Cost and cycle-time spike
Blanket ±0.01 mm on every hole and face forces slow feeds, more setups, and longer inspection — even when only one fit needs that band.
Precision & Tolerance Standards for CNC Parts
Reference for general and process capability tolerances used in CNC work. Project limits are always per drawing and confirmed via DFM — industry tables are not a contract for every PO.
When this guide applies
Use this guide when you are writing or reviewing a CNC drawing and need a clear split between industry reference tables and project delivery defaults.
A tolerance is the permissible variation on a dimension or geometry. Tighter bands need slower feeds, better tooling, and more inspection — cost rises quickly. Specify the loosest value that still meets function.
Zonclair delivery defaults (unless the drawing is tighter):
- Standard: ±0.10 mm
- Milling / turning capability: ±0.01 mm on selected features when material and geometry allow — confirmed via DFM
- Critical 5-axis features: ±0.005 mm only when designated and DFM-confirmed
- Plastics: typically about ±0.05 mm unless reviewed
We recommend ISO 2768-m as the general default on drawings. Call out critical features individually with their own tighter limits and clear datums for CMM. Your PO follows the drawing and agreed quality plan (Quality Control).
ISO 2768-1 linear tolerance classes (reference)
| Class | 0.5–6 mm (±) | 6–30 mm (±) | 30–120 mm (±) | Typical use |
|---|---|---|---|---|
| f (fine) | 0.05 | 0.10 | 0.15 | Precision fits, high-quality mechanical parts |
| m (medium) | 0.10 | 0.20 | 0.30 | General engineering — recommended default for non-critical features |
| c (coarse) | 0.20 | 0.50 | 0.80 | Rough operations, low-precision parts |
| v (very coarse) | 0.50 | 1.0 | 1.5 | Castings, forgings, raw stock reference |
Note
Prefer ISO 2768-m for non-critical features. Halving a tolerance band can multiply machining and inspection cost — apply tight bands only where function demands them. Analyze stack-up on assemblies before freezing every interface.
Process capability vs delivery (reference)
| Process | Typical industry reference | Exceptional (extra care) | Zonclair delivery note |
|---|---|---|---|
| 3-axis milling | ±0.05 mm | ±0.01 mm | ±0.10 mm standard; ±0.01 mm selected features via DFM |
| 5-axis milling | ±0.05 mm | ±0.015 mm class | ±0.005 mm only on designated critical features via DFM |
| CNC turning | ±0.025 mm | ±0.005 mm class | Diameters often tighter than lengths; confirm on drawing |
| Swiss turning | ±0.01 mm | ±0.003 mm class | Small slender parts; linear capability claims need DFM |
| Wire EDM | ±0.01 mm standard class | ±0.005 mm critical class | State standard vs critical features separately |
| Hole diameter (drill / ream) | ±0.05 mm drill | ±0.02 mm ream/bore | Specify ream/bore when fits require it |
GD&T essentials
GD&T (ASME Y14.5 / ISO 1101) controls form, orientation, location, and runout beyond simple ± dimensions. Common controls on CNC parts:
- Flatness — sealing and sliding faces
- Parallelism / perpendicularity — rails, beds, assembly alignment
- True position — hole patterns (prefer over stacked ± where possible)
- Profile of a surface — complex 3D contours
- Circularity / cylindricity — shafts and bearing seats
- Runout — rotating components relative to a datum axis
Always define a clear datum reference frame. Without datums, geometric tolerances cannot be verified consistently on CMM. Prefer true position with datums over a maze of ± callouts on hole patterns.
Surface roughness (Ra) vs process
| Ra | Typical process | Appearance / use |
|---|---|---|
| 3.2 µm (125 µin) | Standard milling / rough finish | Visible tool marks; non-cosmetic structure |
| 1.6 µm (63 µin) | Fine finishing pass | Semi-bright; general precision |
| 0.8 µm (32 µin) | High-quality finish / fine boring | Sealing faces, bearing journals |
| 0.4 µm (16 µin) | Grind / hone / lap | Hydraulic rods, high-precision seats |
| ≤ 0.2 µm (8 µin) | Super-finish / polish | Special applications — confirm scope |
Note
Finishes change size — hardcoat growth is often on the order of ~0.05 mm per surface; mask threads/fits and compensate CAD (see Surface Finishes). Inspection scope follows the drawing and agreed quality plan — not an implied 100% full-dimension check on every commercial job.
What goes wrong with blanket tight tolerances
Ambiguous datums
Tight GD&T without a clear datum frame cannot be measured consistently on CMM, so quotes stall or first articles disagree with the drawing intent.
Finish growth ignored
Anodize, plating, or powder coat moves size after machining. Fits and threads fail if growth and masking were never called out.
Plastic vs metal assumptions
Polymer parts rarely hold metal-class bands without special fixturing. Plan ~±0.05 mm typical unless DFM confirms otherwise.
What to include in your tolerance RFQ
- Preferred STEP model plus a marked 2D PDF/DWG with datums and critical characteristics highlighted.
- General tolerance callout (ISO 2768-m recommended) vs individually tighter features.
- Material grade/temper — plastics need separate expectation (~±0.05 mm typical).
- Surface finish, masking, and whether critical sizes are before or after coating.
- Inspection needs: standard check, sampling, FAI, CMM scope, MTR/CoC.
- Quantity and revision so the quality plan matches the release.
Upload via Upload CAD for DFM Review. With a complete RFQ we typically respond within one business day.
Ready to start your CNC project?
Upload CAD with critical dimensions called out. We typically respond within one business day.