Making plasma cutting easier

Using CNC automation technology

THE FABRICATOR® AUGUST 2003

August 28, 2003

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For many people, the world of plasma cutting is a complex and daunting place, with a cryptic set of rules that can be mastered only by highly trained technicians after weeks of training. For every change of material or thickness being cut, a long process ensues of resetting gas mixtures, tweaking pierce heights and pierce delays, and manually calibrating every last parameter to ensure a reliable result.

For many people, the world of plasma cutting is a complex and daunting place, with a cryptic set of rules that can be mastered only by highly trained technicians after weeks of training. For every change of material or thickness being cut, a long process ensues of resetting gas mixtures, tweaking pierce heights and pierce delays, and manually calibrating every last parameter to ensure a reliable result.

However, technological advancements have automated the calibration process and eliminated the guesswork to make highly precise plasma cutting accessible to large and small fabrication shops alike. By working together to develop their technologies, CNC machinery companies and plasma unit manufacturers have optimized machinery control systems to take full advantage of the speed and power of plasma cutting. CNC technology allows systems to communicate to maintain unprecedented levels of control over the quality of plasma cuts.

This level of control is achieved by linking all parameters of a plasma unit to the same CNC unit that instructs the motion of the head, which allows the controller to compute all factors that might affect the quality of a cut. Anytime the material type, thickness, or plasma process is changed, several parameters must be adjusted, such as feed rate, pierce delay, pierce height, and gas mixture. Instead of adjusting dials and making precise measurements before running a job, the operator simply loads the material to be cut and then selects the material type from a menu-driven list on the machine keypad. The controller then automatically configures the machine and the plasma unit. This greatly reduces setup time and nearly eliminates the margin for human error, naturally improving job efficiency, production time, and job quality.

From the User's Angle

Normally, a user creates a nested sheet of parts in a CAD/CAM system and then saves the machine-ready file on a computer or network server. New systems can have a built-in Ethernet connection and communicate via DNC using standard TCP/IP to the user's computer or to a network server. At the machine, the user loads the material and selects the material type. The controller prompts the user to insert the appropriate torch consumables.

The user then selects the job and starts the cutting. When equipped with a bar code scanner, the computer can produce a printed work order for all jobs to be cut, allowing the operator to commence jobs simply by scanning the appropriate code on the sheet, prompting the job file to be loaded and executed automatically.

The Cutting Process

The process of cutting with plasma involves an electrical reaction, so unlike other cutting techniques, plasma cutters can cut only materials that conduct electricity. When a job file is started, the machine moves to the first pierce or cut position and the torch moves down toward the material. On the end of the torch is a device called an ohmic sensor. Once the ohmic sensor makes contact with the surface of the material, it closes an electrical circuit, informing the machine that it has reached the surface of the material. The torch then lifts above the material to the pierce height. The pierce height is higher than the cutting height to prevent hot metal from shooting directly back into the torch during the pierce process.

Once the pierce is complete, the torch moves to the cut height and begins to cut. The optimum pierce height, cut height, and feed rate all are based on the consumables and the material being cut. On sophisticated plasma cutting systems, all of these parameters are set automatically.

Once the cutting begins, the distance between the torch and the top of the material is maintained by reading the voltage from the plasma arc, in a process known as automatic torch height control (ATHC). Generally, sheets of material do not lie completely flat, especially thin-gauge material. To ensure a high-quality cut, it is important to maintain a constant distance between the torch and the material surface. Maintaining a constant height requires a responsive ATHC system. On some systems, the arc voltage is checked at a rate of 500 times per second, and the readings are used to adjust the Z axis up or down accordingly. The result is a quality cut that does not require user intervention.

Improvements and Advancements

The process described seems simple, but actually it is a result of extensive developments in modern technology. Older plasma systems, as well as many modern ones, do not take advantage of these new developments and require painstaking manual adjustments to produce parts that remain inferior in edge quality and efficiency. As further technological developments are made, the quality gap between less advanced systems and modern ones is widening at a fast pace.

One of the latest advancements in plasma cutting technology is 200-amp fine plasma cutting. This process produces a more constricted plasma jet for a higher arc density and more accurate cutting. The result is a smooth, precise cut that is almost dross-free.

The condition of the plate with regard to rust or reflectivity does not affect the cutting process. A high-quality fine plasma cutting system allows the cutting of holes with diameters nearly equivalent to the thickness of the material being cut. For example, good-quality 0.35-in.-diameter holes can be cut accurately in 0.25-in.-thick material.

Moving Forward

Many companies today are committed to moving this technology forward. In the future, leading companies will continue to work closely together to push the technological envelope, making plasma technology faster, more accurate, and easier to use.

Brad Thompson is marketing director and Kris Hanchette is vice president of sales with MultiCam LP, P.O. Box 612048, Dallas, TX 75261, 972-929-4070, fax 972-929-4071, brad@multicam.com, www.multicam.com.

The FABRICATORwould like to acknowledge the assistance of Hypertherm Inc. in preparing this article. Etna Road, P.O. Box 5010, Hanover, NH 03755, 603-643-3441, fax 603-643-5352, info@hypertherm.com, www.hypertherm.com.



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The FABRICATOR® is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971. Print subscriptions are free to qualified persons in North America involved in metal forming and fabricating.

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