Plasma Cutting - Tips & Techniques
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 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 is superheated, compressed air that ionizes to form a conductive gas. This gas conducts electricity from the torch of the plasma cutter to the workpiece. This is done through a conductive electrode and copper nozzle on the machine, which constrict the high-velocity gas causing a "tornado" effect. This provides energy to the arc that melts and blows away the metal.
What can I use a plasma cutter for and how does it compare to oxyacetylene?
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.
Oxyacetylene cuts by burning through the metal and is therefore limited to steel and other ferrous metals. Plasma cutting produces enough energy to melt the metal and then creates the momentum to blow it away. Because of this, plasma cutting can cut non-ferrous materials, requires a lower skill level, and offers faster travel speeds. In addition, it does not require the use of flammable or explosive materials and is therefore safer to operate.
The only drawback is that plasma cutting machines are more expensive; oxyacetylene does not require access to electrical power or compressed air, making it a more convenient method for some users.
Here's what to look for when purchasing a plasma cutting machine.
Once you have determined plasma cutting is the right process for you, consider 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 1/4"-thick material, you should consider a lower-amperage plasma cutter. If you most frequently cut metal that is 1/2" thick, 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 1/4" machine has approximately 25 amps of output, a 1/2" machine has a 50-60-amp output, while a 3/4" - 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 thicknesses of metal measured in IPM (inches per minute). If the metal you cut most frequently is 1/4", 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 1/4" steel, choose a 1/4" class (60-amp) machine.
If you are performing long, time-consuming cuts, or are cutting in an automated set-up, be sure to check 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 typically rated as a percentage of a 10-minute period. For example, a 60% duty cycle at 60 amps means you can cut with 60 amps output power continuously for six minutes out of a 10-minute period. In general, 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. But this may not be the most advantageous way to initiate the cutting arc as high frequency may interfere with computers or office equipment that may be in use in the area (or it can reduce the life of their internal electronics).
Instead, the lift arc method features a DC+ nozzle with a DC-electrode inside and touching it. When the trigger is pressed, current flows between the electrode and the nozzle. The electrode pulls away from the nozzle and a pilot arc is established. The transfer from pilot to cutting occurs when the pilot arc is brought close to the workpiece. 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 workpiece. 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 the life of that consumable.
Look for a manufacturer that offers a machine with the least number of consumable parts. These parts are considered the wear items of the unit and have to be replaced as they pit and degrade. A smaller number of consumables means 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.
Demo 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 (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 when doing a demo is to lift the plasma torch up from the plate while cutting. See how far you can move the torch away from the workpiece and still maintain an arc. A longer arc means more volts and the ability to cut through thicker plate. Lifting the arc may also be a good indication of how well the machine can gouge.
6. Pilot-to-cut and cut-to-pilot transfers.
The transfer from pilot to cut occurs when the pilot arc is brought close to the workpiece. 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 1/4" away from the work. At the end of the cut, the control system maintains the arc by instantly retracting back into a pilot arc.
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 that oil and water, which reduce cutting performance, are removed from the compressed air.
10. Determine if the machine is easy to operate and feels comfortable.
Look for a plasma cutter that has a big, easy-to-read, intuitive control panel. 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. You basically need to try it out the same way you would test the grip of a golf club.
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 there. Other safety systems can be fooled into thinking the nozzle is in place (i.e., shield cup sensing). In such systems, the nozzle can be left out. Therefore, output can be turned on to expose the operator to 300vDC. In addition, look for a machine that provides a pre-flow sequence. This feature warns user 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.
A. Set-Up Procedures
Before you start, check for the following items: A clean air supply without water or oil. Consumables that wear quickly, or black burn marks on the plate, may indicate contaminated air. Correct air pressure can be checked by looking at the gauges on the unit. Nozzle and electrode correctly in place. A ground connection to a clean portion of the work.
B. 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.
C. Piercing the Work
Many inexperienced users try to pierce the metal by coming straight down perpendicular (90°) to the work. This results in molten metal being blown back into the torch. A better method is to approach the metal at a slight angle (60°) and then rotate the torch to 90°. This way, the molten metal is blown away from the torch.
D. Don't Touch the Nozzle to the Workpiece
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 a metal template is used. In this case, the user drags the nozzle along the template. The result is the same as dragging the nozzle on the work: prematurely worn nozzles.
E. Beginners Should Use a Drag Cup to Facilitate the Cut
Many systems offer a drag cup, which snaps over the nozzle. This allows the torch to be on the workpiece and dragged along to facilitate a consistent cut.
F. 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° 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 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.
G. 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.
H. 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, get ready by the edge of the work before starting the arc so you can get right to cutting.
I. Maintain a Constant Work Distance
Optimally, you should maintain a 3/16" to 1/8" distance from the nozzle to the work. Moving in a weaving, up and down fashion will only hinder your efforts.
J. 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.
K. End with a Push on Thick Material
One trick to use on thicker material is to give a slight push 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.