CNC Plasma Cutting

August 22, 2016

A plasma cutter is a tool for cutting metal, it excels at cutting steel and bigger versions can cut through materials around 40mm thick. It is often used for sheet metal but can also be used for cutting pipes. When plasma cutters are used to form a groove in the material, this is called gouging.

The plasma cutter works by ionising a gas as it flows through a small hole, a large current flows through the gas heating it to to form plasma. The jet of conductive plasma transfers heat and current into the metal which causes it to melt, the flow of plasma pushes the molten metal out of the way.

A plasma cutter can be combined with CNC to produce repeatable accurate cuts. A CNC plasma cutter is a versatile tool and can be used to create a wide range of parts such as signs, art work, furniture or gates. Combined with a welder parts can be cut and welded into 3 dimensional objects such as boxes or chassis.

The parts for a CNC plasma cutter are as follows:

Power supply

The system needs a supply capable of providing high voltage to ionise the gas (pilot) and high current to do the cutting. The size of the supply depends on the thickness of metal to cut and also the type. As with welding, alloy metals typically need a high current. As the CNC machine does not get tired you may need a supply with a higher duty cycle (cutting time to cooling time) than for manual cutting. A CNC machine still needs loading and unloading so it won’t be cutting all of the time.

Torch

The torch for CNC plasma cutting is pretty much the same as for manual cutting. Key differences is that it needs to attach to the CNC machine and does not need a manual control. The high currents associated with thicker materials might require water cooling for the torch. For finer details a precision torch can be used, these use a shielding gas as well as the cutting gas. The addition of the shielding gas means that the plasma jet is finer but with the same energy. That can also lead to faster cutting.

Air supply

Most plasma cutters use air as their cutting gas, this is typically supplied by a compressor. The pressure is typically lower than used for air tools and the flow rate may be different too. To ensure consistent cutting a regulator is used to control these. More sophisticated cutters may use alternative gasses for cutting such as nitrogen, oxygen or an argon/hydrogen mix. These add to the cost and complexity of the system but improve the quality or performance.

Gantry / Mechanics

The part of the machine that moves the cutter is called the gantry. This is quite similar to a CNC router in style. To move the cutter servos are often used as they have better torque and acceleration than the alternative of stepper motors. For improved accuracy encoders are used to determine the cutter’s position. These motors need a power supply although that is often bundled with the controller.

Table

The process of cutting forms hot metal dross and this needs somewhere to go. However, the material also needs supporting. For this reason a plasma cutting table is formed from metal bars arranged in a lattice. Some plasma tables are filled with water to cool the dross or work. Plasma cutting can work underwater which reduces dust and work distortion. Your torch and controller will need to be selected to work with this.

CNC controller and Software

The CNC controller is responsible for turning your design into movements of the cutter. This includes controlling the speed and acceleration of the motion. Too fast or slow and the quality of the cut may be reduced or you may have to post process your material to remove dross (molten metal from the cut). A simple controller will take G-Code from your computer but a top of the range controller may have a screen and can import CAD files directly. Another feature that may exist on your controller is nesting, this is the process of fitting many pieces together on one sheet. Good nesting will reduce waste and mean you spend less time loading blank sheets onto the machine.

Fume extraction and safety

Plasma cutting can generate fumes and dust. These can be reduced by using a down draft table along with a fume extractor. Otherwise the main safety concerns are the same as for welding, heat from the materials, fire risk and the flash from the plasma jet. Welding curtains around your machine could be a good investment otherwise welding helmets will be needed by all in the area.

Reference

How a plasma cutter works (Lincoln Electric)

How a plasma water table works (ESAB)

Plasma Cutter Project ideas

Thanks to Westerman International for providing the pictures of the machines and idea for this post.

By Andy from Workshopshed One Comment

  Posted in: CNC