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FACTORS AFFECTING THE CUT QUALITY

To get the best cut quality results with plasma, it is necessary to be aware of the main factors affecting the cut quality. In this article we are going to describe them.

6 MAIN FACTORS

  • Amperage (cutting current)
  • Gases used for plasma cutting
  • Arc voltage
  • Torch height
  • Cutting speed
  • Kerf width  

AMPERAGE

 Amperage means the measurement of the electron flow in an electrical circuit. A higher current level usually results in less dross for a given material thickness.

Ic = 260 Amp

Thickness 25 mm, mild steel, cutting speed 1685 mm / min. 

A lower current level usually results in better edge quality and squareness for a given thickness, but maximum dross formation.  

Ic = 130 Apm

Thickness 25 mm, mild steel, cutting speed 550 mm / min 

Lower current consumables typically have a longer operating life than higher current ones, especially when cutting with oxygen.

GASES USED FOR PLASMA CUTTING

Purity - It is essential that the following gases be delivered to the plasma 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 is the gas utilized to cut materials (cutting gas). Gas that is ionized in the plasma process exits through the nozzle orifice.

  • Typically used plasma gases:
  • Compressed Air
  • Oxygen
  • Nitrogen 
  • Argon-Hydrogen

Shield or Secondary gas surrounds the arc, and in some torches assists in arc constriction. Further assists in nozzle cooling, creates an atmosphere around the cut edge devoid of oxygen, produces a cleaner cut edge. Shield gas is dependent on choice of plasma gas.

The selection of gases depends on the type of material that is being cut (mild steel, stainless steel, aluminium, and so on). Expected cut quality should also be considered. The combination of gases is predetermined by the manufacturer in the instruction manual. 

ARC VOLTAGE

Arc voltage height control monitors circuit voltage and adjusts the height of the torch accordingly. Height of the torch affects bevel angle and overall quality of the cut. The arc voltage height control maintains stand-off distance on uneven plate.

Arc voltage depends on the cutting speed, and the two are related. As cutting speed increases, arc voltage decreases and vice versa. The cutting speed changes when going in and out of corners, at the beginning and end of a cut, and when cutting circles and contours. This will cause dross in corners and contours and improve bevel angles (when speed decreases). 

  Relation between the arc voltage and cutting speed

THC (torch height control) reacts like this: 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

Piercing height is the initial height of the torch from the plate when piercing. Piercing height should be 1.5-2 times the correct stand-off distance. Cutting height is the torch-to-work distance maintained during a cut cycle.  

How does it work?

1. THC is "blind" before cutting process

2. Torch cap or sensor has to touch the top surface of cutting material to detect zero height (initial position)

3. THC retracts the torch from zero height to piercing height (adjusted according to cutting charts)

4. After piercing of the cutting material the THC moves the torch down to cutting height

5. AVC is turned ON (AVC enabled) 

CUTTING SPEED

Correct cutting speed means dross free cutting. To get the good cut quality you should always follow the cutting charts. The proper operating data (such as amperage, arc voltage, torch-to-work distance and cutting speed) for a given material thickness should be in the mid-range of cutting chart.

Example of plasma arc formation

Low cutting speed / Correct cutting speed / Fast cutting speed 

KERF WIDTH

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