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Tooth thickness of the internal gear can be calculated with equations 9 and 20, but one must remember that the tooth and space thicknesses are reversed, (see Figure 1.35). Also, in using equation 10 to calculate tooth thickness at various radii, (see Figure 1.36), it is the tooth space that is calculated and the internal gear tooth thickness is obtained by a subtraction from the circular pitch at that radius, Thus, applying equation 10 to Figure 1.36,

7.3 Teeth Thickness Measurement

In a procedure similar to that used for external gears, tooth thickness can be measured indirectly by gaging with pins, but this time the measurement is "under" the pins, as shown in Figure 1.37. Equations 11 thru 13 are modified accordingly to yield:
      For an even number of teeth:
        M= 2 ( R_c - d_w )                                                                         (38)
                           2
      For an odd number of teeth:
        M = 2(R_c cos 90º - d_w )                                                              (39)
                            N      2
      inv f₁=inv f + p - T -    d_w  
                           N      D     Dcos f

where:
        R_c = cos f   R
                cos f₁

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