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Tolerancing and Geometric Dimensioning: A Primer

By | Mr Inam

Designing and building any mechanical system is a complex process that needs to properly account for many different planners like cost, material, and manufacturing techniques. One of the key challenges is ensuring that all the parts, once manufactured, will fit together and function as intended. And that’s why tolerancing is an integral part of the mechanical design process. The easiest way to define tolerancing is using the dimensional approach, where drawings depict how much each part’s dimension is allowed to deviate from its nominal value. But dimensional tolerancing only works well in many scenarios because it does not reflect how much will be used. It does not specify that you need a surface to be flat because it has to create a seal with another part. 

It’s a valuable adjunct to dimensional tolerancing since it gives you control over 14 separate geometric properties, each of which may be used to emphasize a different part of your design. The American Society of Mechanical Engineers’ GD&T dimensional and tolerancing system guarantees perfect component integration into subsequent assemblies.


Unlike the traditional tolerancing approach that applies tolerances to dimensions, GD&T applies them to features and stat. For example, a part could be a surface, a hole, or a slot. 

It’s important to differentiate between surface features, just individual surfaces. And features of science are any features with defined dimensions, meaning that they can be measured precisely, whether that’s a feature defined by two opposed parallel surfaces.

Categories of Geometric Tolerancing: 

The characteristics of geometric tolerancing can be split into the following categories.

Feature Control Frame:      

Geometric tolerancing is assigned the features using a feature control frame. These little grids contain all the information needed to fully control a particular geometric characteristic. The components can be applied using extension lines, or they can be attached directly to dimensions for elements of science.

Form Tolerance:

The next category of geometric tolerancing is formed tolerance, which includes straightness, flatness, roundness, and cylindricity. It is applied to individual lines instead of to an entire surface. When applied to a surface feature, any cable on a character in the same direction must be in a tolerance sound defined by two parallel lines. 

Location Tolerances:

There were three location tolerances; position, concentricity, and symmetry. But the concentricity and symmetry tolerances were removed from the 2018 edition of ASME Y14.5-20. 

The position is one of the most commonly used geometric tolerances. It defines the maximum distance that the axis or medium plane of a size feature can be located away from its theoretically exact position. It’s often applied to control the location of a hole.

Profile Tolerances:

Profile tolerances are very versatile. It can be used to control the form, orientation, and locational features simultaneously. The profile of a line is similar to the shape of a surface, but it contains individual line elements of a character instead of the entire surface. The inspection of complex profile tolerances can be difficult without the MMC. In some cases, profile tolerances can be used instead of other tolerance types. 

Runout Tolerance:

Finally, we will discuss the runout category of tolerances. Runout is a term used to describe the eccentricity of a surface relative to a particular axis. There are two runout tolerances; circular runout and total runout. Circular runout controls the roundness of individual cross-sections of a feature. Runout tolerances are often applied to rotating parts because any significant eccentricity relative to the axis of rotation can cause unwanted vibration. 

Good skills in GD&Timprove your communication, resulting in high-quality product design. Furthermore, properly learning OD geometric skills can also save you and your organization time and money.



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