CNC Machining Tolerances: The Ultimate Guide

When you send a CAD file to a CNC machining shop, the first question is not "Can you make this?" — it is "How precisely can you make this?" The answer lies in CNC machining tolerances, the acceptable deviation from the nominal dimension specified in your design.

Understanding tolerances is critical for anyone involved in product development, mechanical design, or manufacturing procurement. A tolerance that is too loose can cause assembly failures. A tolerance that is too tight can double your cost. This guide covers everything you need to know to specify the right tolerance for your CNC machined parts.

What is Machining Tolerance?

In CNC machining, tolerance is the total amount a specific dimension is permitted to vary from its nominal value. For example, if a shaft is designed to be 20.00mm in diameter with a tolerance of ±0.05mm, the actual measured diameter must fall between 19.95mm and 20.05mm to be acceptable.

Tolerances matter because no manufacturing process can produce a part to an exact dimension — there will always be some variation. The tighter the tolerance, the more precise (and more expensive) the machining process must be.

Standard CNC Machining Tolerances

Different CNC machining processes achieve different levels of precision. Here is a reference table of standard and precision tolerances for common processes at Kaierwo's CNC machining service:

Note: These tolerances assume standard part geometries and common materials (aluminum, steel, brass, engineering plastics). Actual achievable tolerances may vary based on part complexity, material, and size.

Factors That Affect CNC Machining Tolerances

Many variables influence how tightly a part can be machined. Understanding these factors helps you set realistic tolerance expectations and avoid over-specifying.

1. Material Type and Hardness

Softer materials like aluminum and brass are easier to machine to tight tolerances because they cut cleanly with minimal tool deflection. Harder materials like stainless steel and titanium generate more cutting forces, which can cause tool deflection and thermal expansion, making tight tolerances harder to achieve.

Plastics present a different challenge — they expand and contract significantly with temperature changes. POM (Delrin) and PEEK are more dimensionally stable than ABS or nylon.

2. Part Geometry and Complexity

Simple geometries — flat surfaces, straight holes, concentric features — are easier to hold to tight tolerances. Complex parts with deep pockets, thin walls, or features that require multiple setups introduce cumulative errors. Each additional setup adds potential misalignment.

5-axis machining helps by allowing multiple features to be machined in a single setup, reducing re-fixturing errors. Learn more about 5-axis CNC machining at Kaierwo →

3. Machine Tool Capability

Not all CNC machines are equal. A precision 5-axis machining center with linear scales and thermal compensation will hold tighter tolerances than a standard 3-axis machine. The machine's condition, maintenance history, and calibration also matter.

4. Tool Wear

As cutting tools wear, they cut less precisely. Carbide tools maintain sharpness longer than high-speed steel (HSS). For production runs, tool wear monitoring and scheduled tool changes are essential to maintain tolerances batch after batch.

5. Workholding Rigidity

How the part is held affects how it deforms during machining. Soft jaws, vacuum chucks, and custom fixtures can distribute clamping forces more evenly, reducing part distortion. Poor fixturing is one of the most common causes of out-of-tolerance parts.

6. Cutting Parameters

Spindle speed, feed rate, and depth of cut all affect dimensional accuracy. Aggressive parameters remove material faster but generate more heat and force, which can push parts out of tolerance. Precision machining often requires conservative parameters.

7. Temperature

Metals expand and contract with temperature changes. A 100mm aluminum part will change approximately 0.024mm per 10°C temperature shift. Precision machine shops control ambient temperature and allow parts to thermally stabilize before final measurement.

Tight Tolerance Machining Tips

If your design requires tight tolerances (±0.01mm or tighter), consider these strategies:

Design for Manufacturability

• Specify tight tolerances only on critical dimensions (mating surfaces, seal interfaces)

• Use general tolerances (±0.1mm or ±0.05mm) for non-critical features

• Avoid deep pockets and thin walls — they deflect under cutting forces

• Design parts to be machined in a single setup where possible

• Provide relief grooves for threaded features

Material Selection

• For metal parts requiring ±0.01mm: aluminum 6061-T6 or 7075-T6 is ideal

• For steel parts: use pre-hardened tool steel (P20, H13) rather than annealed grades

• For plastic parts: POM (Delrin) or PEEK are the most dimensionally stable

• Consider the material's coefficient of thermal expansion (CTE)

Process Selection

• Use 5-axis machining for features that would otherwise require multiple setups

• Use EDM for features that cannot be reached with conventional cutting tools

• Use surface grinding for ultra-flat surfaces and tight parallel tolerances

• Consider hard turning as an alternative to grinding for cylindrical features

Tolerance Standards: ISO 2768 Explained

ISO 2768 is the international standard for general tolerances — tolerances that apply when no specific tolerance is indicated on a drawing. It allows engineers to leave non-critical dimensions un-toleranced while still maintaining defined quality standards.

ISO 2768-1: Linear and Angular Dimensions

This part defines tolerance classes for linear dimensions (lengths, widths, diameters) and angular dimensions. There are four accuracy classes:

When you do not specify a tolerance on a dimension, the "medium" class (m) is typically applied by default. At Kaierwo, we default to the "fine" (f) class for all CNC machined parts unless otherwise specified.

ISO 2768-2: Geometrical Tolerances

This part covers tolerances for straightness, flatness, parallelism, perpendicularity, and symmetry. These are the tolerances that control the shape of features, not just their size.

How Kaierwo Ensures Tolerance Compliance

Holding tight tolerances requires more than good machines — it requires a comprehensive quality system. Here is how Kaierwo maintains tolerance compliance on every part:

Inspection Equipment

• CMM (Coordinate Measuring Machine): Measures 3D coordinates with accuracy of ±0.003mm. Used for complex geometries and tight tolerance verification.

• 3D Scanner: Captures full-surface point clouds for comparison against CAD models. Ideal for freeform surfaces.

• Calipers and Micrometers: Digital instruments for quick in-process checks during machining.

• Pin Gauges: For precise hole diameter verification.

• Hardness Tester: Verifies material hardness meets specification.

Process Controls

• In-process inspection: Operators check critical dimensions during machining, not just at the end

• First article inspection (FAI): The first part of every batch is fully inspected before continuing production

• Final inspection: 100% of parts are inspected before shipping

• Inspection reports: Dimensional reports available on request

Certifications

Kaierwo is ISO 9001:2015 and ISO 13485 (medical) certified. Every process — from material procurement to final inspection — follows documented procedures.

Common Tolerance Mistakes to Avoid

Mistake 1: Over-Tolerancing

The most common mistake is applying tight tolerances to every dimension. This dramatically increases cost and lead time. Only specify ±0.01mm where it matters — mating surfaces, seal interfaces, bearing fits. Let non-critical features use general tolerances.

Mistake 2: Ignoring Material Behavior

Plastic parts specified to ±0.01mm are often unrealistic — plastics move with temperature and humidity. If you need metal-level tolerances, use metal or a very stable engineering plastic like PEEK.

Mistake 3: Not Considering Assembly Stack-Up

If three parts are assembled in a stack, each with ±0.05mm tolerance, the total stack-up could be ±0.15mm. Use tolerance stack-up analysis (worst-case or statistical) to ensure your assembly will function.

Mistake 4: Specifying Tolerances That Cannot Be Measured

If your inspection equipment cannot measure to the tolerance you specified, you cannot verify compliance. Make sure your manufacturer has CMM or other precision measurement capabilities.

FAQ

What is the standard tolerance for CNC machining?

The standard tolerance for CNC machining is ±0.05mm (±0.002"). For precision applications, tight tolerances of ±0.01mm (±0.0004") can be achieved with 5-axis machining centers and appropriate tooling.

What is ISO 2768?

ISO 2768 is an international standard that defines general tolerances for linear and angular dimensions without individual tolerance indications. It has two parts: ISO 2768-1 for linear and angular dimensions, and ISO 2768-2 for geometrical tolerances.

What factors affect CNC machining tolerances?

Key factors include material type and hardness, part geometry and complexity, machine tool capability and condition, cutting tool wear, workholding rigidity, cutting parameters, temperature variations, and operator skill.

Can CNC machining achieve tolerances tighter than 0.01mm?

Yes, with specialized equipment such as jig grinding machines and precision EDM, tolerances as tight as ±0.005mm (±0.0002") can be achieved. However, this significantly increases cost and lead time.

How does Kaierwo ensure tolerance compliance?

Kaierwo ensures tolerance compliance through 100% part inspection using CMM, 3D scanner, calipers, micrometers, pin gauges, and hardness testers. All processes are ISO 9001 certified with documented inspection reports.

Conclusion

Specifying the right CNC machining tolerances is a balance between functional requirements and manufacturing cost. By understanding standard tolerances, the factors that affect them, and when to apply tight vs. general tolerances, you can produce parts that work correctly without overpaying for unnecessary precision.

At Kaierwo, we hold standard tolerances of ±0.05mm on all CNC machined parts and can achieve precision tolerances of ±0.01mm when needed. Every part is 100% inspected with CMM and other precision equipment. Learn more about our quality systems or browse our case studies.

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