Plasma Cutting 101

Posted by pikesan on Sep 27, 2007 in How-To & Tech | 0 comments

Plasma Cutting: Determining if it’s Right for You and What to Look for in a Machine

Introduction
Do you need a cutting tool for occasional repair and maintenance work? Have you recently embarked on a new project that requires higher cutting volumes? Or, are you looking for a new alternative to your current mechanical saw? All of these scenarios provide great reasons to investigate plasma cutting. With the cost of machines on the decline, smaller-sized, portable machines flooding the market and technology offering increased benefits and easier usage — it may be time to take a serious look at plasma for your cutting applications. The benefits of plasma cutting include ease of use, higher quality cuts and faster travel speeds.

What is Plasma Cutting Technology?
In simplest terms, plasma cutting is a process that uses a high velocity jet of ionized gas that is delivered from a constricting orifice. The high velocity ionized gas, that is, the plasma, conducts electricity from the torch of the plasma cutter to the work piece. The plasma heats the workpiece, melting the material. The high velocity stream of ionized gas mechanically blows the molten metal away, severing the material.

How Does Plasma Cutting Compare to Oxyfuel cutting?
Plasma cutting can be performed on any type of conductive metal – mild steel, aluminum and stainless are some examples. With mild steel, operators will experience faster, thicker cuts than with alloys.

Oxyfuel cuts by burning, or oxidizing, the metal it is severing. It is therefore limited to steel and other ferrous metals which support the oxidizing process. Metals like aluminum and stainless steel form an oxide that inhibits further oxidization, making conventional oxyfuel cutting impossible. Plasma cutting, however, does not rely on oxidation to work, and thus it can cut aluminum, stainless and any other conductive material.

While different gasses can be used for plasma cutting, most people today use compressed air for the plasma gas. In most shops, compressed air is readily available, and thus plasma does not require fuel gas and compressed oxygen for operation.

Plasma cutting is typically easier for the novice to master, and on thinner materials, plasma cutting is much faster than oxyfuel cutting. However, for heavy sections of steel (1 inch and greater), oxyfuel is still preferred since oxyfuel is typically faster and, for heavier plate applications, very high capacity power supplies are required for plasma cutting applications.

What Can I Use a Plasma Cutter for?
Plasma cutting is ideal for cutting steel, and non-ferrous material less than 1 inch thick. Oxyfuel cutting requires that the operator carefully control the cutting speed so as to maintain the oxidizing process. Plasma is more forgiving in this regard. Plasma cutting really shines in some niche applications, such as cutting expanded metal, something that is nearly impossible with oxyfuel. And, compared to mechanical mean of cutting, plasma cutting is typically much faster, and can easily make non-linear cuts.

What are the limitations to Plasma Cutting? Where is Oxyfuel preferred?
The plasma cutting machines are typically more expensive than oxyacetylene, and also, oxyacetylene does not require access to electrical power or compressed air which may make it a more convenient method for some users. Oxyfuel can cut thicker sections (>1 inch) of steel more quickly than plasma.

What to Look for When Purchasing a Plasma Cutting Machine
Once you have determined plasma cutting is the right process for you, look at the following factors when making a buying decision.

1. Determine The Thickness of the Metal that You will Most Frequently Cut
One of the first factors you need to determine is the thickness of metal most frequently cut. Most plasma cutting power sources are rated on their cutting ability and amperage. Therefore, if you most often cut ¼” thick material, you should consider a lower amperage plasma cutter. If you most frequently cut metal that is ½” in thickness look for a higher amperage machine. Even though a smaller machine may be able to cut through a given thickness of metal, it may not produce a quality cut. Instead, you may get a sever cut which barely makes it through the plate and leaves behind dross or slag. Every unit has an optimal range of thickness — make sure it matches up with what you need. In general, a ¼” machine has approximately 25 amps of output, a 1/2” machine has a 50-60 amp output while a ¾” – 1″ machine has 80 amps output.

2. Select Your Optimal Cutting Speed
Do you perform most of your cutting in a production environment or in an atmosphere where cutting speed isn’t as critical? When buying a plasma cutter, the manufacturer should provide cutting speeds for all thickness of metal measured in IPM (inches per minute). If the metal you cut most frequently is ¼”, a machine that offers higher amperages will be able to cut through the metal much faster than one rated at a lower amperage, although both will do the job. For production cutting, a good rule of thumb is to choose a machine, which can handle approximately twice your normal cutting thickness. For example, to perform long, fast, quality production cuts on ¼” steel, choose a 1/2” class (60 amp) machine.

If you are performing long, time-consuming cuts or are cutting in an automated set-up, be sure to check into the machine’s duty cycle. Duty cycle is simply the time you can continuously cut before the machine or torch will overheat and require cooling. Duty cycle is rated as a percentage of a ten-minute period. For example, a 60 percent duty cycle at 50 amps means you can cut with 50 amps output power continuously for six minutes out of a 10-minute period. The higher the duty cycle, the longer you can cut without taking a break.

3. Can the Machine Offer an Alternative to High Frequency Starting?
Most plasma cutters have a pilot arc that utilizes high frequency to conduct electricity through the air. However, high frequency can interfere with computers or office equipment that may be in use in the area. Thus, starting methods that eliminate the potential problems associated with high frequency starting circuits may be advantageous.

The lift arc method features a DC+ nozzle with a DC- electrode inside. Initially, the nozzle and the electrode physically touch. When the trigger is pulled, current flows between the electrode and the nozzle. Next, the electrode pulls away from the nozzle and a pilot arc is established. The transfer from pilot to cutting arc occurs when the pilot arc is brought close to the work piece. This transfer is caused by the electric potential from nozzle to work.

Lincoln Electric’s Pro-Cut® line offers patented Dual Winding Technology with separate windings for the pilot and cutting arc. With Dual Windings, the pilot arc is optimized during current transfer for a fast, positive transfer without the use of a resistor. Dual Windings work by creating the electric potential for a transfer – they create a voltage difference to snap the arc to the work piece. Because Lincoln has eliminated the large resistor usually found in plasma cutting machines, it can offer units that are smaller in size with increased portability.

4. Compare Consumable Cost Versus Consumable Life
Plasma cutting torches have a variety of wear items that require replacement, commonly called consumables. Look for a manufacturer that offers a machine with the fewest number of consumable parts. A smaller number of consumables mean less to replace and more cost savings. For example, Lincoln Electric’s Pro-Cut line has only three front-end parts in the torch and only two of those are consumables: the electrode and the nozzle. Lincoln also offers tool-less changeovers when replacing these consumables.

Look in the manufacturer’s specifications for how long a consumable will last – but be sure when comparing one machine against another that you are comparing the same data. Some manufacturers will rate consumables by number of cuts, while others will use the number of starts as the measurement standard.

5. Test the Machine and Examine Cut Quality
Make test cuts on a number of machines, traveling at the same rate of speed on the same thickness of material to see which machine offers the best quality. As you compare cuts, examine the plate for dross on the bottom side and see if the kerf (the gap left by cut) angle is perpendicular or angular.

Look for a plasma cutter that offers a tight, focused arc. Lincoln Electric offers its Pro-Cut line with VORTECH™ Technology consumables which are specially designed to concentrate the plasma swirl, offering a tighter arc and concentrating more cutting power on the work piece.

Another test to perform is to lift the plasma torch up from the plate while cutting. See how far you can move the torch away from the work piece and still maintain an arc. A longer arc means more volts and the ability to cut through thicker plate.

6. Pilot to Cut and Cut to Pilot Transfers
The transfer from pilot arc to cutting arc occurs when the pilot arc is brought close to the work piece. A voltage potential from nozzle to work is mechanism for this transfer. Traditionally, a large resistor in the pilot arc current path created this voltage potential. This voltage potential directly affects the height at which the arc can transfer. After the pilot arc transfers to work a switch (relay or transistor) is used to open the current path.

Look for a machine that provides a quick, positive transfer from pilot to cutting at a large transfer height. These machines will be more forgiving to the operator and will better support gouging. A good way to test transfer characteristics is by cutting expanded metal or gratings. In these instances, the machine will be required to quickly transfer from pilot to cut and back to pilot very quickly. To get around this, they may recommend you cut expanded metal using only the pilot current.

Lincoln’s Pro-Cut products excel at this process because they employ Dual Winding Technology™. This technology utilizes two separate power systems (windings): one tailored for pilot arcs, the other for cutting. This patented configuration creates the nozzle to work voltage potential without a large resistor. Additionally, the control system can rapidly select which winding is required for the task. The result is instantaneous positive transfers from up to ¼” away from the work. At the end of the cut, the control system maintains the arc by instantly retracting back into a pilot arc.

*Pro-Cut 55 and 80 only.

7. Check the Machine’s Working Visibility
As you are working on an application, you want to be able to see what you are cutting, especially when tracing a pattern. Visibility is facilitated by the geometry of the torch – a smaller, less bulky torch will enable you to better see where you are cutting, as will an extended nozzle.

8. Look for the Portability Factor
Many consumers use their plasma cutter for a variety of cutting applications and need to move the machine around a plant, job site or even from site to site. Having a lightweight, portable unit and a means of transportation for that unit – such as a valet style undercarriage or shoulder strap – make all the difference. Additionally, if floor space in a work area is limited, having a machine with a small footprint is valuable.

Also, you want a machine that offers storage for the work cable, torch and consumables. Built-in storage drastically improves portability since these items will not drag on the ground or get lost during machine transport.

9. Determine the Ruggedness of the Machine
For today’s hard job site environments, look for a machine that offers durability and has protected controls. For example, fittings and torch connections that are protected will wear better than those that aren’t. Some machines offer a protective cage around the air filter and other integral parts of the machine. These filters are an important feature since they ensure oil is removed from the compressed air. Oil can cause arcing and reducing cutting performance. Protection of these filters is important as they ensure oil and water, which reduces cutting performance, is removed from the compressed air.

10. Find Out if the Machine is Easy to Operate and Feels Comfortable
Look for a plasma cutter that has a big, easy-to-read control panel that is user-friendly. Such a panel allows someone who does not normally use a plasma cutter to be able to pick it up and use it. In addition, a machine with procedural information clearly printed on the unit will help with set-up and troubleshooting.

How does the torch feel in your hand? You want something that has good ergonomics and feels comfortable.

11. Look for Safety Features
Look for a machine that offers a true Nozzle-in-Place safety sensor. With such a feature, the plasma cutter will not start an arc unless the nozzle is in place. Some safety systems can be fooled into thinking the nozzle is in place (i.e. shield cup sensing), even when it is not. If the output is turned on, the operator will be exposed to 300 VDC, a very unsafe condition. This cannot happen with the Lincoln Nozzle-in-Place safety sensor.

Look for a machine that provides a pre-flow sequence. This feature provides an advanced warning to the use before the arc initiates. In addition, look for a machine which provides a three-second pre-flow safety which gives users advanced warning to make sure all body parts are clear of the nozzle before the arc initiates.

How Can I Make the Most of This Cutting Tool?
After you have selected the plasma cutting machine that is right for you, here are some tricks-of-the-trade that will help beginners make the best possible cut.

1. Set-Up Procedures
Before you start, check for the following items:

• A clean compressed air supply, without water or oil. Consumables that wear quickly, or black burn marks on the plate, may indicate that the air is contaminated

• Correct air pressure – this can be checked by looking at the gauges on the unit

• A nozzle and electrode are correctly in place

• A good connection of the work lead to a clean portion of the work

2. Safety Gear
Some basic safety practices should be observed. You should read your instruction manual thoroughly to understand the machine. Wear long sleeves and gloves while cutting since molten metal is generated during the cutting process. Eye protection such as dark goggles or a welding shield is required to protect your eyes from the cutting arc. Typically a darkness shade of #7 to #9 is acceptable. Finally, follow all safety tips and guidelines that are detailed in your instruction manual.

3. Piercing the Work
Many inexperienced users try to pierce the metal by coming straight down, perpendicular (90 degrees) to the work. This results in molten metal being blown back into the torch. A better method is to approach the metal at an angle (60 degrees from horizontal, 30 degrees from vertical) and then rotate the torch to the vertical position. This way, the molten metal is blown away from the torch.

4. Don’t Touch the Nozzle to the Work Piece
Do not touch the nozzle to the work when using current levels of 45 amps or more. Doing so will drastically reduce the nozzle life as the cutting will double arc through the nozzle. Double arcing can also occur if the torch is guided by dragging it against a metal template. The result is the same as dragging the nozzle on the work — prematurely worn nozzles.

5. Beginners Should Use a Drag Cup to Facilitate the Cut
Many systems offer an insulated drag cup, which snaps over the nozzle. This allows the torch to rest on the work piece and dragged along to facilitate a consistent cut.

6. Travel at the Right Speed
When moving at the right cutting speed, the molten metal spray will blow out the bottom of the plate at a 15 to 20 degree angle. If you are moving too slowly, you will create slow speed dross, which is an accumulation of molten metal on the bottom edge of the cut. When moving too fast, high-speed dross on the top surface is created since you are not allowing time for the arc to completely go through the metal. Traveling too fast or too slow will create a low-quality cut. Typically, low speed dross can be distinguished from high-speed dross by ease of removal. For example, low speed dross can be removed by hand whereas high-speed dross typically requires grinding.

7. Set the Current to Maximum As You Begin
When setting the current, put it on the maximum output of the machine, then turn it down as needed. More power is usually better, except when doing precision cutting or when you need to keep a small kerf.

8. Minimize Pilot Arc Time
Because of the wear it creates on the consumables, try to minimize the amount of time spent in pilot arc mode. To do this, position the plasma torch by the edge of the work before starting the arc so you can get right to cutting.

9. Maintain A Constant Work Distance
Optimally, you should maintain a 3/16″ to 1/8″ distance from the nozzle to the work. Moving the torch in an up and down fashion will only hinder your efforts.

10. Travel in the Direction that will Give You the Best Finished Work
If you are making a circular cut and plan to keep the round piece as your finished work, move in a clockwise direction. If you plan to keep the piece from which the circle was cut, move in a counterclockwise direction.

As you push the torch away from you, the better cut will appear on the metal that is on the right hand side, since it will tend to have a better, squarer edge.

11. End with a Push Angle on Thick Material
One trick to use on thicker material is to rotate the torch slightly, increasing the torch orientation to a push, rather than drag angle as you cut through the last section of material. This increase in the push angle at the finish will cut through the bottom first and get rid of the bottom corner that is usually left at the end of thick plate. Never finish a cut by using the torch to hammer away the last corner of the work.

After finding the right machine for your application and learning some of the tricks of the trade, you should be ready to cut. Remember that plasma cutting offers a number of benefits and should provide you with faster, higher quality cuts.

For more information, visit Lincoln Electric