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Tolerance & Inspection Guide for CNC Machined Parts

A practical guide to understanding CNC machining tolerances, drawing requirements, inspection methods, and dimensional reports before requesting a quote.

Tolerance Basics ISO 2768 Feature Tolerances Drawing Notes Inspection Methods Reports
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Dimensional inspection of CNC machined parts at Meili Machining Inspection & Tolerance
Tolerance is confirmed per drawing — not by a single number for all parts
Critical features need to be called out clearly; the drawing is the final reference
Dimensional inspection reports are available on request — confirm scope before production
Tolerance Basics

What tolerance means in CNC machining

Tolerance defines the acceptable deviation from a nominal dimension. In CNC machining, every feature has a tolerance — whether or not it is explicitly called out on the drawing. Understanding how tolerance works helps avoid over-specifying non-critical features, and ensures that critical fits, threads, and interfaces are controlled where it actually matters.

Tighter tolerance increases cost, cycle time, and inspection burden. It requires slower feeds, more frequent tool checks, and sometimes dedicated fixturing. The right approach is to apply tight tolerance only where function demands it — and confirm feasibility during drawing review before production starts.

General Dimensions
Typical range ±0.1 mm and above, unless tighter is called out. Governed by a general tolerance standard such as ISO 2768-m when no explicit tolerance is specified.
When it applies Clearance holes, bulk stock dimensions, non-mating faces, overall envelope dimensions.
Confirm which general tolerance standard applies before production.
Critical Fit Dimensions
Typical range ±0.01–0.05 mm or tighter for bearing bores, shaft interfaces, and precision assemblies. Feasibility depends on material, geometry, wall thickness, and process.
When it applies Mating bores, precision shafts, locating pins, threaded interfaces, and fit-critical surfaces.
Must be explicitly called out and confirmed per drawing review.
Cosmetic / Non-Critical
Typical range General tolerance applies. Surface appearance is confirmed visually against agreed standard, not by a dimensional callout.
When it applies Exposed faces, fillet radii, non-functional edges, cosmetic recesses or pockets.
Over-tolerancing cosmetic features increases cost without function benefit.
Post-Process Affected
Typical range Anodizing adds 0.01–0.025 mm per side. Hard anodizing up to 0.05 mm per side. Plating varies by process. Dimensions on finish-affected surfaces must allow for coating buildup.
When it applies Any dimension on a surface that receives anodizing, plating, or coating after machining.
Confirm finish impact on fit dimensions before quoting. Machine to pre-finish target.
ISO 2768

General tolerance standard — a practical overview

ISO 2768 defines general tolerances for linear dimensions, angular dimensions, and geometric form. It is useful for controlling non-critical dimensions without individually tolerancing every feature on the drawing. ISO 2768 defines two parts: Part 1 for linear and angular tolerances (grades f, m, c, v), and Part 2 for geometric tolerances.

ISO 2768-m is a common starting point for CNC machined parts — but it is not a default guarantee for every order. The applicable grade must be specified on the drawing title block. For critical features, an explicit dimension tolerance always overrides the general standard. ISO 2768 is not a substitute for GD&T callouts where form, position, or orientation is functionally important.

Grade Designation Typical use What it means in practice Caution
f Fine High-precision machined parts where tight general tolerances are expected across the whole part Tightest general tolerance class. Suitable for precise assemblies where features that are not individually toleranced still need to be held closely. Increases cost and inspection burden across all features — use when function requires it, not by default.
m Medium Standard CNC machined parts — the most common general tolerance reference for machined metal parts Suitable for most CNC milling and turning work. Provides adequate control for non-critical features without unnecessarily tight general requirements. ISO 2768-m is a starting point, not a guaranteed default. Must be confirmed per drawing. Critical features still require explicit callouts.
c Coarse Less precise machining, sheet metal work, or features where generous tolerance is acceptable Relaxed general tolerance. Appropriate for non-precision parts, structural components, or features where function allows wide variation. Should not be applied to functional or mating surfaces. Confirm before use to avoid misunderstanding.
v Very coarse Rough fabrication, castings, or forged blanks before finish machining Very relaxed tolerance class. Rarely used for precision CNC work. Intended for rough-state dimensions where exact values are not functionally important. Not appropriate for finished CNC machined parts. If specified, confirm the intent with the supplier.
Note: The values above describe practical application, not the exact numerical tolerances defined in the ISO 2768 standard. For compliance purposes, always refer to the official ISO 2768 document. Exact allowable deviations depend on the nominal dimension range and the grade selected — consult the standard or confirm with your supplier during drawing review.
Feature Tolerances

Common CNC tolerance situations by feature type

Different features have different achievable tolerance ranges depending on material, geometry, wall thickness, process, and setup. Tighter tolerance should be applied only where function requires it — applying high-precision tolerance to non-critical features increases cost without a quality benefit.

Holes & Bores
What to specify Nominal diameter plus tolerance, or an ISO fit designation (e.g. H7). Depth and position should also be called out if critical.
What affects the result Tool size and runout, material hardness, wall thickness, and fixturing rigidity. Deep holes are harder to hold than shallow holes.
Common inspection method Calibrated plug gauge, bore gauge, or micrometer depending on diameter and tolerance requirement.
Position tolerance for hole patterns should be called out with GD&T if assembly fit depends on it.
Shafts & Pins
What to specify Outer diameter with tolerance, or ISO fit (e.g. h6, g6). Surface roughness on bearing or sealing surfaces should also be specified.
What affects the result Shaft length-to-diameter ratio, material, tool pressure, and whether live tooling or OD turning is used. Long slender shafts are more prone to deflection.
Common inspection method Outside micrometer, ring gauge, or bench-mount indicator for roundness.
Concentricity and runout should be called out where mating or rotating precision is needed.
Threads
What to specify Thread standard (ISO metric, UNC/UNF), nominal size, pitch, thread class or fit (e.g. 6H/6g), and any special requirements such as length of engagement.
What affects the result Tap or die quality, material machinability, and whether thread is cut or formed. Blind threads have a minimum depth limit.
Common inspection method Thread plug gauge (internal) or thread ring gauge (external), matched to the specified thread class.
Incomplete or ambiguous thread callouts are a common cause of nonconformance. Specify fully.
Flatness & Parallelism
What to specify GD&T flatness symbol with tolerance zone in mm, or parallelism with a datum reference. Title-block tolerance does not control these by default.
What affects the result Material stress-relief, part clamping during machining, thermal expansion, and thin-wall deflection. Stress in bar stock can cause bow after machining.
Common inspection method Height gauge on surface plate, indicator sweep, or CMM for complex geometry.
If flatness is functionally important — for sealing, clamping, or mating — call it out explicitly.
Position & GD&T
What to specify True position callout with datum references for hole patterns, boss locations, or features that must align with mating parts. Include material condition modifier if relevant.
What affects the result Machine zero accuracy, fixturing repeatability, and cumulative stack-up from earlier operations. Multi-setup parts are harder to hold in position.
Common inspection method CMM measurement is most reliable for true position. Height gauge and coordinate measurement can be used for simpler cases.
Position callouts require agreed datums and inspection method confirmation before production.
Surface Finish & Roughness
What to specify Ra value in µm and applicable surface(s). Typical CNC milled surfaces: Ra 1.6–3.2 µm. Ground or fine-turned: Ra 0.4–0.8 µm. Indicate on drawing per ISO 1302.
What affects the result Tool type, feed rate, cutting speed, material, coolant, and whether a finishing pass is used. Roughness varies by direction of cut.
Common inspection method Contact profilometer (surface roughness tester). Visual comparison against agreed sample for cosmetic requirements.
Surface finish requirement should be confirmed early — it may affect machining strategy and cost.
Drawing Notes

What to show on your drawing

A complete, unambiguous drawing is the single most important factor in getting a dimensionally correct part. The drawing is the final reference — if something is not on the drawing, it cannot be inspected against. The checklist below covers the most common items that should be confirmed before sending drawings for quote.

  • Critical dimensions Call out all dimensions that affect fit, assembly, or function with explicit tolerances. Do not rely on general tolerance for features that matter.
  • Datum references Define datums clearly when GD&T callouts are used. Datums determine how the part is positioned during inspection — ambiguous datums lead to inconsistent results.
  • GD&T callouts Use GD&T symbols for flatness, parallelism, perpendicularity, position, or runout where angular or form errors could affect assembly. Title-block tolerance does not control these.
  • Thread specifications Include thread standard (ISO metric / UNC / UNF), nominal size, pitch, thread class, and depth for blind holes. Incomplete thread callouts are a common source of nonconformance.
  • Surface roughness Specify Ra value in µm for functional surfaces. Mark applicable faces clearly on the drawing. If a cosmetic standard is required, agree a visual reference or sample.
  • Finish & coating Specify finish type, color, and standard if applicable. Note any dimensional impact — anodizing and plating add material and may affect fit dimensions. Confirm tolerance budget allows for coating.
  • General tolerance standard Specify in the title block: ISO 2768-m (or the applicable grade). This applies to all dimensions without an individual callout. If none is stated, confirm with your supplier before production.
  • Inspection points If specific dimensions must be verified and reported, identify them on the drawing. This allows the supplier to confirm inspection method and report scope before quoting.
Dimensional check of CNC machined part against drawing specification

Critical dimensions are checked against drawing callouts before shipment. Parts without explicit callouts are verified against the general tolerance standard stated in the title block.

Inspection Methods

What tool is used for what

Inspection method depends on the feature, the tolerance requirement, and what is agreed at the time of quoting. Not every tool is suitable for every feature — and the inspection method should be confirmed before production for critical or high-consequence dimensions.

Caliper and micrometer for CNC part dimensional inspection
Caliper & Micrometer
Measures Linear dimensions: OD, ID, length, step heights, slot widths, and overall envelope.
Suitable for Features with tolerances of ±0.02 mm and above. Micrometers provide higher resolution for tighter OD/ID callouts.
When used Routine dimensional check on most machined parts before shipment.
Height gauge for step and depth inspection of machined parts
Height Gauge
Measures Step heights, depth from datum surface, boss or pocket heights, and perpendicularity to the reference surface.
Suitable for Features referenced from a common datum face. Works on a surface plate with part correctly oriented.
When used Critical step, depth, or flatness checks where a caliper reading from the edge would be unreliable.
Thread plug and ring gauges for thread inspection
Thread & Plug Gauge
Measures Thread GO/NO-GO for internal threads (plug gauge) and external threads (ring gauge). Hole diameter GO/NO-GO for plain bores.
Suitable for Verifying thread fit class (e.g. 6H, 6g) and hole diameter tolerance. Fast and repeatable for production runs.
When used All parts with threaded holes or bores, and critical diameter fits requiring a discrete pass/fail check.
Pre-shipment inspection — what is always checked

Every shipment goes through a visual and dimensional check before leaving. The items below are verified on all orders. Critical dimension reporting and surface roughness testing are confirmed at quoting stage when required.

Dimensions against drawing callouts
Thread and hole GO/NO-GO pass
Surface appearance and burr condition
Finish color and uniformity (where applicable)
Packaging condition before export
Quantity and part number against packing list
CMM — Coordinate Measuring Machine
External CMM inspection can be arranged on request for complex geometry, tight position tolerances, or GD&T callouts that require full 3D verification. CMM is not in-house at Meili — it is coordinated through a partner inspection service. If CMM verification is required, it should be specified and confirmed before quoting so lead time and cost can be accounted for.
Inspection Reports

Documentation — what is available and when to request it

Inspection reports document measured values against drawing callouts and provide a record of dimensional compliance. The format, scope, and whether a report is required should be confirmed before production — not after shipment.

Dimensional Inspection Report
Lists critical or called-out dimensions, the measured value for each, and pass/fail status against drawing tolerance. Available on request — specify which dimensions should be covered when sending drawings. Not automatically generated for every order.
First Article Inspection (FAI)
A full dimensional check of the first production article before the full batch is run. Provided when required and confirmed at quoting stage. Useful for new part numbers, new setups, or orders where compliance must be confirmed before committing to full quantity.
CMM Dimensional Report
A formal coordinate measurement report covering position, form, and profile tolerances. Arranged through an external CMM service on request — not in-house. Scope, format, and cost depend on part complexity. Must be requested and confirmed before production.
Material Certificate
Mill test report or material certificate for the raw material used. Available for standard commercial materials. Confirm availability when requesting a quote if a traceability certificate is required for your application or sector.

Confirm scope before production Inspection report format, scope, and cost should be agreed before quoting. Requesting a report after machining may not be possible for all dimensions, and can add cost if parts need to be re-measured or sent to an external inspection service.

Tolerance Planning

How to avoid tolerance problems

Most tolerance problems in custom CNC machining are avoidable with clear drawings and upfront confirmation. These six practices reduce risk before a single chip is cut.

01
Do not over-tolerance non-critical features
Applying ±0.01 mm to every dimension adds cost without benefit. Review the drawing and reserve tight tolerances for dimensions that directly affect fit, function, or assembly clearance.
02
Identify and call out critical fit dimensions
Bearing bores, shaft diameters, locating pin holes, and mating interfaces must carry explicit dimension tolerances or ISO fit designations. General tolerance standard does not replace individual callouts for critical features.
03
Specify threads completely
State thread standard, size, pitch, class (e.g. 6H), and depth for every threaded hole or shaft. Incomplete thread callouts are one of the most common root causes of nonconformance and rework.
04
Account for finish thickness on fit dimensions
If a surface is anodized, plated, or coated after machining, the coating adds material. Confirm the expected thickness and machine to a pre-finish target so the finished dimension meets drawing requirements.
05
Confirm inspection method before production
Some tolerances require specific inspection equipment — position tolerance may need CMM, surface roughness needs a profilometer. Agreeing the inspection method upfront ensures both parties measure the same way.
06
Ask before production if anything is unclear
DFM review and tolerance discussion before production starts is faster and cheaper than rework after. If a tolerance looks difficult or the drawing has ambiguities, raise them at quoting — not after parts are made.

Have tolerance or inspection requirements?

Send your drawing with critical dimensions, tolerance callouts, inspection points, and quantity — we will review manufacturability and inspection feasibility before quoting.

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