Page 27 - MetalForming September 2009

DieProtection for
Lean
Error-proofing concepts for toolmakers and die designers
Manufacturing By Drew Stevens
This new book from the Precision Metalforming Association is written by long-time die-sensor expert Drew Stevens, who shows readers step by step how to develop modern sensing technolo- gies for the pressroom and apply them to a lean- manufacturing environ- ment. “Slow stamping- press speeds and fre- quent die crashes are not acceptable,” writes Stevens, “and the manu- facturers that choose to shift their thinking to new technologies will prevail in the world metalforming market.”
To learn how to protect investments by applying sensors in your metalforming operation and support your lean- manufacturing directives, order a copy of Stevens’ book today.
Call Marlene at 216-901-8800 x127 Online at www.pma.org and visit our Online Store
Pressure stripper
Sheetmetal
Lower die steel
Pressure stripper
Sheetmetal
Lower die steel
Die spring (compressed)
Punch
Punching Action
amount of deflection. This will increase the useful life of the springs and reduce the chances of spring failure.
In a spring stripper system, the springs build up in force as they compress during die clos- ing. Although 50-percent com- pression is possible for medi- um-pressure springs, it is usually routine to limit travel to 30 per- cent of the spring’s free length in order to ensure long life. As a rule, medium-to-high-pressure springs are compressed 25 per- cent while high-pressure and extra-high-pressure springs are limited to 17 percent and 15 per- cent, respectively.
Keep in mind, maximum spring force is realized at maxi- mum compression. This means full stripping force is not avail- able throughout the entire strip-
ping action on the press upstroke. As the die opens, the pressure decreases as the springs lengthen. As a result, springs may not provide enough force for high stripping loads in applications requiring tight cutting clearances, thick sheet materials or high-strength materials. In these cases, nitrogen gas springs may be required.
Nitrogen gas springs offer a host of benefits. They provide on-contact force and require no preloading. They are available in compact profiles that require less die shut height while pro- viding high, repeatable forces and long life cycles. Gas springs enable achieve- ment of desired force with fewer springs. They are easily and economi- cally repaired when necessary. Nitrogen gas springs also may be operated in a linked system, providing pressure con- trol via a control panel located out- side of the die. This provides more accurate pressure control, especially in dies where longer spring deflection is required. MF
Punch
Die spring (extended)
Stripping Action
Fig. 2—Spring pressure stripper
arrangement produces ideal cutting conditions and flatter parts as com- pared to solid, bridge-type strippers.
Stripper plates are typically made of cold-rolled steel that is not hardened. Removable window plates may be machined and inserted into the stripper face to provide easy access to the punch- es during production. Hardened plates also may be inserted into the stripper face to provide a hard surface in high wear areas.
The most common source of strip- ping pressure is a die spring, made of heavy square or round wire and usual- ly rated by the number of pounds required to compress the spring 0.100 in. Die springs are available in a variety of diameters, lengths, wire shapes and wire sizes. (See the article in this issue, pages 22-23.) Common pressure ratings include medium, medium-high, high and extra-high pressure. As a general rule, always use as many springs as the die will accommodate that will pro- duce the required load with the least
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METALFORMING / SEPTEMBER 2009 25