Lavorazione CNC (Produzione sottrattiva) Standard di tolleranza: Una guida pratica

In subtractive manufacturing—especially Lavorazione CNC—tolerance standards are the backbone of consistent, parti funzionali. Immagina di progettare un bullone troppo grande per il suo foro, o un ingranaggio che traballa perché il suo centro non è allineato: questi errori si verificano quando gli standard di tolleranza vengono ignorati o fraintesi. Per progettisti e produttori, sapere quale standard utilizzare (e perché) fa risparmiare tempo, 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 E 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 (tramite fresatura, girando, o perforazione) 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 caratteristiche? You don’t need to calculate tolerances for each one. Standards like ISO 2768 e ISO 286 predefine acceptable variation for common dimensions (per esempio., lunghezza, angoli, 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. Per esempio, 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) e 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 (per esempio., lunghezza, larghezza), rounding radii (per esempio., a curved edge on a bracket), and angular dimensions (per esempio., 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 (per esempio., componenti elettronici).
• ISO 2768-M (Medium Grade): The “default” for most CNC parts (per esempio., mobilia, general machinery).

ISO 2768 Linear Dimension Tolerances (mm)

ISO 2768 Rounding Radius & Chamfer Height Tolerances (mm)

ISO 2768 Angular Dimension Tolerances (°/’)

Esempio del mondo reale: A CNC shop made 100 aluminum brackets for a shelf. The designer specified ISO 2768-M for the 20 lunghezza mm (tolerance ±0.2 mm) and 90° angle (tolerance ±0°30′). All brackets fit the shelf’s slots perfectly—only 2 were rejected (per un 2% tasso di difetto), which is well within industry norms.

2. ISO 286: Tolerances for Mating Parts (Alberi & Fori)

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 (per esempio., strumenti chirurgici, componenti aerospaziali).
• ISO 286-Class 7 (IT7): Stretto (per esempio., parti del motore, ingranaggi di precisione).
• ISO 286-Class 8 (IT8): Moderare (per esempio., componenti della bicicletta, general machinery).

ISO 286 Tolerances for Key Dimension Ranges (mm)

Key Terms to Know:

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

Caso di studio: 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 e ISO 286

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

Fare un passo 1: Does the Part Have Mating Features?

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

Fare un passo 2: Check the Dimension Range

Make sure your dimension falls within the standard’s range. Per esempio:

• 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 (Sotto 50 mm)—larger dimensions (Sopra 1000 mm) often use Class 8.

Fare un passo 3: Confirm CNC Machine Capability

A standard CNC mill can usually hit ISO 2768-M and ISO 286-Class 8. To get Class 6 O 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. clienti, 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

Alla tecnologia Yigu, we see ISO 2768 e ISO 286 as essential tools for making CNC subtractive manufacturing efficient. Troppo spesso, clients overspecify tolerances (per esempio., using ISO 286-Class 6 for a non-critical bracket) and end up paying more. We help them align standards with function: Per esempio, 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, nei tempi e nel budget.

Domande frequenti

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

SÌ! Per esempio, 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. Classe 6 is tighter, but it’s more expensive and harder to achieve. Per esempio, 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 (per esempio., 5000 mm)?

For dimensions over the standard ranges (per esempio., 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.