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unilateral tolerance and bilateral tolerance

Understanding Unilateral Tolerance and Bilateral Tolerance in Machining

In the machining world, precision is paramount, and understanding the nuances of tolerances is the cornerstone of achieving unparalleled quality.

Bilateral and unilateral tolerances are more than just technical jargon; they are critical tools that ensure every machined part meets its intended design specifications with exactitude.

This blog post will dive deep into bilateral and unilateral tolerances, illustrating their pivotal roles in quality control and how they guide the production process toward perfection.

The Role of Tolerance in Quality Control

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Precision CNC Machining

Tolerance in machining is a fundamental parameter that determines the allowable limit of variation in the dimensions of a machined part.

Its role in quality control cannot be overstated, as it directly influences the functionality, compatibility, and performance of the component within its intended assembly or application. By defining precise upper and lower bounds for dimensions, tolerance ensures that parts fit together seamlessly, operate efficiently, and meet stringent quality standards.

In essence, tolerance acts as a guardian of precision, enabling manufacturers to achieve and maintain high-quality outputs, reduce material waste, and enhance product reliability, thereby upholding the integrity of the entire manufacturing process.

What is Unilateral Tolerances?

Unilateral tolerance is a limit on how much a manufactured part’s dimensions can vary in one direction from the standard measurement. It assures that parts fit correctly without affecting functionality by allowing a certain range of variation that is either greater or smaller, but not both, compared to the nominal or target dimension.

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Unilateral Tolerances

Unilateral tolerance in a schematic diagram specifies the acceptable limits of variation in a single direction from a nominal dimension. For the right triangle in your diagram:

  • The distance from the hole center to the side should be 12 mm but can be as small as 11.94 mm, showing a negative unilateral tolerance (i.e., the size can only vary to become smaller than the nominal, down to 11.94 mm).
  • The distance to side b is set at 20 mm. Still, it can slightly increase up to 20.04 mm, indicating a positive unilateral tolerance (i.e., the size can exceed the nominal, up to an additional 0.04 mm).
  • The smallest angle r, ideally 30°, can be up to 31°, another case of positive unilateral tolerance.
  • The hole’s diameter, targeted at 10.00 mm, may be increased by up to 0.05 mm, yet another positive unilateral tolerance.

This clearly indicates that while some measurements can increase, the distance to side can only decrease, not increase, in your schematic’s defined tolerances.

What is Bilateral Tolerances?

Bilateral tolerance in manufacturing defines the upper and lower limits for a dimension, allowing for permissible variation both above and below the nominal value.

This ensures parts can mate effectively, accommodating variations in both directions for optimal fit.

Bilateral tolerance can be symmetric, with equal upper and lower limits, or asymmetric, where the allowed variations above and below the nominal dimension differ.

This approach facilitates precise control over manufacturing variations, ensuring parts meet stringent quality standards.

bilateral tolerances
Bilateral Tolerances

The bilateral tolerances in the schematic define the acceptable range of variation from the nominal size in both positive and negative directions. For the right triangle in the schematic:

  • For side a, the hole’s center can be 12 mm ± 0.02 mm, so it can be between 11.98 and 12.02 mm.
  • Regarding side b, the allowed distance ranges from 19.96 to 20.04 mm, permitting equal variation.
  • The angle r has a tolerance of ±1°, acceptable between 29° and 31°.
  • The target aperture is 10.00 mm, the maximum can be 10.05 mm and the minimum can be 9.98 mm, which is an asymmetric bilateral tolerance, so allows for more variation in the positive direction than in the negative—a case of unequal bilateral tolerance since the tolerance range isn’t symmetrical around the nominal value.

Asymmetric bilateral tolerance(unequal bilateral tolerances) has unequal allowances above and below the nominal size, while symmetric bilateral tolerance has equal allowances on both sides, ensuring consistent variation limits around a specified measurement.

Other Types of Engineering Tolerances

Common Geometric Tolerances Table
Tolerance Type Description
Flatness Ensures a surface is within two parallel planes.
Straightness Ensures a line is straight over its entire length.
Circularity Ensures any point on a circle is equidistant from the center point.
Cylindricity Controls roundness, straightness, and taper of an entire cylindrical feature.
Profile of a Line Defines a uniform boundary along a line on any feature or surface.
Profile of a Surface Controls the 3D form of a surface; it must lie within a tolerance zone.
Parallelism Ensures a feature or surface is parallel to a datum plane or axis.
Perpendicularity Ensures a surface or feature is perpendicular to a datum.
Angularity Sets the defined angle relationship between features or to a datum.
Concentricity Controls the median points of a feature to be concentric to a datum axis.
Symmetry Ensures the balance of two or more features across a datum plane.
Position Controls the location and sometimes orientation of features.

Beyond unilateral and bilateral tolerances, engineering and manufacturing employ various types of tolerances to ensure product quality and performance.

Geometric tolerances control the shape, orientation, and placement of features, critical for complex assemblies.

Fit tolerances dictate the allowable limits between mating parts, ensuring a proper fit for functional requirements, categorized into clearances or interference fits.

Material condition tolerances account for variations in material properties, influencing manufacturing processes and part behavior under different conditions.

Surface finish tolerances specify the roughness and texture of a part’s surface, affecting functionality and aesthetics.

Each tolerance type plays a specific role in the comprehensive quality control system, guiding designers and manufacturers in achieving precision, reliability, and efficiency in product development and production.

How AT-Machining Approaches Tolerances

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Tolerances Measure

AT-Machining adopts a meticulous and strategic approach to tolerances, integrating precision engineering principles with state-of-the-art technology to meet and exceed industry standards.

Recognizing the critical impact of tolerances on product quality and functionality, AT-Machining employs advanced measurement and machining techniques to ensure exact compliance with specified tolerance ranges. The company’s expertise encompasses a broad spectrum of tolerance types, including unilateral, bilateral, geometric, and fit tolerances, ensuring comprehensive quality control across all machining projects.

By leveraging skilled professionals, sophisticated software for design and simulation, and precision machining tools, AT-Machining guarantees that each component not only meets the exact specifications but also aligns perfectly with the overarching requirements of the project, ensuring optimal performance, durability, and customer satisfaction.

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Are you ready to take your machining project to the next level? Contact AT-Machining today to discuss your needs with our experts. Let us help you achieve the precision and quality your projects deserve. Visit our website or give us a call to start the conversation. With AT-Machining, precision is not just a goal; it’s a guarantee.


As we conclude our journey through the intricate world of unilateral and bilateral tolerance in machining, it’s clear that these concepts are not just technical necessities but the backbone of precision engineering. AT-Machining’s expertise in navigating these tolerances showcases our unwavering commitment to delivering unmatched quality and precision in every project. Embrace the precision; let AT-Machining be your partner in excellence.

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