Selecting a Plasma Cutter

Is plasma arc cutting right for you?

What is plasma cutting technology?
In simple terms, plasma is superheated, compressed air that ionizes to form a conductive gas. This gas conducts electricity from the torch of the plasma cutting machine to the work piece. This is done through a conductive electrode and copper nozzle on the machine, which constricts the high-velocity gas and cause a tornado effect. This provides energy to the arc that melts and blows away the metal.
 

What to look for when purchasing a plasma cutting machine?
Once you have determined the 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.
The first factor you need to determine is the thickness of the metal most frequently cut. Most plasma cutting power sources are rated on their cutting ability and amperage. Therefore, if you most often cut thick material, you should consider a lower amperage machine. If you most frequently cut metal that is thick, look for a higher amperage machine. Even though a smaller machine may be given to cut through a given thickness of metal, it may not produce a quality cut. Instead, you may get sever cut that barely makes it through the plate and leaves behind dross or slag. Every unit ahs an optimal range of thickness-make sure it matches up with what you need. In general, a machine ahs approximately 25 A of output, a machine has an output of 50 to 60A and a machine has 80A of output.

2. Select the optimal cutting speed.
Are most of your cutting operations performed 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 inches per minute. If the metal you cut most frequently is , a machine that offers higher amperages will be able to cut through the metal much faster than one rated at lower amperage, although both will do the job. For production cutting, a good rule of thumb is to choose a machine class twice your cutting thickness. For example to perform long, fast, quality production cuts on 1/4 steel, choose a  class(60-A) machine. If you perform long, time-consuming cuts or are cutting in an automated setup, 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 typically rated as a percentage of a 10-min. period. For example, a 60% duty-cycle at 60A means you can cut with 60-A output power continuously for 6 min out of a 10-min period. In general, the higher the duty-cycle, the longer you can cut without taking a break.

3. Compare consumable cost vs. consumable life.
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 most cost savings. Look in the manufacturer's specifications for how long a consumable will last, but be sure to compare the same data when comparing one machine against another. Some manufacturers use different measurement standards; some rate consumables by number of cuts, while others use the number of starts.

4. Test the machine and examine cut quality.
Test 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 fross on the bottom side and see if the kerf (gap left by the cut) angle is perpendicular and angular. Look for a plasma cutter that offers a tight, focused arc for the best kerf. Another test 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.

5. Pilot-to-cut and cut-to pilot transfers.
The transfer from pilot to cut occurs when the pilot is brought close to the work piece. The mechanism for this transfer is a voltage potential from the nozzle to the work piece. Traditionally a larger resistor in the pilot arc current path creates this voltage potential. The voltage potential directly affects the height at which the arc can transfer. After the pilot arc transfers to the work, a relay or transistor switch 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. Cutting expanded metal or gratings will be required to transfer from pilot to cut and back to pilot very quickly. To get around this, the manufacturer may recommend cutting expanded metal using only the pilot current.

6. Check the machines working visibility.
As you work on an application, you want 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 to better see where you are cutting, as will an extended nozzle.

7. Look for the portability factor.
Many consumers use their plasma cutters for a variety of cutting applications and need to move a machine around a plant, job site or even from site to site. Having a trolly mounted unit and a means of transporting it 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 means these items will not drag on the ground or get lost during machine transport.

8. Determine the machines ruggedness.
For today's harsh job site environments, look for a machine that offers durability and features protected controls. For example, fittings and torch connections that are protected will wear better than those that are not. Some manufacturers offer a two stage air filters. These filters are an important feature because they ensure oil is removed from the compressed air. Oil can cause arcing and reduce cutting performance & also it will reduce the life of consumables. This two stage filters is important because it ensures oil and water, which reduce cutting performance, are removed from the compressed air.

9. Is the machine comfortable and easy to operate?
Look for a plasma cutter that has a big, easy to read control panel. Such a panel enables someone unfamiliar with a particular plasma cutting unit 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 setup and trouble shooting. How does the torch feel in your hand? You want something that feels comfortable and has a good ergonomic design. Try the torch out just the same way you would test the grip of a golf club.

10. After sales service.
This is the most important aspect while selecting the plasma cutting system. Before selecting the machine, confirm with the existing clients about the after sales service & availability of the spares provided by the manufacturer.
 

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 even beginners make the best possible cut.
 

Set-up procedures:
Before you start, check for the following items:

	A clean air supply without water or oil.
	(consumables that leave quickly or wear black 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 correctly at place.
	A ground connection to a clean portion of the work.

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

Piercing the work
Many inexperienced users try to pierce the metal by coming down perpendicular (90°) to the work. This blows molten metal 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.
 

Don't touch the nozzle to the work piece
When using higher current levels, 45 A and above, do not touch the nozzle t the work. Doing so will drastically reduce the nozzle life because the cutting will double arc through the nozzle. Double arcing ca 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.
 

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

Travel at the right speed
When moving at the right cutting speed, the molten spray will blow out the bottom of the plate at a 15-20° angle. If you are moving too slow, you will create a slow speed dross, which is an accumulation of molten metal or burr on the bottom edge of the cut. When moving too fast, high-speed dross is created because you are not allowing time for the arc to completely through the metal. This will create a low-quality cut. Typically, ease of removal distinguishes low-speed dross from high-speed dross. For example, low-speed dross typically requires grinding.
 

Maintain a constant work distance
Optimally, you should maintain a 3/16 to 1.8 distance from a nozzle to the work. Moving in a weaving, up-and-down fashion will only hinder your efforts.

Travel in the direction that gives 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 on the right-hand side because it will tend to have a better, more square edge.
 

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. This increase in the push angle at the finish will cut through the bottom first and get rid of he bottom corner that is usually left at the end of thick plate. Never use the torch to hammer away the last corner of the work to finish a cut.
 

You're on your way
After finding the right machine for your application and learning some tricks of the trade, you should be ready to cut. Remember, electro plasma equipments pvt. ltd offers a number of benefits and you should provide you with faster, high-quality and economic cuts.