Page 15 - MetalForming Magazine April 2023

Tooling by Design
By Peter Ulintz
Increasing Press Speed and (In)Efficiency
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 need- ed to punch and trim at higher speeds can increase unloading forces—snapthrough—that shock and break tools, and some- times 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 pro- vides reliable slug control. But as stamping speeds increase, tight clearances can be a source of increased downtime and
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 connec-
tool maintenance due to wear, 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
galling and breakage.
allows the succeeding group of punches to contact the part material before the slugs from the preceding group of punch- es 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 con- dition between the punch and work material with each hit. The resulting interference fit causes friction that gen- erates heat. Increasing press speeds generates more heat and decreases tooling contact time along with any cooling
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His background includes tool and die making, tool engineering, process design, engineering management and advanced product development. As an educator and technical presenter, Peter speaks at PMA national seminars, regional roundtables, international confer- ences, and college and university programs. He also provides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
Shut-height reduction introduces additional stresses impact- ing the large end of the pitman on the crank journals, pro- ducing 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 prob- lems, 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.
“Increasing efficiency involves
much more than simply turning
tions. Depending on the mag- nitude of inertia forces and the rigidity of the machine design, the pitman can elongate, effec- tively reducing shut height.
up the speed.”
12 MetalForming/April 2023
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