The decision to produce a part progressively is usually determined by two factors: the volume of production and the complexity of the part. These two factors are instrumental in the design and construction of the tooling. It is important to address all factors that will contribute to the desired level of part quality, tool maintenance, and tooling life. Trade-offs will be necessary to reach most decisions, and all will affect tooling costs.
The process begins with determining how the part will be run through the die. This is governed by the features of the part and the locations of the datums and critical tolerances. Then, the trade-offs begin.
Optimizing material usage may require rotating the part in the strip, which changes the grain direction of the steel in the part and thus can affect the strength of any forms in the part. Forming with the grain can cause cracking and fatiguing of the metal and make holding consistent form angles more difficult. Therefore, the form will be far more susceptible to problems associated with the chemical makeup of each coil that is run.
For example, Figure 1 shows a part for the computer industry that was rotated in the strip to guard against inconsistent form angles that could be caused by differences between coils. The part contained critical dimensions with 0.025-millimeter tolerances dependent on the forms. Rotating the strip to ensure more consistent forms was not the most efficient use of material. In this case, however, part tolerances won out over optimizing material usage.
Part configuration could provide a second motivation for rotating a part in the strip. If cam forming or piercing is required to make the part progressively, rotating the part may be the best, and sometimes only, option because the cam and driver can take up a significant amount of room. The part typically is rotated so that the cams’ functions are perpendicular to the coil. This provides the easiest and most accessible condition for the cams.
Often, a compromise between rotating a part to optimize material usage and angling the cams to keep them outside of the coil is the final result. This could increase piece part and tooling costs. To produce the part progressively, however, such a compromise may be necessary.
A third consideration that may require rotating the part in the strip is the amount of lift that is needed to carry the strip through the die. Lift can sometimes be reduced significantly or eliminated by properly rotating a part.
If all forms in a part are in the same direction, lift can be eliminated by forming upward. This usually adds to the cost of the die. When the part has forms in opposite directions, compromises must be made among excessive lift, poor material use, and the complexity and cost of the tooling.
Read more: Designing progressive dies