding group of punches to contact the part material before the slugs from the preceding group of punches break out of the strip. The unloading energy from the first group of punches drives the next group through the part material, reducing the unloading loads. Using the shear-band depth for the punch-offset distances—instead of an arbitrary percentage of material thickness as generally employed—also reduces punch entry into the die matrix, which helps to minimize punch wear.

Tight clearances produce hole diameters slightly smaller than the punch-point diameter, creating a press-fit condition between the punch and work material with each hit. The resulting interference fit causes friction that generates heat. Increasing press speeds generates more heat and decreases tooling contact time along with any cooling that might be afforded by the tooling. Smaller-diameter punches also have less ability to dissipate heat and are prone to overheating. This can result in a loss of hardness, reduced wear resistance and dimensional instability. 

Speed Increases Stress on Press Components

Now consider a typical mechanical press cycle: The ram accelerates from rest at top dead center until it reaches a maximum velocity at 90 deg. of rotation. The press slide then begins to decelerate as it approaches the bottom of its stroke. Because the ram motion creates large inertia forces—the greater the press speed the greater the inertia force—the ram wants to continue its downward travel as the press crank tries to accelerate it in the opposite direction on the upstroke. 

This places tremendous stress on press components, especially the pitman connections. Depending on the magnitude of inertia forces and the rigidity of the machine design, the pitman can elongate, effectively reducing shut height. Shut-height reduction introduces additional stresses impacting the large end of the pitman on the crank journals, producing additional frame deflections.

Material fracture during punching and cutting operations produces unloading forces that transfer additional kinetic energy into the press frame in the form of vibrations. As press speeds increase, less time is available to dissipate these vibrations. Vibrations reaching critical levels produce magnified stresses that can create a range of nuisance problems, from nuts and bolts loosening to catastrophic problems such as crankshaft and tie-rod breakage.

Affects Material Feed

Increasing press speed also affects the critical angle for sensor faults. The critical angle refers to the last degree of shaft rotation where a sensor fault can be initiated, and the press will stop before the die closes on the strip. Factors affecting critical angle include press speed (strokes/min.) and press stopping time. Press stopping time increases with press speed, which in turn changes the critical angle. Now the sensor must signal the press to stop earlier in the stroke, resulting in a smaller feed angle. This means that the feed cycle also must be completed sooner, requiring greater feed acceleration. Feed-roll slippage becomes an issue with higher feed accelerations, producing a feed-length distance that measures less than the programmed feed length.

Pilot-release timing offers another consideration. Servo-feed units relying on pneumatic cylinders to lift and close the feed rolls or grippers also must consider the delay time from when the signal is sent to open the feed and the actual time it takes to open. The time required to charge solenoids, activate switches, exhaust air from the pneumatic cylinder and pressurize the opposite side to lift or close the feed roll is quite small, about 35 msec, but can impact pilot-release timing significantly.

A press running at 55 strokes/min. takes an average of 3.1 msec for the crank to turn 1 deg. A lag time of 35 msec equates to 11.3 deg. of lag time, meaning that the pilot-release signal from the controller must be initiated at 11.3 deg. before the required roll-lift angle. Increasing speed to 65 strokes/min. means that only 2.6 msec is required for the crank to turn 1 deg., thus increasing lag time to 13.6 deg., which requires a corresponding signal change from the press control.

Increased press speeds can produce unexpected inefficiencies when punches begin to chip due to high impact, when sensor faults allow the die to close on the strip, or when pilot pins do not position the coil strip accurately or distort pilot holes. Clearly, increasing efficiency involves much more than simply turning up the speed. MF

Industry-Related Terms: Burr Height, Burr, Center, Die, Form, Grippers, Ram, Scrap, Shut Height, Slug, Stroke, Thickness, Transfer
View Glossary of Metalforming Terms

Technologies: Stamping Presses, Tooling

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