Chapter 3 Datums 79 Datum feature symbols are commonly drawn using thin lines with the symbol size related to the drawing lettering height. The triangular base on the datum feature symbol can be filled or unfilled, depending on the company or school preference. The filled base helps easily locate these symbols on the drawing. Each datum feature on a part requiring identification must be assigned a different datum identification letter. Uppercase letters of the alphabet, except the letters I, O, and Q, are used for datum feature symbol letters. These letters are not used because they can be mistaken for numbers. Example 3-1 shows the specifications for drawing a datum feature symbol. Symbol Specifications Examples H = Letter height 2H 2H H H 60° Any needed length Optional shoulder Identification letter Filled Unfilled Example 3-1. Drawing specifi cations for datum feature symbols. Datum Feature The datum feature is the actual feature of the part that is used to establish the datum. When the datum feature is a surface, it is the actual surface of the object that is identified as the datum. Look at the magnified view of a datum feature placed on the simulated datum in Example 3-2. Study the following terms: Actual mating envelope: The smallest size that can be contracted about an external feature or the largest size that can be expanded within an internal feature. Datum feature: The actual feature of the part (such as a surface). Datum feature simulator: The opposite shape of the datum feature. The datum feature simulator is one of two types: 1. The theoretical datum feature simulator is a perfect boundary used to establish a datum from a specifi ed datum feature. 2. The physical datum feature simulator is the physical boundary used to establish a simulated datum from a specifi ed datum feature. The manufacturing inspection equipment associated with the datum feature or features is used as the physical object to establish the simulated datum or datums. Physical datum feature simulators represent the theoretical datum feature simulators during manufacturing and inspection.