What Do Dimensional Tolerances Mean for Metal Parts and Fabrication?

Dimensional tolerances directly dictate functional success in metal fabrication. Factors such as temperature and the need for moving parts to work together can cause slight variations in part dimensions. For this reason, acceptable limits must be defined for features such as length, thickness, diameter and flatness.

In this guide, we’ll explain what dimensional tolerances are, why they matter, how different manufacturing processes affect dimensional accuracy, and what to consider when specifying tolerances for a metal component.

What Are Dimensional Tolerances?

Dimensional tolerances establish the precise limits of acceptable deviation across feature length, diameter and thickness. In complex mechanical assemblies, dimensional and geometric tolerances are used according to established standards to control the size, form, orientation, location and run-out of features.

Whether measured in millimetres or inches, dimensional tolerances define the allowable variation from a nominal dimension while ensuring components remain fit for purpose.

Precise evaluation and control of tolerances are required. Otherwise, mating features constrain specific degrees of freedom, propagating geometric deviations directly to adjacent components and operating equipment.

Dimensional tolerances can apply to many different characteristics, including:

• Length, width and thickness
• Hole and shaft diameters
• Angles and bend dimensions
• Flatness and straightness
• Surface features and machined dimensions

Narrowing tolerance limits requires elevated process capabilities and tighter machine control. This requirement increases machining time, requires improved inspection methods and increases production costs.

How Tolerances Affect Fit, Function and Assembly

Dimensional tolerances are critical when two or more metal components must interface. A component manufactured outside specified tolerance limits can become oversized, undersized or unsuitable for assembly. For example, an oversized shaft or one with surface roughness exceeding the specified limit may fail to engage its mating hole. Consequently, tolerances must be evaluated earlier during design rather than strictly during fabrication. The required tolerance depends on component function and the necessary precision of its mechanical interaction.

Common Metal Fabrication Tolerances by Manufacturing Process

Different manufacturing methods provide different levels of dimensional accuracy. A tolerance that is practical for sawing or forming may not be suitable for precision machining. To achieve the required dimensional tolerance, it is important to know what limits and level of accuracy each method can operate within. In addition, we cannot always say that the tightest tolerance is the best choice. Cost and the working environment should also be taken into consideration.

• Cutting and Sawing: Processes such as band sawing and shearing are commonly used in general fabrication applications to cut metal materials to the desired length and can provide sufficient tolerance limits for most applications. The achievable tolerance varies depending on the quality and type of equipment used, the experience of the person operating the equipment, the mechanical properties of the material being cut, its thickness and the dimensions of the part to be cut.
• Laser and Plasma Cutting: Due to localised heat input and rapid cooling rates, laser cutting minimises thermal distortion compared to abrasive cutting methods. Consequently, it provides more precise dimensional tolerances and is used for sheet metal components that require high precision. Plasma cutting is commonly used for metal plates and sheets; however, cutting accuracy varies depending on material thickness and operating parameters.
• Machining: Except for heavy stock removal operations, milling and grinding achieve tight dimensional tolerances. Controlled material removal yields accurate dimensions for components requiring exact fits or specified running clearances. This is also where oversized metal stock machining can be useful. A metal component may initially be supplied slightly larger than its finished dimensions and then machined to achieve the required tolerance.
• Forming and Bending: Bending and forming operations involve significant metal deformation, posing a high risk of dimensional deviation. Dimensional accuracy is directly governed by material grade and homogeneity, section thickness, bend radius, tooling material, quality and service life, as well as the extent of springback, all of which directly dictate part dimensions and achievable tolerance precision.

Factors That Influence Dimensional Accuracy in Metal Parts

Several factors can affect material dimensional tolerances during manufacturing. These include:

• Material: Each metallic material possesses unique thermal conductivity and formability characteristics. Consequently, achievable tolerances during cutting, machining and forming operations are directly dependent on the material.
• Thickness: Thicker materials can behave differently during cutting and forming than thinner materials.
• Manufacturing process: Each process has its own typical accuracy and limitations.
• Tooling and equipment: Machine condition, tool wear and setup can affect dimensional accuracy.
• Part geometry: Complex geometries can hinder the achievement of high-precision tolerances.
• Temperature: Thermal expansion and contraction can affect measurements and accuracy during and after machining.
• Inspection methods: The equipment and method used to measure a component can also influence the reported dimensions.

Understanding these factors helps engineers establish realistic engineering tolerances rather than specifying limits that are unnecessarily difficult or expensive to achieve.

BS EN, ISO and ASTM Standards for Metal Size Tolerances

When purchasing commercial metal stock or standard stock materials, it is important to understand that the nominal size is not always the exact measured size of the material. Standards define acceptable variations for many types of metal products.

For example, BS EN, ISO and ASTM standards establish dimensional requirements for certain types of bars and other metal products. Similar standards apply to sheet, plate, tubing and other forms of metal stock.

In the UK and Europe, dimensional tolerances are commonly specified using BS EN and ISO standards, while ASTM standards remain widely used in many engineering and manufacturing sectors.

This means that metal size variation is not necessarily a manufacturing defect. A material can measure slightly above or below its nominal dimension while still meeting the applicable material standard.

This is particularly important when designing parts that will be machined from commercially available stock materials. Engineers may need to account for the material’s permitted dimensional variation when selecting stock sizes.

How to Specify the Right Tolerances for Your Project

The correct tolerance depends on the function of the component. Not every dimension needs the same level of precision.

For general fabrication, relatively broad tolerances may be sufficient. However, components that require precise fits, moving interfaces or controlled clearances may need much tighter limits.

When specifying tolerances, consider:

• How the component will be used
• How it will fit with other parts
• Which manufacturing process will be used
• The required level of accuracy
• Inspection requirements
• The cost of achieving tighter tolerances

Rather than applying excessively tight tolerances to every dimension, it is generally preferred to specify the tightest tolerance only where strictly necessary, accepting looser, functional tolerances elsewhere. This approach maintains the required operational performance of the finished component while effectively controlling manufacturing costs.

Understanding Dimensional Tolerance Symbols

Engineering drawings often use standardised symbols and notation to communicate dimensional requirements. In manufacturing operations, unavoidable sources of variation, such as tooling wear and thermal fluctuations, necessitate defining an acceptable tolerance range around the target nominal dimension. Depending on functional assembly requirements, these tolerances may be specified as symmetrical or asymmetrical to skew the allowable deviation. Clear engineering specifications allow manufacturers to isolate critical dimensions, ensuring the selection of appropriate process routes, such as choosing cold-rolled over hot-rolled stock to maintain performance while optimising production economics.

Get the Right Metal for Your Project

Understanding dimensional tolerances simplifies raw material sourcing and process planning. Whether working with mill stock variances or precision components, selecting materials that are compatible with the manufacturing process can help control fabrication costs. Explore commercial stock options through Metal Supermarkets.