What are the commonly used tolerance standards in the CNC machining industry? What are the common types of tolerances? Do you know the common terms and associated parameters? Let Peak Fasten Tech take you to learn more today.

Generally speaking, the machining tolerance of CNC determines the accuracy and production cost of parts. Generally speaking, the metal tolerance standard is generally ±0.13mm, the plastic standard tolerance is ±0.25mm, and the matching parts such as bearings require strict tolerances to ensure the assembly fit is qualified. Reasonable selection of tolerance type can optimize costs and improve efficiency.

CNC machining tolerance refers to the allowable deviation range of part size or nominal value, reflecting the level of precision required in the product manufacturing process. Manufacturers will determine their tolerance standards based on the shape, assembly requirements and functional requirements of the parts to ensure that the product meets the design requirements.

When marking machining tolerances, the "±" symbol is usually used. For example, if a part has a nominal size of 3.000 inches and a tolerance range of ±0.0010 inches is required, the final height of the part must be controlled between 2.999 inches and 3.001 inches to pass quality inspection.

Smaller tolerances represent higher machining accuracy requirements, which are usually called "tight tolerances"; conversely, larger tolerance ranges are called "loose tolerances." Tight tolerance or even zero tolerance machining requires more machining setups, longer machining cycles, and specialized tools, so it is more expensive to manufacture.

Common CNC machining tolerance standards

Typical process tolerance range：

Note: The actual tolerance needs to be adjusted based on material properties, equipment accuracy and part geometry complexity.

International standards include：

‌ISO 2768‌：This is a general processing tolerance standard established by the International Organization for Standardization (ISO) and is applicable to metal parts with a size range of 0.5-3000 mm. It is divided into three categories: fine (f), medium (m), and rough (c). The appropriate tolerance grade is selected according to the actual application and design requirements of the part.

ASME Y14.5‌：The Y14.5 standard published by the American Society of Mechanical Engineers (ASME) is mainly used to control geometric tolerances and contains a series of geometric tolerance symbols and definitions. It focuses on geometric tolerance control, such as flatness and perpendicularity, and is suitable for high-precision assembly requirements.

‌GB/T1804-2000‌：This is the Chinese national standard for the limit deviation values of linear dimensions, fillet radius and chamfer height dimensions.

GB/T 1804-2000 (linear dimensions, chamfer tolerances)

GB/T 1184-1996 (unformed position tolerances)

Of course, since CNC machining involves a variety of processes and uses different types of cutting tools, the tolerance standards for different processes are also different. The following are the standard tolerance ranges for typical CNC machining processes:

Compared with other machining technologies, CNC machining processes can usually achieve tighter tolerance requirements to ensure the accuracy and quality of parts. In actual production, the specific tolerances may be adjusted depending on the material properties, part geometry and the accuracy of the machining equipment.

CNC machining tolerance types

Due to different machining processes and part geometry, the manufacturing industry often uses different types of tolerances to meet CNC machining needs. The following are common CNC machining tolerance types:

1.Standard Tolerance

This is the default tolerance range used in the machining process and is suitable for most conventional products. When the customer does not specify specific tolerance requirements, CNC milling usually uses a tolerance of ±0.1mm.

2. Limit tolerance

The allowable range of part size is specified by using upper and lower limits. For example, if the tolerance range of a CNC machined part is 15 – 15.5 mm, then its size must be between 15 mm and 15.5 mm.

3. One-way tolerance

A dimension is allowed to deviate in only one direction. For example, +0.00/-0.03mm means that the part size can be reduced by up to 0.03mm, but cannot exceed the nominal size. This type of tolerance is often used for parts that require precise assembly.

This type of tolerance is usually used for parts that need to be assembled accurately. If the part is too large or too small, it may not be installed correctly or affect the use function. Therefore, when designing, it should be ensured that the final processing size does not exceed the limit range.

4. Bidirectional tolerance

Bidirectional tolerance allows the dimension to deviate in both the positive and negative directions. For example, ±0.05mm means that the part size can be increased or decreased by 0.05mm based on the nominal size. This tolerance is mainly applicable to the control of external dimensions to ensure that the overall size of the part meets the assembly requirements.

5. Geometric dimensions and tolerances

GD&T is a system for accurately defining and transmitting machining tolerances, which is more complex than traditional tolerance annotation methods. It describes the geometry and dimensional tolerance requirements of parts through the Feature Control Frame.

GD&T not only controls the final size range of parts, but also specifies the geometric characteristics of parts, such as flatness, concentricity, and true position. For parts that require high-precision machining, the GD&T symbol system can ensure the dimensional accuracy of key features and improve the manufacturing and assembly quality of parts.

Reasonable selection of tolerance types can optimize machining costs and improve production efficiency while ensuring part function and assembly accuracy.

The above is some analysis of CNC tolerances.

Explanation of common terms

Surface roughness: refers to the evaluation index of the microscopic roughness of the machined surface, which affects the wear resistance and friction coefficient of the parts. Higher surface roughness requirements are usually applicable to parts that need to reduce friction or withstand high stress. The sampling length and evaluation length must be selected according to GB/T 1031-1995, and the common Ra value sequence is 0.012–0.4μm2.

Geometric tolerance‌: used to control the accuracy of the shape (such as flatness, roundness) and position (such as parallelism, perpendicularity) of parts. Geometric tolerance is particularly important for parts that require assembly accuracy.‌

Choosing an experienced CNC machining service provider can improve machining accuracy, better control tolerances, ensure compliance with design requirements, and reduce the cost of subsequent modifications and rework. Peak Fasten Tech, a professional one stop fasteners CNC parts supplier, can provide you with the best service.

When submitting a manufacturing request, it is recommended to clarify the tolerance requirements so that the processor can optimize the process in advance, improve production efficiency and reduce costs.

In short, when determining tolerances, it is necessary to comprehensively consider costs, part application scenarios, and processing capabilities to ensure that both functional requirements can be met and production costs can be controlled.

Peak Fasten Tech, a professional one stop solution provider for different kind of CNC machining, CNC milling, CNC turning service. Please feel free to ask if any query on CNC machining or bottleneck problems to solve on any high precision parts.