Tolerance
What is tolerance?
In manufacturing industries, tolerance is a definition that specifies the permitted deviation from a part’s nominal dimension. The nominal dimension is the target value defined by the designer, and the tolerance defines its upper and lower limits within which the actual dimension is allowed to vary. Without a precisely defined tolerance, a machined part may not fit together with other components, which can cause malfunctions in the assembly, for example in the fit between a shaft and a bearing or a bushing.
Tolerances are an essential part of engineering drawings and technical documentation in machine shops, and they directly affect the cutting parameters, measurement methods, and machining strategy chosen by the machinist.
Dimensional tolerances in machining
A dimensional tolerance refers to the permissible range of variation associated with a single dimension, such as diameter, length, or width. It can be expressed as a symmetrical deviation, for example ±0.02 mm, or as asymmetrical upper and lower limits, such as 50.00 +0.01 / –0.03 mm.
Concepts related to dimensional tolerances, such as the upper limit size, lower limit size, upper deviation, and lower deviation, precisely define the allowable dimension range. In the machine shop this practically means that measurements taken with a micrometer, caliper or gauge are always checked against these limit values.
General tolerances, in turn, define the default deviations for dimensions that have not been given a specific, explicit tolerance. General tolerances are typically based on ISO standards and simplify everyday work in the shop when not every dimension is individually specified on the drawing.
ISO tolerances and IT grades
ISO tolerances are based on the international ISO system, in which tolerance grades are indicated with IT designations. The IT grade defines the size of the tolerance zone in relation to the nominal dimension. For example, IT6 represents a very high level of machining accuracy, whereas IT10 allows significantly greater variation.
When a tolerance is given in the form Ø40 H7, the letter indicates the position of the tolerance zone relative to the nominal size, and the number indicates the accuracy grade. This system is especially important when defining fits such as clearance fit, transition fit, and interference fit. A correctly chosen ISO tolerance ensures that the shaft and hole work as intended without extra machining or assembly problems.
Geometric tolerances and GD&T
Geometric tolerances complement dimensional tolerances by defining the permissible deviations in a part’s shape, position, and orientation. For example, a diameter tolerance alone does not guarantee that a shaft is straight or that its centerline is in the correct location.
Geometric tolerances cover, among other things, straightness, flatness, circularity, cylindricity, parallelism, and perpendicularity. They are shown on engineering drawings using symbols and datums in accordance with ISO GPS and GD&T principles. For the machinist, this means that measurement may require, for example, a dial indicator, a 3D coordinate measuring machine (CMM), or other precise measurement systems instead of just traditional hand tools.
Geometric tolerances are especially important in precision mechanics, bearing housings, guide surfaces, and other applications where even small form errors can affect function or wear resistance.
Fits and tolerances
Tolerance is closely related to fits. A fit defines the relationship between two mating parts, such as a shaft and a hole. In a clearance fit, there is an intentional gap between the parts that allows movement. In a transition fit, the joint is precise and can be slightly tight or slightly loose. An interference fit, on the other hand, is based on negative clearance, where the parts press tightly against each other.
In a machine shop, achieving the desired fit requires that both parts are machined within the specified tolerances. Even a small deviation can change a clearance fit into a press fit, or vice versa.
The impact of tolerance on cost and production
The tighter the tolerance, the more demanding the machining process. Tight tolerances require a rigid, stable machine tool, well-controlled cutting parameters, high-quality cutting tools, as well as accurate measurement and possibly a temperature-controlled inspection environment.
In the machining industry, cost-effectiveness is based on choosing the right tolerance. Overly tight tolerances increase machining time and inspection costs without providing functional benefits. On the other hand, tolerances that are too loose can lead to assembly problems, noise, vibration, or premature wear.
Measuring tolerances and quality assurance
Controlling tolerances is based on reliable measurement and calibration. Measuring equipment includes, for example, micrometers, calipers, height gauges, dial indicators, and coordinate measuring machines. In addition, gauges such as plug gauges and thread ring gauges are used especially in serial production for rapid acceptance checks.
In quality assurance, the key factors are the repeatability of measurement results and the capability of the measurement system. The process capability of the machine shop determines how reliably production stays within the specified tolerance limits.
Summary
Tolerance defines the permissible deviation from the nominal dimension and is a key factor in machining, fits, and assembly performance. Dimensional tolerances, ISO tolerances, and geometric tolerances together ensure that parts manufactured in a machine shop meet both dimensional and functional requirements. A properly chosen tolerance balances quality and cost-effectiveness, which is crucial for a machine shop’s competitiveness.
