Metal Fabrication Tolerances Guide for Cut Parts
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A bracket can be laser cut perfectly to the drawing and still fail to fit if the drawing does not allow for the way the part will be made, folded, welded or assembled. This metal fabrication tolerances guide explains how to set realistic requirements for sheet metal parts, avoid costly rework and get from drawing to usable components faster.
For most jobs, the best tolerance is not the tightest one. It is the tolerance that gives the part the function it needs without adding unnecessary cost, inspection or delay. That matters whether you need a single replacement plate or a repeat batch of production components.
What a fabrication tolerance actually means
A tolerance is the acceptable variation from a stated size, position, angle or feature. If a hole is specified as 10 mm with a tolerance of plus or minus 0.1 mm, the finished hole can measure between 9.9 mm and 10.1 mm and still meet the requirement.
On a laser-cut sheet part, tolerances may apply to the outside profile, hole diameters, slot widths, hole positions, bend angles, folded dimensions and flatness. They should be considered separately. A part can have an accurate cut profile but move slightly during welding, for example, or a folded return can vary because material thickness changes between batches.
The practical question is always: which dimensions genuinely control fit, movement or appearance? These are the dimensions that deserve a stated tolerance. General dimensions that do not affect performance can usually follow the fabricator’s normal working tolerance.
Typical laser-cut sheet metal tolerances
Fibre laser cutting is accurate and repeatable, particularly for flat profiles cut from good-quality sheet. Exact capability depends on material, thickness, feature shape and part size, but a general laser-cut dimensional tolerance of around plus or minus 0.1 to 0.2 mm is often realistic for straightforward sheet metal parts.
That does not mean every detail on every part should be marked plus or minus 0.1 mm. Small holes, narrow slots and internal corners behave differently from a large outside profile. Heat from the cut, material condition, sheet thickness and the kerf all influence the result.
For a working drawing, it is sensible to identify critical dimensions clearly and use a general tolerance for the rest. For example, a mounting-hole pitch may need to be held closely because it aligns with an existing frame, while the overall width of a cover plate may have more freedom.
Hole sizes and slots
Holes are often the features that cause assembly problems. A nominal 6 mm hole may be fine for a clearance fixing, but it is not necessarily suitable when a dowel, bearing or accurately located pin has to fit. If the fixing needs clearance, specify the clearance rather than relying on a nominal hole size.
As a rule, holes and slots should not be too small in relation to the material thickness. Very small features can be affected by heat and may be less consistent, especially in thicker steel. Where possible, make slot widths and hole diameters at least equal to the sheet thickness, and preferably larger when the design allows it.
If a feature must be undersized for a later process, state that clearly. A laser-cut pilot hole followed by drilling, reaming or tapping can be the right route where a tighter final size is required.
Internal corners and profile details
Laser cutting leaves a small radius at internal corners because the cutting beam cannot create a perfectly sharp inside corner. If another component must sit fully into that corner, add a relief, dog-bone or square-corner feature to the drawing.
Tiny notches and narrow tabs can also be vulnerable. They may distort, carry burrs or become impractical at certain thicknesses. Simplifying a feature by a fraction of a millimetre can make a part easier to cut and more reliable in service.
Why material and thickness affect accuracy
A tolerance that works in 1.5 mm mild steel may not be sensible in 12 mm stainless steel. Thicker material takes longer to cut, has a wider kerf and can show more heat-related movement. Stainless steel and aluminium also react differently from mild steel during cutting and subsequent fabrication.
Sheet itself has tolerances. The stated thickness is nominal, and actual thickness can vary within the material standard. This becomes particularly relevant where parts slide together, where a slot is designed around sheet thickness, or where folded components must meet flush.
Flatness is another consideration. Large, thin parts can move after cutting because internal stresses are released. A wide panel with a closely controlled flatness requirement may need additional processes or design changes, such as formed ribs, folds or a thicker material. Asking for tight flatness on a thin, unsupported plate can increase cost without guaranteeing the result a design needs.
Tolerances after folding and welding
Once a part is folded or welded, cutting accuracy is only one part of the picture. Press brake work introduces variation from bend radius, grain direction, thickness, tooling and springback. A 90-degree bend may be specified with an angular tolerance, while a folded flange should have a realistic tolerance on its finished position.
Dimensions measured from a bend need particular care. The bend line is not a perfect point, and the outside of a folded part stretches. A correctly developed flat pattern must allow for bend deduction or bend allowance. If the material, thickness or bend radius changes, the flat pattern may need to change too.
Welding adds heat, which can pull components out of position. If two laser-cut plates need to remain precisely aligned after welding, consider using tabs, slots, location holes or a fixture. For high-accuracy assemblies, machining critical faces or holes after welding may be more economical than trying to hold tight tolerances throughout the weld process.
How to specify tolerances on a drawing
A clear drawing prevents assumptions. A DXF is excellent for defining a laser-cut profile, but it does not always explain material, thickness, finish, quantity, thread requirements or which dimensions are critical. Supplying a PDF drawing alongside the DXF is often the simplest option for bespoke work.
Put the material and thickness in a visible note. State the required quantity and identify any deburring, countersinking, tapping, folding or welding needed. Then apply tolerances only where they matter.
A useful drawing normally includes these details:
- Overall dimensions and critical hole or slot locations.
- Material grade, thickness and any required finish.
- Hole sizes, thread call-outs and countersink details.
- A general tolerance note for non-critical dimensions.
- Revision number and part number for repeat orders.
For parts with multiple features, use datums. In plain terms, choose the edges, faces or centre lines from which important dimensions are measured. A drawing that says where the part starts is far easier to manufacture consistently than one made up of measurements taken from several unrelated edges.
When tighter tolerances are worth paying for
Tight tolerances are worth specifying when they protect function. Common examples include hole patterns that mate with an existing part, locating features for repeat assembly, bearing fits, slide mechanisms and components that need to interchange across a batch.
They are less useful on cosmetic edges, generous clearances and non-mating cover panels. Over-specifying these areas can lead to extra set-up, more inspection and a higher quoted price, with no benefit to the finished product.
If a part needs a highly accurate feature, it is often better to combine processes. Laser cutting can produce the profile quickly and economically, while drilling, reaming, machining or jigged welding handles the feature that needs closer control. This approach keeps the whole part from being priced as though every dimension were critical.
A practical tolerance check before ordering
Before sending a drawing, check how the part fits in the real assembly. Ask whether holes need clearance for bolts, whether a folded edge needs room around another component, and whether paint, powder coating or galvanising will add thickness. A tight tab-and-slot fit in bare steel may become difficult after coating.
Also consider how the part will be measured. If the requirement cannot be checked easily with standard measuring equipment, define the inspection method or discuss the intended use before production. This is especially useful for repeat work, where agreeing expectations early prevents variation between batches.
At NKD Services, a clear DXF and a short note on material, thickness, quantity and critical dimensions are usually enough to get a custom laser-cut job moving. For simple parts, even a marked-up sketch, photograph or specification sheet can help establish what needs to be made.
The most useful tolerance note is often a simple one: state what must fit, what can vary and what process follows cutting. Give the fabricator that information at the start, and the finished part is far more likely to arrive ready for the job rather than ready for another round of adjustments.