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Unilateral Tolerance: Enhancing Parts Manufacturing Accuracy

In mechanical machining, due to various influencing factors, the manufactured parts’ dimensions rarely match the design dimensions exactly. These influencing factors include machine accuracy, cutting tool wear, material properties, and more. To address this uncertainty, engineers introduced the concept of tolerance, which allows part dimensions to deviate within a certain range from the design value.

Unilateral tolerance is a common way of setting tolerances. It specifies the maximum or minimum range by which part dimensions can deviate from the design value. Unlike bilateral tolerance, unilateral tolerance only restricts the dimension in one direction, meaning it only allows the part dimension to deviate in one direction from the design value.

This article will delve into the application of unilateral tolerance in mechanical machining and how it helps engineers and quality experts ensure product precision and quality.

What is Unilateral Tolerances?

Unilateral tolerance refers to the condition where the basic dimension can only change in one direction, meaning the tolerance can only vary in the positive or negative direction (compared to the reference value + or -). For example, specified as +0.000/-0.005, this is a unilateral tolerance that only allows variation in the negative direction. It indicates that the finished part’s dimensional deviation can increase by 0.005 millimeters in the defined negative direction, but no deviation is allowed in the opposite direction.

unilateral tolerance examples

This type of tolerance is typically used in designs where a part (such as a bearing or gear) must fit into another component. It ensures that the part does not exceed the specified dimensions, as larger dimensions would hinder its installation in the intended position.

The Importance of Tolerance in Quality Control

Tolerance defines the allowable range of variation in part dimensions and is a crucial parameter for measuring machining accuracy.

In quality control processes, setting clear standard or non-standard tolerances on drawings provides explicit guidance for CNC machining services and machinists, informing them of the precision standards that must be met during production. Smaller tolerance values represent higher precision requirements, meaning manufacturers need to invest more resources and effort to ensure the dimensional accuracy of parts. Conversely, larger tolerance values allow for certain dimensional fluctuations, thereby reducing machining difficulty and cost.

On a deeper level, strictly controlling the dimensional accuracy of parts through tolerances helps manufacturers achieve and maintain high-quality product output, reduce material waste, and enhance product reliability and durability. Therefore, throughout the manufacturing process, tolerances play a crucial role and are key to ensuring product quality and competitiveness.

Other Types of Design Tolerance

In addition to unilateral tolerance, there are many other engineering tolerances that can be used to describe specific quality constraints. For example, bilateral tolerance, limit tolerance, and geometric dimensioning and tolerancing (GD&T).

Bilateral Tolerances

In CNC machining, if the tolerance is symmetrical around the basic dimension, it is called bilateral tolerance. This means that when the tolerance is symmetrical around the basic dimension, the deviation of the dimension can be positive or negative, allowing the part to be slightly larger or smaller than the specified size.

For example, if the tolerance is marked as +/- 0.05mm, this indicates that the size of the machined part can increase or decrease by 0.05mm from the specified basic dimension. In other words, if the basic dimension is 100mm, then the finished part’s size can range from 99.95mm to 100.05mm and still be considered within the acceptable range.

Unlike unilateral tolerance, bilateral tolerance allows deviations in both positive and negative directions.

Limit Tolerances

Limit tolerance is represented as a range of values (upper and lower deviation values), defining the acceptable range for a dimension. As long as the measurement falls within this range, the part is considered acceptable.

For example, a limit tolerance marked as 0.70-0.75mm is the limit tolerance for CNC machining, indicating that the part’s dimension must lie between the upper deviation value and the lower deviation value (0.70mm is the lower deviation value, and 0.75mm is the upper deviation value). As long as the dimension does not exceed this tolerance band, it will be accepted.

limit tolerance

Geometric Dimensioning and Tolerancing (GD&T)

GD&T tolerance is a more advanced and complex geometric tolerance system compared to SD&T tolerance. GD&T not only provides part dimensions and tolerances but also includes numerical values with measurement units to specify the dimensions and/or geometric characteristics and/or the location of features on a part. While SD&T specifies the shape of a part, GD&T defines additional geometric features such as flatness, perpendicularity, positional tolerance, concentricity, symmetry, and more. GD&T tolerances are typically used for parts requiring extremely precise dimensions.

Tolerance Design Tips and Guidelines

When designing machined parts, tolerance design is a crucial step. Here are some practical suggestions and tips for tolerance design, aimed at helping designers ensure part performance while optimizing machining efficiency and cost control.

Clear Tolerance Design

Not all parts and their features need strict tolerances. To reduce costs and improve production efficiency, focus on key features of the parts, especially those that interact with or fit into other parts. This approach ensures assembly accuracy and functionality while avoiding unnecessary machining waste.

Avoid Overly Strict Tolerances

Overly strict tolerance requirements not only increase the risk of non-compliant parts but also can lead to higher CNC machining costs. This may involve the need for more advanced measuring tools and fixtures, longer production cycles, and so on. Therefore, when setting tolerances, a reasonable balance should be achieved based on actual needs and machining capabilities to attain the best cost-performance ratio.

Consider CNC Machine Capabilities and Precision

Different plastics and metal CNC machine vary in machining accuracy and stability. Thus, it’s important to set reasonable tolerance ranges based on the characteristics of the selected machines. If you choose a professional parts manufacturer like BOYI, you don’t need to worry too much about the machining capabilities of the equipment. These manufacturers typically have a variety of machining equipment and can find the most suitable machining solution to meet your tolerance requirements.

Considering Material Characteristics

Different materials may have different deformation and shrinkage rates during machining. Therefore, tolerance ranges should be set based on the properties of the materials. For soft materials, which are more prone to bending and deformation during machining, tolerance ranges need to be set more cautiously to ensure part accuracy and stability.

cnc machining center

BOYI Provides Precision Parts Machining Services

To ensure that every product precisely meets the specified tolerance range, BOYI rapid prototyping services tolerances strictly adhere to the international standard ISO 2768. Metal parts are machined according to the ISO 2768-m standard, while plastic parts strictly follow the ISO 2768-c standard.

Additionally, we have a skilled and experienced professional team proficient in various machining techniques and processes, capable of providing comprehensive machining services to our customers.

If you are looking for a partner who can provide exceptional precision parts machining services, BOYI is your ideal choice. Please feel free to contact us to discuss your specific needs with our expert team.


Reasonable selection and setting of tolerances in the design and manufacturing process are of great significance for ensuring product quality, improving production efficiency, and reducing production costs. In this regard, BOYI is your best partner.


What is the difference between unilateral and bilateral tolerance?

The distinction between unilateral and bilateral tolerance lies in the manner they define the permissible deviation from a nominal or basic size. Unilateral tolerance allows deviation in one direction only, while bilateral tolerance permits deviation in both directions from the basic size, typically indicated by a ± symbol.

What is a unilateral tolerance limit?

A unilateral tolerance limit specifies the maximum deviation allowed in one direction from the basic size of a part, either above or below it. It represents the limit size on that single side of the tolerance zone.

Tagged: CNC Machining Guide

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