When it comes to CNC machining, precision is everything. Whether you’re manufacturing aerospace components, medical devices, or custom mechanical parts, understanding tolerances and accuracy is essential to achieving optimal fit, performance, and functionality. At Davantech, we specialize in precision CNC machining and offer guidance to help you define the right tolerances for your application—balancing quality, cost, and manufacturability.
What Are CNC Machining Tolerances?
Tolerances refer to the permissible limit of variation in a physical dimension. In CNC machining, tolerances specify how much a given measurement can deviate from its nominal value without affecting part performance.
For example, if a shaft’s diameter is specified as Ø10.00 mm ±0.05 mm, the actual diameter can range between 9.95 mm and 10.05 mm.
Why Are Tolerances Important?
-
Functional Fit: Tight tolerances ensure that parts fit together correctly in assemblies, especially in high-speed or high-load applications.
-
Performance: In mechanical systems, small deviations can affect performance, efficiency, or safety.
-
Interchangeability: Precision tolerances allow parts to be replaced without modification.
-
Cost Control: Overly tight tolerances increase machining time and inspection requirements. Specifying realistic tolerances helps reduce cost.
Types of Tolerances in CNC Machining
-
Linear Dimensions: Defines how much a machined edge or hole can vary in length, width, or height.
-
Geometric Tolerances: Specifies allowable deviations in form, orientation, position, and runout (e.g., flatness, parallelism, concentricity).
-
Surface Finish Tolerances: Describes the texture or smoothness of the surface, typically measured in Ra (roughness average) µm or µin.
ISO 2768-m: General Tolerances Table (Medium Class)
Linear Dimensions (External & Internal Features)
Applies to features such as widths, lengths, diameters, hole spacing, etc.
| Nominal Size Range (mm) | Tolerance ± (mm) |
| 0.5 to 3 | ±0.1 |
| >3 to 6 | ±0.1 |
| >6 to 30 | ±0.2 |
| >30 to 120 | ±0.3 |
| >120 to 400 | ±0.5 |
| >400 to 1000 | ±0.8 |
| >1000 to 2000 | ±1.2 |
| >2000 to 4000 | ±2.0 |
Standard CNC Machining Tolerances
🔹 1. General Standard Tolerances (Without Special Requirements)
| Process | Typical Tolerance (± mm) | Typical Tolerance (± inch) |
| CNC Milling (3-axis) | ±0.1 – 0.05 mm | ±0.004 – 0.002 in |
| CNC Turning (lathe) | ±0.05 – 0.02 mm | ±0.002 – 0.0008 in |
| 5-Axis CNC Milling | ±0.02 mm or better | ±0.0008 in |
| EDM (Wire or Sinker) | ±0.005 – 0.01 mm | ±0.0002 – 0.0004 in |
| Grinding | ±0.002 – 0.005 mm | ±0.00008 – 0.0002 in |
| Laser Cutting | ±0.1 – 0.2 mm | ±0.004 – 0.008 in |
| Waterjet Cutting | ±0.1 – 0.2 mm | ±0.004 – 0.008 in |
| Sheet Metal Bending | ±0.2 – 0.5 mm | ±0.008 – 0.020 in |
✅ Note: These are general standards. Tighter tolerances are possible with high-precision machines and process control, but may increase cost significantly.
🔹 2. Precision Tolerances (High-End Applications)
| Application | Typical Tolerance Range |
| Aerospace Components | ±0.005 mm to ±0.02 mm |
| Medical Implants/Devices | ±0.005 mm or better |
| Optical/Micro-Mechanical Parts | ±0.001 – 0.005 mm (micron range) |
| Injection Mold Tooling | ±0.01 – 0.02 mm |
🔹 3. Common ISO Standard Tolerance References
| ISO Standard | Description |
| ISO 2768-m | Medium tolerance for general machining |
| ISO 2768-f | Fine tolerance, tighter than standard |
| ISO 286 (IT Grades) | Defines international tolerance grades (IT6 = fine, IT9 = coarse, etc.) |
Factors That Affect CNC Machining Tolerances
- Material Type: Softer metals like aluminum are easier to machine to tight tolerances than hard steels or plastics that may deform.
- Part Geometry: Thin walls, deep pockets, or complex shapes may lead to deflection or vibration.
- Machine Capability: At Davantech, we use advanced 3-, 4-, and 5-axis CNC machines capable of achieving high precision and repeatability.
- Tool Wear & Temperature: Worn tools or thermal expansion can impact dimensional accuracy—mitigated by strict quality control.
Tolerance vs. Accuracy vs. Precision
These terms are often used interchangeably but have distinct meanings:
- Tolerance: The allowed variation in dimension.
- Accuracy: How close a measurement is to the true or intended value.
- Precision: The repeatability or consistency of measurements.
At Davantech, we prioritize both accuracy and precision—ensuring every part meets the defined tolerances consistently.
High-Precision CNC Machining at Davantech
We offer high-accuracy machining services for industries where tolerances matter most.
Our capabilities include:
✅ Tight tolerances as low as ±0.005 mm
✅ Full dimensional inspections with CMM reports on request
✅ Material expertise: aluminum, stainless steel, brass, titanium, and plastics
✅ ISO-compliant processes and quality control
✅ Engineering support for DFM and tolerance optimization
How to Specify Tolerances in Your Drawing
To ensure clear communication and avoid delays, include the following in your technical drawings:
- Nominal dimensions with tolerances
- Tolerance class (e.g., ISO 2768‑f)
- Surface roughness requirements
- Geometric tolerances using GD&T symbols
- Notes for critical features or fits (e.g., press fit, clearance fit)
If you’re unsure, our engineering team can help you define realistic tolerances that balance performance and cost.
Need High-Tolerance Machining? Contact Davantech Today
When precision matters, trust Davantech as your partner for CNC machining in China. We combine cutting-edge technology, skilled technicians, and strict quality control to deliver parts that meet your exact requirements.
📞 Contact us today for a quote or technical consultation. 📧 Send your drawings and let’s discuss your tolerance needs.