CNC Machining (Subtractive Manufacturing) Tolerance Standards: A Practical Guide

In subtractive manufacturing—especially CNC machining—tolerance standards are the backbone of consistent, functional parts. Imagine designing a bolt that’s too big for its hole, or a gear that wobbles because its center isn’t aligned: these mistakes happen when tolerance standards are ignored or misunderstood. For designers and manufacturers, knowing which standard to use (and why) saves time, cuts rework costs, and ensures parts fit right the first time. This guide breaks down the most critical tolerance standards for CNC subtractive manufacturing—ISO 2768 and ISO 286—explains how to apply them, and uses real-world examples to avoid common pitfalls.

Why Tolerance Standards Matter for CNC Subtractive Manufacturing

Subtractive manufacturing removes material (via milling, turning, or drilling) to shape a part—unlike 3D printing, which adds material. This process relies on precision, but “precision” doesn’t mean “as tight as possible.” Tolerance standards solve three big problems:

1. They Simplify Design Work

Designing a part with 50 features? You don’t need to calculate tolerances for each one. Standards like ISO 2768 and ISO 286 predefine acceptable variation for common dimensions (e.g., length, angles, shaft diameters). A designer at a furniture company once spent 8 hours calculating custom tolerances for a table leg—until they switched to ISO 2768-M. They cut design time by 70% and still got perfect-fitting parts.

2. They Align Designers and Manufacturers

A designer’s “tight tolerance” might mean ±0.01 mm to them, but ±0.05 mm to a manufacturer. Standards eliminate guesswork. When a medical device designer specifies “ISO 286-Class 6” for a surgical tool, the CNC shop knows exactly what precision to aim for—no back-and-forth.

3. They Balance Quality and Cost

Tighter tolerances cost more: a part with ISO 286-Class 6 (±0.006 mm) can cost 2–3x more than one with ISO 286-Class 8 (±0.014 mm). Standards help you pick the loosest tolerance that still works. For example, a decorative CNC-machined sign only needs ISO 2768-M (±0.1 mm for small dimensions)—using a tighter standard would waste money for no benefit.

The Two Core ISO Tolerance Standards for CNC Subtractive Manufacturing

Nearly every CNC shop worldwide uses ISO standards for subtractive manufacturing. The two most common are ISO 2768 (for general dimensions) and ISO 286 (for mating parts like shafts and holes). Let’s break each down with clear tables and use cases.

1. ISO 2768: General Tolerances for Linear, Angular, and Rounded Features

ISO 2768 is for “unannotated” features—dimensions where you don’t need a custom tolerance. It covers three key areas: linear dimensions (e.g., length, width), rounding radii (e.g., a curved edge on a bracket), and angular dimensions (e.g., a 45° angle on a frame).

It has two main grades to choose from:

• ISO 2768-F (Precision Grade): For parts that need tighter control (e.g., electronics components).
• ISO 2768-M (Medium Grade): The “default” for most CNC parts (e.g., furniture, general machinery).

ISO 2768 Linear Dimension Tolerances (mm)

ISO 2768 Rounding Radius & Chamfer Height Tolerances (mm)

ISO 2768 Angular Dimension Tolerances (°/’)

Real-World Example: A CNC shop made 100 aluminum brackets for a shelf. The designer specified ISO 2768-M for the 20 mm length (tolerance ±0.2 mm) and 90° angle (tolerance ±0°30′). All brackets fit the shelf’s slots perfectly—only 2 were rejected (for a 2% defect rate), which is well within industry norms.

2. ISO 286: Tolerances for Mating Parts (Shafts & Holes)

ISO 286 is for parts that need to fit together—like a shaft (male part) and a hole (female part) in a motor. It defines “IT grades” (International Tolerance grades) for linear dimensions, with lower numbers meaning tighter tolerances. For CNC subtractive manufacturing, the most common grades are:

• ISO 286-Class 6 (IT6): Ultra-tight (e.g., surgical tools, aerospace components).
• ISO 286-Class 7 (IT7): Tight (e.g., engine parts, precision gears).
• ISO 286-Class 8 (IT8): Moderate (e.g., bike components, general machinery).

ISO 286 Tolerances for Key Dimension Ranges (mm)

Key Terms to Know:

• Rated Dimension: The size on your drawing (e.g., a 10 mm shaft).
• Upper Limit Size: The largest allowed size (e.g., 10.015 mm for a 10 mm shaft with IT7).
• Lower Limit Size: The smallest allowed size (e.g., 9.985 mm for the same shaft).
• Tolerance: The difference between upper and lower limits (e.g., 0.03 mm for IT7 here).

Case Study: A bike manufacturer used ISO 286-Class 8 for their pedal shafts (15 mm rated dimension). The tolerance was 0.027 mm, so shafts could be 14.973–15.027 mm. All shafts fit the pedal holes without wobbling, and the cost was 30% lower than if they’d used Class 7.

How to Choose Between ISO 2768 and ISO 286

The biggest question designers ask is: “Which standard do I use?” Follow this simple flow:

Step 1: Does the Part Have Mating Features?

• Yes (e.g., a bolt that goes through a hole, a gear on a shaft): Use ISO 286. Pick the grade based on how tight the fit needs to be:
• Ultra-tight fit (no movement): Class 6.
• Tight fit (minimal movement): Class 7.
• Loose fit (some movement): Class 8.
• No (e.g., a decorative sign, a standalone bracket): Use ISO 2768. Pick F for precision parts, M for most others.

Step 2: Check the Dimension Range

Make sure your dimension falls within the standard’s range. For example:

• ISO 2768-F isn’t recommended for dimensions over 2000 mm (it’s too hard to achieve).
• ISO 286-Class 6 works best for small dimensions (under 50 mm)—larger dimensions (over 1000 mm) often use Class 8.

Step 3: Confirm CNC Machine Capability

A standard CNC mill can usually hit ISO 2768-M and ISO 286-Class 8. To get Class 6 or 7, you need a high-precision machine (which costs more). A small shop once tried to make ISO 286-Class 6 parts on a standard mill—they had a 25% rejection rate and had to upgrade their machine.

ISO vs. ASME: What About American Standards?

If you work with U.S. clients, you might encounter ASME standards (instead of ISO). The good news is they map to ISO standards—so you don’t need to learn a whole new system. Here’s the key correspondence:

Pro Tip: If a U.S. client asks for “ASME B4.1 Grade 7,” just use ISO 286-Class 7—they’re nearly identical in tolerance ranges.

Yigu Technology’s Perspective on CNC Subtractive Tolerance Standards

At Yigu Technology, we see ISO 2768 and ISO 286 as essential tools for making CNC subtractive manufacturing efficient. Too often, clients overspecify tolerances (e.g., using ISO 286-Class 6 for a non-critical bracket) and end up paying more. We help them align standards with function: for example, a client making industrial valves switched from Class 6 to Class 7 (still meeting safety norms) and cut costs by 25%. We also share our CNC machine capabilities upfront—so designers know exactly what’s possible. Tolerance standards aren’t just rules—they’re a way to build trust and deliver parts that work, on time and on budget.

FAQ

1. Can I use ISO 2768 and ISO 286 on the same part?

Yes! For example, a CNC-machined gear might use ISO 286-Class 7 for its center hole (a mating feature) and ISO 2768-M for its outer diameter (a non-mating feature). Just make sure to note both standards on your drawing.

2. Is ISO 286-Class 6 always better than Class 8?

No—better depends on function. Class 6 is tighter, but it’s more expensive and harder to achieve. For example, a toy car’s axle only needs Class 8 (it doesn’t need precision), but a jet engine’s shaft needs Class 6 (any wobble could be dangerous). Always pick the loosest class that works.

3. What if my dimension is outside ISO 2768/286 ranges (e.g., 5000 mm)?

For dimensions over the standard ranges (e.g., ISO 2768-M stops at 4000 mm), create a custom tolerance. Work with your CNC shop to set a range they can achieve—for a 5000 mm beam, a tolerance of ±3 mm is usually realistic and cost-effective.