Page 65 - MetalForming April 2015

The Science of Forming
preparation for Monday production. Come Monday, the stamping no longer meets dimensional specifications. The required embossment completely changed the deformation of the near- by corner. Located closely enough to the corner, it pulled material from the corner but reduced material flow in other directions. Other problems also were generated. The die was modified without necessary data. It took several days to finally modify the die to pro- duce acceptable stampings.
This and many other problems are being solved before a single change is made to the hard die. Virtual forming (computerized die tryout) provides important data. While not always per- fect, the generated information proves much better than no data. In this case, the computer program can show the current die and the modified die incre- mentally forming the stampings as a video, with the tooling erased. Differ- ences in deformation and input adjust- ments can be compared and modified with simple keyboard strokes.
4) A line of data points beats one data point. An important material property is the work-hardening expo- nent (n-value). This is best obtained through a series of tensile-test meas- urements between a strain of 10 per- cent and tensile strength that are plot- ted on a graph or computed by the tensile-test controller. The slope of the curve represents the n-value. From the n-value one can calculate the percent- age of uniform elongation. The reverse can be accomplished, but the n-value and uniform elongation then would depend on one single data point taken at the maximum load. Given a stress- strain curve with a flat maximum-stress curve, which data point do you take? Working with the new advanced high- strength steels, as a major change in procedure, plotting the instantaneous n-values is required because their mag- nitudes change as the material deforms. MF
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MetalForming/April 2015 63