Understanding Engineering Tolerances

In CNC machining and manufacturing, tolerances are crucial for ensuring that parts meet the precise specifications required for functionality and quality. Every manufacturing process, including CNC machining, inherently introduces some degree of variation in the produced parts. Engineering tolerances define the acceptable range of variation for critical dimensions, ensuring that these variations don’t affect the overall performance, fit, or functionality of the final product.

Without well-defined tolerances, even slight discrepancies in dimensions could lead to components that don’t fit together correctly or fail to perform as designed. Tolerances are essential for balancing the cost, performance, and manufacturability of parts, helping engineers and manufacturers avoid errors and inefficiencies during production.

In this article, we will explore what engineering tolerances are, the various types of tolerances commonly used, and how they are calculated. Understanding these concepts is key to maintaining quality and precision in CNC machining and other manufacturing processes.

What Is Tolerance in Engineering?

In engineering, tolerances define the allowable deviations from specified dimensions in a part or product. They are critical in ensuring that the manufactured part fits and functions as intended, especially when it’s part of a larger assembly. Without proper tolerance specifications, even small deviations can lead to parts that do not fit properly or fail to perform as expected.

Each manufacturing process, including CNC machining, comes with its own set of inherent inaccuracies. Tolerance in engineering ensures that these variations are accounted for, preventing waste and ensuring that parts are usable without compromising the overall design intent. Properly applied tolerances provide clear guidelines for manufacturing processes, helping to reduce errors and improve efficiency.

Dimension Tolerances

In CNC machining, it’s nearly impossible for machines to produce parts with perfect accuracy. As a result, the final dimensions of a product will naturally vary from the nominal or stated dimensions. For instance, when a laser cut is performed, a part that is intended to be a 20 mm hole might end up being slightly larger or smaller due to the heat and precision limitations of the cutting process.

Nominal Value

The nominal value refers to the target or ideal dimension of a part. It represents the exact measurement the engineer intends to achieve. However, due to limitations in manufacturing processes, the actual dimensions will often vary within a specified range.

Lower Deviation

Example of lower-deviation tolerance on a 100 mm nominal dimension. The lower deviation specifies how much smaller than nominal the part can be. It is denoted by a “−” sign. For instance, a 100 mm nominal with a lower deviation of −0.5 mm means the part can be as small as 99.5 mm (but no smaller). Anything below that would be out of tolerance. This ensures the part will not end up undersized due to machining variation.

Upper Deviation

The upper deviation specifies how much larger than nominal the part can be, shown with a “+” sign. For example, a 100 mm nominal dimension with a +0.5 mm upper tolerance allows sizes up to 100.5 mm. In that case the part must not exceed 100.5 mm. Together, upper and lower deviations define the acceptable size range for the feature.

Bilateral Deviation

Bilateral (±) tolerance example: a 100 mm nominal with ±0.25 mm allows 99.75–100.25 mm. A bilateral tolerance allows variation in both directions (positive and negative). It is denoted with a “±” symbol. For example, specifying a 50 mm nominal with a bilateral tolerance of ±0.15 mm permits any size from 49.85 mm to 50.15 mm. This means the total “room for error” is 0.30 mm centered on the nominal. Bilateral tolerances simplify notation by combining upper and lower limits into one symmetric range.

General Tolerances

When a drawing does not give a specific tolerance for every dimension, general tolerances apply. These are default limits defined by standards or notes (rather than by each dimension). A common practice is to include a note like “ISO 2768-m” on the drawing, which tells the manufacturer to use the medium class of ISO 2768 general tolerances for all unspecified linear or angular dimensions. General tolerances can cover linear dimensions, angular dimensions, radii, chamfer heights, etc. and help simplify drawings by providing default limits when precise tolerances aren’t critical.

Here’s the linear dimension table for a better explanation:

For example, ISO 2768‑1 specifies that a 25 mm dimension in the 6–30 mm range has a ±0.2 mm tolerance in the “m” (medium) class. In other words, with ISO 2768-m the 25 mm dimension may legally be between 24.8 mm and 25.2 mm without any additional notes. 

Fits Between Parts

When two parts must assemble (for example a shaft and a hole), tolerances determine the fit between them. The limits and fits system combines the tolerances of mating parts to achieve a desired clearance or interference. The allowance is the maximum difference between hole and shaft diameters. The three basic fit types are:

Clearance fit: The shaft is always smaller than the hole, so there is guaranteed space (clearance) between them. For instance, both the hole and shaft dimensions are specified with tolerances that ensure the shaft’s maximum size is still less than the hole’s minimum size. This fit allows free movement or rotation (e.g. a sliding bearing).

Transition fit: The shaft and hole tolerances overlap such that the assembled parts may either have a small clearance or a slight interference. In this case the maximum shaft size is just above the hole’s minimum, and the shaft’s minimum is below the hole’s maximum. The result is a compromise fit where parts may press-fit or slide lightly depending on actual sizes.

Interference Fit: The shaft is always larger than the hole, requiring force (press fit) to assemble. Even the shaft at its smallest permissible size is bigger than the hole at its largest. This ensures no movement after assembly, useful for parts meant to be permanently joined (e.g. gears on shafts).

Choosing between these fits depends on the application’s requirements for movement, alignment, and ease of assembly. Each fit type is achieved by carefully specifying tolerances on both mating parts.

Geometric Dimensioning and Tolerancing (GD&T)

In addition to linear tolerances on size, engineers use GD&T to control form, orientation, and location of features. GD&T is a standardized symbolic language on drawings that defines tolerances for shape and relational features. For example, a parallelism tolerance might require one surface to be within 0.1 mm of being parallel to a reference surface. In GD&T notation, a feature control frame with the parallelism symbol and a tolerance value indicates exactly how straight or flat a feature must be relative to a datum. GD&T can specify flatness, straightness, position, perpendicularity, circularity and many other characteristics. It helps ensure that even if a part is the right size, its features are located and oriented correctly for assembly and function.

Why Tolerance Matters and Where to Use It?

Tolerances are crucial because real-world manufacturing always has variability. Specifying tolerances tells the maker exactly what variation is acceptable. Without clear tolerances, a manufacturer would use generic defaults, which may not suit the design intent. Uncontrolled variation can cause parts to be too loose or too tight in assembly, leading to failures or scrap. Proper tolerancing ensures reliability and quality while balancing cost. In particular:

Fit and Function: Tolerances guarantee that mating parts will assemble correctly. They set the allowable clearance or interference so components fit as intended. Without adequate tolerances, parts might be too tight or too loose, causing mechanical failures or inability to assemble. Proper tolerances in precision fields (aerospace, automotive, medical) are vital for safety and performance.

Interchangeability: Standardized tolerances make parts interchangeable. When parts are mass-produced with specified tolerances, any part from any batch or supplier will fit with any mating part. This simplifies assembly, maintenance, and repairs. Interchangeability is the foundation of modular design in manufacturing.

Quality Control: Tolerances provide measurable limits for inspection. By defining exactly how far a part may deviate, tolerances give clear pass/fail criteria. This enables quality control processes (gauging, CMM measurements, etc.) to verify parts against the drawing requirements. Using tolerances reduces the chance of defective components reaching customers.

Cost Efficiency: Tighter tolerances generally require more precise (and expensive) manufacturing processes. Over-specifying tight tolerances can dramatically increase cost and scrap rate without real benefit. By setting tolerances only as tight as necessary for function, engineers balance precision with manufacturability. Proper tolerance specification minimizes wasted material and machining time while still meeting design needs.

Ensuring Precision with Geomiq: The Importance of Engineering Tolerances

In the world of CNC machining, precision is non-negotiable. Engineering tolerances are essential for ensuring that parts are manufactured with the necessary accuracy, even when slight variations occur. Whether it's laser cutting, milling, or turning, proper tolerances make sure that parts fit together as intended and perform reliably in their applications.

At Geomiq, we understand the critical role that tolerances play in achieving high-quality, precision-engineered parts. By applying the right tolerances to your projects, we ensure that every component meets exacting standards, saving time, reducing waste, and ensuring that your designs are brought to life with the highest level of accuracy.