Even the best machine, using the best cutting tools, will eventually show some variation in output. This is due to factors such as tool pressure and wear. Therefore, machining tolerance standards are essential in manufacturing. This is especially true for mating parts or parts that must fit together. For example, a bushing that is supposed to fit onto a rod may have a tolerance of ±0.002″. If the inside diameter is cut too small, the bushing will not fit. If it's cut too large, the fit will be sloppy. The part may be unusable.

In terms of machining, the definition of acceptable part variation means：

Less waste: You'll spend a lot of money making unusable products if you have to scrap every part that doesn't meet the exact design specifications. By setting tolerances in the machining design process, you can allow for variation, so you get more usable parts without wasting resources.

Reduce costs: When a tight band is not required, tight machining tolerances can lead to unnecessary expenditure. Increased accuracy is always a positive. However, a tolerance band that is tighter than the project requires can consume significant resources. In some cases, efficiency and costs can be minimised by choosing a slightly looser band.

Faster changeovers: Tolerance standards for machining mean that parts can be interchangeable. If a machine component fails, the part can be easily replaced. This minimises downtime.

Quality assurance: The "wiggle room" that machining tolerances provide is often the focus. However, machining tolerances also establish strict guidelines for when a product is serviceable and when it is not. These standards improve end-user satisfaction. They also prevent problems associated with inconsistent parts.