Short-Run Stampings

As a very rough approximation, it can be stated that for average conditions, the borderline between short-run and regular (middle-volume) production may be assumed to be between 5000 and 10,000 pieces per run or per lot. Probably more important, however, is the total quantity expected to be produced by the tooling over the life of the product. If this quantity is less than 20,000, then short-run methods will probably provide the lowest overall costs. When 10,000 to 20,000 total parts are required, it may be advantageous to have both short-run and conventional tooling quoted so as to enable a comparative-cost study to be made.

Another rule of thumb for differentiating between short-run and regular stampings is the following: When the cost of the dies themselves exceeds the cost of the parts produced, it is a short-run job.

Labor costs per piece are invariably higher per unit for short-run methods. The materials cost also may be somewhat higher in short-run jobs because of poorer stock utilization. Setup time is usually greater, and other secondary factors such as material handling, scrap, and work-in-process inventory are apt to be somewhat higher with short-run methods owing primarily to the greater number of operations involved.

The approach that yields the lowest costs for the total of all these factors over the life of the product is the one that should be used.

It should be noted that the total tooling cost of the separate dies used for short-run methods is always a fraction of that of a multistage stamping die that incorporates all the necessary operations simultaneously. (The estimated cost ratio is as much as 1:6 and up to 1:8 or more.) Very significant is the fact that in a majority of cases short-run methods permit the use of exisiting universal-type dies, especially for hole punching and right-angle bending.

Another advantage of the short-run approach is the shorter lead time required for tooling the short-run variety. It should be noted, however, that the quality of stampings produced with permanent-type tooling is usually superior to that of those produced with temporary dies; thus interchangeability of parts is better.

Short-run stamping methods should be considered under any of the following conditions: (1) For experimental, prototype, or pilot-lot production, particularly when engineering changes are likely to occur and the use of permanent tooling would not yet be advisable. (2) For spare-parts production after the original tooling has been disposed of. (3) For products such as industrial, medical, or laboratory equipment or for other fields in which unit production levels are not large. (4) In cases in which faster delivery of a component is essential for the commercial success of a product (e.g., a seasonal article whose development has been started later or has taken too long). With shortened tooling lead time, production may start earlier. In these cases, the basic economic requirements (low cost and high productivity) must be disregarded in the temporary tooling. These considerations are duly taken into account when permanent-type tooling is developed later. (5) When standby tools are desired for high-production permanent-type dies whose working is essential for the uninterrupted manufacturing process. (6) Last but not least, for low-budget business when it is not considered convenient to invest a large sum in costly permanent tooling.

One rule of thumb is that any material that can be produced in the form of sheet of strip can be press-worked. The exceptions, of course, are brittle nonmetallic materials such as glass and the somewhat similarly brittle, very-high-hardness metals, e.g., those of hardness above Rc 50.

Materials for stamping are classified in three groups: (1) ferrous metals, (2) nonferrous metals, (3) nonmetallics.

Ferrous Metals

By fat the most widely used metal for general stamping applications is cold-rolled steel in sheet, strip, and coil form having a carbon content of between 0.05 and 0.20 percent. The lower-carbon steels are the least expensive for stamping.

Steels with 0.10 percent or less carbon are most suitable for severe forming applications. However, steel with up to 0.15 percent carbon normally can be bent 180°on itself in any direction. If the carbon content is 0.15 to 0.25 percent, steel can be bent 180°over one thickness of material. Use of aluminum-killed steels is also advantageous when the design calls for bending or folding the material down on itself.

Comparatively few materials can be drawn successfully, since the material must have the required ductility and tensile strength. Aluminum-killed (sometimes called special-killed) steel is the preferred material for deep-drawing applications. Drawing quality steel is superior to regular commercial-quality steel for these applications but is not as drawable as the aluminum-killed material. Cold-rolled steel is preferred over hot-rolled for drawing applications.

Of the stainless steels, Types 302 ( S30200), 304 (S30400), and 305 (S30500) have maximum stamping and forming capabilities. However, with all stainless steels, higher tonnages are required and punch and die life suffers because of the tougher material. Since the stainless-steel groups and some steel-alloy groups such as the silicon steels are more difficult to work, especially when forming, liberal tolerances should be specified. For coning applications using stainless steel, the following grades are usable：301 (S30100), 302 (S30200),304 (S30400),305 (S30500), 410(S41000), 430(S30500) (305 is more costly than 301 or 302).

For coining applications, carbon or alloy steels of no more than 0.30 percent carbon content should be specified.

Temper is an important factor in the formability of both ferrous and other materials. Galvanized and other coated steels are normally practical materials.