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PLASMA CUTTING

In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. After sufficient heating a gas dissociates its molecular bonds, rendering it into constituent atoms. However, further heating may also lead to ionization (a loss or gain of electrons) of the molecules or atoms of the gas, thus turning it into a plasma, containing charged particles: positive ions and negative electrons. The presence of a non-negligible number of charge carriers makes the plasma electrically conductive so that it responds strongly to electromagnetic fields.

Plasma cutting is a thermal cutting process in which a constricted arc is used. Polyatomic gases dissociate in the arc and partially ionize; monoatomic gases partially ionize. The plasma beam thus generated has a high temperature and kinetic energy. It melts or partially vaporizes the material and blows it away. Thereby the kerf is produced. The sheet thickness which can be cut is limited since for plasma cutting the whole heat required to liquefy the material has to be made available by the plasma cutting. With plasma cutting a difference is made between transferred and non-transferred arc. For the plasma cutting process, the material to be cut shall be electrically conducting since it forms part of the electrical circuit. This process is suitable for low and high cutting performance, i.e. cutting of thin and thick metal sheets. The plasma gas which is used as function of the material to be cut and of the cutting thickness, is of decisive importance for the energy transfer. On plasma cutting with non-transferred arc, the material is not placed within the electrical circuit. Therefore, electrically non-conducting materials may also be cut by this process. Plasma cutting with non-transferred arc only is suitable for low-cutting performance values as the cutting nozzle serves as anode. Nearly all fusible, electrically conducting metals, such as unalloyed and low-alloy steels, nickel based materials, copper alloys, titanium alloys, aluminium alloys and others are suitable for plasma cutting.

Advantages and disadvantages of plasma arc cutting

Plasma arc cutting has long been seen as a low cost alternative to oxy-fuel and laser profiling where cut angle was not an issue. Recent developments in the high precision / high definition plasma process have significantly improved the quality and capabilities of plasma cutting, making it a more versatile and accurate option than ever before.

Automated plasma arc cutting systems provide several advantages over other cutting methods such as oxyfuel and laser:
Rapid Cutting Speeds: plasma arc cutting is faster than oxyfuel for cutting steel up to 50 mm thick and is competitive for greater thickness. Plasma cutting achieves speeds greater than those of laser cutting systems for thickness over 3 mm. The fast cutting speeds result in increased production, enabling systems to pay for themselves in as little as 6 months for smaller units.

Wide Range of Materials and Thickness: Plasma cutting systems can yield quality cuts on both ferrous and nonferrous metals. Thickness from gauge to 80 mm can be cut effectively.

Easy to Use: Plasma cutting requires only minimal operator training. The torch is easy to operate, and new operators can make excellent cuts almost immediately. Plasma cutting systems are rugged, are well suitable for production environments, and do not require the potentially complicated adjustments associated with laser cutting systems.

Economical: Plasma cutting is more economical than oxyfuel for thickness under 25 mm, and comparable up to about 50 mm. For example, for 12 mm steel, plasma cutting costs are about half those of oxyfuel.

Despite the advantages of plasma cutters, there are some drawbacks. The cutter's electrode and nozzle sometimes require frequent replacement which adds to the cost of operation. Non-conductive materials such as wood or plastic cannot be cut with plasma cutters. Another minor drawback is that the plasma arc typically leaves a 4-6 degree bevel on the cut edge; although this angle is almost invisible on thinner material, it is noticeable on thicker pieces.


The quality tolerances and the geometrical product specifications of the thermally-cut work piece are specified and explained in International Standard EN ISO 9013.

The nominal dimension of cutting part (dimension with allowance) can be obtained from the nominal dimension of the finished part (dimension from the drawing). In order to be able to maintain the nominal dimensions at the finished part, it is necessary, for otside dimensions of work pieces a machining finishing allowance, Bz, to add the perpendicularity tolerance as well as the lower limit deviation and, for inside dimensions of work pieces with a machining finishing allowance, Bz, to subtract the perpendicularity tolerance as well as the lower limit deviation.


A nominal dimension of finished past
B nominal dimension of cutting past
Bz machining allowance
Gu lower limit deviation
Go upper limit deviation
u perpendicularity tolerance
Th work piece thickness

In order to calculate the cutting dimension (B), we have to know the finite dimension (A):

B = A + 2Bz + Gu

The cutting dimension can be calculated by adding to the finite dimension the value of the machining allowance and the cutting tolerance.
The real value of the cutting dimension will be between (B – Gu) and (B + Go).

The dimensions in the drawings shall be taken to be the nominal dimensions, the actual dimensions being determined on the clean surfaces of the cut. The limit deviations for the cut surface quality(perpendicularity tolerance) are treated separately from the limit deviations for the dimensional deviations of the work piece in order to emphasize the different influences on the work piece.


a) Viewed from above                                                         b) Viewed from bottom
CONTOUR CUTTING

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