Which Tolerances Should Be Agreed for a Laser-Cut Part ART-438

Short answer

There is no universal tolerance that applies to every part, grade, and thickness. General drawing tolerances and a separate edge requirement may be sufficient for a simple blank. A mounting plate, enclosure, or bending blank may need explicit datums, critical dimensions, hole-pattern position, and a definition of the condition in which it is inspected.

The phrase “accuracy within 0.1 mm” is incomplete unless it identifies the feature, the datum, and the acceptance method. A dimensional tolerance also does not replace an edge requirement. A part can have the correct length and still carry an unacceptable burr, dross, or taper.

A part tolerance is not the machine's positioning accuracy

Equipment documentation may state positioning accuracy, repeatability, or another property of its mechanical system under defined conditions. These values do not automatically become tolerances for finished parts. The result also depends on actual sheet thickness, material condition, internal stress, geometry, nesting, heat input, support and handling, process settings, and inspection after the part is removed from the sheet.

For that reason, a request should not ask only for “the maximum accuracy of the machine.” It should describe the finished feature: overall size, distance between holes, alignment with a mating part, angle, or suitability for bending. The contractor can then assess whether the requirement is realistic, whether a trial is needed, and which inspection method makes sense.

Characteristics that must be separated

| Characteristic | What it describes | What should be specified | |---|---|---| | Linear dimension | Length, width, diameter, or distance | Nominal value, tolerance, datum, and measurement points | | Position | Hole center, slot, or contour relative to datums | Datum system and functional purpose | | Angle | Direction or inclination between features | Nominal angle, datum, and inspection method | | Form | Contour deviation, ovality, or local distortion | The controlled form and the relevant area | | Flatness | Departure from the required plane | Free-state condition, support surface, and limit | | Cut edge | Burr, dross, taper, or another cut-quality feature | Visual or measurable acceptance criterion |

ISO 1101 provides a common language for geometrical specification. ISO 2768-1 covers general linear and angular tolerances when a drawing correctly invokes it. ISO 9013 describes characteristics of thermal cuts. These standards help formulate a requirement, but they do not assign one automatic value to every part.

Which datum should be used

A datum is an agreed feature from which another dimension or position is established. It is not enough to place a distance between two elements if neither party knows which one controls the measurement. A mounting-hole center, for example, may be related to a functional edge, a centerline, or another hole group.

Without a datum, customer and contractor may measure the same part differently. Measuring from an arbitrary cut edge is especially risky where the edge contains a local lead-in mark or distortion. Each critical feature should therefore be linked to the part's function and the drawing coordinate system.

Show the primary and secondary datums, critical holes, nominal dimensions, tolerances, and inspection method. If the part is symmetrical, define the centerline. If several holes work as a pattern, control their mutual position rather than checking only the diameter of each hole.

Select tolerances by function

Ask what happens if the feature deviates. If the deviation does not affect assembly, clearance, sealing, fastening, appearance, or the next operation, there is usually no reason to make the tolerance unusually tight. If interchangeability or reliable assembly depends on it, treat it as a critical feature and agree how it will be inspected.

Features can be grouped into three levels:

• critical features determine function or assembly;
• important features influence the next operation, appearance, or interaction with another part;
• reference features help orientation but are not rejection criteria unless the drawing says so.

A single part may use different requirements in different zones. Fastener holes may be critical, the outer contour important, and a decorative opening merely reference information. This hierarchy is more useful and economical than applying one very small tolerance to the entire geometry.

General tolerances and special requirements

A general tolerance is convenient for dimensions that do not need individual treatment. It should not silently be assumed to control hole position, contour form, flatness, or edge quality. When a drawing invokes ISO 2768-1, verify that the class is stated and that it is appropriate for the product and process.

Use a special tolerance when a feature has functional importance. A note that a hole is “toleranced” may still be incomplete: is acceptance based on its diameter, center position, distance from a datum, or fit with a mating hole? For a multi-operation part, identify which characteristics are accepted after cutting and which only after bending or assembly.

Do not confuse dimensional tolerance with edge quality

A dimensional tolerance answers how far the actual size may depart from nominal. An edge requirement answers what cut condition is acceptable for the next operation or visible surface. It may cover burr, dross, taper, roughness, or the need for deburring.

A part can meet its linear dimension yet fail the edge requirement. Conversely, a clean edge does not prove that the holes are correctly positioned. Record these requirements separately instead of replacing them with the vague phrase “high accuracy.” For more detail, see the related Knowledge Hub topic ART-439 on specifying cut-edge requirements.

For a critical feature, state the nominal value, tolerance, datum, inspection instrument or gauge, part condition, and inspection stage. A hole position might be measured from two stated datums before bending, not from an arbitrary edge of the assembled product. If the inspection method is unresolved, record it as an open decision instead of assuming that machine accuracy settles the question.

Factors that influence the achievable result

Material and thickness affect the heat balance, cut shape, and possible distortion. Expectations cannot automatically be transferred between carbon steel, stainless steel, and aluminium merely because the nominal thickness is the same. Sheet flatness, scale, protective film, internal stress, and support during cutting also matter.

Geometry changes the task. A long narrow contour, sharp internal corners, closely spaced holes, and narrow webs may behave differently from a compact simple part. A minimum hole size or universal tolerance should therefore not be promised without considering the actual material, thickness, geometry, and proven process.

If the part will be bent or welded, distinguish the requirement for the flat blank from the requirement for the finished assembly. Bending and welding can change position and form. A cutting contractor cannot be held responsible for final assembly geometry unless those subsequent operations and their acceptance criteria are included in the agreed scope.

Agree on the inspection method

A tolerance without an inspection method can still create a dispute. State whether the part is measured freely, on a reference surface, without force, or in a fixture. Pressing a flexible part flat during measurement may create a temporary shape that does not represent its real assembly condition. For a hole, define whether the diameter, center, or functional fit is inspected.

Agree on the sampling plan: first-off part, every part, periodic samples, or another method. If a report is required, define units, measurement points, report format, and responsibility. A single photograph of a caliper rarely proves conformity of a complex geometry.

For a disputed characteristic, decide in advance who makes the acceptance decision and which document governs it. This protects both parties from a new requirement appearing only after production.

Example: datum dimension and edge requirement

Consider a mounting plate whose hole pattern must align with another part. The critical characteristic may be the position of the hole-group center from a functional edge, not merely each hole diameter. The drawing should define the edge or centerline used as the datum and how mutual position is inspected. A diameter-only requirement can produce correct holes in the wrong location.

For another part, the decisive feature may be an edge entering a welded joint. Overall length and edge condition are then two separate acceptance criteria. Measuring length does not establish whether burr or dross will interfere with fit-up. Both expectations must be agreed so that a general dimensional tolerance is not mistaken for a guarantee of every result.

How to show a critical feature on the drawing

Place the nominal dimension, individual tolerance, datum, and any geometrical requirement next to the critical feature. In the notes, define units, the state of the part during inspection, the general tolerance, and separate edge rules. Do not hide a decisive requirement only in correspondence or a verbal agreement.

Before release, confirm that the PDF drawing, CAD file, and specification have the same revision. Conflicting versions can create a deviation that is incorrectly blamed on cutting. Mark a preliminary file clearly if it is not approved for production.

Trial part before a production batch

If the tolerance has not been demonstrated on the actual material, begin with a representative trial part. Agree the file revision, grade, thickness, datums, acceptance features, and next operation before cutting. Inspect the functional characteristics, not only those that happen to be easiest to measure.

A trial proves a defined route under defined conditions. It is not an automatic guarantee for every item. Reconsider the result if material, thickness, geometry, sheet format, or edge requirement changes. Store the trial record with the drawing and inspection report so that the conditions do not have to be reconstructed from memory.

Send a drawing for tolerance review and manufacturing estimate

Include the current drawing or file, revision, material, thickness, quantity, part function, next operation, and list of critical features. Mark datums, holes, edges, and inspection methods if they are known. If the requirement is not yet measurable, describe the function—for example, “must align with the mating plate”—and agree a verifiable criterion before production.

Tolerance agreement workflow

1. Describe the part's function and the next operation. 2. Identify critical dimensions, holes, datums, and joints. 3. Separate linear, angular, geometrical, and edge requirements. 4. Check which dimensions are covered by the drawing's general tolerance. 5. State datum, nominal value, and tolerance for each special feature. 6. Agree the part condition, sample size, and measurement method. 7. Verify representative geometry with a trial part. 8. Record acceptance criteria before production.

Common mistakes

• Writing “high accuracy” without a nominal value, datum, or inspection method.
• Copying a tolerance from another part, grade, or thickness.
• Treating the machine's positioning figure as a finished-part tolerance.
• Measuring a hole without deciding whether diameter, center, or position matters.
• Mixing dimensional tolerance with edge quality.
• Applying the same tight tolerance to critical and reference dimensions.
• Ignoring bending, welding, or distortion in the next operation.
• Accepting a batch from one accidental measurement without a sampling plan.
• Changing process or safety settings without an approved technical procedure.

Checklist

• [ ] The part's function and next operation are described.
• [ ] Critical features are identified separately.
• [ ] A datum is defined for every critical feature.
• [ ] Size, position, form, flatness, angle, and edge requirements are separated.
• [ ] The general drawing tolerance uses a clear reference and suitable class.
• [ ] Special requirements are placed next to the relevant geometry.
• [ ] Part condition and measurement method are defined.
• [ ] Sampling and report format are agreed.
• [ ] Subsequent operations and a trial part are considered.

What cannot be decided without input data

No universal tolerance can be selected without a drawing, datums, material, thickness, geometry, next operation, and inspection method. Values cannot be transferred blindly from another machine or material, and finished-part tolerance cannot be derived from the machine's positioning specification alone.

Standards provide a common technical language but do not replace a part-specific drawing requirement, trial, and acceptance method. A critical feature must be connected to a datum, function, and repeatable inspection. Edge quality and final geometry after bending or welding require separate definitions.

Summary

A realistic laser-cutting tolerance follows from function, datums, material, thickness, geometry, subsequent operations, and inspection. Linear dimensions, hole position, form, flatness, angle, and edge quality should be agreed separately. The machine's positioning accuracy is not automatically the tolerance of the part.

A good drawing does more than show small numbers. It explains which datum controls each critical feature, what is inspected, and how the result is accepted. When the requirement has not been demonstrated, use a representative trial and do not transfer its result to unrelated geometry without evidence.

This approach allows customer and contractor to agree on a result that serves the part's function instead of arguing over an abstract “laser accuracy” figure.

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