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# Introduction of Dimensional Deviation

Mar 23, 2019

A. Dimensional deviation

Dimensional deviation is the algebraic difference of a size minus its nominal size, which can be divided into actual deviation and limit deviation.

1. Actual deviation.

The deviation from the actual size minus its nominal size is called the actual deviation. The actual deviation is denoted by "Ea" and "ea".

2. Limit deviation.

The algebraic difference between the limit size and its nominal size is called the limit deviation. Limit deviation has upper limit deviation and lower limit deviation. The upper limit deviation is the algebraic difference of the upper limit size minus the nominal size, and the lower limit deviation is the algebraic difference of the lower limit size minus the nominal size. The deviation value is an algebraic value, which can be positive value, negative value or zero. The upper limit deviation of the hole and axis is expressed by "ES" and "es" respectively, and the lower limit deviation of the hole and axis is expressed by "EI" and "ei" respectively.

B. Dimensional tolerance

1.  Dimensional tolerance.

Dimensional tolerance is the variation of allowable dimensions. The dimensional tolerance is equal to the absolute value of the algebraic difference between the upper limit size and the lower limit size, and the absolute value of the algebraic difference between the upper limit deviation and the lower limit deviation. Tolerance is the absolute value, cannot be negative, cannot be zero (tolerance is zero, parts will not be processed). The tolerance of hole and shaft is expressed by "Th" and "Ts" respectively

2. Standard tolerance

The tolerance value specified in the national standard to determine the size of the strip is the standard tolerance. Tolerance zone in the tolerance zone diagram, represented by the limit deviation and the lower limit deviation or size limit and lower limit on the size of an area defined by two straight lines. It is determined by the magnitude of the tolerance and its position relative to the zero line as the fundamental deviation (as shown in figure). 