CNC Machining Tolerances: ISO 2768, Fits, GD&T and Inspection

CNC machining tolerances define how much a manufactured feature may vary from its nominal size, shape or position. They control whether parts assemble, move, seal and perform as intended. Tolerances also have a major effect on machining time, inspection effort, scrap risk and price.

A common mistake is to apply the tightest available tolerance to the entire drawing. This rarely improves the product. It usually increases cost and can make a practical design difficult to manufacture. A better approach is to define functional requirements at critical interfaces and use economical general tolerances elsewhere.

This guide explains dimensional tolerances, ISO 2768, limits and fits, geometric dimensioning and tolerancing, surface finish, inspection methods and the information needed to source reliable CNC-machined parts.

What Is a CNC Machining Tolerance?

A tolerance is the permitted variation from a nominal value. If a shaft is specified as 10.00 mm with a tolerance of ±0.02 mm, an acceptable measured diameter falls between 9.98 mm and 10.02 mm.

Tolerances may control size, angle, location, form, orientation or surface profile. They may be expressed as bilateral values, unilateral limits, upper and lower limits, fit classes or GD&T feature-control frames.

Accuracy, Precision and Tolerance

These terms are related but different. Accuracy describes closeness to the target value. Precision describes consistency among repeated results. Tolerance is the allowed variation specified by the designer.

A process can be precise but offset from nominal, producing repeatable parts outside the tolerance. It can also average close to nominal but vary too widely. Process control must address both centering and variation.

Why Tolerances Affect Cost

Tight requirements may need a more stable machine, additional setups, controlled temperature, special tools, light finishing passes and higher-resolution inspection. They also reduce the usable process window and increase the risk that a part must be reworked or scrapped.

The cost effect is especially strong when a tight tolerance covers a large distance, a thin flexible structure, several setups or a surface affected by finishing. Tighten only the features that influence product function.

General Tolerances

General tolerances apply when a dimension does not have an individual tolerance. They prevent every measurement from needing its own plus/minus note. The drawing title block should state the applicable standard or table.

A general tolerance is not a universal promise from every machine shop. Part size, geometry, material and process still matter. The supplier should review whether the selected general tolerance is suitable for the complete design.

Understanding ISO 2768

ISO 2768 is commonly used to define general tolerances for dimensions and geometry when individual values are not specified. It is often referenced through a designation such as ISO 2768-mK, combining a dimensional class with a geometric class.

The standard contains tables that vary tolerance according to nominal dimension range. Designers should reference the correct edition and class rather than writing “ISO tolerance” without detail. The drawing and purchasing documents should use the same requirement.

ISO 2768 Dimensional Classes

ISO 2768-1 includes general tolerance classes commonly identified as fine, medium, coarse and very coarse. The appropriate class depends on process capability and functional need.

Fine is not automatically better. A medium class is often practical for ordinary machined dimensions, while individual critical features receive specific tolerances. Coarser classes may be suitable for noncritical large dimensions, fabricated structures or preliminary features.

ISO 2768 Geometric Classes

ISO 2768-2 addresses general geometric tolerances for features without individual controls. Its classes are commonly identified by letters. The chosen class affects straightness, flatness, perpendicularity, symmetry and runout expectations.

Because standards and customer practices evolve, confirm the referenced edition and contractual interpretation. When a feature is essential to assembly, use an explicit GD&T control instead of relying only on a general class.

Bilateral, Unilateral and Limit Tolerances

Bilateral tolerance

A bilateral tolerance permits variation in both directions from nominal, such as 20.00 ±0.05 mm. It is easy to read and suitable when variation on either side has equal functional effect.

Unilateral tolerance

A unilateral tolerance permits variation mainly or entirely in one direction, such as 20.00 +0.00/−0.05 mm. It is useful when crossing nominal in one direction could cause interference or insufficient material.

Limit dimensions

Limit dimensions show the maximum and minimum acceptable values directly. They can reduce interpretation errors for important features but should still be tied to a clear datum and inspection method when location or form matters.

Limits and Fits

Hole-and-shaft fits control the relationship between mating cylindrical features. A clearance fit always leaves space, a transition fit may produce slight clearance or interference, and an interference fit requires force or thermal methods for assembly.

The fit designation should follow a recognized system and match the functional requirement. The designer must consider operating temperature, lubrication, surface finish, coating, load and service assembly—not only the room-temperature nominal size.

Hole-Basis and Shaft-Basis Systems

In a hole-basis system, the hole uses a standard tolerance position and the shaft changes to create the required fit. This is common because standard drills, reamers and gauges make hole sizes convenient to control.

A shaft-basis system may be useful when standard bar, bearings or multiple components share a shaft. The selection should simplify manufacturing across the entire assembly rather than one isolated part.

Designing Precision Holes

A drilled hole may not achieve the size, roundness, straightness or finish required for a bearing, seal or dowel. Precision holes can require boring, reaming, circular interpolation, honing or grinding.

Specify the actual functional requirements. A close diameter tolerance does not automatically control position or axis orientation. Use GD&T when the bore must align with another feature or assembly datum.

Threads and Thread Tolerances

Thread specifications should include standard, nominal size, pitch, class, engagement length and handedness where applicable. Distinguish full-thread depth from total drilled depth in blind holes.

Coating or anodizing changes thread dimensions. Internal threads may need masking or allowance, while external threads may require controlled pre-finish size. Define whether final acceptance occurs before or after treatment.

What Is GD&T?

Geometric dimensioning and tolerancing controls a feature's form, orientation, location, runout or profile relative to functional datums. It communicates design intent more directly than long chains of coordinate dimensions.

GD&T is not simply a way to make drawings stricter. Correct use can enlarge the acceptable manufacturing zone while protecting assembly function. Incorrect or redundant controls can make a drawing expensive and difficult to inspect.

Choosing Functional Datums

Datums should represent how the part locates in the product or inspection setup. A common scheme uses a stable mounting plane as the primary datum, a bore or edge as the secondary datum and another feature to control rotation.

Avoid choosing a thin, flexible or unfinished surface as the primary datum unless it truly defines assembly. Datum features must be accessible to machining and inspection equipment.

Flatness

Flatness controls the entire surface without reference to a datum. It is often applied to mounting faces, sealing surfaces and precision bases.

Large thin plates can appear flat while clamped and relax after removal. The drawing should state free-state or restrained measurement conditions when flexibility is significant. Extremely tight flatness over a large span can dominate the manufacturing process.

Parallelism and Perpendicularity

Parallelism controls orientation relative to a datum, while perpendicularity controls a 90-degree relationship. Both requirements are influenced by datum quality, setup strategy and part stiffness.

Do not specify tight parallelism when only local thickness matters. Conversely, a thickness tolerance alone does not guarantee that two faces are functionally parallel.

Position

Position tolerance controls the location of holes, pins, slots and other features relative to datums. It is widely used for fastener patterns and mating interfaces.

Maximum material condition can provide bonus tolerance when feature size departs from its worst-case limit, reflecting the actual assembly boundary. Designers and inspectors must use the same interpretation and gauge strategy.

Profile

Profile tolerances control line or surface variation relative to the theoretically exact geometry. They are useful for cast, molded, machined and freeform surfaces.

A profile callout can replace many coordinate dimensions, but the datum reference and tolerance distribution must be intentional. A full-surface profile with a tight value may require dense scanning or CMM measurement.

Runout

Circular runout controls variation at individual cross-sections during rotation around a datum axis. Total runout controls the complete surface. These requirements are common for shafts, bearing seats, sealing diameters and rotating assemblies.

Runout can be influenced by roundness, straightness, coaxiality and setup. The datum axis must represent the functional rotation condition.

Tolerance Stack-Up

Assemblies contain variation from multiple dimensions and components. Worst-case stack-up assumes every feature reaches the most unfavorable limit. Statistical methods may be appropriate for stable production processes and large quantities.

Do not force one component to absorb the entire assembly uncertainty. Review the full chain, identify adjustable interfaces and assign tolerances according to function and process capability.

CNC Milling Tolerances

Milling accuracy depends on machine geometry, tool deflection, holder runout, workholding, thermal stability and feature access. Features created in the same rigid setup are generally easier to relate than features machined after re-clamping.

Deep pockets, long tools and thin walls reduce stability. A drawing should not assume that all milled features can hold the same tolerance regardless of depth and orientation.

CNC Turning Tolerances

Turning is efficient for concentric diameters, faces, grooves and threads. Accuracy depends on chucking, tool condition, spindle stability, stock, part length and support.

Long slender shafts can bend under cutting pressure. Features made from opposite ends or after transfer to a sub-spindle require attention to concentricity and axial relationships.

Five-Axis Machining Tolerances

Five-axis machining can reduce setups and preserve relationships between angled features. However, rotary-axis calibration, pivot-point accuracy, fixture rigidity and tool reach influence the result.

Five-axis capability does not remove the need for a sound datum and inspection plan. Large parts and long tools may still require staged machining or local tolerance strategies.

Material Effects

Aluminum, steel, titanium, copper alloys and engineering plastics behave differently during machining. Residual stress, thermal expansion, hardness, moisture absorption and stiffness affect dimensional stability.

A tolerance proven on a rigid steel block may be unrealistic on a thin plastic frame. Material condition and environmental requirements belong in the tolerance review.

Temperature and Measurement

Materials and measuring equipment change size with temperature. Large dimensions and tight tolerances are especially sensitive. A warm part measured immediately after machining may differ from the same part after stabilization.

Critical inspection should use controlled conditions and an agreed reference temperature. The supplier should allow the part and instruments to equalize before final measurement.

Thin-Wall Distortion

Thin components can move under clamping and spring back after release. They may also distort as internal stock stress is removed. A dimension measured while the part is forced against a fixture may not represent its free condition.

Use staged roughing, balanced material removal, temporary supports and light finishing cuts. The drawing should define the support condition for inspection.

Surface Finish and Dimensional Tolerance

Surface roughness and size tolerance describe different characteristics. A bore can meet diameter while having an unsuitable finish, or have an excellent finish but fail size.

Roughness requirements affect tool selection, feed, finishing passes and inspection. Apply them where sealing, sliding, fatigue, appearance or coating adhesion requires control.

Finishing and Coating Allowance

Anodizing, plating, paint, powder coating, passivation and heat treatment can change dimensions or stability. Coatings may build on surfaces, while some conversion processes have negligible dimensional effect.

Specify which dimensions apply after finishing. Mark masking zones, threaded features, electrical contacts and precision fits. The machining supplier and finishing provider should agree on pre-treatment targets.

Common Inspection Tools

• Calipers for general dimensional checks.
• Micrometers for precise external or internal size.
• Bore gauges for diameter and variation.
• Height gauges and surface plates for location and height.
• Pin and plug gauges for holes and functional acceptance.
• Thread gauges for internal and external threads.
• Dial indicators for runout and comparative checks.
• Surface roughness testers for texture.
• Optical systems for small or complex features.
• Coordinate measuring machines for dimensional and geometric inspection.

CMM Inspection

A coordinate measuring machine can evaluate sizes, locations, orientations and profiles from a programmed measurement routine. CMM inspection is valuable for complex datum structures and documented first articles.

The CMM must have suitable accuracy, probe access and working volume. Measurement strategy, point distribution, alignment and software interpretation can affect reported results. Agree on the method for critical or borderline features.

Functional Gauging

A functional gauge checks whether the part will assemble or meet a boundary condition. It can be faster and more representative than measuring every contributing dimension separately.

Gauge design, calibration and wear must be controlled. For low-volume prototypes, trial assembly with the mating component may provide useful supplementary evidence.

Measurement Uncertainty

Every measurement has uncertainty from equipment, environment, setup, operator and method. If the tolerance is close to the instrument's capability, acceptance decisions become unreliable.

Select equipment with adequate resolution and uncertainty for the feature. Very tight tolerances may require a controlled metrology environment and a documented measurement system.

First-Article Inspection

A first-article inspection report verifies that the initial production process can create the specified part. It may include ballooned drawings, measured results, material certification and process records.

Define the required report format before production. Reconstructing complete first-article data after parts are shipped can be costly or impossible.

Process Capability in Production

A few acceptable prototypes do not prove that a process will remain stable across thousands of parts. Production programs may monitor capability, tool wear, fixture condition and measurement trends.

Tolerance design should consider the expected manufacturing distribution, not only whether one expert machinist can produce a conforming sample.

How to Choose Economical Tolerances

1. Identify the product's functional interfaces.
2. Build the assembly tolerance stack.
3. Select stable, accessible datums.
4. Choose a practical general tolerance.
5. Apply tighter values only to critical features.
6. Match tolerances to material and geometry.
7. Account for coating and operating temperature.
8. Define inspection and support conditions.
9. Request manufacturing feedback before release.

Ways to Reduce Tolerance-Related Cost

• Replace broad tight tolerances with local functional controls.
• Keep related critical features in one setup when possible.
• Use larger, more rigid feature geometry.
• Avoid tight requirements across flexible spans.
• Define fits with recognized standards.
• Use functional gauges for repetitive assembly checks.
• Separate cosmetic expectations from dimensional requirements.
• Specify an appropriate sampling plan.
• Coordinate machining and finishing allowances.

RFQ Checklist for Tolerance-Critical Parts

• STEP model and revision-controlled drawing.
• Applicable tolerance standard and edition.
• Material grade, temper and certification.
• Functional datums and critical features.
• Fit classes and mating-component information.
• Free-state or restrained measurement condition.
• Surface roughness and finish requirements.
• Pre-finish or post-finish dimension rules.
• Inspection method and report format.
• Prototype quantity and production forecast.

Common Drawing Mistakes

• Applying the same tight tolerance to every dimension.
• Using an undefined “standard tolerance.”
• Selecting unstable or inaccessible datums.
• Controlling size but not feature position.
• Duplicating dimensions and creating conflicts.
• Using redundant GD&T controls.
• Ignoring material movement and thin-wall flexibility.
• Omitting the measurement condition.
• Failing to account for coating thickness.
• Requesting inspection without defining the report scope.

Frequently Asked Questions

What is a standard CNC machining tolerance?

There is no single value for every part. Suppliers use their own standard capabilities or a referenced system such as ISO 2768. Geometry, size, material and process still determine what is practical.

Can CNC machining hold ±0.01 mm?

It may be achievable on suitable rigid features with capable equipment and inspection, but it should not be assumed for an entire part. Size, geometry, temperature and setup strategy must be reviewed.

Does a tighter tolerance always improve quality?

No. Quality means meeting functional requirements consistently. An unnecessarily tight tolerance raises cost without improving performance.

What is the difference between flatness and parallelism?

Flatness controls one surface independently. Parallelism controls a surface or axis relative to a datum.

Should dimensions be inspected before or after anodizing?

The drawing should state the requirement. Critical final fits are generally evaluated in their delivered condition, with machining allowance and masking planned accordingly.

Why do suppliers ask which dimensions are critical?

Critical-feature information helps them select the correct process, setup and inspection effort. It also prevents unnecessary cost on nonfunctional geometry.

Request a CNC Tolerance Review

Effective CNC machining tolerances protect assembly and performance while leaving the manufacturer a realistic process window. A clear datum system, appropriate general standard and focused critical controls can improve both quality and price.

Explore Aluprototype's CNC machining and rapid prototyping services, read the high-precision aluminum machining guide, or review large aluminum prototype tolerance planning. Send your model, drawing, quantity, material and inspection requirements through the request-a-quote page for a manufacturing review.