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When understanding the plasma cutting process, we need to appreciate that it is a method of cutting through materials that are conducive to metal, by using an extremely fast jet of very hot plasma.
Often, the materials that a plasma cutting device is used for, include:
- Stainless steel
However other materials can be cut through via the plasma method.
Where is the Plasma Cutting Process Used?
This cutting process is usually found in:
- Fabrication workshops
- Auto repair shops
- Restoration workshops
- Industrial worksites
Salvage and scrap procedures (in fact it is here that Brian first learned how to operate a plasma cutter safely)
Plasma cutting is prevalent in manufacturing and heavy industries because of its efficiency, speed, and low cost. The lower cost has also meant that it has become somewhat of a staple in enthusiasts’ home workshops.
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The plasma cutting process is widely used in automotive restoration and repairs, manufacturing, scrapping, salvaging, and industrial construction.
The cutting process makes use of accelerated jets of hot plasma that cut electrically conductive materials like steel, aluminum, copper, brass, and any other conductive materials.
During the plasma cutting process, an electrical channel of electrically ionized, superheated gas, (plasma from the cutter itself) is created to cut through a piece of material forming a complete electric circuit that leads back to the plasma cutter through the grounding clamp.
The cutting process is achieved by blowing compressed gas such as air, oxygen, inert- and other gases (depending on the type of material being cut) at high speed through a focused nozzle towards the piece being worked on.
This causes the formation of an electric arc within the gas between the workpiece itself and an electrode close to or integrated into the gas nozzle.
Some of the gas is ionized by the electrical arc, forming an electrically conductive channel of plasma.
As the electricity from the plasma cutter travels down the channel of plasma, sufficient heat is delivered to melt through the workpiece.
At the same time, the hot, molten metal is blown away by much of the compressed gas and high-velocity plasma, thereby separating (cutting through) the workpiece.
Plasma cutting is an effective way of cutting electrically conductive materials from thin to thick.
Strong, computer-controlled torches are capable of cutting through steel sheets up to 6 inches (150 mm) thick while hand-held torches can cut through steel plates up 1.5 inches (38 mm) thick.
The very hot, extremely localized ‘cone’ produced by plasma cutters is particularly useful for cutting sheet metal into angled or curved shapes.
How Are Electric Arcs Generated?
The arc is usually produced by a three-step process before coming into contact with a workpiece.
The air within the cutting torch head is briefly ionized by a high voltage spark which makes the air conductive, allowing a pilot arc to form.
The pilot arc is formed by current flowing from the electrode to the nozzle inside the torch head. While in this phase, the nozzle (a consumable part) is burnt up.
The ionized air then blows the plasma through the nozzle, providing a path of current from the electrode towards the workpiece.
As soon as the control system senses the current flowing between the electrode and the workpiece, the electrical connection to the nozzle is cut off.
The current that flows from the electrode to the workpiece causes an arc to form outside the nozzle.
Cutting can then commence without the nozzle burning up. Nozzle life is not determined by cutting time but by the number of arc starts.
A Short History of Plasma Cutting
Welding with plasma in the 1960s gave way to plasma cutting in the 1980s as a very productive method for cutting plate and sheet metal.
It had several advantages over traditional ‘metal-against-metal’ cutting methods that included highly accurate cuts, cutting that produced no metal chips, and providing a cleaner edge than cutting with oxy-fuels.
Early models of plasma cutters were large, expensive, and somewhat slow which relegated it to the process of repeat cutting patterns in ‘mass-production’ mode.
As was the case with many other machine tools, Computer Numerical Control (CNC) technology was applied to plasma cutters from the late 1980s through to the 1990s.
CNC gave plasma cutting machines the ability to cut diverse shapes on-demand, based on a specific set of instructions programmed into the numerical control system of the machines.
However, CNC plasma cutting machines were generally limited to cutting parts and patterns in flat steel sheets by using only 2 axes of motion, generally referred to as ‘X Y Cutting’.
When operating plasma cutting machines proper eye protection and face shields should be used to prevent eye damage from flying debris as well as a condition called ‘arc eye’.
The use of green lens shade #5 is recommended. OSHA recommends the use of shade #8 for arc current that is less than 300A but makes note that:
“these values only apply where the actual arc is clearly visible. Experience shows that lighter filters can be used when the arc is obscured by the workpiece”.OSHA
According to Lincoln Electric, a manufacturer of plasma cutting machines
“Typically darkness shades that range between #7 – #9 are acceptable”.
Another manufacturer, Longevity Global, Inc., offers more specific recommendations for protection at lower amperages in their ‘Eye Protection for Plasma Arc Cutting Table’.
For further protection leather gloves, a jacket, and an apron are recommended to prevent burns from hot metal and sparks.
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