Flexibility with smart flying shear upgrades

Tube Cutoff System

Flexibility is key to profitable tube mill operations in this economic downturn. While once upon a time small-diameter low-carbon steel tubing produced in long lengths was the main profit path, suddenly the game has changed. Now all types of steel tubing, from 40- to 110-KSI-tensile strength and cut-to-length, must be supplied as needed. Smart flying shear upgrades can help provide the flexibility necessary to improve a tubemaking facility's bottom line.

An Efficient, Continuous Operation

A well-managed tube mill operation is inherently efficient. This is because tube mills, by design, can be operated continuously if all of the components are properly installed, maintained, set up, and operated in accordance with the machine builder's instructions.

Carefully arranged slit coil storage and a clear, safe path to the coil processing area of a tube mill entry section facilitate continuous operation.

Coil ends are butt welded automatically while a strip accumulator allows the tube mill to operate without interruption.

Forming, welding, and sizing rolls drive the strip longitudinally through the tube mill, progressively developing the flat strip into a finished round or profiled tube without interruption. The rolls are powered by multiple electric motors with controls that automatically maintain the appropriate strip speed for each motor drive section of the tube mill.

Longitudinal tube welding is achieved with a variety of technologies including high-frequency, laser, and gas tungsten arc welding (GTAW). The nature of these welding technologies and their controls provide automatic, continuous operation of the tube mill.

Inspection systems integral to the tube mill operation can monitor strip width, strip thickness, tube surface integrity, weld quality, and tube size. In some cases, control outputs from the inspection systems trigger paint sprays to identify the fault areas. Some inspection systems are closed-loop and activate automatic fault correction. These systems also can activate flying shear scrap cut controls that limit the length and position/placement of scrap tube, thereby reducing scrap generation while maintaining efficient, continuous tube mill operation.

Tooling

Tooling for all tube mill components is designed by the machine builders to meet the overall tube mill specifications. Tube mill rolls, for example, are designed to deliver tube to the specified OD tolerance with smooth, mark-free surfaces. When maintained and operated according to the tube mill supplier's specifications, tools typically process tens of millions of feet of tube between regrinds. Rolls need to be reground in progression per the OEM design to maintain trouble-free operation of the tube mill, specified tube size, and good surface quality. Close attention to correct maintenance, setup, and operation of the tube mill roll tooling ensures reliable, continuous operation.

Flying Shear Component

The flying shear component of a tube mill presents a completely different set of challenges to a tube mill's continuous operation. With this equipment, a tube severing die set moves in an interrupted fashion from where it is first accelerated from a home position to match the tube speed; then the die set is operated, severing the tube to a preset length; finally, the die set direction is reversed, driving it home while the cut length continues to move out of the flying shear and is discharged for further processing.

Two Case Studies

It is at this point in the tubemaking process—the flying shear operation—where flexibility can be enhanced. Here are two examples:

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2. A tube mill operation produces tubing up to 3 in. OD and up to 12-gauge wall thickness in 20-ft. lengths for offline recutting. Small batches of tube 6 ft. and longer are cut-to-length at the tube mill from time to time. The flying shear is limited to 50 cuts per minute. Length accuracy varies from +/- 1/32 in. to +/- 1/16 in.

   For this tube mill, new business is available for 150,000 lengths weekly of small-diameter, light-wall, cut-to-length tubing from 30 to 48 in. These products could be processed at the tube mill, but current cuts per minute and cut length accuracy limitations prohibit it.

   The existing flying shear mainframe has been well-maintained, and, as such, it is suitable for increased performance. However, the mainframe clutch, die set accelerator, and controls limit production and length accuracy.

   A new flying shear is not an option because of cost and delivery timing. The following upgrades can bring a new level of reliable productivity and tube production flexibility:

• Replacing the pneumatic clutch brake with a hydraulic brake can make up to 75 cuts per minute possible with no interruptions to production from the clutch overheating and other issues related to the old clutch brake (Figure 1 and Figure 2).
  
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• Replacing the old hydraulic cylinder drive with a heavy-duty rack-and-pinion die set drive makes around-the-clock, continuous production of up to 75 cuts per minute possible. (Figure 3)
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• Replacing the old dedicated system controls with new high-cycle flying shear controls, which accept offsets from other areas of the tube mill operation such as inspection and quality control components, makes it possible to minimize scrap cut length and discharge position without interrupting operation.

With these upgrades, the tube mill has the flexibility to produce cut-to-length tubing 30 in. and longer at up to 75 cuts per minute with +/- .032-in. length accuracy on demand. This new flying shear capacity makes flexible scheduling possible whereby tubular products can be produced efficiently by making use of reliable online and offline tube cutoff resources.

2. A tube operation makes tubing up to 4 in. OD up to 7-gauge wall thickness in up to 40-ft. lengths. Most of the tube in this operation is made from mild steel up to 60-KSI tensile strength. It usually is shipped in 20- or-40 ft. lengths, with some products cut-to-length down to 10 ft. long.

   New business is possible for this tube mill for high-tensile steel tubing. Lengths are from 6 to 22 ft. A quick check with the tube mill builder and production trials with the high-tensile material prove the tube mill is capable of running the material without difficulty.

   The flying shear cutoff, however, stalls in spite of extraordinary machine and tooling setup effort. The stalling problem is attributed to insufficient cutting force inherent in the mainframe direct-drive design. In an attempt to increase cutting force, the operator increased the direct-drive RPM, which increases shear blade speed. This causes severe tooling pickup, further affecting the stall condition. This flying shear recently was fitted with state-of-the-art controls.

   A new flying shear system is out of the question because of the unavailability of capital and delivery timing.

• Since the die set accelerator and controls are in good condition and suitable for the application, they are set aside for use with the mainframe upgrade.
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• The existing mainframe is replaced with a new back-geared mainframe fitted with a variable-speed ram drive that is designed to deliver the force necessary to cut high-tensile tube with sufficient die travel at very low shear blade speeds. Automatic blade lubrication and other features are included with the new mainframe. (Figure 4)
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• The existing die set is fitted to the new mainframe.

With this upgrade, high-tensile materials can be cut efficiently with no stalling problems, which makes continuous tube mill operation possible. The shear blade speed can be adjusted to maintain reasonable tooling life and excellent cut quality. This upgrade provides the flexibility to cut a wide range of tube materials with tensile strengths from about 40 to 110 KSI.

As these examples show, reliable flexibility in tube production, a key component of profits in this slow economy, can be achieved using high-quality, properly engineered, smart flying shear upgrades.