CNC machining tolerances can make the difference between a quote that is fast and reliable and one that requires days of clarification. When a drawing says only “tight tolerance required,” a manufacturer must price the uncertainty: extra setups, slower machining, more inspection, and a higher risk of rejected parts.
This guide explains how to specify tolerances so your CNC machining supplier can understand the function of the part, select a suitable process, and provide a more accurate quotation.
What Is a CNC Machining Tolerance?
A tolerance is the permitted variation from a nominal dimension or geometry. For example, a diameter specified as 20.00 ±0.02 mm may measure from 19.98 mm to 20.02 mm and still meet the drawing requirement.
Tolerances can control more than size. They may also define form, orientation, and location—including flatness, perpendicularity, parallelism, position, runout, and profile. The achievable result depends on the complete manufacturing situation: material, feature geometry, wall thickness, tool access, heat treatment, surface finish, batch size, fixturing, and the inspection method.
Why Tighter Is Not Always Better
It is tempting to apply the smallest possible tolerance to every dimension. In practice, unnecessarily tight tolerances usually increase cost without improving part function. They may require additional operations, special tooling, slower cycle times, temperature-controlled measurement, more detailed inspection, or a higher scrap allowance.
A better rule is simple: apply tight tolerances only where the assembly, sealing, motion, alignment, appearance, or service life requires them. Use practical general tolerances for non-critical dimensions.
General Tolerances vs. Critical Tolerances
General tolerances apply to dimensions that do not have an individual tolerance next to them. They are normally stated in the drawing title block or in a general note.
Critical tolerances are assigned directly to features whose variation affects fit or function. Typical examples include bearing seats, sealing diameters, hole patterns, locating faces, precision pockets, datum surfaces, and mating interfaces.
Separating the two helps the machine shop focus process control and inspection on the features that matter most.
What to Include in a CNC Machining RFQ
1. Units, Drawing Revision, and File Priority
State whether the drawing uses millimetres or inches, identify the current revision, and explain which file controls if the 2D drawing and 3D model conflict. The 3D model is useful for geometry, but the 2D drawing should communicate tolerances, datums, surface finish, threads, and inspection notes.
2. A Clear General Tolerance Note
Include a general tolerance rule for unspecified dimensions. If you reference a standard, state the exact standard and edition used by your organization. Do not assume every supplier will interpret a short note such as “ISO tolerance” in the same way.
ASME Y14.5 is widely used for geometric dimensioning and tolerancing (GD&T). ISO 2768-1 covers general tolerances for linear and angular dimensions without individual indications. For general geometrical specifications, note that ISO 2768-2 has been withdrawn and replaced by ISO 22081. Standards evolve, so confirming the edition avoids ambiguity.
3. Identify Function-Critical Features
Mark the dimensions that affect the part’s function. If a bore receives a bearing, a face creates a seal, or a hole pattern locates another component, make that relationship clear. A brief functional note can help the engineering team understand why a tolerance is important and propose a manufacturable solution when necessary.
4. Define Datums and Use GD&T Where Appropriate
Dimensions need a stable reference. A datum system tells the manufacturer how the part is located for machining and how it should be set up for inspection. Without meaningful datums, a tight positional or orientation tolerance may be difficult to manufacture and impossible to verify consistently.
Use GD&T when it communicates functional requirements more clearly than multiple coordinate dimensions. Avoid adding symbols that do not serve a functional purpose.
5. Specify Fits, Threads, and Mating Parts
For shafts and holes, define the required fit or provide the mating dimension and functional intent. For threads, include the thread system, nominal size, pitch, tolerance class, depth, and whether the requirement applies before or after coating. Also identify inserts, press fits, and sealing features.
6. State Surface Finish Requirements
Surface roughness and dimensional tolerance are related but not interchangeable. A part can be within size tolerance and still have an unsuitable surface. Apply roughness requirements only to the relevant faces and include any direction-of-lay or sealing requirements when needed.
7. Define Inspection and Reporting Needs
Tell the supplier which features require inspection and whether you need a standard inspection report, first article inspection, material certificate, CMM report, or another record. Also identify the agreed measurement condition or method for features that can be interpreted differently.
Typical Information by Feature
| Feature | Useful information to specify | Why it matters |
|---|---|---|
| Mating bore or shaft | Diameter tolerance or fit class, surface finish, mating part | Controls clearance, interference, and wear |
| Hole pattern | Datum references, true position, hole size and depth | Controls assembly alignment |
| Sealing face | Flatness, finish, sealing area, coating condition | Controls leakage and contact |
| Thin wall | Minimum thickness, critical zones, acceptable distortion | Guides fixturing and machining sequence |
| Thread | Standard, size, pitch, class, depth, gauge requirement | Prevents assembly and inspection disputes |
Five Common Tolerance Mistakes
- Applying one tight tolerance to the entire drawing. This can make non-critical features unnecessarily expensive.
- Sending files with conflicting revisions. A model and drawing that disagree will delay review and create quality risk.
- Using GD&T without functional datums. The inspection setup may not represent how the part works in the assembly.
- Ignoring the effect of finishing. Anodizing, plating, heat treatment, grinding, and polishing can change dimensions or the condition in which they must be measured.
- Leaving inspection expectations until after production. Reporting and measurement requirements should be included at the quotation stage.
Pre-RFQ Tolerance Checklist
- Confirm units and the latest drawing revision.
- Provide both a 3D CAD model and a controlled 2D drawing when possible.
- State the general tolerance rule and the exact referenced standard/edition.
- Highlight critical dimensions, datums, fits, threads, and surface finishes.
- Specify material, heat treatment, coating, quantity, and delivery requirements.
- Explain the function of unusual or especially tight requirements.
- Define required inspection records and measurement methods.
- Check that the model, drawing, and purchasing notes do not conflict.
For a broader file-preparation checklist, read our guide: How to Prepare CAD Files for an Accurate CNC Machining Quote.
How SkyPu Reviews CNC Machining Tolerances
SkyPu reviews the material, geometry, datum strategy, tooling access, process sequence, finishing, and inspection requirements before confirming a quotation. When a requirement creates avoidable cost or manufacturing risk, our team can discuss the functional need and suggest a practical alternative.
On selected critical features and suitable projects, tolerances down to ±0.001 mm may be achievable after engineering review. Final capability depends on the part design, feature geometry, material, heat treatment, process, quantity, measurement environment, and inspection plan; it should never be assumed from a single number alone.
Explore our precision mold component capabilities and stamping die component capabilities, or send your 2D drawing, 3D model, material, quantity, and delivery requirements to Lily@ksskypu.com.
Need a Tolerance Review Before You Order?
Our engineering team can review your drawings for manufacturability, identify unclear tolerance callouts, and prepare a more accurate CNC machining quote.
Reference Standards
- ASME Y14.5 — Dimensioning and Tolerancing
- ISO 2768-1:1989 — General tolerances for linear and angular dimensions
- ISO 2768, Edition 2 (under publication at the time of writing)
- ISO 22081:2021 — Geometrical product specifications
Standards and customer requirements can change. Always verify the applicable edition and contract requirements for your project.
