The process of contoured tubing manufacturing

How tubemakers are meeting demands

Practical Welding Today January/February 2001
February 28, 2002
By: Werner Wasmer

This article discusses the importance of new technology in the roll forming industry, especially in welded contoured tubing. The author discusses the process of welding contoured tubing all the way through to the end product.

Figure 1:
Contoured tubing requires more forming passes than conventional tubing. The more complex the shape of the tube, the more forming passes required.

Sheet metal part users demand manufacturers to provide higher consistency, lighter weight, and better corrosion resistance. This leads nearly all producers of roll formed parts and contoured tube to face increasing material costs. Very rarely can they count on greater profits.

New manufacturing processes are paving the way for cost savings. Hollow or tubular sections with a suitable shape—a custom shape that is application-dependent—achieve much higher consistency in strength than conventional open profiles. The stiffness against torsion, for example, can be up to 1,000 times higher. This allows for a considerable reduction of strip material thickness and, consequently, material cost savings.

Manufacturing welded contoured tubing with difficult sections also is becoming more important to the roll forming industry, challenging the knowledge, experience, and innovation of manufacturers. Difficult sections require more than 30 forming stations to form a part, as well as high-quality and high-integrity welds.

Contoured tubes are odd-shaped tubes, rather than round, square, or rectangular. The characteristics of contoured tubing—material thickness, part geometry, and overall quality—can be varied and therefore adjusted to the component requirements.

Figure 2:
For contoured tubing, up to 30 forming passes can be used. However, tube welding systems with up to 50 forming passes have been built.

Various punching, notching, or embossing operations can be performed on contoured tubing inline, or before or after forming. Modern contoured tube welding systems can produce products from all standard materials, including the following (see Lead-in photo):

1. Fine-grained steel.

2. Cold-rolled steel.

3. Microcomposition metal.

4. Aluminized steel (steel coated with an aluminum-iron alloy).

5. Hot-dip galvanized steel.

6. Electrogalvanized steel.

7. Stainless steel.

Roll Forming of Contoured Tubing

Because of their complex shapes, contoured tubing requires substantially more forming passes than conventional profiles or tubes (see Figure 1). For contoured tubing, up to 30 forming passes with the corresponding side roll tables can be used. However, tube welding systems with up to 50 forming passes already have been built (see Figure 2).

Given the large number of forming passes that may be needed, most forming units are equipped with a quick-change system.

In these systems, two, four, and six forming stands and the corresponding side roll tables are mounted onto a quick-change cassette. The cassette, an assembly with a quick-change plate with forming stands mounted on it, is loaded and unloaded via overhead crane or a fork truck. It houses the roll form tooling.

It is possible to automate fully the connecting process of the drive to the gearbox. The gearbox drives the work shafts contained in the quick-change cassette. The time required for the coupling process is entirely independent of the number of stands and is estimated to be approximately five minutes.

Figure 3:
An HF unit welds the formed shape into a tube.

Creating the Perfect Weld

After the tube is formed, a high-frequency (HF) unit welds the formed shape into a tube (see Figure 3). The inductor coil of the HF unit is mounted on a three-axis adjustable table and allows optimal positioning of the inductor to the welding table for a high-quality weld. A clear definition or presentation of the strip edges into the welding zone in which edge scarfing or rolling is required is an important prerequisite for a perfect weld.

This definition is obtained with a special edge-conditioning unit at the strip entry. A five-roll weld table is designed to position the strip edges correctly for the tube welding. The vertical bottom roll supports the profile, and the two top rolls ensure the precise guidance of the strip edges by allowing the edges to meet each other in a guaranteed parallel plane.

The horizontal rolls of the welding table, equipped with hydraulic positioning features, press the heated strip edges together to create the weld seam. As soon as the tube welding system comes to a halt, the welding rolls retract to a predetermined and adjustable distance and remove the load from the bearings of the weld spindle supports.

The ability to exchange these units means the best possible welding rolls for each application can be selected.

Putting on the Finishing Touches

Finishing operations on contoured tube typically include scarfing, which eliminates the weld bead that results from the weld process. The hot chips from the scarfing operation are guided into a chip press located in the frame structure of the machine. In this press, chips are collected and compressed into small bundles, which allows for easy and safe disposal.

Because of the high weld temperatures, galvanization of the base material in the area of the weld seam is burned off approximately 1/8 to 1/4 inch from the weld seam.

To ensure corrosion protection for the tubing, the weld seam area needs to be posttreated in the same tube welding line with a system similar to an arc spraying process. Two automatically fed zinc-aluminum wires ignite an arc and melt the wires. Continuously blowing air sprays the liquid metal to the tube surface.

Cutting to Length

Because of the continuous operation of the line, cutting tubes to the correct length has to be done with a flying cutoff machine. It is equipped with a synchronizing position control, through which it moves synchronously to the contoured tubing and enables the cutoff at the predetermined length. A measuring wheel, mounted after the calibration unit directly onto the contoured tubing, determines the length measurement.

When the defined part length is reached, the tool carriage accelerates exactly to the current production speed. As soon as the movement of the carriage and the tube is synchronized and the required product length is reached, the tube is cut.

After the cut has been completed, the carriage returns to maximum speed. Typically, very low length tolerances can be realized, with accuracy of ±0.04 inch. For prepunched contoured tubing, the cutoff is defined in relation to the hole pattern through the use of an optical indexing system. This is designed to ensure a constant distance from the cut edge to the hole pattern.

Since a variety of tubing can be produced on these tube welding systems, it is possible to use different cutoff units. For instance, a circular cutoff saw can be used for cut speeds of up to 5,500 linear inches per second, while a metal circular cutoff saw can reach speeds of up to 12,600 linear inches per minute (IPM).

These cutoff units are mounted on the tooling carriage. Changing a unit takes approximately 15 minutes. The units are prepositioned with index pins, and the couplings establish hydraulic and electronic connections. The system control of the tube welding line recognizes which type of cutoff unit is being used and automatically refers to the corresponding cutoff program.

The End Product

Increased customer demands, higher material costs, and how to manufacture welded contoured tubing with difficult sections are important issues in the roll forming industry. When considering contoured tubing manufacturing processes, it is important to examine the entire process, from the structure of the tube welding system to the cutting and finishing of a final product.

Werner Wasmer is Vice President at Dreistern Inc., 3003 Unionville Pike, Hatfield, Pennsylvania 19440, phone 215-822-7766, fax 215-822-1521, e-mail, Web site Dreistern is a manufacturer of roll forming equipment and high-frequency, gas tungsten arc, and laser welding tube mills.

Werner Wasmer

Contributing Writer

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