Although plasma cutting is desirable for many metal-cutting applications, analyze your specific application before choosing a cutting method. This decision depends primarily on the material cut, thickness, desired cutting speed, intricacy, and quality.
Material Type
Carbon steels, aluminum, and stainless steels are most commonly cut with
plasma arc. Many other metals may be cut with plasma including nickel alloys,
brass, bronze, tungsten, copper, cast iron, titanium, and zirconium.
Material Thicknesses and Cutting Speeds
Workpiece thickness determines whether plasmas cutting speeds will be cost
effective for your application. However, the maximum cutting speed depends not
only on thickness but also on power supply and material type. The table shown
here illustrates representative cutting speeds on various thicknesses of
aluminum, stainless steels, and carbon steels.
Cut Quality
Cut quality is affected by type of metal and cutting speed. Process
variables, such as cutting gas, power, and cutting speed are adjusted to provide the
optimum cut for each metal type. Although the size of the power supply is also a
factor, cuts in metals up to 2 in. thick tend to be smooth while cuts in
thicker sections may be rougher but still clean.
Cutting Specifications
For applications where high-quality cuts are needed, determine your
requirements for a) tolerances, b) amount of bevel, c) dross, and d) heat-affected zone
Plasma cuts to closer tolerances than flame processes like oxyfuel because of the faster cutting speeds heat the workpiece less, resulting in less distortion. Plasma is capable of tolerances to 1/32 in. in materials under ˝ in., but the tolerances achieved depend on material type, thickness, and power supply.
Plasma cutting produces a beveled cut, forming a wider cut at the top of the workpiece than at the bottom. The bevel can easily be corrected or reduced with special techniques or equipment. Generally the amount of bevel is less for thinner materials. The amount of dross or oxidation on the surface of the workpiece depends mainly on cutting speed, type of gas, and arc voltage. Using the manufacturers guidelines for these variables can produce dross-free cuts. The high speeds of plasma cutting minimize the amount of distortion and heat-affected zone (HAZ). HAZ width is affected by material type and thickness, conductivity, and torch design.
Cutting Gases
The cutting gas selected depends on the speeds and quality of cut desired.
Several cutting gases can be used in a plasma system to improve cut quality and
speed. Nitrogen is widely used because it is relatively inexpensive and can be
used on many materials and thicknesses. Special mixtures of argon and hydrogen
can improve cutting speed and quality on thicker metals and those other than
carbon steels. Oxygen is used in combination with other gases to improve cut
quality by increasing heat, improving cutting speed, and/or reducing power
requirements. Compressed shop air is popular for many applications because it is
inexpensive and provides good quality cuts on thicknesses under 1 in., especially
on carbon steels.
Power Supply
The power supply required depends on the material thickness and cutting speeds
desired. Increasing the power increases the cutting speed or enables thicker
metals to be cut without slow down. Power ratings are commonly between 20 and
200 kW.
