Ask the Stamping Expert: Determining stock width for stamping a part in a progressive die

Q: How do I define the width of the metal stock required to stamp a part in a new progressive die?

A: You need to consider a lot of factors when determining stock width for stamping a part in a progressive die. I assume your question on the stock width is so you can estimate the raw material usage (and cost) for stamping the part. Your real concern should be the ratio of scrap to part material usage. After all, the price is determined by the weight of the raw material, not the width. Stock width affects that ratio, but it isn’t the overall driving factor—the strip layout is.

Without seeing the part print, I can’t give you a specific answer, but I can outline some considerations.

Two Main Goals

The first goal is to design a robust, repeatable tool. After the die is built, if the production part data varies because of a poor, nonrobust strip layout, you will pay for the life of the program. In the long run it will cost much more in maintenance, constant tooling adjustments, and quality issues than it would to have spent a few extra cents upfront on material in the piece-part price.

The second goal is to minimize the scrap generated by the strip layout.

Design Considerations

Material Support. The carriers—an added web of material usually running along the outer edges of the part used for strip stabilization and pilot placement—and tie bars between the parts offer strength and stability to the final strip skeleton. Depending on whether the material is rigid and hard or very soft and malleable, skimping on the carriers can result in a strip that develops waves or sag in between pitches.

Attempting to minimize the carrier width using very small pilots, or none at all, may prevent the pilots from maintaining accurate, repeatable strip registration. Areas of heavy forming may tend to pull the material when piloting, resulting in ovaling of the pilot holes and loss of precision.

Part Size Relative to Thickness. A very large part with many forms made from very thin material will require many tie bars within the strip layout, in addition to correctly sized carriers on both sides. This will help keep the strip as rigid as possible as it progresses through the tool. On the other hand, a small part from thicker material will require less.

Part Geometry. Some parts can be laid out so they nest very close, allowing for narrow strip stock and a short feed length. Some have appendages that do not nest efficiently and require wide stock and a long feed, resulting in a greater scrap-to-part ratio.

Sometimes this will allow you to place pilots in between the parts, which can eliminate the need for pilots on the carrier. If volume allows, it might be more efficient to lay out a tool as two parts out with every press stroke. This may increase the coil stock width but allow effective interlocking of the parts so that the actual material usage per piece is less than a one-part-out tool.

Die Type. Single-hit compound dies need very little webbing (or scrap) around the part as do flat blanks in progressive dies. Sometimes holes in the part design can be used for registration as the strip progresses through the tool.

Large progressive dies require the part to be well-supported. I define “large” by the number of stations (or progressions) required in the tool. The more trim and form stations required to make the part, the more robust carrier and tie bars will be required.

A progressive die is designed so the size and shape of the stamping at each progression mates with the subsequent station. If at any station the dimensions of the part do not meet the design intent, the finished part off the tool will not meet the design intent either. Dimensional instability in any station will multiply the problems as the strip progresses through every station of the die. Remember, garbage in yields garbage out.