Conquering Cut Angularity in Plasma Cutting: A Troubleshooting Guide

by Alistair McDonald
Posted on 30/01/2025

Many fabrication shops struggle with parts requiring rework due to dross or dimensional inaccuracies. While some issues stem from machine problems, others are inherent to the plasma process. This guide focuses on the critical process variables influencing dimensional accuracy and how to control them to minimize rework and scrap.

Understanding Kerf and Compensation

Kerf is the width of the material removed by the plasma arc. Because the arc’s size and shape fluctuate with amperage, voltage, gas flow, and cutting speed, the kerf also varies. Nozzle size directly affects kerf width; larger orifices handle higher amperages. A useful kerf width estimate is 1.5 times the nozzle orifice size.

CNC controls use kerf compensation to account for the kerf. The operator enters a value (usually equal to or half the kerf width) so the CNC automatically positions the cut on the waste side of the part. Kerf compensation is often determined through trial and error: estimate or measure the kerf, cut test pieces, measure, and adjust the compensation until the part dimensions are correct.

Troubleshooting Kerf Issues

  • Kerf Too Wide (Part Too Small): Causes include a worn nozzle, high torch standoff (arc voltage), excessive amperage, insufficient gas flow, or low cutting speed. These factors widen the arc. An incorrect (small) kerf compensation value also contributes.
  • Kerf Too Narrow (Part Too Big): Causes include low torch standoff (arc voltage), insufficient amperage, excessive gas flow, or high cutting speed. These factors shrink the arc. An incorrect (large) kerf compensation value also contributes.

Bevel Angle: Achieving Straight Cuts

A 0° bevel means a perfectly perpendicular cut. The clockwise swirl of plasma gas in most torches results in a straighter cut on the right side of the kerf relative to torch movement. Typical bevel angles range from 1-3° on the “good” side and 3-8° on the “bad” side. High-tolerance systems achieve even smaller angles. While some bevel is unavoidable, excessive bevel (over 5°) indicates a parameter issue.

Troubleshooting Bevel Issues

  • (Excessive) Positive Bevel (Top Smaller Than Bottom): Causes include a worn nozzle, high torch standoff, insufficient amperage, or excessive cutting speed. These make the arc lag, concentrating energy on the top of the kerf. Improper cut direction can also contribute. A hard bead of high-speed dross often accompanies positive bevel.
  • Negative Bevel (Bottom Smaller Than Top): Causes include low torch standoff, excessive amperage, or low cutting speed. These remove more material at the bottom. Low-speed dross usually accompanies negative bevel.
  • Irregular Bevel (Positive and Negative on the Same Piece): This usually indicates a failed nozzle, an out-of-square torch, or misalignment between the electrode and nozzle. The arc deviates from a straight path, creating a parallelogram-shaped cross-section or concave/convex cut surfaces. These are signs of severely worn or misaligned parts.

Summary

Cut angularity is rarely a table problem. Much more likely torch height, speed, torch consumables, plasma amperage, poor air quality (moisture) etc. Check air pressure going to the Plasma.

Sometimes relocating earth clamp can help. Plasma is DC current so you have a – and +. The plasma arc can sometimes pull away from or towards the earth clamp.

Find out what material (steel or aluminium etc) and what thickness.
We may try some different settings and cartridge/ amperage settings.

Plasma will almost always give some amount of angularity. It is really all about minimising this but rarely can you actually eliminate it.

A basic guide is; Check on which axis the angularity is on . If it’s on both sides of the cut on the same axis it’s a consumable issue. If it’s on one side of the cut it’s a speed or torch height issue .

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