Plasma Cutting Guide

Intro

In this article we will be explaining what plasma cutting is, the different types of machines and torches, along with the cutting capabilities, prices and power requirements. Later in the article will be going over some tips and tricks of using them. Watch our YouTube video where we talk about all these details and a lot more here.

What is Plasma Cutting

Plasma arc cutting is similar to electric welding, except instead of joining, it separates. In the 1960s, engineers found that they could turn up the gas on electronic welders to increase the speed of welding operations, but they reached a point where the torch ceased welding and began cutting. This discovery led to plasma cutting torches. The plasma arc results from electrically heating a gas, typically air, to a very high temperature. This ionizes its atoms and enables them to conduct electricity. When plasma is directed at high speed toward a piece of metal, the electrons in the plasma collide with the base metal. Consequently, the metal is literally melted and blown away in the form of a cut. Referenced from (Eastwood Tech Article )

A plasma arc torch uses a "swirl ring" that spins the gas around an electrode. The gas is heated in the chamber between the electrode and torch tip, ionizing the gas and creating plasma. This causes the plasma gas to greatly expand in volume and pressure. The small, narrow opening of the torch tip constricts the plasma and accelerates it toward the work piece at very high speeds (20,000 ft./s) and temperatures (up to 30,000° F). The high-intensity plasma jet melts a very localized area. The force of the jet (or arc) pushes through the work piece and removes the molten metal. This jet easily cuts through metals with poor heat conductivity (stainless steel) or excellent conductivity (aluminum). Referenced from (Miller Welds Tech Article)


High Frequency vs. Pilot Arc High Frequency vs. Blow Back

High Frequency - e.g. PT31 & AG60 torches - Early Plasma torch designs used a high frequency discharge (think of a spark plug firing) to ionize the plasma forming gas, allowing the DC arc to transfer to the material being cut. This system requires the operator to “scratch” start, or get the torch very close to material to cut. This style does not work on dirty or painted surfaces. These are common torches used on the cheapest plasma cutters.

Pilot Arc - High Frequency – e.g. Eastwood & Lotos machines - The next iteration of Plasma torches saw the continued use of high frequency but now using a “Pilot Arc” to start (spark plug in the torch).  A High Frequency Pilot Arc allows the torch to fire without being in close proximity of the material being cut and will cut through dirt, rust or paint. These systems are an improvement, but tend to have random torch/consumable styles with limited options.

Blow Back – e.g. Most name brand machines – This style was first designed by Hypertherm, with other brands creating similar styles. Blow back start uses a moving, spring loaded electrode that “blows back” away from the nozzle using cutting air pressure, at the beginning of each cut cycle to start the arc. Blow back style torches do not use a High Frequency, but are still considered a Pilot Arc start. This makes these torches ideal for use in CNC cutting systems as there is no High Frequency to interfere with computer controlling systems. You will find these style torches on most all of your middle of the road and expensive name brand machines that offer drag cutting consumables.  Blow back is far superior to the other types of torches.  Referenced from (Jim Colt of Hyperthem)

 

Choosing the right plasma cutter – Comparing types, uses and price points           

Power and Amperage is important - 110v will cut 1/8” pretty well and can usually cut up to 1/4-3/8” if you go very slow. 220v is recommended for anything 3/16” and up.

General amperage to cut thickness

-30 amp good at 1/8”, max 1/4”

-50 amp good at 1/4", max 3/8”

-60 amp good at 3/8”, max 5/8”

-60 A+   good at 1/2", max 3/4" – these are the name brand machines

-80 to 100 amp 3/4" to 1” and above

 

$150-300 - High frequency – These machines using PT-31 or AG60 torches are very common and are good up to 1/4-3/8”  material. These machines are ideal for thinner material, artistic free hand cutting, or for entry level plasma cutting. They have a limited selection of consumables and need to be mail ordered. While you can drag cut with these machines your consumable life will be negatively affected.

$200-800 – Pilot arc high frequency – These machines we recommend avoiding, due to blow back machines now selling for a similar price point and using a better torch. These will generally cut thicker material, but the torches used tend to vary a lot, are harder to find and still have limited consumables. With these being a High Frequency start they are also unable to be used on a CNC machine.

$250-1200 – Blowback to avoid – S45 and IPT 45, 65 torches found on Harbor Freight Titanium and others. While these machines can be good, it’s the torch that is not. These torches don’t offer any different types of consumables for drag cutting or gouging. Avoid machines with 45, 65 torches.

$600-1000 – Good/cheap blowback – PT/IPT 40, 50, 60, 80 Tecmo style torches are the ones to get. These are used by many different brands such as Everlast, PrimeWeld, YesWelder. With the PT(Tecmo) or IPT(Innotec)  torch we offer a proper drag shield conversion that allows you to use this torch with our Plasma Stencils. We recommend getting a 60 amp machine, which can tackle most of your common cutting needs. Most of these machines come with a CNC port which allows easy plasma table hookup. If you’re looking to be doing more fabrication projects, drag cutting, or using it in combination with a CNC Table. Then you will want to spend a little bit more to get into these  “ middle of the Road” machines.        

$1800 and up - Name brand blowback machines - Hypertherm, Miller, Thermal Dynamics etc. - While these are much more than the other machines, you do truly do get what you pay for. The parts and accessories are generally available at your local welding store and there is much more support for cutting issues. There are many consumable options for normal, drag, gouging and CNC cutting. These are great for professional and industrial plasma cutting along with all the varying the types of cutting they are capable of.  

 

            Different Cutting Tips/Types

Normal/ Stand Off - This is the standard cutting tip that is supplied with most plasma cutting machines. This style of tip is good for free-hand cutting and following lines. Usually a two prong or ring type stand-off guide is used to prevent the tip from making contact with the material being cut. This setup is not electrically isolated which is why you will wear consumables quicker if you use these tips for drag cutting. This is also the  tip to use for CNC table cutting.

Drag Shield - A drag shield is electrically isolated from the cutting tip and has the proper amount of stand-off built in, which allows it to be placed directly on the material and “dragged” without any electrical interference. This allows for consumables to last much longer as well. These drag shields are generally a common size that work great with our Plasma Stencils for accurate hand-held cutting or Offset Blocks for straight cuts. It should be noted that not all Plasma torches have a proper drag shield option available.  This is why we recommend the PT/IPT style torch or just about name brand that offers a drag shield. We do sell “conversion rings” to allow drag cutting with other types of torches.

Gouging - A gouging tip is used when you do not want to completely cut through the material. Generally speaking you will only find these tips available for your name brand machines. A gouging tip allows you to drag at an angle and pull the arc away to better control the amount of material being removed. The gouging tip is great if you need to V out a crack on thick material, or if you need to remove a weld holding a bracket on.


Plasma Cutting Tips and Tricks

Proper air pressure - You will need an air compressor that will run your plasma cutter efficiently. Most plasma cutters need between 60 and 80 psi "cutting" air pressure. The bigger the plasma cutter the bigger the air compressor.  I recommend a compressor that will put out 5cfm at 90 psi or more, at least a 20 gallon tank and top out at 120 psi or more. I also recommend using a 3/8" hose at 120 psi to your plasma cutter for best results.

Clean and dry air is very important for good cutting results. Make sure your air compressor has a water separator, you don't have to spend a lot to get clean air for your plasma cutter as far as filters. Referenced from (Georges Plasma Cutter Shop)

Use the right tip size for the right amperage setting - Lower amperage tips will have a smaller orifice that helps maintain a narrow plasma stream at the lower amperage settings.

- At an 80 amp setting, using a 40-amp tip, it will distort the tip orifice and greatly reduce the consumable life and cut quality will suffer as well.

- On the other hand if you use an 80-amp tip at a lower amperage range, this will lead to a much wider kerf as the tip will not be able to focus the plasma stream. This can also have a negative effect on consumable life as well as cut quality and dross.

Inspect consumables regularly - Worn consumables are usually responsible for poor cut quality and arc starting problems. Excessively worn consumables can cause permanent damage to the plasma torch as well. Change the electrode when the Hafnium divot/pit/hole in the end is 1/16" deep or so. If you do not change the electrode before it loses its center core, it will cause damage to your torch head. The tip/nozzle should be changed with the electrode, or when you can see that the orifice is no longer round. For precise cutting it is good to use a new tip and for destructive/general cutting a more used tip will work. Referenced from (Georges Plasma Cutter Shop)

Cheap vs Good Consumables - The cost of consumables is an area where it is tempting to try and save some money. While the allure of cheap consumables sounds good this can be a double edge blade. Cheaper consumables are generally manufactured with poorer quality materials and to looser tolerances. This results in poorer cut quality, and reduced consumable life. Cheap consumables can also give a lot of random, hard to explain cutting issues, along with error codes during CNC cutting operations. Stay away from cheap consumables.

Practice before cutting - Practicing the movements required along your planned cutting path without pulling the trigger of the torch. This will help you adjust your body position and ensure that you are clear of any obstructions so you can have a nice continuous cut path. Using scrap to make multiple practice cuts will allow you to become more familiar with your machine.  

Pierce Starting - When you have to pierce start it is best to hold your torch at a 45 degree angle away from you or any sensitive items. Once the plasma arc has penetrated the material roll the torch to where it is perpendicular to the material being cut. At this point is where I like to stop the plasma cut and clear off any debris on the top side of the material being cut. This is even more important if you are using a stencil or straight edge guide as any debris can be caught between the torch and template/stencil will cause an inaccurate cut.

Brace your cutting hand - Support your cutting hand with your non-cutting hand. This will help to maintain a smooth, constant, and steady cut. This is even more critical when you are standoff cutting or free hand cutting.

Keep a consistent cut speed  (Be like a Robot) - Keeping a constant travel speed and adjusting to the cutting needs is important as this will have a direct impact on the amount of dross and cut quality.

-Too Fast of a cutting speed may create a splash back and top splatter.

-Too Slow of a cutting speed may create excessive dross and cut taper.

Easily remove dross - Dross forms on the back side of the cut and can be worse if cut speed is too slow                 or the air supply is not dry. The quickest way to remove dross is to use a chipping hammer, but a standard hammer will work just fine too. The best form is to strike the dross at a 45 degree angle from the cut and generally the dross will just pop off. For dross on thinner material, or to remove a lot of dross a stiff cupped wire wheel on a 4.5” angle grinder works great too. Just be careful as the dross will become small projectiles around your shop.

Clean the plasma torch regularly - It is important to keep the internal and external torch threads clean and in good condition, otherwise you could gall the threads damaging the torch. Use electrical contact cleaner or Hydrogen Peroxide and a cotton swap to clean the torch

For specific shapes and straight cuts - If you are looking to make a specific shape like a circle, square or making a straight cut is where the use of a Stencil or straight edge cutting guide can be extremely useful. We will go into more depth on the offering of Stencils and Straight cutting guides in our next article.

            In summary, using the right cutting techniques and the right plasma cutter, as well as utilizing some of these tips and tricks can improve your cut quality, make the job easier, and increase productivity. Applying these simple techniques can save countless hours you may have spent trouble shooting poor cut quality or issues with your plasma cutter. These tips can increase the consumable life which in turn will lower the operating costs. Like with everything your plasma cutting skills will increase with practice.


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