How to buy a plasma cutter

Is plasma cutting right for you? What to look for in a machine

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 is superheated, compressed air that ionizes to form a conductive gas. This gas conducts electricity from the torch of the plasma cutter to the work piece. 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 (see diagram).

Plasma cutting can be performed on any type of conductive metal Ð mild steel, aluminum and stainless are some examples. With mild steel, operators will experience faster, thicker cuts than with alloys.

How does plasma cutting compare to oxyacetylene? 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, has 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 drawbacks are that plasma cutting machines are 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. Once you have determined plasma cutting is the right process for you, look at the following factors when making a buying decision. 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. 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-inch machine has approximately 25 amps of output, a 1/2-inch machine has a 50-60 amp output while a 3/4- to 1-inch machine has 80 amps output.

Is cutting speed 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 1/4-inch thick, 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.

If you are performing long, time-consuming cuts or are cutting in an automated set-up, be sure to check into the machineÕs 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 ten-minute period. For example, a 60 percent 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.

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 work piece. This transfer is caused by the electric potential from nozzle to work.

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 mean less to replace and more cost savings. Look in the manufacturerÕs specifications for how long a consumable will last Ð 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.

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. 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 work piece 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.

The transfer from pilot to cut 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.

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. 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.

For todayÕs 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 arenÕt.

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.

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.

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; output turned on and expose the operator to 300 VDC. In addition, 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 to 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: 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 — this can be checked by looking at the gauges on the unit
• A nozzle and electrode are correctly in place
• A ground connection to a clean portion of the work

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 are 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.

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 a slight angle (60 degrees) and then rotate the torch to 90 degrees. This way, the molten metal is blown away from the torch. When using higher current levels, 45 amps and above, do not touch the nozzle to the work. 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.

Many systems offer a drag cup, which snaps over the nozzle. This allows the torch to be on the work piece and dragged along to facilitate a consistent cut.

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 slow, 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.

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.

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.

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. 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.

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.