How to plasma weld
Part six in our How to Weld series
‘Plasma’ is a state of matter whereby gas or a mixture of gases is heated to an extremely high temperature and ionized to become electrically conductive. In plasma cutting, compressed air is the main source of supply with nitrogen the gas used for commercial or heavy industrial uses.
The electrically conductive arc is forced through a small, constricting orifice / plasma tip to create a highvelocity jet of ionized gas. The parent metal or workpiece is then melted away by the heat of this clean, controlled arc.
To put it into perspective, lightning is probably one of the most recognized forms of plasma. The lightning is looking for an earth in the same way that the plasma machine looks for the earthed workpiece.
The intense heat of the plasma arc is around 22,000°C. Compare this with oxyacetylene at around 3150°C or oxy/LPG at 2850°C, and you will have an idea of the energy created. To
reach this, plasma machines use DC power sources with very high opencircuit voltages.
That can be lethal. As a result, most good machines have a safety system called ‘parts in place’ to prevent accidental shock. If you disassemble the nozzle, the machine will automatically cut the power to the head. But it’s wise to turn the power off before changing electrodes anyway.
What it will cut
Plasma is ideal for cutting both ferrous and non-ferrous metals. Aluminium and stainless steels are the most common, as well as thin steels and zinc-coated alloys. Some types and brands of plasma cutters struggle with brass and copper. So, if you have a need to cut these materials, try the machine first! Heat-affected zones are very narrow in plasma-cut materials, especially in thin metals where heat from oxy/acetylene gas at the cutting edge can cause twisting or distortion.
Plasma is very fast compared with other hand-held cutting methods. This makes it popular in the sheet-metal and panel-repair industry, where this ionized arc is hot enough to burn through paint, rust, and various other metal coatings with ease.
Cutting thicknesses for singlephase machines usually range from 0.5mm up to 12mm, although on some machines a cut at the 12mm mark looks more like a shark bite. My advice if you want to cut up to 12mm is to test the machine. Cutting 10–12 mm all day long with a single-phase machine is a big ask. You are entering into a 50A upwards–sized machine, and these come only as three-phase units.
The big three-phase specialized plasma units are capable of cutting aluminium and stainless more than 50mm thick at almost a metre a minute.
Starting to arc
A high-frequency pilot-arc or a retractable-contact start is required to initiate the arc. This arc is transferred through a handpiece or torch. The head of the torch contains a tungsten or hafnium-tipped arc electrode and a tip with a constricting orifice. The orifice size changes to suit the amperage.
The higher the amperage, the larger the diameter of the orifice.
“As a result, most good machines have a safety system called ‘parts in place’ to prevent accidental shock”
Compressed air at between 50 and 65psi is the main source of gas supply. Straight nitrogen, oxygen, and argon/ helium mixtures can be used for production-type mechanized cutting of stainless steel and aluminium and other specialized metals. However, compressed air is the most common and cheapest form of pressure for the average single-phase plasma machine. Clean air supply is a must when using a compressor. Add a good filter — it will pay for itself in a very short time. If oil or water in the form of dirty air passes through the plasma arc, the electrode and tip will burn out very quickly. The cut quality will also be affected.
Of all the metal-cutting processes, whether mechanical, gas, or electric powered, plasma cutting has the ability to give the operator the most freedom to cut difficult shapes. Plasma is also fast, with minimal heat input or distortion. It has the ability to be used in creative work as well as in commercial or industrial applications.
What to look for
The important questions to ask when buying a plasma cutter are:
• What kind of material do you plan to cut?
• How thick is the material likely to be?
• What are your budget limitations?
• How often will the machine be used?
• What standards does it conform to? This is a really important one. I would certainly recommend, when you are buying a plasma cutter, that you try it out first.
Cutting speed
Finding the correct cutting speed will help eliminate dross. ‘Dross’ is the slag left underneath the cutting edge. Many machines have been blamed for leaving too much dross when the real culprit has been travel speed that is too slow. The leaf I cut out had a bit too much dross but that was because I was concentrating more on following the line. A good flap disc cleans up the edge in no time.
Safety
Like any cutting or welding process, plasma has its share of hazards:
• Fumes: be aware that these metals are being melted at very high temperatures with some very nasty by-products.
• Arc: use a minimum of No. 5 shade for very low amp cutting, Nos 6–9 shade for medium amperage machines, and Nos 10–14 shade for high amps. The instruction book should outline these points.
Welding lens shade numbers refer to the lens’s ability to filter light. All auto-darkening welding helmets that meet the appropriate standard provide 100-per-cent protection against harmful infrared and UV rays. They may range from a No. 8 shade for low amp applications up to a No. 13 shade for high amp applications. Top-line helmets include additional ranges (No. 3 to No. 8) for grinding or cutting.
• Arc burn: wear gloves and a longsleeved shirt; this is not a healthy way to get a tan.
• Sparks: where is the dross going? Keep the cutting zone clear of flammables, rags, etc.
• Wear safety gear, including heavyduty trousers, leather apron, or heavy cotton overalls.
• Use common sense.
A good instruction book will point out all of these hazards in more detail. If it doesn’t, don’t buy the machine.
“Many machines have been blamed for leaving too much dross when the real culprit has been travel speed that is too slow”
Two types
You will find two types of plasma cutter available in the cheaper price range. The cheapest models do not have high-frequency or pilot-arc start. Most of them have a retract contact start or lift arc. This means the handpiece must be pushed down on the metal to initiate contact to start. This does not always work that well, so definitely try starting and cutting numerous times before deciding on this type of machine.
The second type will have a highfrequency or pilot-arc start. This high-frequency start is normally the preferred option. Push the button and you are cutting. It is generally much less hassle and very simple to use, especially for the novice.
Trial
While trying out the plasma cutter, be aware of the cut quality, how well you can see the cutting nozzle, and the cut being made. You need to be able to see where you are going. Try a few different types of materials — aluminium, stainless, maybe a piece of copper or brass. Try the unit on the different thicknesses that the manufacturer specifies. Some models have drag tips that allow you to have the nozzle in contact with the metal being cut. This is ideal for beginners or when cutting using templates. Like most things, you get what you pay for.
The torch
The plasma torch is the secret to the plasma process. There are a lot of plasma cutters shoved into a corner because the torch is inferior. The power source is probably fine. Some cheap imported machines have very short-life parts and can be expensive. Who’s bought a printer and found out the cartridges are a horrendous price?
So ask the right questions before purchasing.
• What is the cost of the consumables?
• What safety features does the machine have? Because of the very high voltage available at the handpiece, a parts-in-place sensor is a must. With this feature, the machine will not start unless the nozzle and torch are in place — to avoid electric shock.
• Air compressor? Yes, you need one of these to supply clean air. The compressor should deliver a volume of 8cfm or more, but I have seen smaller compressors used. A lot depends on the brand and model, and how many times the motor can start before overheating. Plasmas do like a lot of air.
• You will notice that after you cease cutting, post-flow air will continue for another 10–20 seconds. This post-flow cools the consumables.
• Cutting tips? Most brands of plasma will have the facility to use two different types of tip: a stand-off tip for higher amperages, and a contact or drag tip for, say, 25A or under. I like using the drag tips when using a straight edge or a template. The stand-off is good when following a line or for gouging.
Set-up checklist
Before starting, check the following items.
• You will need a clean, compressedair supply, free of water or oil. Tips and electrodes that wear quickly or black burn marks on the plate may mean that the air is contaminated.
• Correct air pressure. All good machines come with a readable gauge on the unit and usually have the correct settings from the factory. Check the instructions.
• Be sure that a nozzle, tip, and electrode are correctly in place.
• Does it have a good earth?
• Read the instruction book.
• Don’t make the mistake of buying a plasma machine because of its low cost. A cheap machine that doesn’t cut well or costs too much to run is certainly no bargain. The machine we used to cut our shapes was the Lincoln Electric Invertec PC210 single-phase plasma machine with a built-in air compressor and the ability also to use external air. Notice the nozzle shape, which makes it easy to see the cutting line. There is a huge range of plasmas available. If you are unsure about an unknown brand, ask who the repair agent is and talk to them.
“Don’t make the mistake of buying a plasma machine because of its low cost”