Page 60 - MetalForming April 2014
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when maximum loads remain below 60 percent of the compressive strength of the punch-body material. Table 3 com- pares point and head pressures for an 0.100-in. punch diameter and equiva- lent sheet thickness (0.065 in.) in HSLA and AHSS materials.
The following example reveals the significance that AHSS materials will have on tooling design, tool-steel selec- tion and heattreatment.
The calculated head pressure ( Table 3) for the HSLA material is low enough to not require a backing pate in the die design. But, head pressure for the AHSS material exceeds 20,000 PSI, requiring use of a hardened backing plate in the die.
The compressive force acting on the punch point for HSLA is sufficiently low enough to allow the use of A2 tool steel heattreated to 60 Rc, without the loads exceeding 60 percent of the A2 compressive strength. Conversely, the
force acting on the punch point for the AHSS material exceeds 273,000 PSI—60 percent of the compressive strength of M4 heattreated to 65 Rc. The M4 mate- rial and heattreat combination is mar- ginal for the application; so punch life is significantly reduced as it may plas- tically deform over time.
This example only considers the compressive forces acting on the punch point. A punch machined from M4 and heattreated to 65 Rc likely would chip or break when cutting a material with 110,000-PSI shear strength. In this case, PM-M4 would be a better choice for improved resistance to chipping and wear, and for application of a surface treatment to further enhance wear resistance.
In general, optimum punch per- formance for blanking, punching and trimming is achieved through a com- bined balance of tool-steel toughness and wear resistance. MF
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MetalForming/April 2014
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Tooling by Design
