Page 14 - MetalForming-Mar-2018-issue

Tooling by Design
By Peter Ulintz
High-Speed Stamping
In the electronic, fastener and lam- ination industries, 500 to 1200 strokes/min. are common speeds. Many consider these processes to be high-speed stamping operations. Large automotive or agricultural dies oper- ating at 60 to 80 strokes/min. also might be considered high speed.
It can be difficult to find agreement on a definition for high-speed stamping based on strokes/min. alone, but we could agree that stamping at higher speeds will require special attention. Higher speeds mean increased stresses that pose significant problems, includ- ing: stock feeding and scrap-removal issues, excessive tool defections, dam- aging tipping-moments and increasing snapthrough forces that shock and break stamping tools, and potentially damage press equipment.
The Effects of Higher Speed
As the press slide (ram) and upper- die assembly increase in velocity, kinet- ic energy also increases. Recalling some high-school physics, kinetic energy is expressed as:
KE = (mv2)/2 Where:
KE = kinetic energy m=mass
v = velocity squared
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His back- ground includes tool and die making, tool engi- neering, process design, engineering manage- ment and advanced product development. As an educator and technical
presenter, Peter speaks at PMA national seminars, regional roundtables, international conferences, and college and university programs. He also pro- vides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA, pulintz@pma.org
Kinetic energy is influenced by mass and speed in different ways. Mass increases kinetic energy linearly. When mass (the upper-die weight) doubles, kinetic energy doubles. If the mass triples, kinetic energy triples. Because the equation’s velocity term contains an exponent, kinetic energy increases proportionally with the square of its speed. When speed doubles, kinetic energy increases by a factor of four. If speed triples, kinetic energy increases by a factor of nine.
“Heat, impact vibrations
and snapthrough loads associated with high-speed stamping operations must be understood and controlled in order to prevent damage to stamping tools and press equipment.”
Why is all of this important? A small portion of this energy transfer is expended to deform the sheetmetal into a metal stamping, something desirable. The rest of this energy—as much as 95 percent—will transform into other less desirable energy forms such as heat, dynamic deflections and vibrations.
Heat, impact vibrations and snap- through loads associated with high- speed stamping operations must be understood and controlled in order to prevent damage to stamping tools and press equipment.
Preventing Press and Tooling Damage
After a mechanical-press slide reaches peak velocity—approximately 90 deg. of crank-shaft rotation—the slide begins to decelerate until it even- tually reaches zero velocity at bottom-
dead center. The slide immediately accelerates again in the opposite direc- tion on the upstroke. Because the ram motion generates inertia—the greater the mass and speed, the greater the inertial forces—the ram wants to con- tinue its downward path. These inertial forces place tremendous stress on press components, especially the pitman connections.
Depending on the magnitude of inertia forces and the rigidity of the press design, the pitman can elongate, effectively reducing shut height. This shut-height reduction causes die dam- age and introduces additional press stresses, affecting the large end of the pitman on the crank journal and caus- ing additional frame deflections.
Slug breaking in high-speed-punch- ing operations can generate significant- ly higher snapthrough loads, transfer- ring additional 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 characterized by magnified stress- es. This creates a range of nuisance problems, from nuts and bolts loosen- ing to catastrophic problems such as crankshaft and tie-rod breakage.
One way to reduce reverse tonnage and associated vibrations: Stagger the punch lengths equivalent to the shear- band width in the holes previously punched. This reduces both impact and snapthrough loads. Staggering punches in this manner allows the next group of punches to contact the work- piece material prior to the first group snapping through. The snapthrough energy from the first group of punched holes drives the next set of punches through the part material.
High-speed stamping generates a significant amount of frictional heat and it decreases the amount of contact
12 MetalForming/March 2018
www.metalformingmagazine.com