September 12, 2006
Stampers and roll formers can look at edge trim, heads, tails, skeletons, and coil damage for ways they can reduce scrap to hold the line on metal costs.
|Minimizing coil processing scrap maximizes yield.|
Nearly everyone who processes metal—aluminum, carbon, or stainless steel—is concerned about scrap loss. Scrap loss in coil processing operations, such as slitting, cut-to-length, and stamping, often is overlooked. Finding ways to minimize scrap loss in coil processing operations will maximize your yield and improve your bottom line. In many cases, automating scrap handling will also improve efficiency and promote a safer work environment.
In coil processing operations the first and last 20 feet or so of material—sometimes called the head and the tail of the coil—often is thrown away. This might be required because the material thickness is off-gauge, or because the coil ends are not square. These imperfections are caused at the rolling mill that produced the coil, and there is not much you, the user, can do about it.
Another reason the heads and tails of a coil are discarded is that they often are damaged during handling. This damage may appear on the outside diameter, the inside diameter, or on the edges of the coil.
To prevent damage on the outside diameter, store the coil on a smooth, clean surface. If the material is thin or soft, store it on a surface that is wider than the coil itself so the edges do not hang over (see Figure 1). To avoid damage on the inside diameter and edges, take care with the C hook or other lifting device so it doesn't bang into the coil. Sometimes you can add a polyurethane cover to the lifting device to reduce its potential to damage the coil.
Thin, soft coils should be supported on a surface
that is wider than the coil itself so the edges do not hang over.
You can cut off head and tail scrap and remove them manually with a tool known as a nibbler, but modern coil processing lines usually are equipped with a guillotine shear. The scrap is cut into lengths of approximately 2 ft. and dropped into a scrap cart for removal. Powered, self-dumping scrap carts can be used to automate head and tail scrap handling and should be considered for use on heavy-gauge machines.
In many coil processing operations and in nearly all slitting lines, the coil sides are trimmed off and discarded (see Figure 2). Trimming is necessary because the width variation of the coil coming from the mill usually is greater than the width tolerance of the finished product. The goal is to waste as little as possible while still meeting the width tolerance required.
Side trim scrap is removed from both sides of the coil with a rotary side trimmer or a packed arbor slitter. The scrap strands can be processed with scrap winders, scrap ballers, or scrap choppers.
Scrap winders pull each strand under tension. They should always be installed in pairs.
Scrap ballers are suitable for light gauges. When scrap ballers are used with an accumulation pit, they eliminate threading and reduce scrap breakage.
Side trim is removed to ensure accurate
width on finished parts.
Self-threading scrap chutes can
automate scrap handling.
Scrap choppers feature automatic, self-threading entry chutes that can reduce downtime and improve safety conditions (see Figure 3).
The amount of side trim can be substantial because it is removed from the entire coil. The actual amount that needs to be removed depends on width variation in the master coil and resolution of the edge-guiding system used in the machine. Newer guiding systems feature photoelectric edge sensors and proportional valves that automatically position the uncoiler and allow trim widths as narrow as 1/8 inch per strand.
Defective material is another source of scrap in coil processing operations. The rolling mill can cause shape defects such as coil set, crossbow, edge wave, and center buckle.
Coil set and crossbow are simple shape problems that you usually can correct with a flattener that has large-diameter, widely spaced rolls (see Figure 4).
Flatteners remove coil set and crossbow.
Roller levelers can remove edge wave and center buckles.
Edge wave and center buckle are more complex shape problems. You can correct them with a roller leveler that features small, closely spaced rolls that bend and stretch the material using narrow, adjustable backup rollers arranged in flights (see Figure 5).
Material defects such as scratches, dings, and dents can and should be eliminated. Your first step is to identify the source of the problem. If the defect is intermittent but consistently spaced, it likely was caused by a defective roll. You can determine which roll caused the defect by dividing the spacing of the defect by 3.142 (p). Once you know the roll diameter, you can determine if the defect is being caused by your line or by the rolling mill, and appropriate measures can be taken to solve the problem.
If the defect appears to be a continuous scratch or scuff, a carryover table probably caused the damage. If you inspect the line closely while the strip is threaded and under tension, you will usually be able to see the problem's source. You can then either lower the carryover table or cover it with Micarta®, polyurethane, or other plastics.
Overlooking the potential to reduce scrap in coil processing lines is an avoidable mistake taken at your own risk because reducing scrap loss and automating scrap handling can improve profit margins, increase production, and promote a safer work environment.
Ken Shoop is southeast regional manager for Braner USA Inc., 9301 W. Bernice St., Schiller Park, IL 60176, 847-671-6210, firstname.lastname@example.org, www.branerusa.com
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