In CNC machining, tolerance is the acceptable deviation from a specified dimension. For most industrial applications, standard tolerance is +/-0.05mm. For precision parts like medical devices or aerospace components, +/-0.01mm or tighter is often required. This guide covers the key factors affecting accuracy and how to specify tolerances correctly on your drawings.
10 Factors That Affect CNC Machining Accuracy
1. Machine Tool Rigidity
Older or poorly maintained machines develop spindle runout and table wobble over time. Newer multi-axis CNC machining centers with linear guides maintain accuracy much longer between calibrations. Regular machine calibration is essential for holding tight tolerances.
2. Cutting Tool Condition
Worn inserts cause built-up edge (BUE) and dimensional drift during the cut. We replace inserts based on wear patterns and material type, not just by cycle count. For aluminum parts, this is especially critical as BUE forms quickly without proper air cutting or appropriate geometry inserts.
3. Workpiece Material Homogeneity
Castings and forgings have internal stress patterns and grain variations that cause inconsistent machining behavior. Raw bar stock machines more predictably. For critical-tolerance parts, we specify annealed or stress-relieved material to minimize distortion during and after machining.
4. Fixture and Clamping Method
Poor clamping causes workpiece shift mid-operation – a major source of out-of-tolerance parts in batch runs. Soft jaws, vacuum tables, and precision fixtures each suit different part geometries. We select clamping strategy based on the tolerance requirement and part wall thickness.
5. Thermal Deformation
Spindle heat and ambient shop temperature changes cause metal to expand and contract. For tight-tolerance work, we machine in climate-controlled conditions and schedule critical feature cuts during thermally stable periods (usually mid-morning in our shop).
6. Cutting Parameters (Speed, Feed, Depth of Cut)
Aggressive cuts generate heat and tool deflection, both of which degrade accuracy. For tight-tolerance features, we typically use lighter depth-of-cut passes, climb milling, and carefully matched surface speed and feed rate per material. The difference between a good cut and a rejected part is often 20-30% reduction in feed rate.
7. Part Geometry and Wall Thickness
Thin walls deflect under cutting forces. Deep pockets cause tool chatter that ripples across the part surface. These geometries often require specialized tooling, slower feeds, or 4-axis strategies to hold tolerance. We flag these in our DFM review before production.
8. Measurement Method and Calibration
Manual micrometers introduce operator error. For first-article inspection, we use calibrated Coordinate Measuring Machines (CMMs) with calibration certificates traceable to national standards (GB/T 1957 for length, ISO 10012 for measurement management systems).
9. Post-Processing and Secondary Operations
Heat treatment distorts parts – often 0.1-0.3mm depending on material and part geometry. Anodizing adds a surface layer (typically 0.01-0.025mm per surface) that shifts critical dimensions. We account for post-processing in our original machining allowance and provide post-HT re-machining when needed.
10. Operator Experience and Process Control
Knowledge of the specific machine, material batch, and job history matters. Our operators maintain detailed setup sheets, tool offset records, and batch measurement logs – providing full traceability from raw material to shipped part.
Common Tolerance Standards by Industry
| Industry | Typical Tolerance | Key Requirements |
|---|---|---|
| General Manufacturing | +/-0.05mm | Standard commercial tolerance |
| Automotive | +/-0.02-0.05mm | Varies by subsystem; critical safety parts tighter |
| Medical Devices | +/-0.01mm or tighter | Often requires CMM report per batch |
| Aerospace | +/-0.005-0.02mm | Full FAI documentation required |
| Electronics / Power | +/-0.025-0.05mm | Flatness often more critical than dimensional |
How to Specify Tolerances on Your Drawings
Use GD&T (Geometric Dimensioning and Tolerancing) symbols where possible. Key tips:
- Specify datum references clearly – a tolerance without a datum is ambiguous
- Indicate which surfaces are critical (bearing seats, mating faces) vs. non-critical
- State surface finish requirements (Ra, Rz) separately from dimensional tolerances
- Include material spec and hardness if relevant to the tolerance
- Note any post-processing (anodizing, plating, heat treatment) that affects final dimensions
Not Sure If Your Tolerances Are Achievable?
Send us your drawing for a free DFM (Design for Manufacturability) review. Our engineers will tell you if any features are unnecessarily tight for the application – and suggest more cost-effective alternatives without compromising function.
This article is provided by CNC Precision Tech – a Dongguan-based CNC machining shop serving buyers in North America, Europe, Japan, and Australia. We specialize in precision CNC milling, turning, and sheet metal fabrication with tolerances as tight as +/-0.005mm.