Page 91 - MetalForming October 2012

Fig. 2—Evaluation strip
punching station. This means that the connecting length at later stations has more impact on strip stability, because the strip becomes less stable at the later station(s) in the die com- pared to the initial stations.
Note: To review additional methods for assessing con- nection length and strip stability, read the Master’s Thesis of Ho Minh Tuan (National Taiwan University of Science and Technology), titled “Pro/Web.Link Programming for Evalu- ating Strip Layout of Progressive Dies.” Tuan also proposes a computer-aided method for ranking progressive-die strips. The thesis is available as a .pdf download.
Weighting Factors
Part size, stock thickness and material strength all con- tribute significantly to strip stability. Progressive-die strips produced from thick material tend to be more stable than the same strip produced from lower-strength and thinner mate- rial. Therefore, a weighting factor (ws) must be applied to the
stability factor when calculating strip evaluation (Ev) scores. Weighting helps prioritize each of the four evaluation factors relative to each another.
For example, if we produce the strip in Fig. 2 from a very thin and soft mate- rial, it might be assigned a maximum weighting factor of 1.00, giving it high-
est priority. Producing the same strip from stronger or thicker material might warrant a weighting factor of only 0.50. The strip stability factor (Fs) is twice as important for the strip produced from thin, soft material as compared to thicker, stronger material.
A high weighting factor should force the strip design to change, to provide adequate stability for thin material. For example, the designer could move one of the large cutouts to after the die punches the round center hole. Or, he might con- sider bending one leg at a time rather than both legs con- currently. In terms of strip stability, this would not be the case for thicker material because of lower weighting factor.
Conversely, the moment balancing factor, Fb, should be weighted higher for the stronger, thicker material due to the high snapthrough forces and potential off-center loading that may occur.
Next month we’ll examine feed height factors. Feed height factor is related to the stock lift distance that is required to clear forming stations. This factor will be discussed in detail. MF

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