Mechanical parts are made so they are interchangeable. In other words, each mechanical part or device is made into a certain size and shape to be suitable for other types of machines. In order to make the parts interchangeable, each part is made into a size to match the corresponding parts in the correct way. This is not only impossible, but also impractical for many parts to be made into one size. This is because the machine is not perfect and the tools wear out. A small deviation from the correct size is usually allowed. The size of the deviation depends on the type of parts to be made. For example, a part may be 6 inches, and the upper and lower deviation is 0003 inches (one third of a thousand). Therefore, the deviation can be between 5997 inches and 6003 inches, and the correct size can still be maintained. This is the deviation. The difference between the upper deviation and the lower deviation is the tolerance.
The machining tolerance of precision parts is the maximum variation of part size, and the basic size is the size limit derived from the allowable variation and tolerance range. Sometimes the deviation only allows one direction change, it allows the tolerance to change on the hole or shaft without seriously affecting the fit. When the tolerance of precision parts changes in both directions, it is called complete deviation (positive and negative). The complete deviation is separate and will be on each side of the basic size. But the limit size only has the maximum size and the minimum size. Therefore, the tolerance is the difference between the two dimensions.