Know your metal

Features, Tools - Features

A comparative analysis to guide you in choosing the right fabrication technology | Plasma arc cutting has been around for half a century, and has attained a high degree of sophistication. Water- jet cutting is slightly more recent while laser beam cutting has been around for half a century too. For evaluating the suitability of a metal cutting process several factors are de facto benchmarks such as – what is the kerf width (or cut width) signifying loss of metal, how straight are the sides of the cut (parallel or slanting), how fast is the cut made, how much consumable material is devoured, how much electric power, and how long the process can work without stopping or the duty cycle. This article makes a rough comparison with these three major metal cutting processes.

Plasma-arc cutting Just after the WWII ended, there was the invention of the jet engine, and the need to make larger airplanes or commercial airlines for international mass transit became quite a pressing issue. Now the quandary was, pre-WWII fighter planes were made from Balsa wood, and a lot of canvass cloth tightly stretched across a tubular air frame, often made of steel. However, overall weight of the aircraft was to be reduced, if more than two passengers were going to fly. This was when the potential of aluminium alloys was realised. These were as strong as steel but eight times lighter. Thus, fabrication began with aluminium tubes as struts and frame members.

One major headache with fabrication of aluminium alloy cutting and welding is that the metals form a refractory oxide soon on melting which requires temperatures in excess of 2700 degrees C to break the film down. Therefore, gas welding which worked at lower temperatures was unsuitable while electric arc welding was too erratic. Things changed overnight in 1959 when Robert Gauge, then an engineer working with the now infamous Union Carbide Ltd., invented the world’s first plasma torch. This was designed for melting of metals, but he quickly adapted it for both welding and cutting. Today, bulk of the structural material is used in assembling airplanes is aluminium tubes, struts, honeycomb material etc.

In terms of cost, the lowest would be that of a plasma cutting system. Depending on type, size, and features, a CNC plasma cutting machine could range anywhere from $15,000 to $300,000. That’s a big range, but the vast majority of CNC plasma machines sold today are well below the $100,000 mark. Also, if one would compare against waterjet and laser, then they would be talking about a real CNC machine, and not a low-cost, entry-level, garage- shop type of machine, so it’s going to start closer to $50,000.

Therefore, higher the sophistication of the work demand, costlier will be the cutting system.

• Today a plasma cutting system would range between $50,000 and $100,000

• CNC waterjet machines require an expensive ultra high pressure intensifier pump, so that is going to range from at least $100,000 to as much as $350,000, depending on size and options

• CNC laser machines are usually the costliest. A new machine is typically going to cost $350,000 and could easily exceed $1,000,000 Now how vital is the cutting speed as compared with operating cost, to you? When it comes to operating cost, one has to take into consideration power, gases, consumables, abrasive, and routine maintenance cost.

• Plasma operating cost would again be the lowest, and is typically estimated at approximately $15/hour

• The cost of laser would be slightly higher, typically estimated at around $20/hour

• Waterjet is usually considered to be the most expensive, typically estimated at about $30/hour

Production rate will depend directly on the continuous cutting speed -yes some degree of automation is taken for granted.

Remember, your total cost per part is going to be determined by the cost per hour to run the machine, divided by the number of parts produced per hour. Production rate is judged by comparing cutting speed. A fair evaluation would also need to include the number of cutting tools, because you can easily have a machine with four waterjet heads cutting simultaneously.

For the purpose of this evaluation, we’ll just compare the process speeds by themselves, assuming that we are talking about a small machine with a single cutting tool.

• This is where plasma beats all the rest, as it can cut many materials anywhere from 60 to 200 ipm (inches per minute).

• The laser is much slower on most thicknesses, typically in the range of 20 to 70 ipm.

• Waterjet is by far the slowest. Depending on thickness and quality level, speeds on metal plate are going to range from 15 ipm on down to a fraction of an inch per minute.

Cut edge quality refers to the squareness of the finished edge, as well as how much dross adheres to the bottom of the cut. The highest quality cut would usually be from Waterjet, which gives a very square cut with no dross, and no pierce spatter. Laser is a very close second place, because it also yields a very square cut, but on thicker mild steel or on stainless steel it can leave some dross and generate some pierce spatter. By comparison with these two processes, plasma cutting would have the lowest cut quality.

Plasma will always have some edge bevel angle and often cause some dross. It also generates much more pierce spatter than laser, due to the larger kerf width.

Cut-Part Precision

Cut part precision is a measure of the actual resulting part size compared to the programmed part size, and also includes consideration of the kerf width, which determines how small of an inside contour can be cut. Heat distortion should also be considered, as it can throw off the finished part size as well as cause the parts to warp.

• Again, waterjet would be the best precision, usually in the neighborhood of +/- 0.005”, and having a kerf width around 0.035”

• Waterjet also causes no heat-distortion

• Laser would be very close second place, with typical part dimensions of +/-0.005”, and average kerf width around 0.025” but laser can cause some heat distortion, especially on a thicker plate

• Plasma comes in the last place, having a typical part size tolerance around +/-0.020”, and a typical kerf width of around 0.150”. Plasma also creates some heat distortion, which can be reduced by cutting under-water.

Sadly there is no simple choice here – no single solution is virtuous enough to provide you a good price, good operational expense, good precision, good quality cut – and lack of distortion. Depending upon the product to be fabricated, one will have to decide.

Max Babi

Adjunct Professor

Plasma Technology


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