CNC Machining Tolerance Calculator
Determine achievable tolerances for your machined parts. Select your process and material to see standard, precision, and ultra-precision tolerance ranges with cost impact analysis.
Most common process. Good for prismatic parts with features on accessible faces.
Excellent machinability. Easy to hold tight tolerances. Stable material.
Find Shops That Can Achieve ±0.0050"
Search cnc milling (3-axis) shops with aluminum (6061, 7075) capability
Quick Reference: All Processes Comparison
Standard achievable tolerances (with Aluminum (6061, 7075))
| Process | Standard | Precision | Ultra-Precision |
|---|---|---|---|
CNC Milling (3-Axis)Selected | ±0.0050" | ±0.0010" | ±0.00050" |
CNC Milling (5-Axis) | ±0.0030" | ±0.0010" | ±0.00030" |
CNC Turning/Lathe | ±0.0050" | ±0.0010" | ±0.00020" |
Swiss CNC Turning | ±0.0020" | ±0.00050" | ±0.00010" |
Wire EDM | ±0.0020" | ±0.00050" | ±0.00010" |
Sinker/Plunge EDM | ±0.0030" | ±0.0010" | ±0.00020" |
Surface Grinding | ±0.0010" | ±0.00050" | ±0.00005" |
Cylindrical Grinding | ±0.0010" | ±0.00030" | ±0.00005" |
Jig Grinding | ±0.00050" | ±0.00020" | ±0.00005" |
Understanding Machining Tolerances
What is Machining Tolerance?
Machining tolerance is the permissible variation in a dimension. It defines the acceptable range within which a machined feature must fall. Tighter tolerances require more precise equipment, slower cutting speeds, and additional quality checks—all of which increase cost.
Tolerance Classes Explained
Achievable with standard CNC equipment. No cost premium. Suitable for most non-critical features.
Requires precision equipment and slower speeds. 20-50% cost premium. Common for fits and interfaces.
Specialized equipment (grinding, lapping). 100-300% premium. Reserved for critical applications only.
Factors Affecting Achievable Tolerance
- Material: Softer materials like aluminum are easier to hold tolerance. Hard materials like titanium and hardened steel require specialized approaches.
- Feature Size: Larger features are generally easier to hold tolerance. Small holes and thin walls are more challenging.
- Feature Type: External dimensions are easier than internal. Through-holes are easier than blind holes.
- Part Geometry: Thin walls, deep pockets, and complex geometries make tolerance harder to hold.
- Temperature: Parts expand/contract with temperature. Critical tolerances require temperature-controlled environments.
Cost Impact of Tolerances
A common rule of thumb: every 10x tighter tolerance roughly doubles the cost. This is why over-specifying tolerances is one of the most expensive mistakes engineers make.
Only specify tight tolerances on features that truly require them. Use ±0.005" (standard) as your default, and only tighten for functional interfaces, fits, and critical dimensions.
Industry-Standard Tolerances
| Industry | Typical Tolerance | Critical Features |
|---|---|---|
| General Manufacturing | ±0.005" (±0.127mm) | ±0.002" (±0.05mm) |
| Aerospace (AS9100) | ±0.002" (±0.05mm) | ±0.0005" (±0.013mm) |
| Medical Devices | ±0.001" (±0.025mm) | ±0.0002" (±0.005mm) |
| Automotive | ±0.003" (±0.076mm) | ±0.001" (±0.025mm) |
| Optics/Semiconductors | ±0.0005" (±0.013mm) | ±0.00005" (±0.001mm) |
Best Practices for Specifying Tolerances
- Default to standard tolerances: Use ±0.005" unless you have a specific functional reason for tighter.
- Identify critical features: Only tighten tolerances on mating surfaces, bearing fits, and functional interfaces.
- Consider the stack-up: Multiple tight tolerances compound. Account for tolerance stack-up in assemblies.
- Communicate with your shop: Ask what they can hold. A good shop will tell you where you're over-specifying.
- Use GD&T wisely: Geometric tolerancing can be more precise than ± tolerances for controlling form and position.
Find Shops That Can Hold Your Tolerances
Search our directory of verified machine shops by capability, certification, and precision level.
Frequently Asked Questions
What is a standard CNC machining tolerance?
Standard CNC machining tolerance is typically ±0.005" (±0.127mm) for most features. This is achievable with standard equipment without cost premium. For precision work, ±0.001" (±0.025mm) is common, while ultra-precision work can achieve ±0.0001" (±0.003mm) with specialized equipment.
How much do tighter tolerances cost?
As a rule of thumb, each 10x tighter tolerance roughly doubles the cost. Moving from ±0.005" to ±0.001" might add 20-50% to machining cost. Going to ±0.0001" (ultra-precision) can add 100-300% and often requires secondary operations like grinding.
What tolerances can 5-axis CNC achieve?
5-axis CNC machines typically achieve ±0.001" to ±0.0005" positional accuracy. The advantage of 5-axis is not tighter tolerance per se, but the ability to machine complex geometries in fewer setups, which improves feature-to-feature accuracy and reduces cumulative tolerance stack-up.
What tolerance can I achieve with aluminum vs steel?
Aluminum (especially 6061-T6) is one of the easiest materials to machine to tight tolerances due to its stability and excellent machinability. Steel can achieve similar tolerances but may require slower feeds and more careful tool selection. Hardened steel (>40 HRC) typically requires grinding for tolerances tighter than ±0.001".
How do I specify tolerances on a drawing?
Use a general tolerance block (e.g., "Unless otherwise specified: ±0.005") for most dimensions. Call out specific tolerances only on critical features. For geometric relationships, use GD&T symbols (position, concentricity, parallelism) rather than ± tolerances. Always include surface finish callouts for critical surfaces.