Progressive die stamping is a cost-effective and safe method of producing components. Careful design and construction of dies will ensure optimum performance.

There are six elements that should be addressed when designing and building a progressive die to maximize its performance:

*Interpreting the part print

*Starting material into the die

*Part lifters and part feeding

*Flexible part carriers

*Upper pressure pads

*Drawn shells

A progressive die performs a series of fundamental sheet metal operations at two or more station in the die during each press stroke. These simultaneous operation produce a part from a strip of material that moves through the die. Each working station performs one or more die operations, but the strip must move from the first station through each succeeding station to produce a complete part. Carriers, consisting of one or more strips of material left between the parts, provide movement of the parts from one die station to the next. These carrier strips are separated from the parts in the last die station.

Interpreting the Part Print

The first step in the proper design of a progressive die is to correctly analyze the part print. The tool designer must interpret the print to determine the function of the part by looking for such things as the type of material, critical surfaces, hole size and location, burr location, grain direction requirements, surface finish and other factors.

The die designer must understand the part well, particularly if it has irregular shapes and contours. However, modern computer-drawn prints make this more difficult because computer-drawn part data can be downloaded directly to the die-design computer. As a result, the designer may not become thorough familiar with important part features.

Also, many computer-drawn parts are more difficult to understand, because often, only one surface is shown and it may be the inside or outside surface. Computer drawings often show all lines, including hidden features, as solid lines instead of dotted lines. This leads to interpretation errors, which in turn leads to errors in the building of the die.

To better understand complex part shapes, it is helpful to build a “sight” model of the part using sheet wax, rubber skins or wood models. Dimensional accuracy is not critical for these models, as they are used primarily to visualize the part. Rubber skins and sheet wax also can be used to develop preform shapes and the develop the best positions for the part as it passes through each die operation in the progressive die.

Starting Material in the Die

“Lead-in”gauges must have large leads and a ledge to support the lead end of the coil strip when it is oke of the press.

Care must be taken to ensure that the strip is started correctly into the die. Improper location of the lead end of the strip will do more damage to the die in the first 10 str than the next 100,000 strokes.

Part Carriers

A common feature in all progressive stamping dies is the material that transports the parts from station-to-station as it passes through the die. This material is known by various terms, such as carrier, web, strip, tie, attachment, etc. In this instance, we will use the term carrier, of which there are five basic styles:

Solid carrier—The required work can be accomplished in the part without preliminary trimming. The part is cut off or blanked in the final operation.

Center carrier—The periphery of the part is trimmed; leaving only a narrow tie near the middle of the part. This permits work to be performed all around the part. A wide center carrier permits trimming only at the sides of the part.

Lance and carry at the center—The strip is lanced between parts, leaving a narrow area near the center to carry the parts.

This eliminates scrap material between parts.

Outside carriers—The carriers are attached to the sides of the part so that work can be done to the center of the part.

One side carrier—The part is carried all the way or part of the way through the die with the carrier on one side only. This permits work on three sides of the part.

The type of shape of the carrier will vary depending on what the part requires as it progresses from station to station in the die. The stock width may be left solid if no part material motion is required during die closure or it can be notched to create one, two or even three carriers between the parts.

The carriers can be straight, form a zig-zag pattern or have loops between the parts depending on where attachment points to the part are available or to accommodate whatever clearance may be required by the die tooling. As the part is formed, flanged or drawn into a shell, the carrier may have to move sideways or up and down as the die closes and opens.

When die operations cause the carrier to move, it usually will be required to flex or stretch. Regardless of carrier flexing, their key function is to move the parts close enough to the next station so that pilots, gauges and locators can put the parts into their precise location as the die closes.

If the carrier acquires a permanent stretch, the parts may progress too far to fit on the next station, or in the case that the die has two carriers, one carrier may develop permanent stretch with no stretch in the other carrier. This will create edge camber in the strip, causing it to veer to one side. This results in poor part location.

A stretched carrier can be shortened to its correct length by putting a dimple in the carrier. If a center carrier or one-sided carrier develops camber, the strip can be straightened by dimpling or scoring one side of the carrier. Construct the dimple and scoring punches so that they are easily adjusted sideways for position and vertically for depth.

Edge camber of the material as it is delivered from the coil can cause the strip to bind in the running gauges that guide the material during the feed cycle. This binding may cause the carriers to buckle, which results in short feeds. It often helps to relieve the guide edge of the gauges in between stations and have tighter gauge control at the work station.

Another option is to eliminate camber by trimming both sides of the material in the beginning of the stamping die. By adding stops at the end of these trim notches they can be used as pitch control notches to prevent progression overfeed.