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READING THE CUT

The following article focuses on the interpretation of key factors that affect plasma arc cut quality.

The main cut quality characteristics are bevel angle, lag lines, dross levels and dimensional accuracy, they are addressed individually so that the influence of change can be assessed. 


CUT ANGLE

A cut part whose 4 sides average less than 4°of bevel angle is considered acceptable.

Negative cut angle
top dimension  is greater than the bottom dimension.
The main causes are typically low torch to workpiece distance, misaligned torch, bent or warped material, slow cutting speed, excessive power, worn or damaged consumables.

Solution – increase the arc voltage to raise the torch or increase the cutting speed to square up the cut.

Positive cut angle
top dimension is less than the bottom
dimension

The main causes are typically high torch to workpiece distance, misaligned torch, bent or warped material, fast cutting speed, worn or damaged consumables.

Solution – decrease the arc voltage to lower the torch or increase the cutting speed to square up the cut.

Combination of positive and negative cut angle
This can be the result of the cut speed being too fast or a malfunction of the torch height control.


LAG LINES & DROSS LEVEL

Lag lines are the ripples present on the edge of the cut that show the variation in cutting speed. Reading the lag lines on a cut surface is an excellent way to determine proper cutting speed. Normal lag lines are curved and slanted at about 15° and the cut is dross-free.

Good quality stainless steel cut

Good quality aluminium cut 

Dross is a by-product of the cutting process. It is the undesirable material that remains attached to the part. In most cases, dross can be reduced or eliminated with a correct torch and cutting parameter set-up.

The appearance of dross is relevant to the cutting speed. Using the correct cutting speed can deliver a dross free cut. The proper set up of operation data for a given material thickness is noted in the mid-range of cut chart.

Low speed dross

Globular dross that forms in large deposits. It has low adhesion properties and is quite easy to remove, in sizable pieces.

High speed dross
Thin sharp edged material that welds itself to the bottom edge of the workpiece. It is difficult to remove, normally requiring grinding or other 2nd ops dressing. There are clearly defined "S" shaped lag lines shown on the cut edge of the piece.

Top dross (spatter)
Top dross or spatter can form on the top edges of the workpiece and parent material. It is normally formed when the consumables are spent, incorrect torch height or where the cut speed is too fast. The dross is typically easy to remove.

Intermittent (non-continuous) dross

Worn or spent consumables can contribute to the formation of intermittent dross along the top or bottom edges of the cut piece.

Other factors that contribute to the formation of surface dross are, temperature of parent material, heavily mill scaled or corroded material and high carbon alloys.


DIMENSIONAL ACCURACY (KERF WIDTH)

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

Kerf width is critical to the dimensional accuracy of the part. A wider kerf is typically caused with a slower cut speed or excessive power. The kerf width equals to 1.5 – 2.0 x nozzle orifice diameter.

It is essential that the parameters of the cut charts are adhered to. Cutting performance can differ across installations and material composition, systems have an in-built facility that enables the override of the cut chart parameters.

Example


FINAL RECOMMENDATION

  • Cutting issues? Prior to making any alterations, check the condition of the consumables, check the part numbers are correct and the cutting variables match the manufacturers recommended settings.
  • Small adjustments to the cut charts settings may be needed to compensate for a change in material quality, material temperature or a specific material type.
  • Make small incremental changes when adjusting settings. Adjust the Arc Voltage up or down in 2 volt steps as required. Increase or decrease the cutting speed by 5% until the cut quality improves.
  • Using the recommended cut chart values for cutting speed and arc voltage maximises cutting performance.
  • The cut chart variables are interdependent, changing one setting affects all others and may influence the overall cut quality. 
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