June 1, 2008
Looking at the results of a process—in this case, the strip produced in the die—certainly can lead to good data indicating the root cause of a problem. This Part II of a two-part series discusses hard marks, poor die design, and coil-related problems.
Editor's Note: This is the second part of a two-part article about reading progressive-die strips. Part I covers the causes of pilot hole distortion and mismatched cuts.
Hard marks, otherwise known as coin marks, occur when metal is severely squeezed between two mating die surfaces. Depending on your part requirements, these marks may or may not be a cosmetic blemish.
If the part is required to have a Class A or exposed cosmetic surface, a hard mark is a serious blemish. Cosmetic part or not, hard marks can tell a great deal about the die's timing, as well as the adjustment of the press's shut height.
Some hard marks are easily visible as areas on the metal that are shiny on both sides of the part (see Figure 1). To find those that are not easily visible sometimes requires painting both sides of the strip or part with special water-soluble ink commonly referred to as spotting ink. This ink, usually blue or red in color, is kind of like paint that never dries.
After painting the strip, place it back into the die and cycle the press. After you strike the part in the die, look for areas on the strip where the ink has been squeezed out on both sides of the metal in the same area. Wherever the ink is not present on both sides of the metal, coining has taken place.
I strongly recommend that you keep the last strip that was made in the die to serve as the baseline for comparison. Look for hard marks on the strips and compare them to each other.
For example, let's say that your quality control department has rejected a part because the bend angle on a weld flange is not correct—it is open 1 degree. Before adjusting the die to create more bending action, take a look at the strip and see if any distinct hard marks are present in one strip that aren't in the other. Pay very close attention to the bend radius.
Many toolmakers use the coining method to obtain a 90-degree bend angle. In those cases, the metal must be coined between the lower die and upper forming section. Doing so requires the shut height of the press to be precisely adjusted. If you can see a hard mark in the reference strip but not in the problem strip, chances are that something needs to be adjusted.
Before shimming or grinding the die sections, remember that there are many possible causes for the missing necessary coin mark:
1. The press is not adjusted to the proper shut height. Make sure the ram of the press is coming down the proper distance. Perform lead check readings on your stop blocks. Unless you have a new press with an ultraprecise shut height indicator, avoid using the counter on the press to determine the shut height.
2. The ram of the press is not coming down parallel. If you can see hard marks on one end of the strip but not on the opposite end, the press ram may have a parallelism problem. Check the press ram for parallelism.
3. Some of the die sections have been ground and incorrectly shimmed or timed. This is a common problem. The timing of a progressive die is critical. Any time you grind a forming section, keep a precise record of the amount that was ground off and the amount that the section was shimmed.
Often die sections get shimmed in an effort to solve one problem, and a different problem results. For example, assume a die maintenance person overshims the draw punch of station No. 3 in a progressive die to coin the top surface of the part flat. While he likely will improve part flatness, he might also prevent the station that is bending the part from coining the radius, thus resulting in an open bend angle. In such a case, he might need to lower the draw punch slightly so that it is still coining the top of the part, but not so much that it keeps the die from fully closing.
4. Debris in the die. This one is usually quite obvious. Look for coin marks that look like slivers or slugs. Even a small sliver can keep a tool from fully closing.
Galling is the abrasive or adhesive breakdown of metal cutting and forming sections. It is the result of intense friction between the sheet metal being cut and formed and the tool steel that is doing the cutting and forming.
Scoring is the result of galling (see Figure 2). Tooling galls, and parts get scored. Take the time to examine your strip carefully for signs of galling. Galled sections may need to be polished or replaced. Galling in a tool can result not only in cosmetic product defects, but also in poor metal flow, higher punch stripping forces, and an array of other problems.
Literally hundreds of problems can be identified and solved by analyzing a progressive-die strip. Take the time to study the strip carefully; look not only for hard marks and distorted pilot holes, but also for imperfections of any size that are not present in the reference strip.
Be aware of not only the effects of failure, but also the causes. Something as simple as a galled forming section can be the difference between success and failure.
Until next time Best of luck!
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