Arc cutting is a vital skill for welding professionals, especially in Kenya's growing manufacturing and construction sectors where precision and efficiency are essential. Mastery of arc cutting parameters and processes enables welders to perform clean, accurate cuts on metal components, facilitating fabrication, repair, and maintenance tasks. This chapter focuses on understanding the critical settings and executing arc cutting for metal thicknesses up to 6 mm, a common requirement in workshops, fabrication units, and repair garages across the country.

2.1 Arc cutting parameters

Arc cutting parameters decide the quality, speed, and safety of a cut. The welder sets them to suit the metal, its thickness, and the equipment, so that the cut is clean and the waste and rework are small. The main settings are the current and the electrode, together with the voltage, polarity, and cutting speed that go with them.

2.1.1 Setting

Setting means adjusting the machine and choosing the consumables before the cut starts. Each parameter is chosen for the metal type and thickness, and a trial cut on scrap confirms the choice. Example: a welding workshop cutting 4 mm mild steel plate may set the current, fit a carbon electrode, and make a short test cut on an offcut before starting the job.

2.1.1.1 Current

The current (amperage) controls the heat of the arc, and so it affects the cutting speed and the depth of penetration. For thin metals up to 6 mm, lower settings are used, typically between 40 and 80 A, to prevent excessive melting and warping. Too low a current gives an unstable arc and an incomplete cut, and too high a current burns the edges and wastes the electrode. Example: a workshop cutting mild steel sheet may select a current that gives a stable arc without burning through the metal.

• Heat input: A higher current gives more heat and a faster cut, but a wider and rougher kerf.
• Thickness: Thicker plate needs a higher current than thin sheet.
• Voltage: A range of about 20 to 40 V suits most thin metal cutting, since too high a voltage gives a long, hard-to-control arc and too low a voltage gives an unstable arc.
• Cutting speed: The electrode is moved steadily at a speed that matches the current and the thickness, to give a clean cut without excessive dross or slag. Typical speeds are of the order of 50 to 150 mm per minute for 6 mm plate.
• Trial cut: Adjust the current in small steps on scrap until the cut is clean.

2.1.1.2 Electrodes

The electrode carries the current to the arc, and its type and polarity decide the quality of the cut. For arc cutting, carbon or graphite electrodes are widely used because of their high melting point and the arc stability they give. Direct current electrode positive (DCEP) polarity is often recommended for efficient cutting, since it gives deeper penetration and a concentrated arc. Example: a workshop may use a carbon electrode on DCEP to gouge and cut mild steel plate.

• Carbon and graphite electrodes: They withstand the heat of the arc and keep it stable.
• Polarity: Connect the electrode as the manufacturer advises, usually DCEP for deep, concentrated cutting.
• Diameter: Choose the electrode diameter to suit the current, with a larger diameter for a higher current.
• Shielding gas or air: Some setups use air or oxygen to improve the cut quality and reduce electrode wear, as in air carbon arc cutting.
• Condition: Keep the electrode dry and the tip clean, and replace it when it is too short to hold safely.

Chapter Summary

This chapter explored the critical parameters involved in the arc cutting process, emphasizing the importance of correctly setting the current and selecting appropriate electrodes to achieve optimal results. It detailed the step-by-step procedure for carrying out arc cutting on materials up to 6 mm thick, highlighting both the technical execution and practical applications of the process. The discussion extended to edge finishing after arc cutting, outlining the types of finishing techniques suitable for thin materials, the procedures to follow, and their relevance in maintaining the quality and usability of the cut edges. Together, these topics provide a comprehensive understanding of how to effectively perform arc cutting and subsequent finishing to meet industry standards and application needs.