Tolerances in CNC machining define how much variation is allowed between a part's actual dimensions and its design specifications.
They play a critical role in ensuring functional fit, mechanical performance, and assembly reliability. Whether you're producing aerospace parts, medical components, or high-precision tools, understanding how tolerances affect design and production is essential.

This guide breaks down what CNC tolerances are, how they’re specified, and how they impact cost, lead time, and manufacturing complexity.

What Are Tolerances in CNC Machining?

Tolerances indicate acceptable limits of dimensional variation in manufactured parts.
They define how closely the finished part must match the design drawing.

For example, if a shaft’s diameter is specified as Ø10.00 mm ±0.05 mm, the acceptable range is 9.95 mm to 10.05 mm. Anything outside this range may result in the part being rejected or causing fitment issues during assembly.

Types of tolerances include:

• Linear tolerances (length, width, diameter)

• Angular tolerances (angles between surfaces)

• Geometric tolerances (flatness, concentricity, parallelism)

• Surface finish tolerances (roughness in µm or Ra)

Why Do Tolerances Matter?

Tolerances are crucial for parts that must interact with other components, especially in high-performance or safety-critical applications.

Improper tolerances can lead to:

• Loose fits or binding

• Premature wear or failure

• Excessive vibration

• Assembly rejection

• Product recalls

On the other hand, overly tight tolerances may increase cost, machine time, and scrap rate without adding real functional value. Understanding the balance is key.

Common Tolerance Ranges in CNC Machining

The achievable tolerance depends on material, machine type, tool wear, and part complexity.

Standard tolerance capabilities:

For rotational parts or threaded components, explore CNC turning services where tighter concentricity and diameter tolerances are often required.

Types of Tolerances and Their Definitions

1. Unilateral Tolerances

Variation is allowed only in one direction (e.g., +0.000/-0.010 mm)

2. Bilateral Tolerances

Variation allowed in both directions (e.g., ±0.005 mm)

3. Limit Dimensions

Upper and lower bounds are explicitly stated (e.g., 9.98–10.02 mm)

4. Geometric Dimensioning & Tolerancing (GD&T)

Specifies tolerances for form, orientation, location, and runout. Examples:

• Flatness: Maximum allowable deviation from a perfectly flat plane

• Parallelism: Distance between two surfaces that should remain parallel

• Concentricity: Axis alignment between circular features

• True position: Exact location of holes or features relative to datums

GD&T improves design intent clarity and is widely used in aerospace, automotive, and precision manufacturing.

How Tolerances Affect Manufacturing Cost

The tighter the tolerance, the higher the cost.
This is due to increased machine time, tool wear, setup precision, and inspection requirements.

Key cost drivers:

• Tighter tolerances require slower feed rates and more frequent tool changes

• Special fixturing and jigs may be needed to maintain alignment

• More passes and finer tools may be used to reach final dimension

• Advanced inspection equipment like CMMs must verify critical dimensions

For example:

• A standard ±0.1 mm tolerance may be 30–40% cheaper to produce than ±0.01 mm

• A part requiring ±0.005 mm may require climate-controlled machining environments and specialized tooling

Always define the tightest tolerances only where absolutely necessary.

Designing with Tolerances in Mind

Design engineers should balance function with manufacturability.

Best practices:

• Use standard fits and tolerances (e.g., ISO, ANSI) for shafts, holes, and mating parts

• Apply tighter tolerances only to critical features

• Avoid stacking multiple tight tolerances in assemblies

• Work with your machinist to determine feasible tolerances based on the material and design

• Use GD&T to specify functional requirements, not just arbitrary precision

Material Considerations

Different materials react differently to machining and affect the achievable tolerances.

Thermal expansion, tool deflection, and part warping are more common in plastics and thin-walled metals. Compensating for material behavior is part of holding tight tolerances.

Tolerances in CNC Turning

CNC turning is often used for parts with round geometries, shafts, bushings, and threads—where tolerance control is critical.

Typical tolerance features in turning include:

• Outer diameters (OD)

• Inner diameters (ID)

• Thread pitch and depth

• Roundness and runout

Advanced CNC turning services use live tooling, C-axis control, and in-process gauging to ensure these tolerances are met consistently in production environments.

Inspection and Quality Control

Accurate measurement tools and inspection methods are required to verify CNC tolerances.

Common metrology tools:

• Micrometers for external dimensions

• Calipers for quick measurements

• Height gauges for flatness and parallelism

• Coordinate Measuring Machines (CMMs) for 3D inspection

• Optical comparators for profile analysis

In regulated industries like aerospace and medical, full inspection reports and Statistical Process Control (SPC) are often required for traceability.

Summary: What You Need to Know About CNC Tolerances

Final Thoughts

Understanding tolerances in CNC machining isn’t just for engineers—it’s essential for anyone involved in product development, quality control, or manufacturing strategy.
When used correctly, tolerances ensure parts function properly, fit together seamlessly, and last longer in operation. When misapplied, they increase costs and reduce manufacturability.

Whether you’re developing tight-fitting aerospace components or high-precision medical tools, defining the right tolerances can make or break your project.

To ensure your parts are machined with the precision they require, consider partnering with experienced CNC turning services capable of meeting strict dimensional standards and quality expectations.