CNC Machining Tolerance Chart

Complete reference for achievable tolerances across CNC milling, turning, wire EDM, 5-axis machining, and surface grinding. Includes ISO IT grade specifications.

Tolerances by CNC Process

Process	Standard	Precision	Ultra-Precision	Notes
CNC Milling	±0.05 mm (±0.002")	±0.01 mm (±0.0004")	±0.005 mm (±0.0002")	Most common process. Precision depends on machine rigidity and tooling.
CNC Turning	±0.025 mm (±0.001")	±0.01 mm (±0.0004")	±0.005 mm (±0.0002")	Cylindrical parts. Tighter tolerances achievable on diameters than lengths.
Wire EDM	±0.01 mm (±0.0004")	±0.005 mm (±0.0002")	±0.002 mm (±0.00008")	Best for hardened materials and complex profiles. No mechanical forces.
5-Axis Machining	±0.025 mm (±0.001")	±0.01 mm (±0.0004")	±0.005 mm (±0.0002")	Complex geometries in single setup. Reduces cumulative error from re-fixturing.
Surface Grinding	±0.01 mm (±0.0004")	±0.005 mm (±0.0002")	—	Flatness and parallelism. Often used as a secondary finishing operation.

* Ultra-precision tolerances require specialized equipment and controlled environments. Additional cost and lead time apply.

ISO IT Grade Reference Table

ISO 286 defines standard tolerance grades. The table below shows tolerance values in micrometers for common nominal dimension ranges. CNC machining typically achieves IT6 to IT9 depending on the process.

IT Grade	Description	≤10 mm	10–25 mm	25–50 mm	50–100 mm
IT5	Precision grinding	6 μm	9 μm	11 μm	13 μm
IT6	Precision machining	9 μm	13 μm	16 μm	22 μm
IT7	High-quality machining	15 μm	21 μm	25 μm	35 μm
IT8	Standard machining	22 μm	33 μm	39 μm	54 μm
IT9	Moderate machining	36 μm	52 μm	62 μm	87 μm
IT10	General machining	58 μm	84 μm	100 μm	140 μm
IT11	Rough machining	90 μm	130 μm	160 μm	220 μm
IT12	Sheet metal / casting	150 μm	210 μm	250 μm	350 μm

How to Specify Tolerances on Your Drawings

1. Use GD&T (Geometric Dimensioning and Tolerancing)

GD&T per ASME Y14.5 is the industry standard for communicating tolerances unambiguously. It defines form, orientation, location, and runout tolerances using standardized symbols. This eliminates interpretation differences between your design and our manufacturing team.

2. Specify Only Critical Tolerances

Apply tight tolerances only to functional features: mating surfaces, bearing bores, seal grooves, and alignment pins. Non-critical dimensions should use the general tolerance block (e.g., “Unless otherwise specified: ±0.1 mm”). Over-tolerancing increases cost by 30–50%.

3. Include a Tolerance Block

Every drawing should have a general tolerance block in the title block specifying default tolerances for linear dimensions, angular dimensions, and surface finish. For example:

Linear: ±0.1 mm (1 decimal), ±0.05 mm (2 decimal)

Angular: ±0.5°

Surface: Ra 3.2 μm unless specified

4. Consider Material Behavior

Different materials behave differently during machining. Aluminum has higher thermal expansion than steel, making ultra-tight tolerances harder to maintain. Plastics are even more susceptible to thermal changes. Factor material properties into your tolerance choices.

5. Provide a 3D STEP File

Always include a STEP file alongside your 2D drawing. STEP files carry precise geometry data that prevents dimension interpretation errors. Our CAM team uses the 3D model for programming and the 2D drawing for tolerance and finish callouts.

Frequently Asked Questions

What is a standard CNC machining tolerance?

Standard CNC machining tolerance is typically ±0.05 mm (±0.002") for milling and ±0.025 mm (±0.001") for turning. These tolerances are achievable without special tooling or processes and cover the majority of functional parts.

How tight can CNC tolerances be?

CNC machines can achieve tolerances as tight as ±0.005 mm (±0.0002") for milling and turning, and ±0.002 mm (±0.00008") for wire EDM. Ultra-precision work requires specialized equipment, controlled environments, and significantly increases cost.

What factors affect CNC machining tolerance?

Key factors include: machine rigidity and calibration, cutting tool condition, material properties (thermal expansion, hardness), part geometry and wall thickness, fixturing method, and ambient temperature. Harder materials generally hold tighter tolerances.

What is the difference between tolerance and surface finish?

Tolerance defines how much a dimension can vary from its nominal value (e.g., 10.00 ±0.05 mm). Surface finish (Ra) measures the texture of the surface. A part can have tight tolerances but a rough surface finish, or vice versa. Both should be specified independently.

When should I specify tight tolerances on a CNC part?

Specify tight tolerances only on critical mating surfaces, bearing fits, seal grooves, and alignment features. Over-tolerancing non-critical dimensions increases machining time and cost by 30-50% without functional benefit.

What is an ISO IT grade?

ISO IT (International Tolerance) grades define standardized tolerance bands from IT01 (tightest) to IT18 (loosest). CNC machining typically falls in the IT6-IT9 range. IT7 corresponds roughly to ±0.01-0.025 mm for common shaft sizes.

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