Tooling by Design
Progressive Dies and Tipping Moments
The most important step in designing a progressive die is the development of a strip layout. The strip layout directly affects die size, cost and maintenance, press selection and maintenance, repair costs, stamping costs, in-process reliability, dimensional accuracy and the cost of poor quality.
A strip layout largely depends on the designer’s personal experience and the customary practices of the press shop. The final strip design and approval often is determined by a team of skilled and experienced individuals from various engineering and manufacturing disciplines. Unfortunately, this experience-based approach can result in strip layouts that position the center of the progressive die with the center of the press slide.
Arbitrarily positioning the progressive-die centerline with the slide’s centerline ignores the fact that when two or more die stations perform their task on the strip, the forces may not occur simultaneously or be equal across all die stations. Uneven force distribution across the press slide will cause the slide to tip and move laterally in the direction of the greatest force. If this movement occurs while punches are engaged with other die components, excessive wear or damage may occur. Die damage and wear increases die-maintenance costs and compromises dimensional consistency of the stampings being produced.
All stamping presses have maximum off-center loads established by the press manufacturer. This represents the maximum loading that a press can safely handle without suffering long-term damage. Since a strip layout serves as a master plan that determines nearly every decision made during the die-design process, slide tipping—also known as tipping moments—must be addressed during strip layout.
To calculate tipping moments, multiply the cutting and forming forces by the distance from the center of the press at which these forces occur. Using the center of the press slide as the origin, distances to the left would have negative values while the distances to the right would have positive values. In a perfect world, the sum of the moments would equal zero, indicating balanced forces across the press slide.
The strip layout in the illustration above represents an eight-station progressive-die process. Table 1 provides the cutting and forming forces for each of the stations and their distance from the center of the strip. The last column in the table shows the tipping moment for each die station.
In this example the sum of the moment forces total -121.5 in.-tons, indicating unbalanced loading across the slide. The die designer could improve the loading conditions by shifting the strip layout to the right of the press centerline by 1.5-in. (Table 2).
Shifting the die strip may require a slight increase in the length of the die shoe. If this is not possible due to limited press-bed space, the die designer could strategically apply cutting shear to select punches to help balance the loads. For example, the cutting force in station two could be reduced by approximately 50 percent by adding metal-thickness shear to the cutting punch in that station. The resulting reduction in force at that position (Table 3) balances the loads across the press slide.
It is not necessary to perfectly balance all of the forces across the slide. The press structure and die-guiding systems can handle some off-loading conditions. It also may be impossible to balance loads for many processes, especially in progressive dies. Even under ideal conditions where the sum of all moments equals zero, there still can be some slide tipping because all of the forces are not generated at the same time or at the same slide position. This is because the forces generated by forming processes may occur ½ in. or more above the bottom of the press stroke, while the forces generated by punching and blanking occur near the bottom of the press stroke.
Correcting slide-tipping problems with an existing progressive die is possible. One could shift the die in the press or introduce a dummy load somewhere in the die. A dummy load may consist of an added pressure system at one end of the die to counter the tipping moment, or adding a coining station in an area that eventually ends up in the scrap. When the die cannot be shifted in the press due to a lack of room on the press bed, adding a dummy load often is the only recourse.A better solution: Address the problem during strip layout. MF
Related Enterprise Zones: Tool & Die
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