Mechanical Component Tolerance
Dimensional tolerance is related to, but different from fit in mechanical engineering, which is a designed-in clearance or interference between two parts. Tolerances can be applied to any dimension. The commonly used terms are:
- Basic size: the nominal diameter of the shaft (or bolt) and the hole. This is, in general, the same for both components.
- Lower deviation: the difference between the basic size and the minimum possible component size.
- Upper deviation: the difference between the basic size and the maximum possible component size.
- Fundamental deviation: the minimum difference in size between a component and the basic size. This is identical to the upper deviation for shafts and the lower deviation for holes. If the fundamental deviation is greater than zero, the bolt will always be smaller than the basic size and the hole will always be wider. Fundamental deviation is a form of allowance, rather than tolerance.
- International Tolerance grade: this is a standardised measure of the maximum difference in size between the component and the basic size (see below).
For example, if a shaft with a nominal diameter of 10 mm is to have a sliding fit within a hole, the shaft might be specified with a tolerance range from 9.964 to 10 mm (i.e. a zero fundamental deviation, but a lower deviation of 0.036 mm) and the hole might be specified with a tolerance range from 10.04 mm to 10.076 mm (0.04 mm fundamental deviation and 0.076 mm upper deviation). This would provide a clearance fit of somewhere between 0.04 mm (largest shaft paired with the smallest hole, called the "maximum material condition") and 0.112 mm (smallest shaft paired with the largest hole). In this case the size of the tolerance range for both the shaft and hole is chosen to be the same (0.036 mm), meaning that both components have the same International Tolerance grade but this need not be the case in general.
When no other tolerances are provided, the machining industry uses the following standard tolerances:
| 1 decimal place | (.x): | ±0.2 |
| 2 decimal places | (.0x): | ±0.01 |
| 3 decimal places | (.00x): | ±0.005 |
| 4 decimal places | (.000x): | ±0.0005 |
Read more about this topic: Engineering Tolerance
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