Metal Cutting Processes: Mechanical, Thermal and Arc Methods: Variables That Affect Plasma Cutting

Variables That Affect Plasma Cutting

• Type and pressure of the gas
• Gas flow pattern
• Current level
• Size and shape of the nozzle orifice
• Nozzle-to-work surface distance

Noise and Fume Reduction

For mechanized plasma cutting, the workpiece is often submerged in water. This technique:

• Dramatically reduces noise
• Reduces fumes
• Virtually eliminates oxidation of cut surfaces

Precision Plasma Arc Cutting

A significant advancement uses a magnetic field in the cutter head to stabilize the plasma arc:

• Lorentz forces cause the arc to spin faster and tighter on the electrode tip
• The magnetic field confines the spinning plasma
• Result: Narrower kerf without sacrificing cutting speed
• Performance is comparable to laser cutting in many applications
• Combined with CNC, it produces precision blanks for stamping

Edge quality by material:

• Galvanized and aluminized steel: Clean, burr-free edges
• Mild steel: Some slag may cling to edges

Laser Cutting: The Focused Edge of Light

The Final Frontier of the practitioner's Education

The last process the practitioner studied was laser cutting—and it was the one that most challenged his assumptions. A beam of light, invisible to the human eye, cutting through steel with kerf widths smaller than a credit card is thick. Accuracy measured in thousandths of an inch. Cuts so clean they need no secondary finishing.

LASER stands for Light Amplification in the supplied reference of Radiation. When directed against a material surface, the concentrated energy is high enough to cause localized melting, vaporization, or combustion—with minimal distortion to surrounding material.

Types of Industrial Lasers for Cutting

CO₂ Lasers:

• Gaseous medium: Helium, nitrogen, and carbon dioxide
• Wavelength: ~10.6 µm (far infrared)
• Energized by electric discharge
• Higher power available; overcomes metal reflectivity through raw energy density
• Maximum practical cutting thickness in steel alloys: 25 mm (1 in.)
• Most economically efficient range: up to 12.5 mm (0.49 in.)

Nd:YAG Lasers (Neodymium-doped Yttrium Aluminum Garnet):

• Solid-state crystal medium
• Wavelength: 1.06 µm (near infrared)
• Excited by flashlamp(s) in a reflective cavity
• Shorter wavelength = better absorption by metals = less reflectivity problem
• Suited for thinner materials and precision work

Cutting Metal with Lasers: The Melt Ablation Process

For ferrous alloys, the laser works in concert with an oxygen assist gas:

1. Focused laser beam creates a molten puddle on the steel surface
2. Concentric oxygen jet combines with the heated metal (exothermic reaction)
3. The jet burns through the entire thickness of the steel
4. Gas pressure ejects molten metal from the kerf

For highly alloyed steels (stainless): Pulsed CO₂ laser beams are used. High-pressure, non-oxidizing gas jets with the nozzle on the surface minimize or eliminate clinging dross.

Kerf Widths in CO₂ Laser Cutting

The narrow kerf allows cut patterns to be nested as close as one beam diameter apart. This is critical for material utilization on expensive alloys and for producing sharply contoured, profiled cuts—even in narrow-angle locations.

Cut Edge Roughness (with Oxygen Assist)

Heat-Affected Zones in Mild Steels

Key insight: Pulsed CO₂ laser cutting reduces the HAZ to less than 0.1 mm (0.004 in.), making it the preferred mode for applications where HAZ is critical.

CO₂ and Nd:YAG Cutting Speeds for Nonferrous Metals

Important caveat: Published cutting rates are developed under ideal laboratory conditions with technician-operated equipment. Shop floor rates using semi-skilled operators on complex shapes may vary dramatically from these figures.

Laser Cutting of Nonmetals

Lasers cut nonmetals through three mechanisms depending on the material:

• Cellular materials (paper, wood): Vaporization by combustion
• Thermoplastics: Melting + gas jet expulsion
• Thermosets (epoxies, phenolics): Combustion or chemical degradation (faster than thermoplastics)
• Composites: Generally easy to cut, but quality depends on heat sensitivity of components

Compressed air is commonly used as the assist gas for plastics cutting—widely available and inexpensive.

CO₂ Laser Cutting Rates for Nonmetals:

Limitation for composites: High-pressure fluid jet cutting (waterjet) has proven more effective than lasers for many composite materials due to the thermal sensitivity problem.

Laser Drilling

Three methods exist for laser drilling:

Direct Drilling:

• Single pulse creates a hole
• Maximum depth: 8 mm in metals
• Maximum diameter: 0.5–0.75 mm (Nd:YAG), 1.0 mm (CO₂)
• Aspect ratio: Under 10:1 in metals, up to 15:1 in nonmetals
• Hole taper: Up to 25% diameter change (entrance to exit)
• Recast layer: ~0.1 mm depth
• Diameter tolerance: ±10%

Percussive Drilling:

• Rapid sequence of pulses (higher quality than direct)
• Maximum depth: 25 mm (1 in.)
• Maximum diameter: 1.5 mm (0.06 in.)
• Aspect ratio: 50:1
• Recast layer: 0.5 mm
• Taper: Under 10%
• Diameter tolerance: ±5%

Trepanning:

• Focused beam moved around the hole circumference by rotating mirror
• Maximum depth: 10 mm (0.39 in.)
• Maximum diameter: 2.5 mm (0.1 in.)
• Recast layer: 25 µm (best of all three methods)

Laser Heat Treatment and Surface Processing

Heat Treatment with Lasers

The defocused beam from a CO₂ laser can be used for surface heat treatment, but with an important catch: at room temperature, steel reflects over 90% of the beam energy at the power densities used for heat treatment (less than 10⁴ W/cm², compared to 10⁵+ W/cm² for cutting).

Solutions to the reflectivity problem:

• Surface roughening: Creates tiny craters that trap beam energy long enough to raise temperature
• Surface coatings: Most common method; applied before treatment

Laser Cladding

Laser beams can deposit material onto metal surfaces for wear resistance, corrosion protection, or dimensional restoration—a process similar to hard-facing but with the precision and minimal heat input that only a laser can provide.

Laser Marking

Two methods:

• Mask marking: Beam passes through a stencil mask
• Scanned-beam marking: Beam is deflected by mirrors under computer control to trace characters and patterns

The Process Selection Matrix: Choosing the Right Cut

the practitioner's Framework

After months of study, the practitioner built a decision framework that his entire team could use. Here it is—the distillation of everything in this guide into a single actionable reference:

The Decision Flowchart

Ask these questions in order:

1. Is the material conductive?

   • No → Laser or mechanical only
   • Yes → Continue

2. How thick is the material?

   • 12 inches → Flame cutting (oxyhydrogen)

   • 1–12 inches → Flame cutting or plasma
   • ¼–1 inch → Plasma, laser, or mechanical
   • <¼ inch → Laser, plasma, fine blanking, or press work

3. What tolerance and edge quality do you need?

   • Ultra-precision (±0.001 in., smooth edges) → Fine blanking, wire EDM, or laser
   • Standard (±0.005 in.) → Press work, plasma, or laser
   • Rough (structural cuts) → Flame or plasma

4. What is the production volume?

   • 10,000 parts → Press work or fine blanking (amortize tooling)

   • 500–10,000 → Laser, plasma, or press work
   • <500 → Steel rule die, laser, plasma, or wire EDM

5. Is the material extremely hard (carbide, hardened tool steel)?

   • Yes → EDM (sinker or wire) is likely your only option

Hard-Facing: Extending the Life of Cut Surfaces

When Cutting Is Only Half the Battle

No discussion of metal cutting is complete without addressing what happens after the cut. Components that have been cut, machined, or formed often need surface protection—especially at wear points, cutting edges, and high-friction interfaces.

Hard-facing materials include:

Chromium Plating for Cutting Tools

Hard chromium plating is widely used to extend the life of cutting tools and dies:

• Thickness range: 0.0001 to 0.002 inches
• Static coefficient of friction (steel on chromium-plated steel): 0.17 vs. steel on steel: 0.30
• Sliding coefficient of friction (steel on chromium-plated steel): 0.16 vs. steel on steel: 0.20
• Temperature resistance: Remains bright up to 1,200°F; light adherent oxide forms above that, remaining intact even above 2,000°F
• Chemical resistance: Resists attack by almost all organic and inorganic compounds except muriatic and sulfuric acids

Applications showing greatly increased life after chromium plating:

• Stamping dies
• Drawing dies
• Hot forging dies
• Die-casting dies
• Plastics molding dies

Files and Rotary Burs: The Manual Side of Metal Cutting

When Machines Cannot Reach

Even in the most automated shop, there are places where hand files and rotary burs are the only practical tools. Deburring after blanking. Fitting precision assemblies. Blending weld surfaces. These operations are metal cutting at the most fundamental, tactile level.

Rotary File and Bur Speed Reference

High-Speed Steel Burs (Medium Cut):

Carbide Burs (Any Material):

Key fact: At equal speeds, HSS and carbide burs remove approximately the same amount of metal. But at their most efficient speeds, carbide burs can remove up to 4× the material of standard burs. A carbide bur can last up to 100× as long as an equivalent HSS bur.

Power Brush Finishing: The Final Metal Cutting Step

Power brush finishing uses wire, elastomer-bonded wire, or non-metallic brushes in automatic machines, semi-automatic machines, and portable air tools to:

• Smooth or roughen surfaces
• Remove surface oxidation and weld scale
• Remove burrs
• Produce edge radii

How brushes cut: Each wire point acts as an individual cutting tool. The brush is effectively a multiple-tipped cutting tool that also imparts a cold-working effect through impact action.

Surface finish improvements achievable:

• Parts at 24 µin. Ra → 15 to 10 µin. Ra
• Parts at 10–12 µin. Ra → 7 to 4 µin. Ra

Engineering takeaway

the practitioner started with a problem: stainless brackets with tight tolerances and a broken punch. He ended with something far more valuable than a production fix. He ended with understanding.

Here is what that understanding means for you:

Metal cutting is not one skill. It is a discipline spanning chemistry (flame cutting), plasma physics (plasma and laser), electrical engineering (EDM), and mechanical engineering (press work and fine blanking). Mastering any one process is valuable. Understanding how they all relate—their trade-offs, their sweet spots, their failure modes—is what separates a technician from an engineer, and a job shop from a world-class fabrication operation.

The right process for the job is determined by physics, not habit. Material type, thickness, required tolerance, edge quality, production volume, and cost per part—these are the variables that select your process. Not "what we've always done."

Every process has hidden variables. Clearance in press work. Duty cycle in EDM. Assist gas pressure in laser cutting. Flushing strategy in spark erosion. These are the details that determine whether you produce precision parts or expensive scrap.

The data exists. Use it. Every table in this guide is a shortcut—the result of thousands of hours of testing, failure, and optimization by engineers and researchers across the manufacturing world. You do not need to rediscover these numbers. You need to apply them.

Your Next Step

Pick one process from this guide that you currently use but do not fully understand. Go to the relevant section. Read the data tables. Calculate the values for your specific application. Then walk out to your shop floor and check the actual settings against what the engineering says they should be.

The gap between what you find and what the data recommends is the gap between where you are and where you could be.

Close it.

The Complete Guide to Flame, Arc, Plasma, and Laser Cutting Processes

How Metals Are Actually Cut: The Four Pillars

Before you pick up a torch, flip a switch, or program a CNC path, you need to understand a fundamental truth: all thermal metal cutting operates on one of two principles — oxidation or melting.

• Oxidation-based cutting uses a chemical reaction between oxygen and heated metal to disintegrate the workpiece. The heat is the igniter; the oxygen is the fuel. This is how flame cutting works on steel.
• Melt-based cutting uses concentrated energy (electrical arc, plasma jet, or laser beam) to melt or vaporize the metal, then blows the molten material out of the kerf with a high-velocity gas stream.

Every cutting process in this guide — flame, arc, plasma, and laser — is a variation on these two themes. Understanding which principle dominates each process tells you why certain metals cooperate with certain torches and why others fight back.

Flame Cutting: The Original Metal Separator

The Science Behind the Spark

When iron or steel is heated to a high temperature, it develops a powerful affinity for oxygen. It combines readily with oxygen to form various iron oxides, and this reaction causes the metal to disintegrate and burn with remarkable speed.

This is the principle that makes flame cutting possible. A cutting torch doesn't "melt through" steel the way most people imagine. It oxidizes the metal — it literally burns it away in a controlled chemical reaction.

Here's the sequence:

1. Preheat: A torch tip delivers a flame (oxyacetylene, oxyhydrogen, or another fuel-gas/oxygen combination) to raise the metal to white-hot temperature
2. Oxygen jet: An auxiliary jet of pure oxygen hits the red-hot metal
3. Oxidation cascade: The oxygen combines with the heated iron, generating tremendous additional heat that sustains a self-feeding reaction
4. Kerf formation: The oxidized metal (slag) is blown out of the cut path, leaving a kerf resembling a saw cut

Key Insight: The cutting torch doesn't need to melt steel. It only needs to raise the surface temperature high enough for the oxygen to ignite a chemical reaction. That's why flame cutting is so energy-efficient on carbon steel — the metal's own oxidation provides most of the cutting energy.

The Cutting Torch: Anatomy and Operation

The ordinary cutting torch consists of a heating jet and an auxiliary oxygen jet. The heating jet can use any combination of gases that produces sufficient heat:

• Oxyacetylene — the most common
• Oxyhydrogen — produces a longer flame, capable of cutting thicker material
• Oxypropane, oxynatural gas — alternative fuel options

Some cutting torches feature multiple preheating flame ports surrounding the central oxygen port. This design ensures a preheating flame precedes the oxygen regardless of the torch's direction of travel — a critical advantage for mechanically guided operations.

The rate of cutting depends on three variables:

• Thickness of the steel
• Size of the torch tip
• Oxygen pressure

Adjustment and Use: Getting the Cut Right

When cutting steel plate, the sequence is precise and unforgiving:

1. The preheating flame contacts the edge of the plate
2. The edge is raised to white-hot temperature
3. The oxygen valve is opened
4. Pure oxygen contacts the heated metal
5. Rapid oxidation begins — the metal burns and the cut progresses

This is where beginners fail. They open the oxygen too early (before the metal reaches ignition temperature) or too late (wasting fuel gas and overheating the surrounding material). The timing of that oxygen valve is everything.

What Can You Flame Cut — and What Can't You?

This is the question that cost the practitioner his first batch of material. Not every metal responds to flame cutting the same way, and some metals actively resist it.

Low-Carbon Steel and Wrought Iron

Verdict: Excellent. These materials cut readily and cleanly. The iron content oxidizes efficiently, the slag flows freely from the kerf, and the result is a clean, narrow cut. This is the bread-and-butter application for flame cutting.

High-Carbon Steel

Verdict: Requires preheating. The higher the carbon content, the more preheating you need before the oxygen jet can sustain a clean cut.

Why? Carbon in steel changes the oxidation dynamics. More carbon means the metal needs more energy input before the oxygen reaction becomes self-sustaining.

Stainless Steel

Verdict: Requires flux injection.

Here's the problem: the same elements that make stainless steel "stainless" — primarily chromium — produce oxides that obstruct the flame cutting process. With conventional oxyacetylene equipment, cutting stainless steel degrades into a painfully slow melting-away process rather than a clean oxidation cut.

The solution: flux-injection cutting.

A suitable flux powder is injected directly into the cutting oxygen stream before it enters the torch. This flux reacts with and removes the obstructing chromium oxides, allowing the cut to proceed at speeds practically identical to mild steel cutting.

• Portable flux feeding units use vibrator-type dispensers with rheostat control for precise flow regulation
• The method works for both machine cutting and hand-controlled torches
• Operating procedure and speed are essentially the same as for mild steel

Cast Iron

Verdict: Practicable but expensive.

Cast iron can be cut with the oxyacetylene torch, but it cannot be cut as readily as steel. The ease of cutting depends largely on the physical character of the iron — counterintuitively, very soft cast iron is more difficult to cut than harder varieties.

Why is cast iron harder to cut? Two reasons:

1. Carbon in graphite form hinders the oxidation reaction. The chemical action that makes steel cutting so efficient is partially blocked.
2. The process is partly a melting operation. The slag from a cast-iron cut contains considerable melted cast iron, whereas slag from a steel cut is practically free from metal particles.

The cost penalty is real. Cast iron requires a larger preheating flame and significantly higher oxygen consumption than the equivalent thickness of steel.

Pro tip for cast iron cutting: Feed a steel rod (approximately ¼ inch diameter) into the top of the cut, beneath the torch tip. This rod generates a large amount of hot slag that flows over the cut and raises the temperature of the cast iron, increasing speed and decreasing cost. Special torch tips designed for the larger heat and oxygen demands are recommended.

Brass and Bronze

Verdict: Indirect cutting only. These metals cannot be directly flame-cut because they don't oxidize in the same self-sustaining way as iron and steel. However, brass and bronze plates can be cut by interposing them between steel plates — using the steel's oxidation reaction to carry the cut through the nonferrous metal.

Thickness Limits: How Deep Can the Flame Go?

The maximum cutting thickness depends on the gas combination and the oxygen pressure, which can reach as high as 150 lbf/in².

The oxyhydrogen advantage is purely geometric. Its longer flame can reach the bottom of a deep kerf, keeping the entire column of oxidized material molten so it flows out cleanly. The oxyacetylene flame, being shorter, struggles to maintain full-depth heating in very thick material.

Critical factors for thick-section cutting:

• Mechanically guided torches cut thick material far more satisfactorily than hand-held torches, because the flame stays straight and doesn't wobble
• Kerf width increases with thickness — light material may produce a 1/16-inch kerf, while heavy stock can produce 1/4 to 3/8-inch kerf widths
• Cut accuracy decreases with increasing thickness

Mechanically Guided Torches and CNC Cutting

When precision matters — or when cutting openings in plates, blocks, or parts to a definite outline — torches are guided mechanically or by numerical control.

Three main categories:

• Pantograph-guided torches — trace outlines from a pattern or drawing, ideal for complex shapes
• Straight-line guides — designed specifically for linear cuts
• Circular cutting rigs — purpose-built for cutting round openings

The move from hand-guided to mechanically-guided cutting is one of the most impactful upgrades any fabrication shop can make. It transforms flame cutting from a craft skill into a repeatable manufacturing process.

Cutting Steel Castings: The Blowhole Problem

Steel castings present a unique hazard that even experienced operators sometimes learn about the hard way.

The danger: blowholes.

When the cutting flame strikes a blowhole (a gas pocket trapped in the casting during solidification), molten oxide splashes into the cavity and diverts the flame. The result is an erratic, uncontrolled cut that can ruin the workpiece.

How to detect blowholes during cutting: Excessive sparks indicate the flame has penetrated a blowhole.

The recovery procedure:

1. Immediately move the torch back along the cut
2. Direct the flame at an angle to strike the metal beneath the blowhole
3. Burn away the metal below and beyond the cavity if possible
4. Resume cutting in the normal position once past the blowhole

This procedure requires real-time judgment and experience. It's one reason why skilled flame-cutting operators remain valuable even in an age of CNC automation.

Arc Cutting: When Oxidation Won't Cooperate

Why Arc Cutting Exists

Here's a fact that defines the entire arc cutting process: not all metals oxidize willingly.

Steel cuts beautifully with a flame because the oxygen-iron reaction is thermodynamically favorable and self-sustaining. But cast iron, stainless steels, manganese steels, and nonferrous materials resist oxidation. They don't burn in an oxygen stream — they just sit there and slowly melt.

For these reluctant materials, arc cutting offers an alternative: forget about chemistry. Use brute electrical energy to melt through.

The Fundamental Difference

The trade-off is straightforward: Arc cutting is more versatile but produces rougher cuts. Flame cutting is cleaner but chemically limited to ferrous metals that oxidize efficiently.