Revealing the Magic — Using data and conducting experiments to solve metal forming problems

By Art Hedrick

December 14, 2008

Effective stamping professionals rely on scientific principles and not magic to determine and correct production problems. Find out how to troubleshoot wrinkles and rips and take corrective action by following a basic procedure that can be modified to troubleshoot almost any stamping issue.

If you've ever attended one of my die troubleshooting clinics or seminars, chances are you heard me vent my frustration about how the United States views the tool and die industry. All too often, the process of designing, building, and troubleshooting a die is portrayed as an art form. This portrayal elevates my blood pressure. I hope you are not making die-related decisions based on an inspiration you had on your way to work, but are making these decisions based on the laws of physics.

Besides hearing me vent my frustration regarding this misperception, chances are attendees saw a little magic show during the training session. I perform the magic demonstration for a couple of reasons. First, tooling professionals often are viewed as magicians. A problem occurs; we are asked to go to the press and work our magic to make it go away. Also, magic makes the training session fun. I believe we learn more when we enjoy the training. Following the magic, I usually reveal how the simpler tricks were done. Why? To demonstrate that there is no such thing as magic, and despite what you think you see, it's nothing more than simple physics. Once you understand the physics, the magic goes away. Die troubleshooting is a physics-based science.

Science Versus Voodoo

Dr. Edward Deming, the father of statistical process control, once said "In God we trust; everybody else, bring data." That's good advice.

To troubleshoot a stamping operation successfully, you must approach the operation scientifically and lose the black-art, trial-and-error techniques that have plagued the tooling industries for centuries. To do this, you must use a systematic process based on data to help solve problems, and you must have a good understanding of physics.

In our industry, assumptions can lead to disaster. Don't change tooling permanently simply because that's the way it worked on a previous job. Find out why the technique worked. Collect data to help you arrive at the root cause. There is nothing wrong with tricks of the trade as long as you understand why they work. Take the time to understand your material's unique behavior. What works for steel may not work for aluminum.

Is there such a term or profession as a dieologist? No, but there should be. I define a dieologist as someone who solves sheet metal stamping die problems through a systematic process of collecting data, using applied physics, and conducting data-based experimentation.

In other words, approach die-related problems as a dieologist—not a black-art specialist.

Matrix

Step 1—Identify the Problem with the Part. Obviously, this is one of the easiest steps in the process. Some questions to ask are:

• Is the part wrinkling, buckling, or oil canning (signs of loose metal gathering)?
• Is it tearing or necking (signs of excessive stretching)?
• Is it dimensionally incorrect?

Step 2—Check Process Parameters to See if They Comply with the Setup "Recipe" or Control Plan.

Every die/press/feeder should have documented guidelines for setup. Before making any changes in the tooling, make sure that the die/tooling is set up correctly and that all of the incoming variables are correct. Some questions to ask are:

• Is the incoming sheet material correct? Don't make assumptions.
• Is the material thickness correct?
• Is the blank/strip size correct?
• Are the mechanical properties correct?
• Is the lubricant correct? Mixed properly? Applied correctly?
• Is the nitrogen or cushion pressure set to the recipe?
• Is the progression or pitch set correctly? (Transfer or progressive dies only)
• Are the gauges fitting the blank correctly?
• Are there excessive burrs on the blank?
• Is the blank being loaded correctly?
• Is the press shut height set correctly?
• Is the die placed in the proper location?
• Is the tooling free from debris?
• Is the die setting flat on the bolster plate and ram?
• Do the tonnage monitors read the same as the recipe says they should?
• Is the counterbalance pressure set correctly?

Step 3—Reassess and Qualify Setup. Simply verify setup parameters. If parameters are OK, proceed to Step 4. If process parameters are not correct, readjust and qualify to recipe.

Step 4—Look for Interactive/interrelated Problems. Physically examine the defective product. Some questions to ask are:

• Are rips causing wrinkles? When metal fractures in tension, the plastic flow of "free" material can create wrinkles.
• Are wrinkles causing rips? When pulled over the die entry radius and into the wall of the draw cavity, a wrinkle is forced to unwrinkle. This attempt to unwrinkle the metal may cause a tremendous amount of tensional force elsewhere in the drawn product, which leads to tearing.
• Is galling present? Galling can easily restrict metal flow and cause excessive metal stretching.

Step 5—Prioritize Work. Decide the basic item to be addressed first. If rips are causing wrinkles, address the rip first, and the wrinkle most likely will go away. If wrinkles are causing rips, address the wrinkle, and the rip most likely will disappear.

Step 6—Determine Corrective Action through Experimentation. Remember, you must act as a scientist. Scientists conduct experiments and derive solutions based on the result of their experimentation. Also, keep in mind that at this point, you still haven't made any permanent changes to the tooling. Permanent changes do not take place until you have completed your experimentation.

The following experiments are for data collection only. By conducting them, you can determine where in the die you may have to make permanent changes. You also can reasonably approximate how close you are to failure. If the following experiments do not yield concrete conclusions, they at least will enable you to come up with good hypotheses.

Wrinkles—too much metal present in a given area.

• Conduct circle grid analysis. This will help you determine the strain direction and the direction the metal must be stretched to remove the wrinkle.
• Increase the blank size in any given area. This will increase the surface area of material held between the die face and blank holder, resulting in a higher retraining force similar to that of a draw bead.
• Use grit cloth on areas of the blank. Placing grit cloth or sandpaper between the sheet metal and the blank holder also simulates the effect of a draw bead. Do not use a grit cloth on coated dies.
• Remove lubrication from blank holder, die face, and blank. This increases friction.
• Increase the blank holder force. This also increases friction.

Rips or tears—insufficient metal present in fractured area.

• Conduct circle grid analysis. This tells you the metal failure deformation mode and strain direction. Fractures often can be eliminated by stretching the metal in the opposite direction as the strain. Solving a wrinkling problem also can help you better understand how the metal reacts. Remember, to solve problems like rips and wrinkles, you must think like metal.
• Decrease the blank size in any given area. This decreases the surface area of material between the blank holder and the die face and causes a more inward material flow.
• Use polyfilm, thin-film barrier lubricants, soap, or wax to reduce friction and increase metal flow. Apply these by hand in specific areas of the blank to reduce friction.
• Reduce blank holder pressure. This reduces friction and may increase metal flow.
• Increase equalizer gap. This also reduces friction and may increase metal flow.

Step 7—Implement Permanent Corrective Action. In this step, you make changes to the tooling and/or process.

Solving for wrinkles. You must now find a permanent way to increase friction in a given area. Permanent solutions may be:

• Add a draw bead. This increases friction by forcing the metal to bend and unbend before entering the draw cavity.
• Increase the blank holder force. This increases friction and reduces metal flow into the draw cavity.
• Decrease the die entry radius. This increases friction and reduces metal flow into the draw cavity by forcing it to bend and unbend over a smaller radius.

Solving for rips or tears. Permanent solutions may be:

• Increase the die-entry radius. This decreases friction and reduces metal flow into the draw cavity by forcing it to bend and unbend over a larger radius.
• Reduce the blank holder force. This reduces friction and increases metal flow into the draw cavity.
• Change the lubricant. This reduces friction and increases metal flow into the draw cavity.
• Decrease blank size/shape. Make sure that you still have enough metal for the finished product and that you don't cause other problems such as wrinkling or poor part control.

Step 8—Fine-Tune Your Corrective Action. In this step, make fine adjustments to your process changes, If you are using circle-grid analysis, verify and fine-tune it to be robust.

Step 9—Adjust Control Plan and Recipe.

Document any changes that you have made to the setup process, recipe, or control plan. This helps prevent setting up the job to outdated parameters in the future. (QS-9000 people love this step.)

Troubleshooting matrices won't reveal solutions for all drawing problems, but they may give you an idea of the science that some perceive as a black art. Meet with your toolroom professionals to discuss—as a team—developing a data-based troubleshooting process. Fit it to your company's resources and time constraints. Remember that people will support a world that they are allowed to create.

Above all, use data to make your decisions. Lose the magic.