May 13, 2008
According to OSHA and BLS data, safety in metal manufacturing has been improving. Workplace injuries and injury severity fell from 2000 to 2006. The biggest improvements have been in severe injuries, so minor injuries have gotten more attention lately. One such minor injury is cutting. Roll formers can do their part to prevent workplace cuts by focusing on deburring.
According to data gathered by the Occupational Safety and Health Administration (OSHA) and the Bureau of Labor Statistics (BLS), the steel industry's safety record improves every year, especially in injury severity. Because the incidence of lost-time(severe) injuries has fallen more quickly than that of minor injuries, cuts have become one of the most common work-related injuries. Many of these cuts are caused by burrs.
Burr, an edge defect, is a handling hazard that can contribute to workplace problems. Burrs can hinder the assembly process, scratch finished products, and cut the operators' or customers' hands. This leads to a question: What is the best way to eliminate the burred edge in the slitting process, in the roll forming process, on semifinished parts, and on final products? Following are some common, practical methods.
A hem is a flange that has been bent more than 180 degrees. Hemming is used widely in sheet metal operations. Four types of hems are the flattened hem, open hem, teardrop hem, and rope hem (seeFigure 1). Hems do more than just eliminate sharp edges; they also increase the part's edge stiffness and improve the edges' dimensional accuracy.
Hemming comes with a cost because additional metal is needed to make the hem. The hem size should be 10 times the metal's thickness. It can be very difficult to bend a small hem—the smaller the hem, the stiffer it is. Also, a small hem wears the tooling faster than a larger hem. As the tooling wears, the hem grows. When bending radii are worn, additional metal goes into the hem to maintain the section profile.
Additional forming passes are needed for the hemming method. Four passes (45 degrees, 90 degrees, 135 degrees, and the final shape) are suggested.
Burrs can arise during slitting. Normally this results in a nick (shiny) and a break (dull) area on the slit edge. If the line is straight between the nick and the break 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 (seeFigure 2).
The primary cause of burr is a horizontal knife clearance. An incorrect clearance always results in a poor-quality edge and usually a burr. If the clearance is to loose, the metal stretches around the knife and tears. If the horizontal clearance is too tight, more slitting force is required by the lower knives, which produces a burr. The suggested horizontal clearance (experimental) for low-carbon steel varies from 10 percent of the metal's thickness to 26 percent of the metal's thickness (see Figure 3). Incorrect clearance isn't the only cause of burrs; dull or damaged knife edges can cause them too.
Stationary carbide knives or flat files for smoothing the edges are easy to incorporate into a low-speed operation. For sheet metal, support rolls are needed near the knives or files.
Grinding uses driven side rolls (see Figure 4). Pinch rolls stabilize the sheet metal in the grinding zone. Usually two sets of grinding rolls are necessary for high-speed operations, with one set in use and the other in reserve.
COPRA® FEA roll forming simulation software illustrates the squashing method (seeFigure 5).
Because forming rolls can't bend a small, stiff lip, the rolls squash the lip. The lip width should be about equal to the metal's thickness. This method needs two alternating squashing passes for both the top and bottom edges. It can be used for high-speed operations.
Some processes usually don't produce burrs and therefore don't need burr removal. For example, embossing often results in a dulled edge (seeFigure 6). Embossing tooling uses concave and convex elements to deep-draw the metal. The result is a burr-free edge.
Another way to squash the edges is with idle side rolls. This method is suitable when the edges are in fairly good condition and need only a small amount of work. This is similar to the grinding method shown in Figure 4.
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