• June 21, 2024
  • Shatou, Changan, Dongguan, CN
International Standard Tolerance

The Complete Guide to Standard Machining Tolerances

Standard machining tolerance is the pre-determined range of allowable dimensional deviation in the manufacturing process to ensure the quality and dimensional accuracy of the part. The size of the tolerance is usually determined according to the usage and process requirements of the part, such as length, diameter, flatness, etc. Standard machining tolerances are a necessary part of the manufacturing standard to ensure that the parts meet the design requirements and to guarantee the interchangeability of the parts.

This article details the different types of machining tolerances, their importance and definitions, and more.

International standards for machining tolerances

The international standards for machining tolerances include ISO 2768ISO 8015ISO 14405-1 etc. These standards stipulate the requirements and methods for machining accuracy, tolerance selection, measurement, calculation, etc.

Different types of machining tolerances

1. Dimensional tolerance

The range between the maximum dimensional error and the minimum dimensional error allowed by the part. It is divided into two types: basic tolerance and tolerance distribution.

2. Shape tolerance

The control tolerance of the position and orientation accuracy required for parts assembly. Divided into position tolerance and orientation tolerance.

3. Surface quality tolerance

This is the control tolerance of the surface quality of the parts. Divided into two kinds of finish tolerance and roughness tolerance.

4. Combustion cutting tolerance

In the combustion cutting machinery, processing tolerances are expressed as cutting specifications, surface roughness and cutting shape interval, etc.

5. Machining dimensional tolerance

Machining dimensional tolerance refers to the difference between the actual size before and after machining during the machining of the workpiece. Machining dimensional tolerance is the material, process, and machine tool accuracy jointly determined by.

6. Surface shape tolerance

Surface shape tolerance refers to the maximum height difference between the reference surface and the actual surface within a certain area.

7. Contour tolerance

Contour tolerance is in the part contour line and the datum line, the maximum distance and the minimum distance between the range.

8. Tolerance fit

Tolerance fit is the fit relationship determined by the machining dimensional tolerance and form tolerance between the parts. Common are fit, interference, and gap fit.

The importance of machining tolerances

Machining tolerance is the maximum range of deviation allowed in the manufacturing process. It has a great impact on the quality, reliability and cost of the product. The following are its importance:

1. Ensuring product size and geometry

Machining tolerances are an important factor in ensuring that product dimensions and geometry are as required. If machining tolerances are too large or inaccurate, the geometric and functional characteristics of the product will be affected, which may lead to product failure or non-compliance with requirements.

2. Improving product quality and reliability

Proper machining tolerances improve product quality and reliability by ensuring that the product meets the design requirements. Good machining tolerances provide a repeatable manufacturing process that ensures the production of a high-quality consistent product.

3. Reduced production costs

Machining tolerances in the manufacturing process can have a significant impact on production costs. If machining tolerances are too tight, they will result in increased manufacturing costs, while too large a machining tolerance can result in unusable products. Proper machining tolerances can reduce manufacturing costs and ensure product reliability.

4. Guaranteed delivery times

In standardized production, machining tolerances are an important parameter to ensure product reliability and productivity. Proper machining tolerances can ensure that products can be delivered on time while ensuring production efficiency and program control.

In short, machining tolerances play an important role in ensuring product quality, improving production efficiency, reducing manufacturing costs and ensuring product reliability.

Standard machining tolerance symbols and definitions

The following are common standard machining tolerance symbols and their definitions:

1. “±”: indicates the positive or negative direction of the tolerance. For example, “±0.01mm” means that the part size can be 0.01mm larger or smaller than the required size.

2. “H7”: Indicates the upper dimensional limit. For example, “50H7” means that the upper diameter of the part is 50 mm.

3. “h7”: Indicates the lower size limit. For example, “50h7” means that the lower diameter of the part is 50 mm.

4. “IT6”: Indicates the dimensional accuracy class. For example, “IT6” means that the dimensional accuracy of the part reaches the international standard grade.

5. “R0.1”: indicates the radius tolerance of a circle. For example, “R0.1” means the radius tolerance of the part is 0.1mm.

6. “M6”: Indicates the thread tolerance. For example, “M6” means the thread tolerance of the part is 6 mm.

7. “A-B”: indicates the tolerance between two dimensions. For example, “10-20” means the size of the part is between 10 and 20.

8. “⊥”: indicates the tolerance of verticality. For example, “⊥0.1mm” means that the error of the plane of the part perpendicular to the axis is not more than 0.1mm.

9. “∠30′”: indicates the angle tolerance. For example, “∠30′” means that the angular error of the part does not exceed 30 points.

Inter-part tolerance and stacking tolerance

Inter-part tolerance refers to the dimensional difference between different parts, which is caused by manufacturing errors, design tolerances and other factors. The stacking tolerance refers to the phenomenon that the dimensional accuracy of the final product decreases due to the superposition of tolerances of multiple parts during processing and assembly.

In product design, it is necessary to consider the influence of inter-part tolerance and stacking tolerance and carry out reasonable tolerance fit and product structure design. A reasonable tolerance design can ensure the accuracy requirements of the product in different processing steps, while a reasonable structural design can reduce the impact of stacking tolerances on the accuracy of the product.

Tolerance analysis method

The tolerance analysis method is a method used to evaluate the differences, uncertainties and variations in product size, shape and position. The method consists of the following steps:

1. Determine the measured dimensions

Select the part to be analyzed for tolerance and measure it carefully to ensure the accuracy of the dimensions.

2. Define tolerances

Determine the range of tolerances allowed to ensure product function and quality.

3. Modeling from tolerances

Using mathematical models, describe the shape and position of the product and calculate dimensional differences and uncertainties.

4. Performing sensitivity analysis

Analyzing and determining the impact of the differences in size, shape and position of the product, as well as correcting possible deviations.

5. Optimize the solution

Optimize the product design to meet the required dimensions and quality by evaluating the results of the tolerance analysis.

6. Inspection and verification

The actual inspection of the product is carried out to verify that the product meets the required dimensions and quality.

The tolerance analysis method is an effective quality management method to help producers and manufacturers understand the differences in product dimensions and quality and achieve continuous improvement of product quality.

Best Practices for Tolerance Design

1. Set a reasonable tolerance range

Before developing a tolerance scheme, make a comprehensive analysis of the function, importance, machining process, and usage environment of the part to determine a reasonable tolerance range.

2. Group tolerances step by step

Divide the dimensions of different machining processes into different tolerance groups to avoid causing difficulties in machining.

3. Adopt the principle of least tolerance

Under the premise of ensuring the design requirements, give priority to adopting the least tolerance.

4. Adopt statistical tolerance

In the occasion of high precision and high-quality requirements, statistical tolerance can be adopted.

5. Increasing the inspection means

Increase the inspection means, especially the use of CMM, to improve inspection accuracy and inspection efficiency.

6. Use CAD/CAM technology

Using CAD/CAM technology for tolerance analysis and design can reduce manufacturing costs and processing difficulty.

7. Emphasize quality management

In the manufacturing process, strengthen quality management to avoid poor manufacturing that leads to tolerances that do not meet design requirements.

8. Ensure error-free communication

Designers should fully communicate with production and inspection personnel to ensure consistent intent of understanding and implementing tolerances.

Tolerance measurement and control methods

The tolerance measurement and control method refers to the method of measuring and controlling the product’s dimensional tolerance in the manufacturing process. Common tolerance measurement methods include measuring tool measurement, optical measurement, image measurement and machine vision measurement, etc.; tolerance control methods include design tolerance control, machining process control, assembly control and test control, etc.

The followings are the specific tolerance measurement and control methods:

1. Measuring tool measurement method

Using a measuring tool to measure the size and shape of the parts, such as a micrometer, vernier caliper, height gauge, etc.

2. Optical measurement methods

use optical measuring instruments, such as microscopes, projectors, etc., to achieve high-precision size, shape and other aspects of measurement.

3. Image measurement method

Using cameras, light sources and other equipment to process the image of the product to obtain the size, shape and other parameters of the product.

4. Machine vision measurement method

Using a machine vision system to measure and inspect the product, can realize real-time monitoring and quality control of the product in the process.

5. Design tolerance control

Before product design and manufacturing, according to the product performance requirements and production process conditions, the product dimensional tolerances are specified and controlled.

6. Processing process control

Through the optimization of processing technology, such as forming, cutting, welding and other process parameters control, to achieve the control of dimensional tolerance.

7. Assembly control

Using special tooling and measuring equipment to control and measure the assembly of parts to ensure the control of product dimensional tolerance.

8. Test control

Before the finished product leaves the factory, a comprehensive function and performance test is conducted on the product to ensure that the product quality meets the design requirements.

Examples of Machining Tolerance Applications


1.Machining tolerances for auto parts

Automotive parts must be manufactured precisely to ensure safety and performance. For example, piston rings must have the proper film thickness to ensure the containment of compressed gas in the cylinder and proper wear.

2. Precision mechanical parts machining tolerances

Precision mechanical parts must meet specific standards to ensure high-precision operation and reliability. For example, in the manufacture of precision bearings, the tolerance between bearings must be as small as possible to ensure bearing accuracy and life.

3. Electronic component processing tolerances

Electronic components must have accurate dimensions and precision in order to function properly and remain stable. For example, when manufacturing circuit boards, the dimensional and positional tolerances of printed circuits and components must be strictly controlled to ensure the performance and reliability of the boards.

4. Building accessories processing tolerances

Building accessories must strictly comply with the design requirements to ensure the building’s structure and performance. For example, in the manufacture of steel connections, the geometric dimensions and tolerances of the connections must be controlled to ensure their strength and reliability.

5. Medical instrument processing tolerances

Medical instruments must be manufactured with precision to ensure efficient diagnosis and treatment. For example, when manufacturing artificial joints, manufacturing tolerances and component fit tolerances must be controlled to ensure the stability and longevity of the artificial joint.


Machining tolerances play an integral role in the manufacturing process. While their extent varies from project to project, their presence can hardly be completely ignored. Proper attention to machining tolerances can help save costs and achieve better quality results. If tolerance concepts are difficult to understand or calculate, Jiman can help.

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