Internal stresses within a live section are not consistent, causing problems in secondary operations, like curving or bending. Two pieces that are roll formed by different operators using the same set of tools can look the same, but behave differently. One person adjusted all the forming passes so each pair of rolls did its share of the work, while the other used the last passes to crunch the metal into shape.
Although a rare exception, sometimes the operator cannot obtain a good cross-section without excessive pressure in the last driven passes because of a poor roll design. In other words, the rolls are not capable of forming the product. An experienced operator can determine the capability of a set of roll tooling by setting the roll gaps to do pure bending, and closing them slightly to compensate for the springing apart of the roll spindles under load. The set of tools will have a different capability for each material and material thickness combination that is processed. If the correct shape must be forced into the profile at the end of the forming machine, then part consistency will be difficult to maintain, and will show up in statistical checks or secondary operations.
Many companies that form a variety of shapes and have a choice of machines that can be used often switch the rolls from one machine to another. The operator usually has no input in choosing the machine, because its selection is treated as a production/scheduling problem. However, the person who does the scheduling does not know that forming rolls are designed for a machine with a specific horizontal center distance, and running them at any other distance will affect how well they work. This basic principle explains why operators are heard to say: "This set of rolls works on Machine A, but we have problems when running it on Machine B." The term "horizontal center distance" is the most misunderstood phrase in roll forming. It refers to distance from the centerline of one forming station to the next (see Figure 1).
Does changing the rolls to a machine with a different horizontal center distance mean they will not work? Not necessarily. However, if the horizontal distance is shortened, it is possible that one or more portions of the metal strip will be stretched going from one forming station to another and be permanently wrinkled. The term for this case is "forming too fast."
If the horizontal distance is increased, there is a chance that the profile will spring open between some passes. Wherever a profile opens, the next station must close it back to its original position in addition to doing its share of the forming. It usually is not possible to close the profile to its original position because of localized work hardening of the metal. This phenomenon is called "losing control of the section."
Each metal has a different set of formability characteristics, which includes springback. A set of roll tooling that is designed for a low carbon steel might not work on one of the stainless steels or any metal that has more springback. As a guideline, the only way to determine how much lengthening or shortening the horizontal machine center will affect the performance of a set of forming rolls is to try the rolls at different horizontal distances using the metal in question.
The trend in rollforming is to reduce the thickness of the metal in the part, and maybe change the metal. As a result, the formability of the metal changes. The Purchasing Department orders the new raw stock, and the Sales Department notifies their customers of the availability of an improved product. The Production Department receives notification, so it can plan for a change in volumes for its products.
The operators, however, are not told of the change to the product. Thinner material, with different formability, is brought to the roll forming line, and the operator is expected to produce parts at least as good as the ones they have been making. None of the people involved in making the change is aware that a change in metal thickness affects the performance of the roll tooling.
The design gaugeThe rule is: roll forming tooling is designed and manufactured for one metal thickness, which is the maximum thickness that should be passed through the rolls. That thickness is called the design gauge. For instance, if a set of roll tooling is designed for 14 gauge steel, the actual thickness of the steel can vary from .0741 inch to .0753 inch. The gap between the top and bottom rolls is based on a metal thickness of .0753 inch.
In this example, if any metal thickness greater than .0753 inch is processed, there is a danger of cracking or breaking out some parts of the rolls. At the very least, parts of the metal will be coined, a thinning caused by excessive pressure. Alternately, if thinner metal is roll formed the radii will be larger than that obtained when processing the design thickness.
Isolated thinning of the metal sometimes is necessary. For instance, one way to get sharp corners is to coin the metal along the corner before it is formed. Some products, like ball bearing slides, require a smooth surface in the ball track, and it can be obtained by coining the metal with excessive roll pressure. Note that in these cases the rolls have been designed specifically to coin portions of the metal, and the machine is sized to handle the extra pressure.
When there are problems roll forming an acceptable profile, the tendency is to make roll adjustments at the exit end of the forming machine, which we have seen only make it worse. There are two related rules that address this problem. The first is to look to the front passes on the forming machine when trouble shooting. The other is to adjust all the tools with the straightener guide off the machine. Only when tools are producing the best section possible with roll adjustments is the straightener to be mounted in position. The profile must match the opening in the straightener. If it does not, the rolls should be adjusted until the profile is correct, or the straightener should be replaced. The straightener is designed to take out defects along the length of the part, like bow and twist, and using it to modify the profile is the ultimate in bad practice.
Parts that are symmetrical around their vertical centerline might be bowed coming out of the last pass of rolls, but they should never twist. If there is a twist in them it means the roll tooling should be readjusted, starting from the entry of the rollforming machine. Unsymmetrical profiles usually will show some twisting, which can be removed only with a straightener.
Forming rolls rotate to drive the part in a straight line. While all parts of the piece being formed are traveling at the same speed, different parts of rotating rolls travel at different speeds. This means that only one spot on each roll is traveling the same speed as the part. All other spots on the rolls are moving at different speeds, some faster and some slower than the part exiting the rollformer. Figure 2 showed the metal being gripped by the rolls at the bend point. There is clearance at either side of the center of the bend. In this case, the clearance resulted from processing metal thinner than the design gauge. Most roll manufacturers will purposely grind clearances in the rolls next to the bends so the rolls can slide along the part where there is a speed differential between roll and part. Often, these clearances are put in the rolls when they are initially tested, and do not appear on the design drawings.
Roll clearances are very slight, usually one or two degrees, and are designed to lessen the pressure of the roll rubbing against the metal. Because roll-to-metal contact is not eliminated, it is a good idea to use a roll lubricant whenever possible. Lubricants have many functions, but our concern as operators is to reduce the friction that could mark the surface of the metal, and to eliminate pickup. Pickup occurs when part of the metal or its finish, like paint or galvanized coating, sticks to the rolls. Once pickup starts, it continues to build up on the working surfaces of the rolls and starts to scratch the surface of the metal. Some pickup, like the zinc from galvanized stock, actually cold welds to the rolls, and can be removed only with machining tools.
(Note: this article is excerpted from a technical paper, "What nobody tells you about operating a roll forming machine," delivered at FabTech 2000. The information also came from the "Operator's Guide to Roll Forming" published in 1996 by Ronald Czerski.)