June 8, 2004
It is accepted that, because tube production is a highly competitive industry, many tube producers stay up nights thinking of ways to increase output and improve quality with less labor. Three obvious strategies are to increase mill speed, minimize downtime, and eliminate secondary operations (recutting, finishing, or both). Although the cutoff press has a major role in achieving these goals, it is important to look at the entire mill, and identify the components that influence overall performance.
Producing quality tube is the overriding concern of any tube producer. It doesn't do any good to increase output while sacrificing quality, so a good starting point is an overview of the factors that affect tube quality. Running quality tube requires good material, skilled operators, and a closely aligned and clean mill. Consider the following steps to be the cornerstone of manufacturing quality tube:
A double-cut die set cuts tube in two operations—a partial cut followed by a complete cut—and the finished tube is considered dimple-free.
After verifying that you have the basic foundation in place for producing quality tube, it is necessary to look at output. Reducing downtime and increasing line speed are two approaches to increasing a tube mill's output.
Scheduled Maintenance. After you have determined that your mill is in good condition for producing quality tube, establish a preventive maintenance program to keep it that way. Document failure rates at time of breakdown, purchase spare parts, and replace components before they fail. A thorough maintenance program includes lubrication, oil filter replacement, and weld box maintenance. A well-organized maintenance program minimizes surprise breakdowns and results in greater mill output without an increase in mill speed.
Line Speed. Next, determine what component of the mill is limiting line speed. In most cases, the limiting factors are mill drive power, mill reducer speed, weld power, and the cutoff press. Secondary factors are uncoiler speed, strip accumulator capability, and the tube cooling section throughput. Tube handling and bundling after cutoff can limit a high-speed mill's output.
So you do all of the above and run a high-quality tube. Now we get to the cutoff press. In many cases, this line component is the mill speed's limiting factor and the reason for secondary recut operations. To evaluate the effectiveness of the cutoff press:
1. Look at cutoff press speed, especially when cutting short lengths. Presses have a maximum cycle time, which is limited by maximum number of strokes per minute or, more likely, limited by die accelerator performance. The press stroke time is generally about 20 percent of the total cycle time, which means that the accelerator speed is the major influence on the overall cycle time.
The accelerator must get the die set up to the speed of the tube mill before it makes the cut. For longer cut lengths—that is, for longer lengths of time between cuts—the accelerator has more time to get up to speed to make the cut. As the cut lengths decrease, the accelerator has less time between each cut.
You may want to consider upgrading to a digital controller and larger servomotor to minimize the cut cycle time. Digital controllers provide the additional benefit of a closer cut-length tolerance. The faster press cycle rate allows you to cut shorter lengths and may eliminate secondary recutting because the initial cut is more accurate.
2. Another means of eliminating recutting is using a double-cut die set. This device makes a cross cut in the tube wall at the area of parting blade entry. The resulting cut is considered dimple-free, which eliminates the need for secondary dedimpling, recutting, or both. However, be aware that a double-cut die does not eliminate shearing burrs, so a deburring operation is necessary. This usually can be done continuously directly after the cutoff press.
Bob Jackson is president of Accu-Cut Inc., 64 Privet Lane, Northfield, OH 44067, 330-468-0322, fax 330-468-0318, firstname.lastname@example.org.
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