Arc cutting is a fundamental skill for welding professionals in Kenya, especially in industries requiring precision metal fabrication such as manufacturing, construction, and repair workshops. Mastery of arc cutting enables efficient separation of metals with minimal distortion, which is critical in environments like county government fabrication units or hotel maintenance workshops. This chapter focuses on the technical parameters and practical procedures involved in arc cutting, ensuring that welders can perform clean cuts on metals up to 6 mm thickness safely and effectively.

2.1 Arc Cutting Parameters

Understanding the parameters that influence arc cutting is essential for achieving precise and efficient cuts. In Kenyan welding workshops, where resources might vary, optimising these parameters ensures consistent quality and reduces material wastage. Parameters such as current, arc length, electrode type, and cutting speed must be carefully set to suit the metal thickness and type being cut.

2.1.1 Setting

The setting of arc cutting parameters involves adjusting machine controls and selecting appropriate consumables to match the cutting task. Proper settings improve cut quality and reduce electrode consumption, which is critical for cost control in any welding and fabrication workshop. The current is set first, and the arc length, electrode choice and cutting speed are then matched to it.

2.1.1.1 Current

Current is the most important setting on the machine because it controls the heat that melts the metal along the cutting line. In arc cutting the current is normally set higher than for welding with the same electrode, since the metal must be melted right through the plate and blown or pushed out of the kerf. For example, a welding workshop cutting 4 mm mild steel plate with a 3.2 mm covered electrode would start at the upper end of the range printed on the electrode packet and then fine-tune on a scrap piece.

• Setting Range: The correct current depends on the electrode type and diameter, so use the range printed on the electrode packet or given by the manufacturer. For cutting thin plate up to 6 mm, start at the upper end of that range so that the metal melts through quickly.
• Effect on Cut Quality: Too low a current gives poor penetration, a sluggish arc and rough edges, and the electrode tends to stick. Excessive current overheats the electrode, widens the kerf and causes burn-back of the coating, which wastes both metal and electrodes.
• Adjustment Method: Use the amperage control dial or the current setting lever of the machine. Start at the upper end of the recommended range and adjust in small steps while watching the cut on a test piece.
• Influence of Metal Type: Different metals conduct and lose heat differently. Mild steel, stainless steel and cast iron each need different settings, and thicker or more conductive material needs more current.
• Impact of Electrode Diameter: Larger diameter electrodes need higher current to maintain a stable arc. For example, a 3.2 mm electrode requires more current than a 2.5 mm one.
• Polarity: Use the polarity (DC electrode positive or negative, or AC) recommended for the electrode being used. Wrong polarity gives an unstable arc and poor cutting action even when the current value is correct.

Procedure for setting the cutting current:

1. Identify the thickness and type of metal to be cut, for example 4 mm mild steel plate, and select an electrode suited to cutting.
2. Read the recommended current range and polarity on the electrode packet or manufacturer's data sheet.
3. Connect the work clamp firmly to clean metal and check that the cables and electrode holder are in good condition.
4. Set the polarity and set the current dial at the upper end of the recommended range.
5. Strike an arc on a scrap piece of the same thickness and make a short test cut, noting how quickly the metal melts through and how wide the kerf is.
6. Reduce the current slightly if the kerf is too wide, the electrode overheats or the edges are burnt. If the cut is still sluggish or does not penetrate, check the electrode, polarity and travel speed before raising the current further within the machine's limits.
7. Repeat the test until the cut penetrates fully with a narrow, even kerf, then record the setting for similar jobs.
8. Recheck the current during long jobs, because a hot electrode and a warming machine can change the arc behaviour.

Arc length

Arc length controls how concentrated the heat is on the cutting line. A trainee in a welding workshop will notice at once that a drifting arc gives a ragged, wide cut.

• Definition: Arc length is the gap across which the arc jumps between the electrode tip and the workpiece.
• Recommended Length: Maintain a short arc length, approximately equal to the electrode diameter, to ensure a concentrated and stable arc.
• Consequences of Incorrect Arc Length: A too-long arc causes a wide kerf and poor cut quality; a too-short arc can stick to the workpiece, damaging the electrode.
• Technique: Welders should develop a steady hand and maintain consistent arc length through practice and visual feedback.
• Environmental Factors: Wind or drafts in open workshops can affect arc stability, so use screens or move the work to a sheltered position.

Electrode selection

The electrode carries the current to the arc and also affects how the metal is melted and cleared from the cut. Choosing the right electrode saves time and consumables. A fabrication workshop cutting openings in steel sheets would choose a different electrode from a workshop cutting cast iron brackets.

• Electrode Types: Common electrodes for arc cutting include carbon electrodes and special covered steel electrodes designed for cutting, each suited to different metals and cutting conditions.
• Carbon Electrodes: These do not melt into the cut but heat the metal by the arc, and they usually need direct current. They are suited to rough cutting and to metals such as cast iron, and they leave a rough edge that needs finishing.
• Covered Steel Electrodes: They can be used with an ordinary MMAW machine and cut mild steel, alloy steels and cast iron, so no special equipment is needed. Their coating helps to stabilise the arc and drive molten metal out of the kerf.
• Electrode Coating: Coated electrodes can improve arc stability and reduce spatter, which is beneficial in humid or dusty workshops. Damp coatings cause erratic arcs, so electrodes should be stored dry.
• Cost and Availability: Selection often balances performance and affordability, and small workshops may choose locally available electrodes provided they suit the metal being cut.

Cutting speed

Cutting speed must match the current: the heat supplied has to be enough to melt through the plate at the pace the operator travels.

• Definition: Cutting speed is the rate at which the electrode is moved along the cutting line.
• Optimal Speed: Should be fast enough to avoid excessive heat build-up but slow enough to ensure complete penetration through the metal.
• Effect on Cut Quality: Too slow causes excessive slag and a rough edge; too fast leads to incomplete cutting and the need for multiple passes.
• Operator Skill: Experience plays a critical role in maintaining appropriate cutting speed, especially in site work such as cutting out damaged steel sections during maintenance.
• Use of Guides: Mechanical guides or jigs can assist in maintaining consistent speed and straight cuts in repetitive tasks.

Chapter Summary

This chapter covered the essential parameters involved in the arc cutting process, focusing particularly on how to set the current correctly to achieve efficient cutting results. It detailed the step-by-step procedure for carrying out arc cutting on materials up to 6 mm thick, emphasizing safety and precision throughout the operation. The chapter also explored various applications where arc cutting is practical and commonly used, highlighting its versatility across different sectors. Following the cutting process, attention was given to edge finishing techniques suitable for materials of similar thickness, explaining the types of finishing required to ensure smooth and clean edges. The finishing procedure was outlined clearly, demonstrating how to improve the quality and usability of the cut materials. Finally, the chapter discussed practical applications of edge finishing, reinforcing its importance in preparing materials for further fabrication or assembly.