How to avoid slit-in coil slitting problems

Good coil slitting tooling and practices result in good edge quality.

Many problems can arise during metal slitting. These include poor edge quality, edge burr, edge wave, camber, crossbow, knife marks, and slit width that is out of specification.

When slitting problems arise, the edge is the first place to inspect.

How can you recognize a good edge? It's not as hard as it sounds. Look at the edge! On most common slit thicknesses, a 30x microscope is ideal for this purpose. Normally there is a nick (shiny) and a break (dull) area on the edge. If the line is straight between the nick and the break and the fracture is clean, it is a good edge (see Figure 1). If the line is uneven or the break is rough, it is a bad edge.

Some slitting problems can be attributed to poor metal quality; however, it is much more common for the problems to be caused by other variables and factors during the slitting process. Slitting encompasses many variables, and most of them can be controlled. To achieve a good slitting result, each variable that can be controlled must be controlled.

Figure 1 If the line is straight between the nick (shiny) of an edge and the break (dull area) and the fracture is clean, it is a good edge. If the line is uneven or the break is rough, it is a bad edge, caused by improperly set cutting knives.

Slitting has been thought of as an art, but it is increasingly becoming recognized as a science. Because of advances in equipment, tooling, and software, it is now possible to control the variables and to improve the slitting process to achieve results never before possible.

Controllable Variables

Knife Clearance. Horizontal knife clearance is the most significant of these variables (see Figure 2). In the "old days," the clearance was almost always set to 10 percent of the metal thickness. For example, a 0.008-inch clearance was used to slit 0.080-in.-thick metal. There are a few metal thicknesses and types on which the 10 percent rule happens to work; however, 10 percent is rarely the correct clearance.

The thinner the metal, the harder it is to achieve the correct clearance. Conversely, on thicker metal, the old rules are even less applicable. The newer types of slit metals were designed for specific purposes and have vastly different mechanical properties than the older, low-tech metals. This is true whether you are slitting steel, aluminum, copper, brass, a composite, or any newly engineered metal. Each metal has an optimal clearance, and even the same metal from a different source or thickness might (and usually does) require a different clearance.

Excessive vertical knife clearance, or overlap, may be the culprit in a multitude of strip defects, including edge wave, crossbow, camber, and deep knife marks. It also can damage tooling, such as the knife and stripper rings, and can even wear out the slitter.

Figure 2 Horizontal knife clearance is the most significant coil slitting variable you can adjust to improve slit edge quality.

How do you know which clearance is correct? The answer is simple! The one that gives you the best result. That may sound trite, but slitting—and shearing, for that matter—is a very complicated process. There are no published formulas or equations that work in every case—or even in most cases. Therefore, it is essential to look at the results, and then adjust as necessary.

Tooling. Tooling is a big part of the equation. It will either contribute to a good setup or cause a bad one. When the tooling is manufactured and cared for properly, not only can it create good edges, but also keep knives sharper longer. If the tooling is in poor condition or not manufactured to the correct specification and/or plastic shims are used, it is very hard—if not impossible—to get accurate clearances between the knives.

Operators become very skilled at "fixing" the setup to get it to run. However, if the setup did not need "fixing," they would be a lot further ahead. So much time is wasted fixing setups. It is always better and far less costly to do it right the first time.

If you want good edges, have the right tooling, keep it in good shape, and use it correctly. When spacers are damaged (nicks and dings), they will not seat properly. This will cause inaccurate horizontal clearances. In addition, the knives might wobble, causing clearance variations, which will result in an edge that is either of poor quality or of uneven quality.

Figure 3 Damaged tooling can cause the knives to clash, producing chips and nicks on the knife's cutting edges. This, in turn, causes poor-quality coil strip edges.

Damaged tooling can cause the knives to clash, producing chips and nicks on the knife's cutting edges (see Figure 3). Tooling that is not manufactured to the correct tolerances for the application will act the same as damaged tooling. Shimless tooling means no shims are required, so if shims must be used, something is wrong. Shimless tooling has been available for more than 20 years, so some of it may need to be replaced.

In addition, lightweight spacers can help reduce operator fatigue and errors (see Figure 4).

Quality tooling ensures good setups, controlled edges, and long knife life. If you encounter slitting problems, and you suspect your tooling is the cause, check it for damage or have it inspected by a reputable manufacturer.

Machine. Still, the best tooling will not be a panacea. If the machine has not been maintained, the bearings are loose, or the shoulders are damaged, you still will get poor results. Training and inspection by an expert can help.

Burrs. Burr is an edge defect that can arise during slitting. The primary cause of burr is a horizontal knife clearance that is too tight or too loose. An incorrect clearance automatically causes a poor-quality edge. In fact, one of the characteristics of a poor-quality edge is burr.

If horizontal clearance is too loose, the metal gets stretched around the knife and tears.

Figure 4 Lightweight spacers such as these are designed to be easier than solid metal spacers to lift and handle during setup.

If horizontal clearance is too tight, a lot more force is required to cut the material. The only way you can apply more force is to lower the knives, and this produces burr.

In addition, a dull knife automatically creates burrs. The material wraps around the dull corner of the knife and tears. Therefore, it is important to use the right knife for your slitting needs. A knife that dulls too quickly will cause burring. On the other hand, if the knife chips because it is too brittle, you will need to redo the setup and you will lose time. It is preferable to use the most wear-resistant knife possible so that the edge remains sharp for the longest period of time. The less control you have on the setup and machine, the tougher the knife has to be.

A vertical knife clearance that is set too deep (too much overlap) also causes burr.

Edge Wave. Although edge wave can be caused by stresses in the metal, most edge wave is "slit-in." Too much vertical clearance, or overlap, causes edge wave. Other contributing factors are poor stripper ring practice. If the stripper ring ODs are too small, you will tend to lower the knives to overcome slippage. If the rings are too big, the metal will become stretched at the edges. If they are out of parallel, this also will contribute to edge wave.

Knife Marks. Knife marks on the strip almost always are caused by poor stripper ring practices. In the old days the stripper rings were all the same size and hardness. Even today some operators wrap tape around knives so the noncutting edge will not mark the strip. If you do this, please stop! It is dangerous to wrap tape on the knives; it also contributes to poor edge condition, burrs, and edge wave. Proper stripper ring practices include utilizing male and female rings in different sizes, colors, and hardnesses. Size and hardness are not absolute constants and may vary from machine to machine. The size should vary from one metal and thickness to another.

Camber. Camber can result from stresses in the master coil. If the camber is always in one direction, a good way to check if the camber is caused by material defects is to slit the master coil upside down. If the camber is in the opposite direction, the camber has been caused by stresses in the master coil. If not, the camber is not from the virgin metal; it has been slit in.

Slit-in camber happens when narrow strips are slit and the horizontal clearance is different from one edge to the other. The edges of a slit-in cambered strip are different from side to side. For example, one edge might be good but the other too tight.

Crossbow. Like camber, crossbow might be caused by a defect in the master coil or from the slitting process. Usually crossbow is caused by too much overlap (vertical clearance) or by stripper rings that are the wrong size. If the male rings are too big or the female rings are too small, crossbow may result.

Slit Width. Slit width can be controlled. Some variables that cause slit width to be out of specification are improper knife clearance, incorrect or worn tooling, a machine that is not maintained, and poor stripper ring practices. If the horizontal clearance is not right, the width will change. If the stripper rings are not being used correctly, the width will change. Different metals react differently to being slit.

In general, if you are trying to slit a 12-in.-wide mult, you must set a space wider than 12 in. between the female knives. How much wider depends on material thickness, slit width, the type of metal being slit, and its physical characteristics. Width variation will tend to be more pronounced on heavy-gauge metal than on light-gauge metal. In addition, arbor deflection, knife deflection, setup accuracy, stripper ring practice, and the condition of the machinery all affect the width variation. In today's demanding manufacturing world, it is not unusual to find width tolerances within 0.001 in.

Finally, each machine, metal type, and tooling set might follow different rules. Therefore, you should record the results of each slitting operation and continuously improve by monitoring the results.

The more the tooling manufacturer knows about your operation, the better equipped it will be to help you pick the proper knife metal grade and advise you on the optimal knife clearances. If you don't understand good stripper ring practice, it will be difficult to set up the machine correctly. There are literally hundreds of scenarios for correct stripper ring practice, so it is also important to consult with your supplier on the proper application. It helps to have training on proper coil slitting practices, as one scenario will not work for every setup.

Al Zelt is director, sales and marketing, ASKO Inc., American Shear Knife Division,501 W. Seventh Ave., Homestead, PA 15120, 412-461-4110, fax 412-461-5400, info@askoinc.com, www.askoinc.com. He is also on the Fabricators & Manufacturers Association, Intl., Coil Processing Council.