Page 50 - MetalForming August 2012

Tooling by Design
By Peter Ulintz
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 way 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
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.
Staggered Punches
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 con- dition between the punch and the workpiece. The tight fit results in high frictional forces during stripping, which gen- erates 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 real- ized when punch to die clearances are increased. These engineered clearances—typically 10 percent per side and greater—keep burr height to a minimum while significant- ly 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
Cutting punches, if not
properly engineered, can
fail prematurely in higher-
speed stamping opera-
tions. A common practice
has been to use a relative-
ly tight cutting clearance
of 5 to 8 percent of stock
thickness per side. This
clearance produces an
acceptable burr height and
when stamping speeds are increased, these tight cutting clearances can cause increased downtime and tool mainte- nance due to wear, galling and breakage.
provides reliable slug control, but
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
Peter Ulintz has worked in the metal stamping and tool and die industries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His background includes tool and die making, tool engineering, process engi- neering, engineering management and product devel- opment. He is vice-president of the North American Deep Drawing Research Group. Peter speaks regularly at PMA seminars and conferences and maintains the website, www.ToolingbyDesign.com. The site serves as a web-based source for the transfer of modern metal- forming technology and the advancement of “Perfor- mance-Based Die Engineering Strategies.”
Peter Ulintz
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Tooling Technology
can shock and break tools.
Sudden unloading of compressive stress generates a reverse (tensile) shock that can break punch heads.
48 MetalForming/August 2012
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