Tolerance is the minimum and maximum limit within which a dimension could exist on a physical part. It is nearly impossible to create a physical part meeting the exact dimensions, hence a tolerance is specified in the drawing / CAD data to let the operator know the dimension limits. Also, the manufacturing cost is directly proportional to the tolerance required. The closer the tolerances, the higher will be the cost and vice versa. During the inspection, if the manufactured part is found to be beyond the defined limit, it is entirely rejected or sent to rework.
Tolerances in CNC machining
Generally, there are two types of tolerances used in CNC machining, one w.r.t to tolerance capability of the CNC machine and the second is the dimensional tolerance limit within which the dimensional parameter could exist. Tolerance w.r.t to CNC machine means the minimum tolerance or accuracy of the dimensional parameters that the CNC could achieve. Nowadays, with the advancement of technology, certain special CNC machines can achieve part dimensional accuracy within 10 microns. Generally available CNC machines, on average, can achieve an accuracy of around 25 microns. CNC machine manufacturers specify the accuracy achieved by their machines in the technical specification guide.
Tolerances in product design are defined by the product designer within which the dimensional parameter should exist in order to be acceptable. Part dimensional tolerances are determined by the designer according to the application, fitment and overall form of the design. Dimensional accuracy is more significant in a multi-part assembly where different parts are mating with other components. For example, a car engine is an assembly of multiple individual components such as shafts, pistons, and valves, and each part is manufactured to the closest possible tolerance to achieve desired performance requirements. Engine parts are made with much tighter tolerances than a hand brake handle because the performance requirements of engine parts are much higher than the hand brake handle. Part tolerance is generally captured closer to the concerned dimensions in the drawing and as a 3D annotation in the CAD file of the part.
Type of tolerances
- Limit tolerance
- Unilateral tolerance
- Bilateral tolerance
- GD&T method
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