To get the best quality plasma arc cutting results, it is necessary to be aware of the main factors that affect cut quality. This article explains what those factors are!

7 MAIN FACTORS:

• Consumables, correct rating/condition​
• Amperage (Current)​
• Plasma arc and Shield gases​
• Arc voltage​
• Torch height​
• Cut speed​  
• Kerf width

CONSUMABLE

Good cut quality can not be expected if consumables from an incorrect rating are used, cut charts give good advice on what is the best amperage for a given thickness. The condition of consumables is critical, ensure that all parts are fit for purpose

AMPERAGE (CURRENT)

Amperage is the strength of a current of electricity expressed in amperes. Current is the movement of electrons through an electrical circuit. A higher current level usually results in less dross for a given material thickness. 

I_c = 260 Amp
Thickness 25 mm, mild steel, cutting speed 1685 mm / min.

I_c = 130 Apm
Thickness 25 mm, mild steel, cutting speed 550 mm / min

GASES USED FOR PLASMA ARC CUTTING

Purity – It is essential that the following gases are delivered to the plasma arc system in the listed levels of purity. Failure to do so could result in poor cut quality, poor consumable life, and overall degradation in system cut performance.

O2 and N2 with oxygen processes:

• O2 = 99,5 %, N2 = 99,99 %
  

N5 & H35 with inert processes:

• H35 – 99.995 % pure (H35 = 65% Argon, 35% Hydrogen)
• N5 – 99.98 % pure (F5 = 95% Nitrogen, 5% Hydrogen)

Air: clean, dry and oil-free

Methane: 93 % pure

Plasma gas – this is ionised gas/gases used during the cutting process of conductive materials, the plasma gas exits the nozzle orifice in the form of a plasma arc cutting stream.

Typically used plasma arc gases:

• Compressed Air
• Oxygen
• Nitrogen
• Argon-Hydrogen 
  

Shield or Secondary gas – surrounds the plasma arc, in some torches it assists in arc constriction, assists in cooling the nozzle and shield. Can be used to create an oxygen free atmospheric zone around the cut edge, producing a cleaner cut edge. The shield gas used is dependent upon the choice of plasma arc gas.

The selection of gases depends on the type of material that is being cut (mild steel, stainless steel, aluminium, etc.) and the desired cut quality. Recommended gas combinations are published in the operator manual by the manufacturer.

ARC VOLTAGE

Arc voltage control (AVC) monitors the circuit voltage and adjusts the height of the torch in accordance with the pre-set values. The torch height to work has an effect on the bevel angle, and overall quality of the cut. The arc voltage control maintains the pre-set torch to work distance on uneven surfaces.

There is a relationship between Arc voltage and cut speed. As the cut speed increases, the arc voltage decreases and vice versa. Mechanised cutting programs have speed percentage values of the straight line speed, this enables the cut speed to automatically reduce when cutting circles, irregular shapes and change of direction at corners, also the “Ramp up” and down at the start and end of a cut, this can cause the formation of dross in slower speed areas of the cut but with a better bevel angle. 

Relationship between arc voltage and cut speed

THC (torch height control) reacts to travel speed: The torch will lower as speed decreases or rise as speed increases. THC must be turned off or “AVC Disabled” when speed decreases.

TORCH HEIGHT – PIERCING AND CUTTING

With the plasma arc torch in the resting position, the first stage of the cutting process is where the torch moves to the torch to work height for piercing, the piercing height is pre-programmed to suit the material thickness, in most cases the piercing height is between 1.5- 2 times the cut height. Cut height is the torch-to-work distance maintained during a cut cycle.

How does it work?

1. The THC is “blind” prior to the cutting process

2. In order to establish the zero height point, the torch cap or sensor is required to touch the upper surface of the workpiece.

3. The THC lifts the torch from the zero height to the piercing height in accordance with the cut chart settings.

4. After piercing of the workpiece, the THC lowers the torch to the programmed cut height

5. AVC is turned ON (AVC enabled)

CUTTING SPEED

Correct cut speed can produce dross free cutting. To obtain the “Best” cut quality from a system, use the cut chart data provided by the manufacturer. The precise operating data (such as amperage, arc voltage, torch-to-work distance and cut speed) for a given material thickness should be in the mid-range of the cut chart.

Example: How cut speed affects cut quality

Cut speed is too slow (Thick heavy dross)/Cut speed is correct (Dross free) /Cut speed is too fast ( Thin sharp dross)

KERF WIDTH

Kerf width is relevant to the dimensional accuracy of the part. A wider kerf is usually caused by a slower cut speed or excessive power. Kerf width = 1.5 – 2.0 x orifice diameter of the nozzle.

Kerf is the void created by the cutting process (or the volume of material removed).

We hope the article was useful for you. We also recommend another article related to the theme, “Reading the cut”.

If you need any further help, please contact us at the following email: support@thermacut.cz

Text:        Thermacut, k. s., Jan Košárek

Photos:   archive of Thermacut, k. s.