Basics of Coordinate Metrology
Unit 6:  Basics of Metrology - Dimensions and Tolerances

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To make sure that the parts of the technical drawing can be manufactured correctly, the design engineer enters the individual dimensions on the drawing.
Since the design engineer knows that it is impossible to manufacture with 100% precision, he additionally enters the tolerances for the dimensions:
Figure 1: Figure 2:  
The following dimensions are used:
   
Figure 1 (left): 		    
Figure 2 (right): 
19.8 mm Min. dim. 20.1 mm
20.0 mm Nom. dim. 20.0 mm
20.2 mm Max. dim. 20.2 mm
0.4 mm Tol. width 0.1 mm
This means that the part may have
  • in the left figure: a side length of at least 19.8 mm and at most 20.2 mm.
  • in the right figure: a side length of at least 20.1 mm and at most 20.2 mm.