March 7, 2006
As a consultant to the industry, the author has had the opportunity to discuss coil straightening and leveling with many people. In this article, he reviews 12 common misconceptions he has heard from those people.
Those of us who have worked for equipment builders have presumed that our customers knew and understood our technology. Five years ago I became a consultant in the pay of and loyal to the people to whom we previously sold equipment.
While they know their manufacturing technology very well, their understanding of ours—straightening and leveling—is not always what we thought it was. There is nothing wrong with that; it is as it should be. In retrospect, some of the things we've heard and seen smart people say and do have been rather humorous. Here are a dozen common misunderstandings about coil straightening.
We have heard this many times. Of course, we also hear the opposite: "The supplier says it's flat, and it isn't."
The salesman tells you what his manager said, which is what their operators said. What they might really have meant is "It looks flat" or "It is as flat as we can get it."
In these day of tightening tolerances, manufacturers still are told to send the material back if they don't like it. But that doesn't return your lost time or clients.
Some plants don't have flatness-measuring systems, while others have highly sophisticated flatness-measuring systems. Accurately measuring and describing the degree of flatness of rolled sheets or plates is one thing; producing flat metal is another.
One production plant recoils in the hot condition, so operators cannot measure what the flatness will be at ambient temperature. They know it is good material from experience, but on any given day or for a specific coil, they cannot know for sure.
You should have your own method for checking flatness, such as a flatness table. Then, to put that data to good use, you need to describe how flat is flat enough for your processes. More people are using I-units, which measure wave height and wavelength, to describe flatness.
Getting it right may not even be possible. Some variables might be beyond the supplier's control. Here are several interesting possibilities:
First scenario: Your steel or aluminum coils probably have some coil set simply because they have been coiled. That is a fact of life. The processor might not be able to do anything about it. If the material is dead flat going into the recoiler, then coil set should be the only problem. A simple, non-backed-up spread-center straightener is all you need.
Second scenario: Most mill coils have some crossbow. Slitting also may put alternating up-down crossbow into the cut strands at each male/female knife setup. This, too, is one of the facts of life. In a perfect world, with perfect setups every time, there would be no such crossbow. Perfect slitter setups more often are found in the operator's manual than on the shop floor. The result can be that one slit strand gives you good parts, while the next strand produces too many rejects.
One stamper told me his coils had no crossbow. A short check of his inbound coils with a straight edge proved that wrong. He also said he had too many rejects on about half of his coil strands. It took some patience, but we found the pattern was in every other coil.
Crossbow requires straightening equipment that can stretch the strip surface three times more than is needed to remove coil set, and that takes more vertical force and more horsepower. You will need a supplier with a leveler in its slitting line following the slitter or a backed-up close-center straightener in your press feed line.
Third scenario: Buckles and waves may be a problem with wide coils or sheets. If your coil processor has a leveler in its slitting line, buckles or waves should be under control, unless your stamping or roll forming dies are causing the shape problem. In that case, you will need a leveler in your line, not for flat coils but for shape control and consistent product quality. This requires equipment with controlled work roll bend that also can stretch the metal surface three times more through more vertical force and horsepower.
To say a leveler is an expensive straightener is like saying a pair of binoculars is an expensive hammer. They are two entirely different tools for entirely different purposes. The question should be, What is the right tool for the job?
Truthfully, we seldom actually hear a fabricator put it this way. What we have heard is "We cannot afford better equipment." At the time we were standing next to a 250-ton straight-side press processing millions of dollars' worth of product.
The cost of a leveler is high compared with a spread-center straightener. The cost of a leveler is very high if you don't need it. But it is probably not high relative to the cost of a good press and its die sets. And the cost is low if it can significantly reduce rejects or tinker time.
Here's where the builder and user really speak a different language. The builder's serviceperson generally is a great mechanic who has never run one of these machines in production. He may or may not be a good teacher. He probably never had any formal training in straightener or leveler operation himself.
He probably sees his job as teaching you how to operate the equipment, and you want your operators to understand how to get flat material. These are not the same goals.
In many cases, the builder's start-up technician handles the training. At one shop, the trainer ran only one coil for the line operator, because that's all that was available. At another shop, by the time the builder's technician got the equipment going, there was only about an hour left for actual training time.
One builder recently told us that he doesn't provide operating manuals. His rationale was that if the operators are good, they won't need them.
We hear this a lot. A few months ago I met a man who claimed 38 years of experience running the same two lines. The plant superintendent stood behind me as we watched. After a few minutes and several questions, the superintendent turned to me and whispered in horror, "He hasn't a clue. And he trained every other operator we have."
One senior operator, assigned to train new hires, told me that the machine's gap is adjusted for hot-rolled and the entry gap for cold-rolled. Nonsense.
The same operator said he doesn't use the manually adjustable backup rollers because he doesn't need them. The truth was that they were very hard to adjust and he did not want to do it. Incoming coils for his line were prime, and the application wasn't critical. He simply didn't know how to adjust the machine. The new trainees saw through it and wouldn't work with him.
Another operating manager actually told us, "The factory trained our people when we bought the machine." They bought it 15 years ago. Even if the same operators were still running the line, some ongoing training should have been in place.
One well-respected operation has established a best practices program. Each crew has certain tasks that they do better than other crews. Once a month production crews from all shifts get together to trade ideas. There is no finger-pointing. Managers sit on the sidelines and are there only to support the good ideas and approve process corrections. The improvement in productivity and quality has been astounding.
We heard that comment earlier this year. I said it myself 20 years ago. While it might have been true when all we used was mild steel, now we use yield strengths from 30 kilopounds per square inch (KPSI) to more than 100 KPSI. We process steel and aluminum on the same machine, and they all require very different settings.
Each range of yield strength and material type needs a different gap setting to produce dead-flat and stable material. In this day of improved productivity and reduced scrap loss, we no longer can rely on trial-and-error methods. We need to get different materials flat the first time.
Straightener and leveler operators should have computer-generated entry and exit roll gap setting charts for each range of yield strength and each material, steel or aluminum. Most machine builders can supply them.
We just heard this again, this time from a plant superintendent. Certainly the coils came out flat, as the operator had pointed out.
However, the operator told us that the material got wavy if he worked it harder. Something looked odd about the operation—the machine's work roll gap settings were so light that the operator wasn't stretching the material past its yield strength. In fact, the work rolls barely touched the metal.
The good strip coming out of the machine was the same shape as when it was going in. If he closed the roll gap, the material became out-of-flat because he did not know how to run the machine.
Does it matter? Yes. The purchasing manager told us he was having trouble buying steel that their equipment could flatten. I'm sure that cost them money. They had the equipment to run material of less-than-perfect quality, but the operator didn't know how to.
Straightening and leveling machines are very sensitive to their upper capacity limits. One plant had been leveling 3/8-inch hot-rolled plate on a machine built for 1/4-in. material. The frames were sprung out of line and the machine had to be scrapped. "But we got the plate flat!" according to the senior operator.
At another plant, operators complained that they could not remove edge wave from their coils. They were overloading a non-backed-up straightener never designed for wave or buckle control. Their unsupported straightener rolls were deflecting, and they were actually putting edge wave into the material.
One plant had been running 3/8-in. aluminum on a machine built for 5/16-in. material with no damage to the machine. The operator wasn't closing the entry roll gap enough to level the plate. Had he done so, the machine would have been overloaded and something would have bent or broken.
When asked about machine capacity, some supervisors tell production control, "Ask old Charlie to see if he can run that thickness." What happens if he can't? Presumably he breaks a shaft or bends a roll pin. It's better to check the manufacturer's data if capacity limits are the issue.
When everything was that mythical "mild steel" at 60-KPSI tensile and 40- KPSI yield, we could build those numbers into our settings. Now we have increasingly higher ranges of strengths. We see light steel plates with 180-KPSI yield, and 80 to 90 KPSI is easy to find. The settings and thickness capacities are not the same at all.
If you run prime material, the yield strength should be on the mill test reports.
One straightener manufacturer explained that by putting football-shaped work rolls in the straightener, he could stretch the center of the metal. He said he could stretch the edges by overloading the entry roll gap and bending the work rolls.
Of course, doing this will throw all the bearings and drive out of line. He seems to have forgotten that roll gap adjustment also is critical for proper coil set and crossbow control. Precise control would be impossible. We need the right tools for the job, and this idea is a poor substitute.
This person is doing exactly the opposite of what he should be doing. He needs to stretch the short part (the part between the "hills") of the sheet or coil.
Within months we heard the same incorrect explanation from operators in two other plants on opposite ends of the continent. This misconception may be more common than we thought.
If you've been in this business awhile, some of these stories will be familiar. No doubt, you could add a few. If some hit a chord, have a laugh and fix it tomorrow.
If there is a pattern, it is in the old crutch, "I didn't look at it closely because we have always done it that way!"
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