Page 57 - MetalForming September 2012

higher construction costs and increased tolerance accumulations.
Station-number factor Fn deter- mines a strip layout’s effectiveness in terms of the number of stations it includes. An Fn value of 100 (best pos- sible) represents a minimum number of stations—two. In contrast, an Fn of 10 represents the maximum number of stations, typically the total num- ber of punches used for cutting and bending.
Fn =100–90x N–Nmin Nmax – Nmin
N = total number of stations in the strip layout; Mmax = total number of punches (cutting and bending); and Nmin = the possible minimum number of stations, Nmin = 2
Tipping Moment
Also required is a moment-balanc- ing factor, Fb. When two or more die stations perform their task on the die strip, the forces simultaneously act on the strip at different points. If the reac- tion forces are unbalanced relative to the press centerline, ram tipping occurs. The moment-balancing factor indicates how near to center the equiv- alent reaction forces are to the axis of the press ram. Since the center of the die usually aligns under the center of the ram, this factor considers tipping- moment severity—seldom considered in strip layouts.
All stamping presses have a maxi- mum tipping moment, established by the press builder, that represents the maximum off-center loading condi- tion that the press can safely handle without suffering long-term damage. Designers use this rating to establish a maximum off-center loading parame- ter, Dmax.
To calculate the moment balancing factor:
Fb = 100 – 90 x d Dmax
When d = 0, the center of the ram and of the stamping loads are com-
pletely matched, so Fb = 100 (best con- dition). When d > Dmax, the deviation is so serious that Fb = 10 (worst condi- tion). Similar factors can be established for strip stability and strip lift. Then the designer assigns corresponding weighting factors based on their impor- tance to the strip layout.
The resulting evaluation score (Ev)
provides die and process engineers with a performance-based numerical rating to evaluate each strip layout. The evaluation score has relative mean- ing for different layouts producing the same part. It therefore can be used to find the best solution for that particu- lar part.
Or, we can continue guessing. MF
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