Plasma cutting is easy to learn and use (in fact it's so easy that many users remove the unit from its packaging and go right to work). However, that approach may not optimize your plasma cutter's capabilities. Here are some tips and practices that will increase your efficiency and precision and prolong the life of your plasma cutter and consumables.
Before You Start
Having purchased the right plasma cutter for your needs, the first and most important step is to read the owner's manual, according to Don Keddell, training specialist, Miller Electric Mfg. Co.
"Reading the owner's manual is an important step and one that many users fail to take," Keddell says. "It pays to take the time to read the manual thoroughly and familiarize yourself with your particular plasma cutter. It contains important information on safety and getting the most from your plasma cutter."
The following information does not replace a thorough reading of the manual. Read the manual then keep it handy so you and other operators can reference it as needed.
Keddell suggests developing a "preflight routine" for using your plasma cutter, starting from the back and working forward.
1. Check the power cord to make sure it's in good condition and that it is plugged into the correct type of primary power supply. Some units, such as those using Auto-line(tm) technology, allow you to plug them into any power supply from 208 to 575 volts. Other units require a specific voltage and it's up to the user to make sure they are plugged into the correct power supply and any power selection switches are correctly set.
2. Check your air supply to make sure you have the correct air flow and pressure entering the machine. Your owner's manual should contain the air requirements.
3. Dry air is important for plasma cutting and maximizes cutting capacity. Keddell recommends installing an air filter and/or dryer on the machine, if it does not come equipped with one. Check, clean and replace the filter and or dryer in accordance with the manufacturer's recommendations. It's important, Keddell says, to have some type of sealant on the connection to decrease the risk of an air pressure leak.
4. Connect the torch, if it's not already connected.
5. Make sure all of your consumables are in place and correct for the job and that they are snug and secure. Keddell receives many calls from people who over tighten the cup and experience intermittent cutting as a result.
6. If applicable, select the proper process. On some higher output machines, a switch allows the user to choose between cutting and gouging.
7. Turn the machine on.
8. Check the air setting. The PSI setting may have different settings for cutting and gouging. On some plasma cutters, the operator turns the amperage down to zero, which turns on the air flow and allows the operator to set the correct pressure
9. Set the output to the correct setting. (More on this below.)
A cutting speed chart showing the rated cutting speed for the power source for mild steel, aluminum and stainless steel of various thicknesses.
10. Check the ground connection to the work piece. Although plasma can cut through painted metal, it does require a solid connection on a clean part of the workpiece as close as practical to the work area.
11. Make sure you have all of your safety gear in place. Your owner's manual will have more details, but generally you want the same type of protective gear as when welding. If the table is wet and you lay your bare arm on it, you can become part of the circuit and receive a shock, so make sure you are wearing welding leathers, along with proper gloves and eye protection. Usually a #5 shade is the minimum eye protection with other shades required depending on amperage. A face shield is also recommended.
12. Make a sample cut on the same type of material as your workpiece to check your settings and travel speed.
13. It is difficult to cleanly stop and continue a long cut. Rehearse the cut beforehand to make sure you have adequate freedom of movement to make one continuous cut.
A clean cut relies on several factors: travel speed, technique, distance from the work surface and consumables. Tips focus the plasma stream to the work piece. The machine that Keddell uses for demonstration utilizes 80-amp and 40-amp tips plus offers extended versions of those for increased reach as well as gouging tips.
Use a higher-amp tip when cutting thicker material. Because a lower-amp tip has a smaller orifice, it maintains a narrow plasma stream at lower settings for use on thinner material. Using a 40-amp tip at an 80-amp setting will distort the tip orifice and require replacement. Using an 80-tip on the lower settings will not focus the plasma stream as well and lead to a wider kerf. Use extended tips to reach out further for cutting in corners, using patterns or in tight areas.
Gouging tips spread the plasma stream and remove a lot of material at once, enabling the arc to remove a large about of material in a single pass.
Keddell noted that if you're using clean, dry air and making longer cuts, you can expect to get approximately two hours of continuous cutting time from your consumables. Continuously restarting the pilot arc by triggering the torch will shorten the life of your consumables. Touching the tip to the material without a drag shield in place, will cause double arcing as the electrical current tires to find the shortest path to the workpiece. This will decrease consumable life.
When you're not using a drag shield, maintain a 1/16- to 1/8-in. distance between the tip and the workpiece. As noted, touching the tip to the work surface will affect your cut quality and consumable life. However, moving the tip further away from the workpiece effectively reduces the cutting capacity of the unit.
Many plasma cutters come with a drag shield that is placed on the cutting surface during operation and maintains an optimal 1/8-in standoff, ideal for unsteady hands, longer tip life and pattern cutting.
|Maintaining a 1/16- to 1/8-in. standoff will increase the longevity of your consumables, give a cleaner cut and maximize your machine's cutting capacity.
If you're not using a drag shield, or for more precise cuts, Keddell recommends placing your non-cutting hand down as shown in Fig. 1 and using it as a guide for the other hand. This rest hand position gives freedom of movement in all directions while helping to maintain a constant standoff and steady your hand.
|Fig. 1 Using your non-cutting hand as a brace helps to maintain standoff and provide a cleaner cut.
As noted, Keddell recommends rehearsing your cut beforehand to make sure it can be made in one continuous movement. It's difficult to start an arc in the middle of a cut and maintain a clean cut-line.
When cutting, keep the torch perpendicular to the workpiece whenever possible (except when beveling).
When piercing thin material to start a cut, the arc may be able to punch through with little or no backsplash. However, on thicker material, start with the torch on a 45-degree angle so the first blast of metal has somewhere to go. If not, the metal will splash back and quickly wear your consumables.
Direction of Travel
Human mechanics makes it easier to pull a torch than push it. A plasma cut contains a beveled edge and a straight edge. If this is important to your project, plan accordingly. Plasma swirls as it exits the tip, biting on one side and finishing off on the other. Keddell says an easy way to remember it is to think of the plasma torch as a car without reverse. Whichever way the torch is traveling is forward and the passenger side is always the straight side.
Amps and Travel Speed
Your owner's manual should contain a chart that compares material thickness to travel speed in inches per minute (see Cutting Performance Chart).
"The faster you move (especially on aluminum), the cleaner your cut will be," Keddell says. "On thicker material, set the machine to full output and vary your travel speed. On thinner material, you need to turn down the amperage and change to a lower-amperage tip to maintain a narrow kerf."
To determine if you're going too fast or too slow, visually follow the arc that's coming from the bottom of the cut. The arc should exit the material at a 15- to 20-degree angle opposite the direction of travel. If it's going straight down, it means you're going too slow and, you'll have an unnecessary build-up of dross or slag. If you go too fast, it will start spraying back. Since the arc will be trailing at an angle, at the end of a cut, slow your cutting speed and angle the torch into cut through the last bit of metal.
|If you're maintaining the proper travel speed, the sparks will exit the workpiece at a 15- to 20-degree angle.
|At the end of a cut, angle the torch forward to cut the last bit of metal.
For marking the material to be cut, use either a black marker or white chalk. Either extreme makes the marks easier to see.
For more precise cutting, a straight edge may be clamped to the workpiece to guide the torch. If you'll be making multiple cuts of the same shape, create a template.
Make sure the guide isn't flammable. Keddell prefers aluminum because of its smooth surface, but added that due to its electrical conductivity the aluminum should touch the torch cup or drag shield and not come into contact with the tip
Straight and circle guides are also available to help ensure precise cutting. (See photos.)
||A center punch is used in preparation for using a circle cutting guide.
||A circle cutting guide helps with clean, repeatable circular cuts.
|A straight guide used with a straight edge.
While it may be easy enough to unpack your plasma cutter and begin cutting, take the time to familiarize yourself with its proper operation. As Keddell noted, read the owner's manual first and keep the preceding tips in mind. It will help you optimize your plasma cutting for better fit-up, increased productivity and lower operating costs.
For more important safety tips as well as information on selecting and using your plasma cutter, visit http://www.millerwelds.com/products/plasma/.