Page 38 - MetalForming September 2010

Tooling Technology TOOLING BY DESIGN
PETER ULINTZ
Peter Ulintz has worked in the sheetmetal-forming industry since 1978. His background includes tool and die making, tool and process engineering, engineering management and product devel- opment. Peter also operates the website ToolingbyDesign.com, a source for the transfer of modern metalforming and tool-and-die technology, and which promotes the use of “Performance-Based Die Engineering Strategies.”
Peter speaks at PMA seminars and roundtables focusing on tool and die design, die maintenance, deep drawing, stamping simula- tion, tooling for stamping high- strength steels and problem solv- ing in the press shop.
Peter Ulintz pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Question: A couple of months ago you wrote a column about shaving (A Close Shave, July 2010). The information in your column related primarily to two-step shaving opera- tions; punching a hole first and then shaving it in a second progressive die station or die operation.
One of our customers wants to improve the punch-hole condition in one of the stampings that we currently supply but we do not have room in the die to add an additional shaving station. Are you aware of a reliable method(s) for punching and shaving in a single press stroke?
Answer: Over the years I have seen some single-stroke punch and shave tools, often referred to as step punches, that have worked well, and many that have not. The primary problems with step punches have been slivers in the die, slug-ring ejection and increased tool wear. Satisfactory results usually come only with great effort and much trial and error. Step punches in high-vol- ume stamping applications also will require an increase in the amount and type of die maintenance required.
For the most part, each step punch application is its own R&D project and, therefore, developing an empirical for- mula or standard designs for general use has proved extremely difficult, if not impossible.
Many of the problems associated with step punches are due to the elastic behavior of the sheetmetal being punched. Remember, when punching holes using conventional punch-to-die clearance (approximately 5 to 8 percent per side) the punching stress forces the material around hole-edge periphery outward into compression. When the slug breaks free, the compressive stress-
Fig. 1—A dual-head punch.
es relax and the punched hole “springs back” (inward) toward the punch point. The opposite occurs when engineered cutting clearances (approx. 9 to 20 per- cent per side) are applied. In this case the punching stress pulls the material around the punch hole-edge periphery inward in tension and then the hole springs outward (away) from the punch point when the slug breaks free.
When a punched hole changes size during the punching stroke, it is difficult to accurately recut the hole and main- tain the necessary shave clearances. Worse yet, because the hole often changes shape during the second cutting (or shaving) step, problems such as chipping, wear, galling and/or adhe- sion arise in as little as a few hundred hits.
One solution to this problem has been to extend the punch point for the first cut to allow the hole to change shape before the second cut starts. The problem with this solution is that the punch entry in the die matrix becomes excessive, especially with thicker mate- rials, resulting in dramatically reduced die life.
One company, M.O.M. Tools, has developed a promising single-stroke dual-cut punch that takes advantage of
34 METALFORMING / SEPTEMBER 2010
www.metalformingmagazine.com
A Single Punch and Shave Tool