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Engineering Tolerance can be defined as the allowable variation for any given size in order to achieve a proper function. This means that there are certain qualities such as dimension or property, which may vary within the limits of a design without significantly affecting the function of the equipment or process. In architectural CAD courses, tolerances are useful for determining the physical property of a manufactured object, or the physical distance of space necessary between a boat and bridge or a train and a tunnel. There are several considerations which an engineer must make when setting tolerances, in order to ensure they are accurate and successful.

Measurement Error

An engineer cannot guarantee that a machinist will build parts which are the exact measurement which they have laid out. A machinist is also not aware of how machine parts will interact nor are they supposed to. It is the engineer—in many cases using 3D CAD training—who must create tolerance bands which account for any manufacturing issues, even if it is just a few thousandths off of the required dimensions. Measurement error is an important tolerance to account for because, without the proper fit, a design will not function.

Tolerance Analysis

Tolerance analysis is the approach to understand how imperfections in parts and assembly affect the overall capability of the product. In performing tolerance analysis, there are two different analysis tools: worst-case analysis and statistical analysis. Worst-case tolerance analysis puts individual variables at their tolerance limits, in order to predict the maximum expected variation of the measurement. Statistical tolerance analysis takes the variation of a set of inputs to calculate the expected variation of an output. CAD drafting courses experiment with something called digital tolerance, which uses the model-based definition to annotate computer-aided designs with tolerancing information, allowing engineers to make changes right on the 3D model.

Experimenting with Tolerances

In the world of engineering tolerances, a design of experiments is design for the purpose of information-gathering in the face of variations. This is for most cases a controlled experiment using various factors which could affect the quality or function of the engineering product. For example, you may need to test your machine under heat, to see if you must allocate for expansion in your design. You may also need to take into account any finishing process which will be applied to the final product, which could also affect tolerance, even if just by a decimal. When making a machine part, the machine shop may have a standard tolerance of three decimal places, meaning a shaft you have built for one size may be suddenly three decimal places too big or small and cannot fit into another part. To avoid this issue, it is best for machine shops to have a standard of verifying non-toleranced dimensions. As you can see, fit is one of the most crucial components regarding tolerance, and the proper tolerance calculations mean the design will fit and be functional.