Choosing a Cutting Process

Four processes do most of the profile cutting in metal shops: oxy-fuel, plasma, laser and waterjet. Each one has jobs it does best. The wrong choice can mean a weak edge, slow work, wasted money or parts that have to be cut again.

What You Will Be Able to Do

• Say how each process cuts.
• Choose a process by material and thickness.
• Compare edge quality, kerf and heat effects.
• Explain when a heat-affected zone matters.
• Spot the main hazard each process brings.

How Each Process Cuts

• Oxy-fuel heats steel until it can burn, then blasts it with pure oxygen. The cut is a chemical reaction.
• Plasma forces an electric arc through a jet of gas. The arc melts the metal and the gas blows it out.
• Laser focuses a light beam on a tiny spot. The beam melts or vaporizes metal, and an assist gas blows it out.
• Waterjet pushes water through a tiny orifice at very high pressure, usually with garnet abrasive added. The abrasive wears the material away.

Side-by-Side Comparison

What each process can cut:

What the cut is like:

Oxy-fuel and hand plasma travel to the work. Laser and waterjet machines stay in the shop.

Actual thickness limits depend on the machine, so treat these as general guides.

When to Use Each One

Oxy-Fuel

Use it for thick carbon steel, field cutting, demolition, and when you have no electrical power. The equipment is cheap and portable, and it is the common choice for steel several inches thick or more. It only works on metals that burn in oxygen, so it cannot cut stainless steel or aluminum. It is slow on thin plate and leaves a wide heat-affected zone.

Plasma

Use it for carbon steel, stainless and aluminum in sheet and plate, from thin gauges up to around 1 to 2 inches, depending on the machine. It is faster than oxy-fuel on thinner steel and cuts any conductive metal. It will not cut wood, plastic, glass or anything that does not conduct electricity. Edges have more bevel and dross than a laser cut. Hand plasma is common for repair and fit-up work, and CNC plasma tables are common in plate shops.

Laser

Use it for accurate, fast cutting of sheet and medium plate, especially in high volumes with small features. A fiber laser is the usual choice for metal, including reflective metals such as aluminum, brass and copper. A CO2 laser also cuts many non-metals, such as acrylic and wood. It has a narrow kerf and a small heat-affected zone, so it holds tight tolerances. The machines cost much more, and thick plate is slower.

Waterjet

Use it when heat is a problem, when you are cutting a material the others cannot, or when the material is thick and the edge must not change. Waterjet cuts metals, stone, glass, composites, rubber and foam. Since it is a cold process, it does not leave a heat-affected zone, so it does not distort the part or change its hardness or temper. That is why it suits heat-treated metals, titanium and nickel alloys. It is slower than laser or plasma on thin steel, and the abrasive adds cost.

Terms You Will Hear

• Kerf is the width of the cut. A wide kerf wastes more metal and affects how close parts can be nested.
• Heat-affected zone (HAZ) is the metal near the cut that did not melt but was changed by heat. It can become harder, which can affect later machining, bending or welding.
• Dross is melted metal that sticks to the bottom edge of a cut. It costs you time in cleanup.
• Bevel or taper is when the cut edge is not square to the surface.
• Nesting is arranging parts on a sheet to reduce waste. Spacing between parts keeps heat from distorting them, leaves clearance for the tool, and keeps the sheet stable as it is cut.

Choosing by the Job

Ask these questions in order.

1. Is the material conductive metal? If not, only waterjet or a suitable laser can do it.
2. Is it carbon steel, and thick? Oxy-fuel or plasma are likely the cheapest.
3. Must the edge stay unchanged by heat? Waterjet.
4. How tight is the tolerance? Tight tolerances and small holes point to laser or waterjet.
5. How many parts? High volumes on sheet favor laser.
6. Where is the work? In the field, it is oxy-fuel or hand plasma.
7. What happens next? A welded edge may need to be clean, and a bent edge should not be hard.

Worked Examples

Edge Quality and What Comes Next

A cut edge is rarely the end of the job. Oxygen-cut laser edges and oxy-fuel edges carry oxide, which can cause trouble with paint and welding. Clean them as your procedure requires. Hard edges from thermal cutting can crack when bent or dull a drill. A plasma edge may need grinding before welding. Waterjet edges have no heat change but can have a slight taper, especially on thick material at high speed.

Main Hazards

Each process has its own safety rules. Do not assume that what is safe on one machine carries over to another. For example, laser eyewear is matched to a wavelength, and welding lenses will not do that job.

Common Mistakes

• Trying oxy-fuel on stainless or aluminum. Those metals do not burn the way carbon steel does, so the cut fails.
• Sending a heat-sensitive part to plasma. The edge may harden or the temper may change.
• Picking a laser for thick plate by habit. Plasma or oxy-fuel may be faster and cheaper.
• Ignoring kerf in the layout. Parts come out undersized or too close together.
• Assuming a clean-looking edge is ready to weld. Oxide and hardness may still need attention.

Check Your Understanding

A customer needs 40 pieces of hardened steel plate, and any change in the hardness is a problem. Which process do you choose?

Waterjet, because it has no heat-affected zone and will not change the temper.

You need to cut a 1/4 inch aluminum bracket in the field, with a generator nearby. Oxy-fuel is on the truck. Will it work?

No. Oxy-fuel does not cut aluminum. Use a plasma cutter, or a saw if one is suitable.

A job calls for 2,000 parts from thin steel sheet, with small, tight holes. Which process fits best?

A fiber laser. It is fast on thin sheet and holds tight tolerances on small features.

Why does nesting need some space between parts?

Spacing helps prevent heat distortion, leaves clearance for the tool, and keeps the sheet stable during the cut.

Key Takeaways

• Oxy-fuel is for thick carbon steel and field work.
• Plasma cuts any conductive metal, up to medium thickness.
• Laser is for fast, accurate sheet and medium plate cutting.
• Waterjet cuts nearly anything, with no heat-affected zone.
• Let the material, thickness, tolerance, volume and next step guide your choice.
• Each process has its own hazards, so learn the rules for the machine you are on.