Selecting the proper tool steel, design software
August 1, 2007
To stay competitive in today’s market, stampers must build dies from materials that adequately perform with as little maintenance as possible. Appropriate tool steels can be cold-rolled steel to solid carbide. The key is to know when to use a lower-grade steel and when to use a higher-grade tool steel.
Editor's Note: This article is Part II of a two-part series that discusses how to cut tooling costs without sacrificing die quality. Part I defined cost versus quality and fit and function.
You might laugh to think of a die completely made from cold-rolled steel, yet, dies made from cold-rolled steel, solid carbide, or cast iron can function properly with minimal maintenance. The key is to know when to use a lower-grade steel and when to use a higher-grade tool steel.
Consider fit and function when selecting a tool steel. When your press is down because of a tool steel failure such as galling or cracking, does the whole die fail? Probably not. If you're like most stampers, your die has a weak spot, such as near compression flanges or an area where severe ironing, bending, or cutting takes place.
These areas usually are relatively small when compared to the remainder of the tool. To save the tool, consider machining pockets in less expensive tool steel or cast iron and inserting small amounts of premium-grade tool steels in these areas (see Figure 1). Not only will you reduce tooling costs, but you also will provide a better, less costly means of maintenance.
High-grade tool steels have greater wear resistance and toughness, so the probability of surface breakdown and cracking is less likely. However, in the event that the premium tool steel's surface breaks down, it will most likely need to be coated. The process of coating die sections is expensive, and the larger the die section, the greater the coating cost. If the section is small, the coating cost will be less.
Also, high tool steel grades usually have higher tempering temperatures than less expensive tool steels like d-2. This makes the inserts better candidates for a high-temperature coating process. Inserting a premium-grade die section also allows a die maintenance technician to keep a spare insert on the shelf for quick replacement.
Remember, there is nothing wrong with building a die from hardenable cast iron, 4140, or other inexpensive tool steels as long as the areas subjected to high wear are inserted with higher-quality tool steel. This is a simple way to reduce tooling costs while improving performance and simplifying maintenance.
Die design and finite element analysis (FEA) software has revolutionized the way toolmakers process, design, and build stamping dies (see Figure 2). Although software cannot create a part geometry, it can provide feedback data and expedite the design process.
FE Analysis. In the die-building industry, the most common type of software used is forming simulation software. This software helps you develop parts using the most economical process and material. Simulation also can help you develop processes by showing the results of a "virtual" die. Software reduces time spent on trial and error after the die has been assembled.
For example, to change a die entry radius from 0.500 inch to 0.625 in. in a forming simulation may take 30 seconds. To do the same on the floor may take a diemaker several hours or even the whole day, depending on the size of the tool. Simulation software can identify and change problem areas, such as excessive thinning, fractures, or wrinkling. Software also allows you to predetermine blank size and shape.
Remember, FEA software won't design the die for you; it shows you the results of your die design based on your data. Many toolmakers invest in this technology thinking that it will design the die for them. Not true. Some of my clients have reduced their die tryout time as much as 80 percent by using FEA.
Because the initial cost of FEA software can be substantial, it's important to compare that cost with your anticipated costs and efficiency savings over the long term. It would be surprising if your calculations do not reflect a good return on investment. Some toolmakers have saved millions using this software.
Significant training is required to run the software properly. An average training period is approximately two months if training is done every day. However, the amount of training needed depends on the person learning the software. Using the software and training appropriate personnel to run it effectively can provide a quick ROI.
Die Design Software. Die design software also can expedite the tool design process. Software like solid modeling allows you to create die geometries in a fraction of the time that it took 15 years ago. Items such as basic die components can be inserted as a 3-D solid model from a model database.
More advanced software also can assist the machining process by transferring surface data into a toll path from a wire burn machine or mill. Solid modeling also helps the die builder to interpret easily what the process engineer and die designer intended.
These are just a few methods that can help you reduce the time and cost associated with processing, designing, and building dies. Until next time … best of luck!