Page 33 - MetalForming May 2015

piece. Minimize adhesive wear by increasing the toughness of the tooling material, and by the proper selection and use of lubricants, tool steel and tooling surface treatments to min- imize friction.
In general, the best overall tool performance is achieved through a combined balance of tool-steel toughness and wear resistance.
Guidelines for Cutting, Punching and Trimming
• Adequately support trimming steels and use keyed, pocketed or heeled tools where possible to avoid flexing and tipping due to side thrust.
• Consider upgrading tool steels to at least one grade level higher compared to stamping HSLA steels.
• Consider powder-metallurgy tool steels, as they often are more economical in the long run due to their low wear rate. Ceramic surface coatings will maximize tool life.
• Avoid tight corner clearances that can lead to excessive burring.
• Provide a shear angle of two- to four-times material thickness over 12 in. of trim length to reduce press tonnage and extend die life.
• Balance shear angles over the length of the cut to prevent material from being moved sideways by a single-angle shear. • To reduce die maintenance, avoid knife-edge condi- tions in trim steels. Perform all trimming 90 deg. to the trim
surface. Design irregular shapes or notch conditions with inserts, and consider these perishable.
• Pierce holes equal to or less than sheetmetal thickness must be pierced 90 deg. to the surface. Hole diameters greater than material thickness should be pierced no more than 10 deg. off-angle.
Guidelines for Forming
• Design small replaceable sections in high wear areas.
• Die sections must have adequate aspect ratio (width-to- height ratio) with heels and keys to resist thrust forces dur- ing forming.
• Higher grades of die material, at least one tool steel grade higher, should be considered. Coat flange steels—a vanadi- um carbide process generally is recommended.
• Higher holding pressures are required to suppress wrin- kles, which in turn increases die wear. Consider higher grades of die materials for pressure pads.
• Plan for several recuts when designing draw-form sta- tions by increasing the thickness of the pad, post, insert, etc.
Lubrication
Lubricant breakdown can be a significant problem with higher strength steels due to higher blankholder and punch forces that result in increased friction and interface temperatures. The greater energy-absorption characteristics
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