Die Science: Recognizing the need for tooling changes

Whether caused by process failure or die design error, stamping problems often require tooling changes, but they must make financial sense. Getty Images

In my career as a professional consultant, I have seen numerous stamping processes that needed a major overhauling. Some dies needed just minor adjusting, while others needed to be completely redesigned and remade. In all cases, though, the stamping problems occurred as a result of one or both of the following basic issues:

1. Process failure
2. Die design error

Die Design Versus Process Design

Don’t confuse process design with die design. Process engineers determine the steps in which a part is to be made. In the case of a progressive die, they typically determine how many stations are required to make the stamped part to the specifications and tolerances determined by the product designer. Once the process has been determined, the process engineer may design a strip layout or process layout. This layout serves as the base around which the tooling is designed.

The process or strip layout not only defines the number of stations required to make the part, but also defines the pitch or progression of the tooling. The process or strip layout also shows in detail how the part is to be formed or cut in each station. It also might show how the scrap will be cut away and how the part will be ejected from the die.

A die designer determines the mechanical means by which the predetermined process will be executed. In other words, the die designer designs the tool to perform accurately all the steps determined by the process engineer. The die designer determines the tool geometry and the best tooling materials to use for the application.

Process Failure

Of the two causes of stamping problems, process failure is the more catastrophic and more expensive to correct. Simply put, if the process for making a part is not correct, no matter how the die is designed, it will not function properly.

For example, if you were attempting to make an automotive oil pan and you decided to use a solid forming die instead of a drawing die for the first forming operation, the part would wrinkle and most likely wad up into a deformed, double-metal mess during forming. No matter how you design the die, if a solid forming process is used, it will fail. Items like oil pans and deep formed shapes typically need to be drawn or stretch-formed.

Other examples of process failures that I have seen are an insufficient number of drawing stations to make a part, incorrect binder shape for a drawing and stretching operation, improper forming sequences, and incorrect part tip angles. Unfortunately, when a process error occurs, it usually requires that a whole new die be built. However, in some cases, some of the secondary operations can be salvaged.

Die Design Error

Although some die design errors can be costly, most of them can be corrected by making a mechanical change in the design of the tool. In other words, the process for making the part was correct, but the die was not capable of achieving the results.

For example, let’s say that the pressure in the blank holder or drawing pad pressure system is maxed out, and the part is still wrinkling in the binder area. This failure would indicate the die design engineer designed the tool with an insufficient number of gas cylinders or perhaps used cylinders with insufficient force. The solution would be to add more cylinders or use higher-pressure ones.

Other classic examples of die design errors are incorrect tool steel selection, poor locating methods on die sections, poor pad guidance methods, incorrect forming and cutting clearances specified, excessive die shoe deflection, poor cam designs, and incorrect stop block locations.

Time for a Change

When you have a process failure, no matter how you change the die (unless it reflects a process change), you can expect little or no improvement. Stampers often want to make significant part improvements but are unwilling to make any major process changes that could cost short-term productivity. They want to do something quick and inexpensive to the existing dies to get the desired results, but often they need an entirely new tool using a completely different process and are reluctant to invest the time and money to make a new die.

In most cases, the money lost because of downtime, scrap, rework, overtime, and risk far outweighs the cost of a new tool. Nobody wants to retool a job, and sometimes the cost of correction can be greater than the profit that can be gained. So the best course is to focus a great deal of time and effort at the beginning stages of your project. Be as certain as you can that you have the correct process in place to make your stamped part.

If for some reason you discover later that you selected the wrong process, analyze the cost of correction and the cost of not correcting the problem, and fix it when it makes financial sense.