Page 31 - MetalForming April 2010

strength x drawing circumference/2000 Let’s look at the blanking forces first. Although not technically correct, the formula would probably be okay as is. Substituting shear stress into the for- mula is more technically correct than the use of tensile strength. But since shear stress data can be hard to come by, applying tensile strength will simply provide a margin of safety in your results. This safety margin compensates for stripping forces not included in
your calculations.
The drawing formula, however, has
some serious problems. First, it is imperative that tensile strength be used in the formula because work hardening will continuously increase flow stress up to the ultimate tensile strength of the workpiece material. Since tensile strength can be double the yield strength or more for some materials, the calculated results would greatly under- estimate the required tonnage.
A significant component missing in your formula is the draw pad (blank holder) tonnage. Draw pad force is gen- erally one-third the drawing force, a significant force that cannot be over looked.
I recommend using the following formula for calculating drawing force: Blank thickness x tensile strength x
drawing circumference/2000 + blank- holder force
After calculating the required blank- ing and drawing forces, compare the results with the derated tonnage for the press being used. The tonnage curve on the previous page illustrates the con- cept of derated tonnage. In this example, a 600-ton press deep drawing 2 in. above the bottom of the press stroke has less than half the rated press force avail- able to carry out the drawing process. Therefore, calculated tonnage must be less than this derated tonnage.
Next month we’ll address press ener- gy, which will likely prove to be the real source of your problems. MF
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