Most tolerance disputes are not caused by a machine cutting badly. They are caused by a drawing that never said what mattered, and a supplier who assumed something different from the customer.

This guide sets out what accuracy a CNC fiber laser realistically achieves, what makes it drift, and how to specify a part so it fits first time.

What Tolerance Can a Fiber Laser Actually Hold?

On typical sheet thicknesses, a well-maintained fiber laser holds approximately +/- 0.1 mm on cut features. That figure is reliable for material up to around 6 mm and loosens as thickness increases.

The Four Things That Move a Dimension

1. Kerf width

The beam removes material. That removed width — the kerf — is typically 0.1 to 0.4 mm depending on thickness and parameters. The machine compensates by offsetting the toolpath to one side of the line, so the finished part matches the drawing.

This works well, but it means the software has to know which side of the line is scrap. On an open contour or an ambiguous geometry it can guess wrong, and you get a part half a kerf undersize.

2. Kerf taper

The cut is very slightly wider at the top than at the bottom, because the beam is a cone converging to a focal point. On 2 mm sheet this is undetectable. On 30 mm plate it is measurable and visible.

Taper matters most on parts that must seat flush against another surface, and on holes that a shaft passes through.

3. Heat input and distortion

Cutting puts heat into the plate. Thin sheet with long cuts, or dense nesting where parts sit close together, can bow or ripple. A part that measures correctly but will not lie flat is still a rejected part.

4. Material condition

Sheet arrives with its own tolerance. Mill thickness varies within grade. Plate can carry internal stress from rolling that releases when a shape is cut from it, moving the part after it leaves the bed. None of this is the laser, but all of it lands in your measurement.

Cut Tolerance Is Not Assembly Tolerance

This is the distinction that causes the most trouble. A laser cuts a flat blank accurately. What happens afterwards has its own error budget, and it is usually larger:

• Bending. Press brake tolerance on a bent flange is typically wider than the cutting tolerance, and bend allowance varies with material batch.
• Welding. Heat pulls assemblies out of position by amounts that dwarf +/- 0.1 mm.
• Coating. Powder coat adds measurable thickness, which matters on close-fitting parts and inside holes.

If a fabricated assembly has to hit a tight overall dimension, the cutting tolerance is rarely the constraint. Specify where it counts and leave the rest open — we cover the practical consequences in how CNC laser cutting works.

ISO 9013 and Edge Quality

Thermal cut quality has a formal standard: ISO 9013, which grades perpendicularity, surface roughness and dimensional deviation for laser, plasma and oxy-fuel cuts.

Most commercial work never invokes it, and it is not needed for a bracket. But if you are working to a customer specification that calls out an edge quality class, put it on the drawing at enquiry stage rather than at inspection. It changes process choice, assist gas and feed rate — and therefore price.

How to Specify Tolerance So You Get What You Need

Four practical rules, in order of how much trouble they save:

1. Do not put a blanket tight tolerance on every dimension. It raises price on features nobody measures and dilutes attention on the ones that matter.
2. Call out the critical dimensions specifically. Two or three flagged features get real attention; forty do not.
3. Distinguish between hole position and hole diameter. Position across a bolt pattern is usually the thing that decides whether parts assemble; diameter often has more room than the drawing implies.
4. State what happens after cutting. A part heading for bending, welding or coating needs different allowances from one going straight into service.

What to Check When Parts Arrive

A quick incoming check catches problems while they are still cheap to fix:

• Overall length and width against the drawing.
• Centre-to-centre distance across the widest bolt pattern, not individual hole positions.
• Flatness — lay the part on a known flat surface and look for rock.
• Edge squareness on thick plate, where taper is expected.
• Burr on the underside, which indicates parameters were off.

RG Tech cuts to approximately +/- 0.1 mm on typical sheet, on an 8000 x 2500 mm bed in Chennai. If your drawing has critical features, flag them when you send it through our contact form and we will confirm what is achievable before you commit to an order.

Accuracy also depends on what you are cutting — see materials that can be laser cut for how thickness limits vary by metal.