CHOOSING THE RIGHT PLASMA SYSTEM FOR YOUR MECHANIZED PLATE CUTTING REQUIREMENTS

I often am asked questions from potential buyers of mechanized plasma cutting equipment, along the lines of ?What is the cut speed on ¼ in steel?? or maybe ?How long do plasma cutting torch consumables last?? Sometimes it seems that plasma cutting system performance is lumped together as if the process was a commodity and all plasma systems, regardless of manufacturer and color and price, produced the same speed, the same cut quality, and used the same amount of torch consumables per shift of plate cutting.

In reality, however, plasma cutting systems are not all equal in terms of performance. Dramatic differences can be realized in terms of cut quality, cut speed and operating cost when comparing different technologies, models and manufacturers of plasma cutting systems. These different capabilities can make it difficult to answer questions pertaining to speed, cut quality and operating cost without having more information about the exact application and equipment available for use.

Even though plasma cutting technology has been in heavy commercial use since the early 1960s, the technology is evolving at a pace faster than in its early years.

Many technological breakthroughs with mechanized plasma cutting systems have affected cut speed, cut quality, and operating cost, along with ease of use and system reliability. This evolution of development and obsolescence compares with desktop personal computers: remember the old 286, 386 and 486 processors, and now the Pentiums? Obsolescence is not quite as severe with plasma cutting, but if you have a 5- to 10-year-old mechanized plasma on your shop floor, rest assured that new technology provides better cut quality with higher throughput at a lower cost per part cut.

Here are a few samples of recent technology advancements:

Long life oxygen consumables technology: Microprocessor-controlled gas and power ramping at the beginning and ending of each cut cycle have allowed for plasma cutting costs that are a fraction of systems from the past, making the plasma cutting process the best choice for most plate applications.

High definition class plasma systems dramatically improve edge squareness, provide for a finer kerf, and produce tighter cut part accuracies have revolutionized plate cutting applications.

Advanced torch height control systems have allowed for thicker plate piercing capability as well as longer consumable life and more repeatable edge squareness by controlling the torch to workpiece distance with a high level of accuracy and automation.

PC-Based CNC controls have reduced the learning curve for CNC machine operators. These controls also have used the processing power of the PC to communicate directly with the plasma, the torch height control system as well as with the cutting machines? drive systems to automate many of the parameters and settings involved with these systems ? formerly the job of the operator. This level of accuracy and automation minimizes the risk of set-up mistakes while improving the repeatability of the plasma cutting process. If you know how to check your email on a PC, then you have the basic skill sets to operate a CNC cutting machine with this technology.

CAM software formerly was used to ?post process? CAD drawing files of parts that needed to be cut. This post processing added the lead in and lead outs, adjusted the drawing dimensionally to compensate for the plasma kerf width, and was used to nest multiple parts on the raw plate. The best CAM software today adds a lot more technology by taking over the machine code generation (the code that directs motion and tells the plasma torch when to start and stop). It also has the ability to manipulate this code for each material type and thickness so that cut quality and fine features in the part being cut are optimized.

Consumable parts design and manufacturing has dramatically improved. Manufacturers now design new consumables and processes using advanced thermal modeling software that gets their design close to perfection, then fine-tunes consumable and cutting performance in state-of-the-art laboratories. Once process designs are finalized, the consumable parts manufacturing is performed using the highest precision CNC equipment. Finished parts often go through CTF (critical to function) inspection on computer-controlled coordinate measuring equipment, ensuring that the CNC plasma equipment provides consistent performance with each new set of consumable parts used.

THREE BASIC CLASSIFICATIONS FOR MECHANIZED PLASMA OFFERINGS

If you look carefully at the plasma cutting systems now available from major manufacturers and understand their performance and capabilities, you can start to distinguish some price/performance/operating cost classifications that seem to define mechanized plasma into three distinct classes. Due to a lack of industry standard terms, I will name these plasma categories Entry level air plasma, Conventional Oxygen class plasma, and High Definition class plasma. Let?s take a look at defining each of these types of plasma systems.

Entry Level Air Plasma

This level of plasma cutting systems is comprised of systems that were designed primarily for the hand plasma cutting market. The systems available in this market today use inverter-based power supplies that were developed for portability, allowing easy movement around the shop floor or easy transport and movement for field use.

Many of these systems have quick disconnect torches that can easily be replaced with a straight barrel machine torch that fits into a standard (1-3/8 in diameter) oxy-fuel machine torch holder. These systems are limited to the use of compressed air or nitrogen as the plasma and cooling gas (the majority of entry level systems use gas-cooled torches for portability, as opposed to liquid cooled torches) and also have limited duty cycle ratings when used at full output current.

These systems have cutting capability ranges from 45 to 120 amps and can pierce and cut materials from gauge to ¾ in with reasonably good cut quality. System costs start as low as $2,500 and consumable life is normally in the $200 to $800 start range, depending on the materials and power levels.

Typical mechanized usage: low to medium tolerance CNC cutting machines, pipe cutoff and coping machines, robotic 3-dimensional applications, track burners, etc. Cut quality is quite good, although dross is common and air plasma tends to nitride-harden cut edges, which can affect machineability and weldability of these surfaces.

Conventional Oxygen Class Plasma

Oxygen plasma produces a dramatically superior edge quality in terms of dross formation, squareness and metallurgy as compared air plasma, although some oxygen plasma systems suffer from relatively short consumable life while others with newer power supply and gas flow technologies provide incredibly long parts life.

The systems I include in this class have 100 percent duty cycle ratings, liquid cooled dual gas torches, and the ability to use a variety of different gas combinations in order to fine-tune edge quality on materials other than carbon steel. Cut quality in terms of edge angularity is acceptable on gauge to ¼ in thicknesses, while on sections from ¼ in and thicker the angularity improves as the material gets thicker. Power levels in this class range from 130 to 400 amps (oxygen cutting) and go as high as 1000 amps for cutting non-ferrous materials up to 6 in thickness with either nitrogen or argon/hydrogen mixes as the plasma gas.

These systems are the workhorses in general metal fabrication applications that need a high volume of parts produced from steel, stainless and aluminum . . . without extremely fine features . . . and tolerances in the ±.025 in to .030 in range. Operating cost is low and cut speeds are very high. The majority of applications are on CNC X-Y cutting machines.

High Definition Class Plasma

This newest technology was first introduced in the mid-1990s when plasma process engineers discovered methods to increase the energy density in a plasma arc by as much as four times. This resulted in a narrower columnar arc that produces better edge squareness, especially on thinner materials. With this introduction, plasma gained a wider high quality cutting range . . . starting on thin gauge materials . . . and with today?s 400 amp high definition systems, plasma can now pierce up to 2 in thick steel.

The first high definition plasma systems required an experienced operator as there were as many as eight different gas pressure settings, as well as much tighter control of cut speed and torch to work distance in order to extract the best cut quality from this process. Now the newest high definition class plasma systems are almost completely automated by using communication with the PC-based CNC control on the cutting machine to set all of the critical parameters based on the operators input of material type and thickness.

Further, the high definition torch technology has been refined to actually provide faster speeds, longer consumable life, as well as superior cut quality as compared to conventional oxygen plasma systems. Today a single 400 amp high definition torch can cut from 28 ga to 2 in steel and can switch from plate marking (at less than 15 amps using argon) to cutting at 400 amps with oxygen in 2.4 seconds ? the market process uses the same nozzle orifice as the cutting process.

In its newest version, high definition provides a very easy-to-use precision plasma process with extremely high versatility, with the highest cut speed and by far the lowest operating cost per part in the metal fabricating industry. Fine features and holes are dramatically improved over the other plasma processes, removing much of the art involved with tweaking a plasma system for best performance and replacing it with computer-controlled science.

QUICK SUMMARY

There is need in industry today for all of the capabilities listed above. We must also remember that mechanized plasma systems cannot cut any metal without being mounted on some sort of motion control device, be it an X-Y cutting machine, a robot, a track burner or a pipe cutoff machine. Entry level plasma systems are typically installed on lower tolerance, lower cost machines while the conventional and high definition plasma systems are installed on larger, tighter tolerance machines with more capabilities and precision and, of course, a higher purchase price.

The bottom line here is that entry level systems produce great plasma cutting performance for lower power, lower duty cycle applications, at a lower capital equipment cost ? but with a higher operating cost.

At the other end of the spectrum, if you need the highest throughput, excellent cut quality and the lowest operating cost along with extreme ease of use (almost as easy to operate as your desktop computer!), look to the latest high definition class plasma technology. These systems are more expensive from a capital equipment perspective, but if used for multiple shifts, they will prove to be less expensive to own

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Jim Colt is the strategic account manager of Hypertherm, Inc., Etna Road, P.O. Box 5010, Hanover, NH 03755, www.hypertherm.com, [email protected].