Die Science: The basics of die design, Part I – Choosing a die set

New to dies? Consider this.

According to The Fabricator:

Like a house or a building, a stamping die needs a solid foundation. In my career as a consultant over the last 40 years, I have witnessed a plethora of poor foundations: die sets too thin, poor guide pin selection, poorly fitting dowels. The list goes on and on.

First, we’ll address how to choose the best die set.

What Is a Die Set?

A die set comprises two flat ground metal plates that serve as mounting surfaces for all working die components. Each plate contains either a guide pin or bushing that maintains perfect plate alignment. This alignment is very important because all the working components mounted on the upper and lower plates must interface with proper, precise clearances.

Not all dies use a die set. Larger dies, such as those for cutting and forming automobile hoods, doors, and bumpers, use precisely guided cast-iron foundations rather than metal plates. The cast-iron foundations are designed and cast into custom shapes specific to each part. Features such as a means for handling the die also can be incorporated into the casting.

Most castings are made using the lost Styrofoam method. In this method, a large Styrofoam model of the casting is made, then packed in a hardenable sand mixture. After hardening, molten cast iron is poured into the hardened cast molding, replacing the Styrofoam by disintegrating it. After the cast iron has solidified, the hardened sand mixture is removed, leaving a cast replica of the Styrofoam model.

When more conventional die shoes or plates are used, the die engineer must carefully choose a die set thickness and size. Die sets range in size depending on part size, the force required to cut and form the metal, and the number of cutting and forming stations needed to produce the part. Made of steel or aluminum, they serve as the foundation on which all the working die components are mounted. They must be machined, by milling or grinding, to be parallel and flat within a critical tolerance. Although grinding is the most popular, a milled surface can be obtained that is as accurate as a ground surface.

Steel or Aluminum Die Sets?

In the world of conventional stamping, steel die sets are by far the most popular and common type. Steel is a good choice because it’s fairly inexpensive and can be machined easily into flat, precise plates. If a screw or dowel hole is located improperly, steel die sets can be plugged and welded fairly easily. Most conventional steel die sets are made of low-carbon steel.

Aluminum die sets are also quite popular, especially for high-speed stamping. At one-third the weight of steel, aluminum can be machined very quickly and can be made with special alloys for greater compressive strength than low-carbon steel. These special grades include QC-7, QC-10, and Fortal.

Aluminum is a good choice for blanking dies because of its shock adsorption and because a blanking die made with an aluminum die set will produce much less noise than one made of steel. Aluminum is very good at both heat distribution and dissipation, and for companies with limited die-handling capability, its light weight is a benefit.

Aluminum die sets are more expensive than steel die sets, however, and they are more difficult to plug and repair if a screw or dowel hole is in the wrong spot.

Die Plate Thickness

Unfortunately, there is no quick and easy formula for determining the ideal die set thickness because so many factors must be considered.

The amount of force needed to cut and form the material is very important, as higher-tonnage applications usually require thicker die sets. The die engineer also must consider whether the die shoe is going to be sitting flat on the bolster plate and fully supported, or whether parallels will be needed. And those parallels must be far enough apart to allow cutting scrap to drop, but that can cause the die set to deflect under load. So, if there is a long span between parallels, the engineer must choose a thicker die set. In addition, the engineer has to consider the shut height requirements of the press. A heavy die with a very thick die set might not fit within the shut height adjustability of the press.

Choosing the proper die set foundation is a critical part of die engineering. Insufficient die shoe thickness can result in numerous deflection-related issues such as cutting sections breaking, misalignment of working die components, punches chipping and breaking, and the inability to make a qualifying part.

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