Views: 0 Author: ZHE Publish Time: 2025-10-09 Origin: Site
In the field of CNC Machining service (even a deviation of 0.1 millimeter can cause parts to fail to fit properly), consistency must be ensured. The specific tolerance standards marked on the technical drawings for each dimension are both cumbersome and chaotic, and prone to errors.
An important international standard has come into play: ISO 2768. It sets default tolerances for dimensions, angles, and geometric shapes, eliminating ambiguity for CNC workshops, engineers, and global suppliers.
Ensure that the CNC machining brackets manufactured in Japan are perfectly matched with the components from Canada, reduce the cost of over-design, and simplify the production process by matching the tolerance level with the actual requirements.
Whether it is medical equipment or furniture hardware, understanding this standard is crucial for providing reliable and compatible components. This guide elaborates on its purpose, structure, application in CNC machining, and common misunderstandings.
This standard was developed by the International Organization for Standardization (ISO). It defines general tolerances - these tolerances apply to all unmarked features on the part, unless specific tolerance values are indicated on the drawing. The core objective is to simplify the drawings and ensure consistency.
Before this standard existed, manufacturers had relied on the default tolerances set by their respective companies. For instance, a CNC processing factory in Germany might set the tolerance for a 50-millimeter length at ±0.2 millimeters, while a factory in the United States would set it at ±0.5 millimeters - this led to problems with parts not matching. This framework addressed this issue by establishing common rules: everyone understood that the meaning of the size tolerance was the same.
For CNC machining services, this standard is a cost-saving method. CNC machines can achieve extremely small tolerances of ±0.001 millimeters, but most parts do not require such high precision. For instance, the handles of garden tools do not need high accuracy. This framework enables manufacturers to avoid wasting time on unnecessary precision processing steps, thereby shortening the production cycle and reducing material waste.
In short, it bridges the gap between precision and practicality—making it indispensable for modern manufacturing.
This standard is divided into two parts, each covering different tolerances. It also defines clear precision grades to meet different application requirements. Proficiency in this is crucial for CNC machining.
| Standard Part | Primary Focus | Tolerance Categories Included | Precision Classes (Tightest to Widest) | Key Use Case in CNC Machining |
|---|---|---|---|---|
| Part 1 | Linear & Angular Dimensions | Lengths, widths, diameters, radii, chamfers, angles | f (fine), m (medium), c (coarse), v (very coarse) | Sizing a CNC-drilled hole (e.g., 10mm diameter) |
| Part 2 | Geometric Tolerances | Flatness, straightness, perpendicularity, parallelism, runout | H (high), K (medium), L (low) | Ensuring a CNC-milled plate lies flat |
Part 1 applies to "dimensional" features - those whose dimensions are measured using a caliper or micrometer. The tolerance value here depends on the size of the feature (the target size). For example, for the same product, 20 mm has a smaller tolerance than 200 mm.
This part is of crucial importance for CNC turning and milling processes. In these processes, the dimensions of the parts are clearly specified (for example, a 50-millimeter shaft, a 150-millimeter bracket). By using the first part, engineers can avoid marking tolerances for each length or diameter.
Part 2 deals with "shape and position-based" characteristics - these characteristics affect the way components interact with each other. For instance, the base fabricated by a CNC machine must be flat to accommodate the motor; and the drilling must be perpendicular to the surface to match the bolts.
Unlike Part 1, geometric tolerances aren’t tied to nominal size. Instead, they’re based on the part’s overall dimensions (flatness tolerance for a 300mm plate is larger than for a 50mm plate). This part is essential for CNC parts that need smooth assembly, like automotive components or industrial machinery.
Applying this framework correctly requires coordination between engineers (who specify tolerances), CNC programmers (who set machine parameters), and quality control (who verifies compliance). Below is a practical, shop-ready workflow.
The class depends on three factors: part function, material, and CNC process capability.
Function: Precision parts (e.g., medical sensors) need “f” (fine) or “m” (medium) classes. Non-critical parts (e.g., decorative trim) use “c” (coarse) or “v” (very coarse).
Material: Metals (aluminum, steel) hold tight tolerances well, so “f” is feasible. Plastics (ABS, POM) expand/shrink during machining, so “m” or “c” may be needed.
CNC Process: CNC mills/turning centers handle “f” classes easily. CNC routers (for wood/plastics) work better with “c” or “v” due to material flexibility.
To indicate compliance, add a single note to the drawing—e.g., “General Tolerances: Part 1, m; Part 2, K.” This tells the CNC shop that all unmarked features follow these classes. If a feature needs a stricter tolerance (e.g., an 8mm hole with ±0.05mm instead of “m” class), mark that specific tolerance directly on the feature.
CNC programmers adjust cutting parameters to meet the chosen class:
For “f” (fine) classes: Slow feed rates (50–100 mm/min for aluminum), sharp carbide tools, and a finishing pass to reduce vibration.
For “c” (coarse) classes: Faster feed rates (200–300 mm/min for aluminum) and fewer passes to cut cycle time.
| Nominal Size Range | Tolerance (Class f: Fine) | Tolerance (Class m: Medium) | Tolerance (Class c: Coarse) | Tolerance (Class v: Very Coarse) | Typical CNC Application |
|---|---|---|---|---|---|
| 0–30mm | ±0.1mm | ±0.2mm | ±0.5mm | ±1.0mm | Small electronic housings |
| 30–120mm | ±0.15mm | ±0.3mm | ±0.8mm | ±1.5mm | Automotive brackets |
| 120–400mm | ±0.25mm | ±0.5mm | ±1.2mm | ±2.5mm | Industrial machine bases |
| 400–1000mm | ±0.4mm | ±0.8mm | ±2.0mm | ±4.0mm |
Even with the right class selected, CNC shops can miss tolerances if they ignore material behavior, machine capability, or measurement practices. Below are three key factors to monitor.
Different materials respond differently to CNC cutting, altering tolerance consistency:
Metals: Aluminum and steel have low thermal expansion (aluminum expands ~23 μm/m·°C) and stay stable. This makes them ideal for “f” classes.
Plastics: ABS and POM expand significantly when heated by tools (ABS expands ~108 μm/m·°C). A CNC-turned plastic bushing machined to “f” class (±0.1mm) may shrink to a too-tight fit after cooling—so “m” class is safer.
Composites: Carbon fiber composites are stiff but prone to delamination. CNC routing of composites often uses “c” class to avoid material damage.
Not all CNC machines can meet every tolerance class. Older machines with worn ball screws or loose spindles may struggle with “f” classes, leading to deviations. To avoid this:
Test machine capability with a “calibration part” (e.g., a 50mm shaft with “f” class tolerance) before production.
Use newer CNC machines (with ±0.001mm repeatability) for “f” class parts, and older machines for “c” or “v” class parts.
Using the wrong tool to verify tolerances renders the framework meaningless. Match tools to the class:
Class f (fine): Digital micrometer (accuracy: ±0.001mm) or coordinate measuring machine (CMM, accuracy: ±0.0005mm).
Class m (medium): Digital caliper (accuracy: ±0.02mm) or surface plate (for flatness checks).
Class c (coarse): Dial caliper (accuracy: ±0.05mm) or ruler (for large sizes).
Quality control teams should document measurements—e.g., recording 10mm hole diameters as 9.98–10.02mm for “f” class—to prove compliance.
Incorrect information can lead to serious mistakes: over-designing components, refusing to use available components, or creating incompatible components combinations. Here are four myths about professionals in numerical control machine tools.
False. The “f” (fine) class is tight enough for many precision applications. For example, a CNC-machined sensor housing with a 15mm length and “f” class tolerance (±0.1mm) meets most electronic device needs. The framework isn’t about “low precision”—it’s about using the right precision for the job.
False. If there is no such standard, the factory might adopt internal standards, and these standards vary greatly. For instance, a factory in China might set the tolerance for a 50-millimeter length at ±0.3 millimeters, while a factory in Germany would set it at ±0.1 millimeters. This results in the inability to assemble the parts correctly.
False. Geometric tolerances ensure that the parts can be correctly assembled and operate normally. The "m" grade straight line dimensions of the plates processed by CNC milling fully meet the requirements, but the flatness does not meet the standards.
Once installed, they will shake and disrupt the assembly effect. These two parts in the standard are equally important for functional parts.
False. As mentioned earlier, plastics and composite materials require a more lenient classification to account for situations such as expansion or delamination. For plastic components processed by CNC milling, using the "F" category will increase the scrap rate, as the material will contract beyond the tolerance range after processing. Always adjust the category based on the material properties.
Below are answers to common questions from engineers, CNC programmers, and buyers, focused on the framework.
This standard is designed for processing. For 3D printing, ISO/ASTM 52921 is more applicable as it takes into account unique challenges such as layer height and post-processing shrinkage. However, some 3D printing factories use this standard as a reference for manufacturing simple components (such as plastic brackets).
Q2:How do I choose between Part 2 classes (H, K, L)?
Base the choice on how the part interacts with others:
H (high): Parts needing tight alignment (e.g., aerospace engine components).
K (medium): General-purpose parts (e.g., automotive transmission components).
L (low): Parts with no critical assembly needs (e.g., decorative covers).
Yes. ISO 8015 defines “basic tolerances” for individual features, while this standard covers general tolerances. For example, a drawing might use Part 1, m for most dimensions but ISO 8015 for a critical 6mm hole needing ±0.03mm tolerance (tighter than “m” class).
It depends on the part’s “fit.” If the deviation doesn’t affect assembly or function (a 50.6mm length in a “c” class part allowing ±0.8mm), the part may be acceptable. If it causes a fit issue (e.g., a 10.1mm hole in an “f” class part allowing ±0.1mm), the part must be scrapped or reworked.
Yes, but adjust the class. Wood and acrylic are less stable than metals: CNC-routed wood may warp, so “c” or “v” class is best. Acrylic is brittle and prone to chipping, so “m” class balances precision and yield.
Calibrate machines every 6–12 months, or after heavy use (e.g., machining hard metals like titanium). Use a calibration part (with known tolerances from the standard) to test accuracy—if the machine fails, adjust ball screws, spindles, or tool holders.
It’s not legally mandatory, but it’s industry-standard. Most global buyers require compliance to ensure parts from different suppliers fit together. Without it, parts may be rejected at customs or during assembly.
Yes. For high-volume CNC production (e.g., 10,000 brackets), you may loosen the class slightly (e.g., from “m” to “c”) to reduce cycle time and cost—if the part’s function allows it. Always confirm with the buyer before adjusting.
International tolerance standards - they are the foundation for achieving efficient, consistent and global manufacturing. For CNC professionals, they simplify tolerance management, reduce costs, and ensure that parts can be assembled as intended. By understanding its structure (linear/angle sections in Part 1, geometric tolerance sections in Part 2), choosing appropriate grades for materials and functions, and avoiding common pitfalls, the quality of the product can be improved.
In a world where CNC parts are transported across continents and assembled into complex machines, this standard serves as a common foundation. It ensures that CNC factories in Brazil, designers in the UK, and purchasers in Australia can use the same "tolerance language" - thereby eliminating misunderstandings and building trust.
Whether it's processing precise medical components or simple products, this standard should be your preferred tool when seeking to balance precision and practicality. Correctly applying it will not only meet customer needs but also optimize your CNC processing workflow, leading to long-term success.
