Tooling by Design
Progressive-Die Strip Evaluation--Part II
To select the optimum strip layout for a given progressive-die stamped part, metalformers must compare and rank each possible layout using a relevant scoring system. Last month we looked at four primary evaluation factors, among the several factors influencing the cost and quality of a progressive die:
• Station number factor, Fn
Fig. 1—Connection length reduction
• Moment balancing factor, Fb
• Strip stability factor, Fs
• Feed height factor, Fh
We can calculate an evaluation score (Ev), first proposed by Lin and Sheu (International Journal of Production Research, 2010), based on the four evaluation factors and their corresponding weighting factors wn, wb, ws, wh:
Ev = (wn x Fn) + (wb x Fb) + (ws x Fs) + (wh x Fh)
The designer or process engineer specifies the weighting factors after determining the contribution of each evaluation factor to the final strip-evaluation score. The four evaluation factors are formulated to range from a total of 10 to 100—a higher score indicates better efficiency in cost and production.
The resulting evaluation score (Ev) provides the die designer and process engineers with a performance-based numerical rating for evaluating each strip layout. The evaluation score has relative meaning for different layouts producing the same part. Therefore, it can be used to find the best process solution for that particular part.
Discussed in last month’s column: station number factor (Fn), which determines strip-layout effectiveness in terms of the number of die stations required; and moment balancing factor (Fb), which accounts for how near to center the equivalent reaction forces are to the axis of the press ram.
The strip stability factor (Fs) describes how reliably the strip feeds, in terms of the connecting material that remains to carry the parts as the strip progresses through the die. When the progressive-die strip travels from one die station to the next, a portion of the coil stock is punched out to create the part blank and carrier strip. The connecting length between the blank and carrier strip is at its maximum length (L1) before any cutting or punching takes place, and becomes smaller as cutting, notching and punching occur. It eventually reaches zero as the finished part is cut free from the carrier strip (Fig. 1).
Fig. 2—Evaluation strip
For ranking purposes, we identify three connection length reductions:
Lideal seldom exists, but in terms of strip stability it is the best way to process a progressive-die strip. This occurs when the connecting length is reduced linearly with the number of die stations. A 10-station die, for example, would remove 10 percent of connecting material at each station until a finished part separates from the strip in the final station. An ideal length reduction would provide a stability ranking of 100.Lneg represents the worst condition, where connecting length is reduced quickly and the blank may become unstable and difficult to feed (Fig. 2). Expect a ranking between 10 and 50 for this type of reduction.
Lpos is a desirable length reduction. The decreasing trend occurs gently, and part stability of the attached part is optimum. Thus, blank positioning should have higher accuracy and precision. A ranking of 50 to 90 would be appropriate in this case.
It is more common, however, for the connection length to be reduced in waves, higher than Lideal in some stations and less than Lideal in others. In addition, the connecting length of a given station often is the residual length from a previous 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 compared to the initial stations.
Note: To review additional methods for assessing connection 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 Evaluating 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.
Part size, stock thickness and material strength all contribute 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 material. 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 material, it might be assigned a maximum weighting factor of 1.00, giving it highest 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 consider bending one leg at a time rather than both legs concurrently. 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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