Cutting Processes

Cutting Processes

Introduction to Cutting Processes

The metal fabricator (welder-fitter) must master a complete range of cutting processes to prepare, fit, and finish metal parts. Cutting is not limited to separating a material: it determines the quality of the assembly, dimensional accuracy, and the safety of the entire operation. This chapter covers the four main thermal cutting processes (oxy-fuel cutting, plasma cutting, laser cutting, arc gouging) as well as mechanical processes, their parameters, their applications, and the Canadian standards that govern them.

For the Red Seal exam, you must not only know the theoretical principles, but also be able to calculate cutting parameters, select the appropriate process based on material and thickness, and identify the specific hazards associated with each method.

Oxy-Fuel Cutting (Oxy-Acetylene Cutting)

Fundamental Principles

Oxy-fuel cutting is a thermal process that uses the combustion of metal in a stream of pure oxygen. Contrary to popular belief, the metal is not melted by the flame: it is oxidized (burned). Iron has an ignition temperature of approximately 870 °C (1600 °F), well below its melting point of 1538 °C (2800 °F). It is this fundamental difference that makes oxy-fuel cutting possible.

The process occurs in three simultaneous steps:

The main chemical reaction is: 3Fe + 2O₂ → Fe₃O₄ + heat. This reaction is exothermic and releases considerable heat that sustains the combustion. In practice, approximately 70% of the energy required for cutting comes from the oxidation reaction itself, and only 30% from the preheating flame.

Equipment and Gases

Oxy-fuel cutting equipment includes:

Table 1: Cutting Gas Characteristics

Critical Rule: Acetylene must never be used at a pressure greater than 103 kPa (15 psi). Above this, it becomes unstable and can decompose explosively. This rule is absolute and is the subject of frequent exam questions.

Cutting Parameters

The choice of tip and pressures depends on metal thickness. The following table gives reference values:

Table 2: Oxy-Fuel Cutting Parameters for Carbon Steel

Cutting speed is a critical parameter. A speed that is too fast produces an irregular kerf, oblique drag lines, and leaves uncut metal at the bottom of the plate. A speed that is too slow produces a wide kerf, vertical drag lines, and excessive overheating that can distort the workpiece.

Cut Quality

The criteria for a quality oxy-fuel cut are:

Common defects and their causes:

Applications and Limitations

Oxy-fuel cutting applies exclusively to carbon steel and low-alloy steels. It is not suitable for:

The practical cutting thickness ranges from 3 mm to 300 mm. Beyond this, other processes are more efficient.

Plasma Cutting

Fundamental Principles

Plasma cutting uses a constricted electric arc through an orifice (nozzle) to create a jet of ionized gas at very high temperature (15,000 to 30,000 °C). This plasma melts the metal, and the high-velocity gas (600 to 900 m/s) evacuates the molten metal out of the kerf.

The principle: a gas (compressed air, nitrogen, oxygen, argon-hydrogen) is ionized by an electric arc established between an electrode (tungsten or hafnium) and the workpiece. The constriction of the arc by the nozzle increases current density and temperature, creating a plasma jet capable of cutting all conductive metals.

Equipment and Plasma Gases

Equipment includes:

Table 3: Plasma Gases and Applications

Plasma Cutting Parameters

The essential parameters are:

Table 4: Typical Plasma Parameters (compressed air, carbon steel)

Plasma Advantages and Limitations

Advantages:

Limitations:

Plasma Cutting and Electrical Safety

The Canadian Electrical Code, Chapter V (CSA C22.1-21) applies to the installation of arc cutting equipment. Key requirements include:

The plasma torch operates at voltages of 100 to 400 V DC. The return circuit must be securely attached to the workpiece, close to the cutting zone, to prevent current from taking unintended paths (bearings, cables, structures).

Laser Cutting

Fundamental Principles

Laser cutting uses a coherent, monochromatic light beam focused onto a very small surface (0.1 to 0.3 mm diameter). Power density reaches 10⁶ to 10⁹ W/cm², causing instantaneous melting or vaporization of the metal. An assist gas (oxygen, nitrogen, air) evacuates the molten material.

The three types of lasers used in manufacturing:

Laser Cutting Parameters

The critical parameters are:

Table 5: CO₂ Laser Parameters for Carbon Steel (4 kW power)

Laser Advantages and Limitations

Advantages:

Limitations:

Laser Safety Standards

CSA Z386 (Safety of laser installations in healthcare facilities) does not apply to manufacturing. For industrial lasers, CSA C22.2 No. 601.1 and Z434 (Management of industrial laser safety) apply. Key requirements:

Arc Gouging (Air Arc)

Fundamental Principles

Arc gouging (or air arc) is a process that uses an electric arc between a carbon electrode and the workpiece to melt the metal, while a stream of compressed air (400-700 kPa) evacuates the molten metal. This process is primarily used for:

Gouging Parameters

Table 6: Air Arc Gouging Parameters

The electrode angle relative to the workpiece is generally 30° to 45°. A shallower angle produces a deeper and narrower gouge; a steeper angle produces a wider and shallower gouge.

Practical Considerations

Defects and Corrections

Mechanical Cutting Processes

Shearing

Shearing is a mechanical process that separates metal through plastic deformation and fracture. The upper blade descends onto the sheet placed on the fixed lower blade. The clearance between the blades is critical: it must be 5 to 10% of the sheet thickness.

Rule of thumb: Clearance (mm) = 0.05 to 0.10 × thickness (mm). For a 6 mm sheet, the clearance will be 0.3 to 0.6 mm.

Typical capacities:

Limitations:

Sawing

Sawing uses a toothed blade to remove metal by cutting. The three main types:

Sawing parameters:

Table 7: Recommended Cutting Speeds for Band Saw (bimetal blade)

Drilling

Drilling is a material removal process using a rotating drill bit. The critical parameters are:

Calculation example: To drill a 12 mm hole in mild steel (V = 25 m/min):

N = (25 × 1000) / (π × 12) = 25,000 / 37.7 = 663 RPM

Table 8: Cutting Speeds for Drilling (HSS drill bit)

Cutting Process Selection

Selection Criteria

The choice of process depends on several interrelated factors:

Table 9: Comparison of Thermal Cutting Processes

Decision Tree

Economic Considerations

The total cutting cost includes:

For a production run of 1000 pieces of 6 mm steel, laser is generally the most economical despite its high equipment cost, due to its speed and precision which reduce finishing operations.

Cutting Process Safety

Specific Hazards

Oxy-Fuel Cutting:

Plasma:

Laser:

Air Arc Gouging:

Personal Protective Equipment (PPE)

Ventilation and Fume Extraction

CSA W117.2 (Safety in welding, cutting, and allied processes) is the reference standard in Canada. Its key requirements:

Rule of thumb: If the workpiece is less than 300 mm from the operator's face, local ventilation is required. In a confined space, an air-supplied respirator is mandatory.

Gas Cylinder Handling

Applicable Canadian Standards

CSA W117.2 — Safety in Welding and Cutting

This standard is the primary reference for cutting operation safety. It covers:

CSA B149.1 — Natural Gas and Propane Code

This standard applies to combustible gas installations, including acetylene and propane used for cutting. Key requirements:

Canadian Electrical Code, Chapter V (CSA C22.1)

Applies to electrical installations of cutting equipment:

CSA W48 — Filler Metals and Allied Products

Although primarily focused on welding electrodes, this standard also covers carbon electrodes for air arc gouging.

Pitfalls to Avoid

Summary

The essential points to remember for the exam:

Mastery of cutting processes is essential for the metal fabricator. Each process has its strengths and limitations, and the informed choice of the appropriate process is a key skill assessed on the Red Seal exam. Practice identifying the right process for each situation quickly, calculating cutting parameters, and applying CSA W117.2 safety rules.