January 16, 2003
Using orbital welding equipment led to productivity gains in one of the nation's first fusion-welded underground pipelines.
Thermal cutting processes that can produce beveled edges are not new. In fact, they probably have been around nearly as long as oxyfuel cutting. Their use has been popular in the shipbuilding industry for many years. However, until recently, the high capital investment and the trial-and-error nature of their application have limited their use.
High productivity is imperative for metal fabricators in today's marketplace. Skilled laborers who want to work in a fabricating environment are hard to find, and unskilled laborers at reasonable wages are equally unavailable in many areas of the country.
One way the productivity of many fabrication applications can be enhanced is to produce an edge with a single bevel in one thermal cutting operation.
For successful welding, the edges of the parts to be joined with a butt joint often require a beveled edge to allow adequate deposition and penetration of the weld. While the strength requirements of the joint dictate the actual joint design required, butt welding of material 3/8 inch or more thick often requires some sort of edge preparation before welding.
Other joint configurations also might require edge preparation, such as a corner joint, T joint, lap joint, and edge joint.
In some applications a beveled edge is a functional requirement for the end use of the part, such as a cutting edge for an earth engagement tool or a closure edge on a gate valve.
Regardless of the application, the beveled edge usually must meet certain requirements that are dictated by the application, such as dimensional consistency and adequate surface finish. A weld joint with an inconsistent bevel angle and bevel depth will, at the least, waste weld material and increase welding time. At most, it will result in a compromised weld joint. The advent of robotic welding has increased the need for consistent weld joint dimensions because robotic welding is less forgiving of weld joint inconsistency than manual welding.
Common characteristics of beveled edges are shown in Figure 1. The top bevel's angle typically is 0 to 45 degrees; the land typically is 3/16 in. or greater; and the bottom bevel's angle also is 0 to 45 degrees.
Other important dimensions are the land location and the part dimension, which usually is an outside dimension.
The most common bevel configurations are shown in Figure 2.
Beveled edges can be created in a variety of ways.
Manual grinding was one of the first methods used to produce a bevel on an edge. Clamp-on mechanical milling machines are used sometimes for cutting a bevel on noncurved edges. Some shipbuilders have employed large-gantry milling machines to provide a bevel edge.
Manual thermal cutting also can produce a beveled edge, as well as tractor-type torch carriers.
For many applications, the most productive way to create a beveled edge on a metal part is to use a CNC gantry motion system carrying either an oxyfuel or plasma torch mounted on a torch manipulator on the motion system.
Each process has advantages and disadvantages.
Plasma often is used for automated bevel cutting because of its potential for high cutting speeds and cut quality. The plasma torch usually is mounted on a manipulator that rotates and tilts the torch to achieve the proper bevel angle.
In turn, this manipulator is mounted on an X-Y motion system that moves the manipulator to achieve the required part profile. Bevel angles up to 45 degrees or more can be created.
Application Considerations. Application considerations for plasma bevel cutting are outlined in Figure 3. Plasma can cut mild steel, stainless steel, and aluminum. However, this process can't be used to cut mild steel thicker than about 1.25 in., and its single torch arrangement means that multiple passes are required for Y, X, and K bevels.
Hardware Configuration. Plasma beveling systems use a manipulator with one plasma torch. While some early experimentation was done with multiple plasma torches on a manipulator, the high capital cost made this configuration unattractive. Because only one torch is mounted on today's manipulators, multiple passes are required to obtain a combination bevel-land edge.
Plasma bevel cutting usually is done with systems that have output arc currents in the range of 200 to 400 amps. Mild steel generally is cut using the new oxygen plasma systems, while nitrogen is used for stainless steel and aluminum. For underwater cutting, an air muffler is used with a water-injected oxygen plasma system to enhance cut quality and reduce dross formation. The so-called dry oxygen plasma process produces good surface finishes on mild steel with a thickness of 0.50 in. and greater.
In addition to the rotary, or C, axis, a tilt, or A, axis can be added for contour plasma bevel cutting. The CNC commands the A axis to tilt the plasma torch to the appropriate bevel angle. High-powered CNCs, with five or more axes of control, may be required to command the torch manipulator for bevel cutting.
As with oxyfuel bevel cutting, the C axis either has finite rotation, in which hoses and cables to the plasma torch require unwrapping after a certain amount of rotation; or infinite rotation, in which slip rings and torch bearings allow continuous, unlimited rotation.
Current plasma bevel manipulators use one of two designs to provide the A-axis tilt: an arc sector or a compound-skew axis (see introductory photo). The compound-skew design uses pure rotary motion to tilt the plasma torch out of plane. The CNC coordinates this A-axis motion with simultaneous C-axis motion, providing the proper torch angle while maintaining the plane of the torch axis perpendicular to the direction of motion.
The most sophisticated systems allow the bevel angle to be changed while the torch moves along a line element, commonly referred to as bevel angle change on-the-fly (BACF). This feature can be useful when the cut part with the beveled edge subsequently is rolled into a shape other than a flat plate, with the bevel angle needing to be constant after the rolling or forming operation. Shipyards often require this capability for cutting their hull sections.
Collision Protection. The high velocities and accelerations used with plasma cutting increase the torch's chances of unintentional collisions with tip-ups, support table sidewalls, and other obstructions. Because it's not practical for the operator to be at the machine control console constantly, torch collision detection often is used.
Torch collision protection usually is provided either by a breakaway torch mounting or by a mount with some compliance to allow the torch to be moved without damage. In either case, a torch collision causes the protection device to stop machine motion and allow the operator to remove the obstruction before re-engaging the torch and restarting the machine motion.
Height Control. Initial torch standoff height can be set by manually controlled, motorized, vertical torch motion. However, automated systems generally use either a torch touch-and-retract system or an inductive probe for setting initial standoff. This operation is done automatically at the beginning of the torch-cutting sequence.
During the cutting process, nearly all systems use arc voltage to maintain proper standoff height, because arc voltage and torch standoff are directly related. The voltage across the plasma arc is measured and compared to a set point. The most accurate systems use a high-performance proportional drive system with a zero dead band to move the difference in arc voltage always toward zero in a controlled fashion. The result is accurate torch height control.
Just as in oxyfuel cutting, accurate and stable torch standoff is needed during plasma bevel cutting, so it's important to use stiff, high-gain proportional height-sensing systems.
Thermal bevel cutting has been used in shipbuilding, pressure vessel construction, farm equipment, railroad equipment, and transmission tower manufacturing. Metal service centers also have begun to offer thermal beveling, especially plasma beveling, as a processing capability.
Ronald W. Schneider, P.E., is marketing manager for Messer-MG Systems & Welding Inc., N94 W14355 Garwin Mace Drive, Menomonee Falls, WI 53051, phone 262-255-5520, fax 262-255-5542, e-mail email@example.com, Web site www.messer-mg.com. Messer-MG Systems & Welding Inc. manufactures CNC thermal cutting machines using plasma, laser, and oxyfuel processes.