Page 25 - MetalForming April 2009

and wear resistance.
General guidelines for cutting,
punching and trimming:
• Trimming steels should be ade-
quately supported and keyed, pocketed or heeled whenever possible to avoid flexing and tipping due to side thrust.
• Consider upgrading tool steels to at least one grade level higher as com- pared to mild steel.
• Consider powder-metallurgy tool steels, as they often are more econom- ical in the long run due to their low wear rate, and consider tool-surface coatings to maximize tool life.
• Avoid tight corner clearances that can eventually lead to excessive bur- ring.
• 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 sideways move- ment of material by a single-angle shear.
• Reduce die maintenance by avoid-
ing knife-edge conditions in trim steels —perform all trimming at 90-deg. angles to the trim surface.
• Design unusual shapes or notch conditions with inserts, and consider those inserts perishable.
• Pierce holes that are equal to or less than material thickness should be pierced at 90-deg. angles to the sur- face; holes greater than material thick- ness should be pierced no more than 10 deg. off-angle.
• Keep punch-head pressures below 40,000 psi to minimize head breakage, and use hardened backing plates when punch-head pressures exceed 20,000 psi.
• Keep punch-point pressures below 60 percent of the compressive strength of the punch body to minimize punch- point breakage and deformation (con- sult your punch manufacturer for details).
• Avoid use of urethane strippers with higher-strength materials; these materials require hardened strippers
with mechanical or nitrogen-gas springs due to the higher stripping and holding forces.
• Develop trim edges whenever pos- sible, as trimming after forming great- ly increases tool wear due to high workhardening.
Bending, Flanging and Forming
All metalforming includes some form of bending—often the predomi- nant feature in a stamping. Elastic springback always occurs after bending and forming at room temperature, and can be considerable with higher- strength steels. Small bend radii reduce the ratio of elastic to plastic strains within the bend, thereby reducing the magnitude of springback. But exercise caution, as a smaller radius also increas- es the amount of force required to deform the material and also increases forming severity throughout the thick- ness of the material.
Wiping dies require the use of pres- sure pads to keep part material from
write no. 17
write no. 18
APRIL 2009 23