Material—Is it always a problem?
If the problem with your roll forming operation lies in your material, here are some tips in getting to the heart of the problem.
Have you ever heard the following?
"If we just would purchase the right material, I would have no production problems at all."
"My setup is right there, man! You just have to get me the right material!"
In most cases bad material is the reason a good production run or setup suddenly does not produce the expected results. Right?
Well, not quite. I have to admit that sometimes material is the reason for a sudden production problem. But most material variances can be reduced or even eliminated if you inspect the material, preferably when it arrives at your door. In addition, with continuing improvements at mills and slitting houses, material problems are more and more a thing of the past—that is, if you have a reliable steel supplier that knows your process. If you buy on the open market, you must keep a close eye on the material specifications and the potential quality variances you might encounter.
But let's say you buy the good stuff. Even in this case you need to keep in mind that standard steel specifications permit a variance of at least 10 to 15 points on the Rockwell hardness B scale. In addition, your material specification probably calls out standard slit width tolerances, material thickness, camber, and burr. If you need to deviate from the standards, you probably will face higher costs.
The Ones to Watch
The following are some general pointers for you to start investigating a material problem. Most of these already have been documented, but they seem to be largely forgotten in the daily struggle.
Wall Thickness. Rolls normally are designed for a specific cross section with a maximum wall thickness. An operator adjusts the mill to accommodate the material thickness, thus changing the clearance between the top and bottom roller dies and producing a part with a dimensional variance shifted toward the top roll.
Material that is too thick therefore changes the clearance and changes the cross section's dimensions accordingly. A too-thin material may yield waves in the section.
Mechanical Material Properties. Variances in mechanical material properties introduce changes in springback and forming ratios. As long as the material properties remain within specification (this is, the 10-HRB range), the roll design should accommodate the variance.
Keep in mind that a drastic change in physical properties has a direct effect on the minimum bend radius. Also note that a hardness test does not identify a material's properties; it gives you only an indication of its tensile strength.
Slit Width. Slit width seems to be a problem mainly when you begin producing a new rollformed product or when you begin using a new material supplier. On average, you should be able to obtain a slit width tolerance of ± 0.005 inch without any problem. However, a reduced tolerance range will add to the cost of raw material.
Camber and Burr. Depending on your material supplier and the material specification, camber and burr can, but should not, be a problem. However, camber can have a devastating effect on material feeding, getting a cross section to specification, and the performance of flying cutoff tooling.
A slit burr normally is not very visible in the end product, because the roll forming pressure breaks the burr during cross-section forming. However, this burr can end up on the strip, adhere to the roll, and leave a mark on the product.
This defect sometimes is hard to find if the metal sliver is floating on the strip. Controlling these features is entirely up to the slitting house, so check its work and decide whether you feel comfortable with their quality.
To sum up, specify your material according to the finished product and your process; then carefully select your raw material supplier. By doing this, you eliminate the likelihood that material will be a problem.
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