November 8, 2011
The plasma cutting process is best-known for its simplicity and its ability to cut virtually any metal. These qualities plus the productivity it yields have made plasma cutting a universally accepted metal cutting process with a range of capabilities and applications. Engineering achievements throughout the last 10 to 15 years have contributed to the improved performance of he process.
The plasma cutting process is best-known for its simplicity and its ability to cut virtually any metal. These qualities plus the productivity it yields have made plasma cutting a universally accepted metal cutting process with a range of capabilities and applications. While the plasma cutting process has been used commercially since the early 1960s, it is interesting to note that the highest levels of engineering achievements that have contributed to the process’s improved performance have occurred in the last 10 to 15 years. That trend continues today.
The flexible nature of the plasma cutting process lends itself well to many applications and capabilities. Motion-control devices such as CNC cutting machines, punch plasma machines, industrial robots, and pipe cutting machines aside, you can categorize the plasma process into three distinct categories: air plasma, conventional mechanized plasma, and high-definition plasma.
Air Plasma. These systems, designed for hand-held torch cutting, are available in power level outputs as low as 12 amps with a maximum cutting thickness of 1⁄8 in. with a hand torch, up to 120 amps. Most of these systems use inverter power supply technology, which makes them portable. Many can be purchased with a machine torch and have electrical interfaces that allow them to be used in mechanized cutting applications as well.
Conventional Mechanized Plasma. Typically, the plasma systems in this category are available only with machine-mountable torches. Also, they generally have more complex interfaces to provide better performance when used in modern CNC cutting machine applications. Power levels for conventional mechanized plasma systems are from 130 amps to as high as 1,000 amps.
Designed for high productivity with midlevel tolerances for cutting nonferrous (stainless and aluminum) materials up to 61⁄4 in. thick, these systems are the workhorses of steel service centers, shipyards, and heavy-equipment manufacturers. While some manufacturers of these conventional plasma systems have put the engineering effort into improving this class of systems, such as technology designed to extend the life of oxygen consumables and sophisticated interface systems that can communicate with the PC-based CNC used on most of today’s cutting machines, these systems largely remain similar to conventional industrial cutting machines that have been used for the last couple of decades.
For the most part, this class of machine requires an astute operator who can pay attention to the multiple setup parameters required for consistent cut quality from day to day. These systems require constant monitoring of arc voltage, gas flow, and pressure, as well as a dozen other parameters that must be set correctly to produce the best cut quality as power level, material thickness, and consumable parts in the torch change (see Figure 1).
High-definition Plasma. High-production sheet and plate cutting is the category that is receiving the most attention in terms of process research and development. As a result, high-definition plasma cutting systems have made huge strides in quality, speed, power level, operating costs, and, most recently, ease of use over the past 10 to 15 years.
In high-definition plasma cutting, developed in the early to mid-1990s, the plasma arc is forced through a smaller nozzle orifice, taking full advantage of the laws of high-temperature physics. This makes cleaner, squarer cut edges while maintaining acceptable consumable parts life in the torch. The earliest systems were limited in amperage and thickness capacity (70 amps with a 3⁄8-in. maximum thickness on steel) and required an expert machine operator to monitor and adjust multiple parameters that affected the cut quality. Even at its beginning stages, however, high-definition plasma technology ranked as one of the top three or four developments in the history of plasma cutting.
Today’s class of high-definition plasma systems are available in amperages from 130 to 800 with cutting thickness capacity from 26 gauge to 3 in. on carbon steel and up to 61⁄4 in. on stainless and aluminum. Consumables life and cut quality and consistency have improved dramatically over the years as well, making these systems a primary metal cutting method of choice for metal fabricators worldwide. Low operating cost, high cutting speeds, and improved quality are the results of high-level engineering efforts and have greatly increased demand.
While R&D efforts continue today, many of the recent breakthroughs in cut quality, consumable life, productivity, and ease of use are based on what used to be considered as “external systems” in the mechanized plasma cutting process.
While it has always been known that machine motion (accuracy, acceleration, and smoothness), torch height control (pierce height, cut height, collision avoidance, and cycle times), and CAM software (postprocessing for kerf width, lead-ins, lead-outs, and nesting) play major roles that affect any mechanized cutting operation in terms of cut part accuracy, operating cost, and throughput, the parameters associated with each of these external systems were controlled by the programmer and the machine operator. A cutting operation with an expert programmer and an experienced operator could be expected to produce higher quality than an operation with less attentive or less experienced staff.
Armed with the knowledge that controlling more tightly and consistently the dozens of critical operating parameters affecting all levels of performance in a high-definition plasma system would further improve the cutting processes on the shop floor, system engineers went to work. The manufacturers of these systems worked intimately with the providers of the machine CNCs, the torch height controls, and the CAD software (often called nesting software). After a few years of development work, the most recent high-definition plasma cutters use the full suite of CNC cutting machine components to fully automate and coordinate the functions that affect cut quality.
These plasma systems can now accept the same AutoCAD®-format drawing file inputs that older machines use and incorporate newer CAM software to analyze features on the part drawings such as holes, external features, shape, material type, and thickness. This analysis is then used to nest the parts; insert the best lead-ins, lead-outs, cut speed, amperage, and gases; and set all of the cutting parameters that were once controlled by the machine operator. The result is high-quality plasma-cut parts; round, taper-free holes; consistent cut quality; less downtime because of plate collision-avoidance technology; less scrap because of fewer or no setup errors by the operator or programmer; and faster cut-to-cut cycle times (see Figure 2). CNC technology advancements employ Windows®- based touchscreen operator controls that are simple to use, greatly reducing a new machine operator’s learning curve.
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