Three processes, three physical principles
Cutting a sheet means separating the material along a contour. The three most common processes do not work in the same way at all, which explains their differences in accuracy, materials and cost.
- Laser concentrates a light beam that melts or vaporises the metal, blown away by an assist gas (oxygen, nitrogen or air). The fibre laser dominates metal sheet; the CO2 laser is mainly used for non-metallic materials.
- Plasma passes an electric arc through a strongly heated, ionised gas, which melts the metal and expels it at high speed. It cuts only conductive materials.
- Waterjet erodes the material with very high-pressure water, loaded with an abrasive (garnet) for hard materials. It is a cold cut, with no heat.
To these is added oxy-fuel cutting, which burns steel with a flame and oxygen: it is limited to unalloyed or low-alloy steel, but remains economical on very heavy thicknesses. Many sheet metal and cutting workshops have several processes, others specialise: the right question is not “which is the best process?”, but “which suits my sheet?”. The guide on the price of laser cutting and bending complements this one on price and design.
Thicknesses and materials: the range of each process
| Process | Materials | Common thicknesses (orders of magnitude) | Good to know |
|---|---|---|---|
| Fibre laser | Metals: steel, stainless steel, aluminium, brass, copper | from 0.5 mm to about 20 mm for steel, slightly less for stainless steel and aluminium in a standard workshop | Most at ease on thin and medium sheet |
| Plasma | Conductive metals only | from a few millimetres to around fifty millimetres, more with very powerful sources | Competitive on heavy thicknesses |
| Waterjet | Almost anything: metals, stone, glass, ceramics, composites, rubber | from 1 mm to a hundred millimetres or more, depending on the material | No thermal effect, but slower |
| Oxy-fuel cutting | Unalloyed or low-alloy steel | from about 5 mm to over 200 mm | Very heavy steel thicknesses |
These ranges depend on the machine power, the assist gas and the edge quality expected: confirm them with the workshop that will make your parts. Between 15 and 30 mm of steel, several processes are possible; the edge quality required and the quantity then decide. As sheets are delivered in standard sizes, the layout of parts on the sheet (nesting) determines the scrap, whatever the process. Laser and plasma can also cut bevels (angled edges, with a tilting cutting head) to prepare V or X welds: mention it if your part is to be butt welded.
Accuracy, taper and edge quality
Accuracy depends on the process, but also on the machine, the material and the thickness: the thicker the sheet, the wider the gap. The values below are orders of magnitude to be confirmed with the workshop.
| Criterion | Fibre laser | Plasma | Waterjet |
|---|---|---|---|
| Typical accuracy | in the order of ±0.1 to ±0.2 mm on thin sheet | in the order of ±0.5 mm or more | in the order of ±0.1 to ±0.2 mm at low thickness |
| Edge | Almost perpendicular, fine striations | Slightly angled, rounded top edge, dross possible | Matt, finely striated, slight taper |
| Possible rework | Rarely | Grinding or machining if tight tolerance | Rarely |
The waterjet is slightly wider at the entry than at the exit: this taper increases with speed, and the jet “lags” in curves. Modern machines compensate for it in part with a tilting head. For the quality of thermal cuts (laser, plasma, oxy-fuel), the ISO 9013 standard classifies dimensional tolerances, perpendicularity and roughness; for waterjet, workshops speak rather of quality levels, from the slowest and finest to the fastest and roughest (often noted Q1 to Q5).
The accuracy of the machine is not the tolerance your part requires: specify a tight tolerance only on functional dimensions, and state the general tolerance class of the drawing (the ISO 2768 tool helps you choose it).
The heat-affected zone and its consequences
A thermal process heats the metal on either side of the cut: this is the heat-affected zone (HAZ). Its width and effects depend on the process, the material and the thickness.
- Laser. The HAZ is narrow, because the beam is highly concentrated and the speed high. Cut with oxygen (thick steel), the edge is oxidised, which can hinder welding or painting; cut with nitrogen, it is clean, but the gas costs more.
- Plasma. The HAZ is wider. Depending on the gas and the steel, the edge can be hardened or nitrogen-enriched, a possible source of porosity in welding or difficulty in tapping: grinding or a machining allowance solves the problem.
- Waterjet. No HAZ, no hardening: the material properties do not change. It is the choice for heat-sensitive materials, such as composites or certain alloys, and for titanium, whose edge becomes contaminated when hot. In return, the abrasive can embed in soft materials, and the starting pierce can delaminate a composite or crack a brittle material: the workshop adapts its starts.
The laser assist gas is therefore part of the request: oxygen speeds up the cutting of mild steel but oxidises the edge, nitrogen gives a clean edge on stainless steel and aluminium at the price of more expensive gas consumption, and compressed air is an economical compromise on thin sheet when edge appearance is secondary.
Heat also distorts: a thin, heavily perforated or long, narrow part can warp in thermal cutting. If flatness matters, state it in your request.
Relative cost: what really counts
We give no prices: the cost per metre varies too much with thickness, material, gas, machine and the workshop’s workload. We can, however, describe the trends.
| Sheet | Fibre laser | Plasma | Waterjet |
|---|---|---|---|
| Thin (up to about 6 mm) | The most economical: high speed, clean edge | Possible, but less accurate than laser | More expensive: slow process |
| Medium (about 6 to 20 mm) | Economical, speed falling with thickness | Competitive if accuracy is not critical | More expensive, but no rework |
| Heavy (beyond 20 mm) | Slow or out of reach depending on the machine | Often the most economical (oxy-fuel cutting for steel) | Slow, but no HAZ or distortion |
Waterjet consumes abrasive and energy for the high-pressure pump, which explains a generally higher cost per metre. But the price of a part is not just the cutting: it adds up material (hence scrap), rework (grinding, deburring, finish machining) and handling. A process that costs more per metre can give a cheaper part if it removes a rework; an economical process can cost more if it requires machining to hold the dimension.
Which process for which sheet? Eight real cases
| Scenario | Process often chosen | Why |
|---|---|---|
| Mild steel 1 to 6 mm, precise parts, production runs | Fibre laser | Speed, accuracy, clean edge. |
| Stainless steel 2 mm, clean edge for welding or appearance | Fibre laser with nitrogen | Edge without oxidation. |
| Aluminium 8 mm | Fibre laser or waterjet | Depending on the edge expected and the quantity. |
| Steel 30 mm to be welded (plate fabrication, structure) | Plasma or oxy-fuel cutting | Cost; the edges will be reworked at welding. |
| Steel 80 mm | Oxy-fuel cutting, powerful plasma or waterjet | Thickness outside the range of standard laser. |
| Titanium, heat-sensitive alloy | Waterjet | No thermal contamination of the edge. |
| Composite, stone, glass, rubber | Waterjet | No heating. |
| Blank to be machined with tight tolerance | Plasma or oxy-fuel cutting with allowance | Final machining sets the dimension. |
This table gives trends, not rules: the workshop chooses the process it masters and has available. Cutting is often followed by bending: the guide on the price of sheet metal bending explains how it is quoted.
What to specify in your quote request
- the material, grade and thickness, with the material certificate if necessary;
- the files: clean DXF (1:1 scale, closed contours) and dimensioned PDF drawing; the guide on CAD file formats details what to send;
- the tolerances: critical dimensions and general tolerance class;
- the edge quality required and the later use: welding, painting, tapping, machining;
- the thermal constraints: heat-sensitive material, flatness to be met;
- the quantity in several steps, and the lead time;
- the associated operations: deburring, bending, tapping, surface treatment;
- the inspection and documents expected.
If you hesitate between two processes, let the workshop propose its own, stating the edge quality and tolerance required: you will then compare equivalent offers. On Meaflow, posting a request is free and commission-free: you receive quotes from several workshops and talk by messaging before choosing.
Frequently asked questions
Laser or plasma: which should you choose for a 15 mm steel sheet?
At this thickness, both are possible. Laser gives a cleaner edge and finer tolerances; plasma is often more economical per metre if accuracy is not critical and the edge will be reworked or welded. State the tolerance, the use of the edge and the quantity: the workshop will propose the suitable process.
Does waterjet heat the part?
No: it is a cold cut, with no heat-affected zone or thermal distortion. It suits heat-sensitive materials and heavy thicknesses, but it is slower and costlier per metre. The kerf is slightly tapered, which some machines compensate for.
How accurate is laser cutting?
On thin sheet, around ±0.1 to ±0.2 mm is commonly achieved, depending on the machine, material and thickness. Accuracy degrades as thickness increases. Confirm with the workshop and specify a tight tolerance only on functional dimensions.
Can you weld a laser- or plasma-cut part directly?
Often yes, but the edge may be oxidised (oxygen laser) or nitrogen-enriched (plasma), which can cause weld defects. Light grinding or nitrogen laser cutting is generally enough. State the planned weld in your request: the workshop will adapt the cutting gas and the rework.