Improving metal forming performance

What if your employees approached metal forming the same way athletes approach performance?

Successful athletes understand that excellence comes from disciplined practice, continuous skill improvement, and passion for learning. Performance is directly related to what a competitor understands about his or her weaknesses and capabilities.

In a sport like football, a successful athlete must also be dynamic and understand how to respond to changes in weather, field conditions, crowd noise, and a variable player lineup. The dynamics of materials, tooling, equipment conditions, and employee knowledge create similar challenges in manufacturing. The more your employees understand properties and dynamics of their “field of play,” the greater their probability of excelling in creating formed components.

Today’s athletes use strength, conditioning, and skills coaches, along with biometric measures to determine their current physical state. These measures also are helpful in discovering the right mix of training to achieve peak performance. Designers, engineers, and tool and die professionals also can benefit from the knowledge of others and performance metrics. Metrics measure materials and forming capabilities to accomplish peak performance on press lines. While practicing on a press is too costly to be practical, tools are available to measure and improve press line performance.

The ability to form components reliably is founded in the knowledge of materials, geometries, and forming technologies during product design and production. Metal formers have a number of standardized tests available to predict sheet metal behaviors in the press and in service to customers. Forming simulation allows stampers to practice using designs and materials in an affordable and nondestructive virtual environment. Virtual simulation allows manufacturers to practice forming components. The virtual plant, however, must closely resemble the characteristics of the selected materials, geometries, and presses to be meaningful. If a stamping operation is able to make the virtual environment accurately resemble reality, it has a greater probability of reducing expensive die tryouts and production failures.

The accuracy and usefulness of simulation practice sessions depend on the depth of understanding your employees have of their material stock. If your supplier provides coils from different mills or has process variation in their operations, you must be aware to avoid unexpected production failures. You can best prepare for production with a deep understanding of the characteristics and variability of your playing field. This is accomplished with a reliable test plan.

The material properties that directly or indirectly influence your material’s formability include yield strength, ultimate tensile strength, Young’s modulus, ductility, hardness, strain hardening exponent, and plastic strain ratio. A comprehensive periodic test plan can give you the knowledge to anticipate material behavior during deformation and understand the variability in your material properties to prevent forming failures. This is your version of practice for performance!

Examples of important material measures include the following:

• Stress-strain curve. A tensile test is used to obtain this information. Tensile tests pull a standardized sample specimen along a single axis until failure. The result of this test is a graphical representation of the results, called the stress-strain curve. The stress-strain curve demonstrates the material's yield strength and ultimate tensile strength. It also measures ductility.
• Strain hardening. This is also determined with a tensile test. Measuring the material’s stress-strain response in its plastic region prior to necking determines the strain hardening exponent (n-value). The greater the n-value, or the difference between ultimate tensile strength and yield, the more the material can be stretched before necking begins.
• Forming limit curve. The forming limit curve is determined using a circle grid analysis or digital image correlation. The forming limit curve maps the failure criterion of a material at a combination of major and minor strains.
• Plastic strain ratio. This measures the ability of the sheet metal to resist thinning during a deep draw.
• Surface hardness. Hardness tests are comparative tests to measure the material's resistance to permanent indentation.

Too often manufacturers test their materials only after they encounter problems on the press line. Specifications for input stock may be based on standardized or mill representations, rather than deeply practiced knowledge. The industry also relies on many rules of thumb, some of which have been around for 50 or more years. In an era of rapid change in technology and materials, we must do better to maintain a competitive advantage.

One solution to deal with a changing environment is practice. Practice in the form of testing and simulation develops your manufacturing muscles, builds knowledge, and creates adaptive and confident employees. With more practice, your probability of building parts of consistent high quality increases. Also, you usually can get parts from design to production faster, and you are able to resolve issues more quickly.

Learning, practicing, testing, and mentoring should be in your culture to ensure continuous improvement and agility on your press floor.