Dimensional tolerance, referred to as tolerance for short, refers to the absolute value of the difference between the maximum limit size and the minimum limit size, or the difference between the upper deviation and the lower deviation. This is the allowable size variation. Dimensional tolerance is an unsigned absolute value. Limit deviation = limit size-basic size, upper limit deviation = maximum limit size-basic size, and lower limit deviation = minimum limit size-basic size.
In the manufacturing process of parts, due to the influence of processing or measurement, there will always be some errors in the actual dimensions after completion.
In order to ensure the interchangeability of parts, the actual size of parts must be controlled within the allowable range, which is called dimensional tolerance.
Example:
1, basic size design given size: 30mm.
2, the limit size allows two limit values of size change:
Maximum limit size = 30+0.0 1 = 30.0 1 mm.
Minimum limit size = 30-0.0 1 = 29.99 mm.
3. Limit deviation, the algebraic value obtained by subtracting the basic size from the limit size. That is, the algebraic difference between the maximum limit size and the minimum limit size MINUS the basic size is the upper deviation and the lower deviation respectively, which are collectively called limit deviation. The upper and lower deviations of holes are indicated by capital letters es and EI respectively:
Upper deviation es = 30.01-30 =+0.01.
Lower deviation EI=29.99-30=-0.0 1.
4. The size tolerance allows the size change, that is, the maximum limit size minus the minimum limit size is also equal to the algebraic difference obtained by subtracting the upper deviation from the lower deviation.
Dimensional tolerance is an unsigned absolute value.