Statistical tolerancing

Statistical tolerancing is the assigning of tolerances to related components of an assembly on the basis of sound statistics (such as the assembly tolerance is equal to the square root of the sum of the squares of the individual tolerances).

  1. Application to assemblies
  2. Identification

Application to assemblies

The tolerances assigned to component items of an assembly are determined by arithmetically dividing the assembly tolerances among the individual components of the assembly. Where tolerances assigned by arithmetic stacking are restrictive, statistical tolerancing may be used for increased individual feature tolerance. The increased tolerance may reduce manufacturing cost, but shall only be employed where the appropriate statistical process control will be used. For application see appropriate statistics or engineering design manuals.


Identification

Statistical tolerances on dimensions are designated as illustrated in Figs. 2-24 through 2-26.

(a) A note such as the following shall be placed on the drawing: “FEATURES IDENTIFIED AS STATISTICALLY TOLERANCED SHALL BE PRODUCED WITH STATISTICAL PROCESS CONTROLS.” See Fig. 2-24.

(b) It may be necessary to designate both the statistical limits and the arithmetic stacking limits where the dimension has the possibility of being produced without statistical process control (SPC). A note such as the following shall be placed on the drawing: “FEATURES IDENTIFIED AS STATISTICALLY TOLERANCED SHALL BE PRODUCED WITH STATISTICAL PROCESS CONTROLS, OR TO THE MORE RESTRICTIVE ARITHMETIC LIMITS.” See Fig. 2-25.

CAUTION: Where using the statistical tolerancing symbol, the necessary statistical indices should be specified.


  • References: