Automated plasma cutting basics

Automated CNC plasma cutting is an effective process for cutting complex metal material parts. CNC plasma cutting machines has several factors or input variables to control (e.g., current, cutting speed, torch height, etc.) and the process requires compliance with a variety of part quality characteristics or response variables (e.g., flatness, clean cut, bevel angle, etc.). it is difficult to find a machine setting that improves the overall quality of the manufactured parts.

 This process works with high velocity gas jet and resistive heating. The cutting speed of plasma is four times faster than any other oxy-fuel processes and the cuts are also clean as it uses dry air in most of its applications. An even cleaner and more accurate cut can be produced when oxygen is used as ionised gas, but the operation costs would be higher than using nitrogen. In the plasma cutting system, a microprocessor is employed, which controls electric current and gas flow simultaneously during the cutting process.

In recent years, plasma CAM has combined plasma-based cutting technology with computer numerical control (CNC) systems to perform complex cuts in a short period of time. The plasma CAM system uses 2D or 3D CAD drawings of the part geometry to generate a cut path. The computer-generated cut path is then used to control the movement of the plasma torch (i.e., a component of the automated plasma cutting tool that is installed on a robotic XY table). However, parts quality is not always as high as required because wrong settings are typical and they result in quality problems such as deformation of the sheet metal, bad part cut, accumulation of metal residuals underneath the work piece, inaccurate dimensions and cut sloppiness. Preliminary experiments revealed that the quality of parts is directly related to the machine settings.

So lets talk about these settings and how to properly choose them.

The cutting power was determined by the current, as voltage was constant. Cutting power is dependent only on the type and thickness of the material being cut. The range of variation allowed by the CutMaster™ A80  was 20 A to 80 A. The 20 A to 40 A range is used for drag tip cutting where the torch tip touches the work piece. However, this cutting range was not used because it was more proper for the thinner sheet metals and for the format that the tip touches the work piece. The 40 A to 80 A range is employed for standoff cutting and the torch tip did not touch the work piece. The experimental levels studied for current were 40 A, 60 A and 80 A. Typical results from insufficient cutting power proved to be cuts that did not penetrate all the way thru the thickness of the work piece.
Typical results from too much cutting power were:
1. Kerf width was too big.
2. Excessive dross build up due to extreme heat.
3. Poor cut surface quality (Thermal Dynamics, 2007).

Pressurised air serves two purposes in plasma cutting. The primary one is to supply gas to fuel the plasma reaction and the secondary one is to blow melted material away while cooling the tip. Pressure is determined by torch model, cutting power and torch leads length. As torch model and torch lead length were kept constant, the only factor that remained was cutting power. A maximum pressure input of 125 psi (8,6 bar) was also listed. Operating pressure was found to be 70 psi (5,1bar).
Typical results from excess pressure were:
1. Poor cut surface quality.
2. Excessive top spatter.
3. Poor bevel angle.

Cut speed
The cut speed is the speed at which the torch moves in the X-Y plane while the torch is cutting. Cut speed varies depending on material type, material thickness and input power. Material thickness and type were constant and cut speed was dependent only on input power. Typical results from high cut speed were high speed dross, poor bevel angle and cuts that did not go completely through the thickness.
Typical results from low cut speed were:
1. Slow speed dross.
2. Unstable arc.
3. Loss of arc.

Torch height
Torch height is the distance between the tip of the torch and the work piece. Standoff distances of 3 mm to 9 mm were proposed in the 1Torch™ instruction manual. A typical result from cutting too close was that the tip would touch the work piece triggering a safety mechanism built into the Cut Master™, which would drop the current to 40 A.
Typical results from cutting too far away were:
1. Excessive top spatter.
2. Poor bevel angle.
3. Cuts that did not go completely through the thickness.

Slower on curves
The machine has the ability to slow down when going around corners to obtain a finer cut. If the setting value for ‘slower on curves’ is greater than zero, then the machine reduces its speed when cutting curves and circles. This means that straight cuts differ from circular, semi-circular or curved cuts.