DRAWING & INSPECTION PLANNING
CNC Machining Tolerances and Inspection Requirements
Clear tolerances help a machining supplier understand what controls part function and how the finished component should be verified. There is no single tolerance that applies safely to every CNC-machined feature. Material, geometry, feature size and depth, datum strategy, setup count, surface treatment, order quantity and measurement method all affect manufacturability.
The Drawing Controls the Requirement
The current approved drawing should identify every functional requirement. A 3D model helps with geometry, but it does not replace dimensions, tolerances, datums, material, finish, revision and inspection notes on the drawing. If a dimension is especially important to fit, sealing, motion or assembly, mark it clearly instead of assuming that the supplier will infer its purpose.
SYD Precision reviews stated requirements during quotation. Capability for a specific tolerance is confirmed only after the complete part, process and inspection plan have been evaluated.
CNC Machining Tolerance Chart: ISO 2768 Reference
ISO 2768-1:1989 defines general tolerances for linear and angular dimensions without individual tolerance indications. The ISO catalogue still lists the 1989 edition as the current published version; a revised second edition is under publication. Always confirm the standard and edition stated on the customer drawing before manufacturing.
| Nominal dimension (mm) | Fine (f) | Medium (m) | Coarse (c) | Very coarse (v) |
|---|---|---|---|---|
| 0.5 to 3 | ±0.05 | ±0.1 | ±0.2 | — |
| Over 3 to 6 | ±0.05 | ±0.1 | ±0.3 | ±0.5 |
| Over 6 to 30 | ±0.1 | ±0.2 | ±0.5 | ±1.0 |
| Over 30 to 120 | ±0.15 | ±0.3 | ±0.8 | ±1.5 |
| Over 120 to 400 | ±0.2 | ±0.5 | ±1.2 | ±2.5 |
| Over 400 to 1,000 | ±0.3 | ±0.8 | ±2.0 | ±4.0 |
| Over 1,000 to 2,000 | ±0.5 | ±1.2 | ±3.0 | ±6.0 |
| Over 2,000 to 4,000 | — | ±2.0 | ±4.0 | ±8.0 |
Important: This chart is a general reference, not a universal statement of machine or supplier capability. It applies only when the drawing invokes the relevant standard and class and the dimension has no individual tolerance. A specific tolerance callout takes precedence.
For a project-specific review, mark the features that control fit or function. See how tolerance requirements affect a CNC machining quote.
Requirements to Define on a CNC Machining Drawing
| Requirement | What to communicate | Why it matters |
|---|---|---|
| Size and location | Nominal dimensions, individual tolerances and the features they reference. | Controls fit and the relationship between machined features. |
| Holes, bores and fits | Diameter, depth, through/blind condition, fit class when applicable and mating-part function. | Guides tool selection, process order and measurement. |
| Form and orientation | Flatness, straightness, parallelism or perpendicularity only where the design requires them. | Defines surface behavior without over-controlling unrelated dimensions. |
| Position and rotation | Datum references, true-position requirements, runout or other functional relationships. | Connects features to the way the part locates or rotates in assembly. |
| Threads | Standard, size, pitch, class, depth and any insert or gauging requirement. | Prevents ambiguity and supports the correct inspection method. |
| Surface condition | Required roughness, cosmetic zones, edge condition and directional texture where relevant. | Separates functional surfaces from general as-machined areas. |
| Post-finish dimensions | Dimensions controlled after anodizing, polishing or another treatment, plus masking areas. | Allows the machining allowance and inspection stage to be planned. |
General vs Critical Tolerances
A general tolerance note can cover non-critical dimensions, while tighter individual controls should be reserved for features that affect function. Applying the tightest value to every dimension can increase setups, inspection time, scrap risk and cost without improving the finished assembly.
Highlight critical-to-function features and explain the mating relationship when useful. This helps the manufacturing review focus on the correct risks.
Datum Strategy and Part Function
Datums should reflect how the part is located, assembled or inspected. A practical datum structure connects the drawing to stable physical surfaces. If the datum scheme conflicts with machining access or the functional relationship, clarification may be required before quotation. Axis count alone does not determine accuracy; see our 3-axis vs 5-axis CNC machining guide for the role of feature access, setup strategy and datum transfer.
Use GD&T only when it supports the design intent and inspection plan; unclear symbols can create more ambiguity than a direct dimension.
Inspection Planning
The measurement method should suit the feature and the stated requirement. Depending on the drawing, suitable tools may include calipers or micrometers for appropriate dimensional checks, a height gauge for suitable relationships, and coordinate measuring equipment for defined geometry when requested and appropriate.
Inspection scope is agreed before production. State whether you need first-article results, a dimensional report, a defined sampling plan or another record. The report format, inspection frequency, feature list and acceptance criteria should be confirmed with the quotation instead of being added after parts are complete.
Learn more about our quality and inspection approach.
Material and Geometry Matter
Thin walls, long unsupported sections, deep pockets, small tools and heat-sensitive features can influence stability. Aluminum and stainless steel behave differently during cutting. The final review considers the complete geometry, not only the tolerance value printed beside one dimension.
Finishing Can Change the Result
Anodizing, polishing and other treatments may add or remove material or change edges and holes. Mark which dimensions apply after finishing, identify masking zones and state whether appearance is functional or cosmetic. Inspection before and after treatment may require different controls.
What to Submit for an Accurate Review
- Current revision of the 2D PDF drawing
- Matching STEP or other agreed 3D model
- Exact material grade, condition and standard
- Critical dimensions, datums and functional relationships
- Surface finish and any dimensions controlled after treatment
- Thread, fit and edge requirements
- Required inspection scope and report format
- Order quantity and any approved revision notes
For general preparation advice, use our CNC machining drawing checklist. Process-specific information is available on the CNC milling and CNC turning pages.
Frequently Asked Questions
Can every dimension be held to a tight tolerance?
Not automatically. Feasibility depends on the material, feature geometry, datum scheme, machining sequence and measurement method. Mark the dimensions that control function so they can be reviewed individually.
How is a requested tolerance confirmed?
It is reviewed against the complete drawing, model, material, quantity, finishing and inspection requirements. Any assumptions or required changes should be resolved before manufacturing.
Can you provide inspection reports?
Inspection documentation can be reviewed as part of the project scope. Required features, sampling, report format and timing must be agreed before an order is placed.
Get a Drawing-Based Machining Review
Send the current drawing, model, material, quantity and inspection requirements. We will review the stated tolerances and manufacturing route before confirming a quotation.