PRECISION MEASURING AND GAGING - OD1642 - LESSON 1/TASK 2
(n) Chordal Thickness (tc): (1) The thickness of the tooth measured
at the pitch circle. (2) The section of the tooth that is measured to see
if the gear is cut correctly.
(o) Chord Addendum (ac): The distance from the top of a gear tooth
to a chord (pitch chord) subtending (to extend under) the intersections of
the tooth thickness arc and the sides of the tooth (used for setting the
gear tooth vernier calipers for measuring tooth thickness).
(p) Diametral Pitch (DP): (1) The most important calculation, it
regulates the tooth size.
(2) The ratio of the number of inches of the
pitch diameter. (3) The number of teeth that will go into each inch of the
pitch diameter evenly.
(q) Number of Teeth (NT): The actual number of teeth of the gear.
(r) Backlash (B): The difference between the tooth thickness and
the tooth space of the engaged gear teeth at the pitch circle.
(3) Use and Application.
When selecting gear cutters for a gear
having 24 teeth, a number 5 cutter would be used. The reason for this is
that this particular cutter will cut all gears containing from 21 to 25
However, for measuring these teeth, certain values must be known.
To check the dimensional accuracy of gear teeth, a gear tooth vernier
caliper is used (figure 51, view B, on page 80).
The vertical scale is
adjusted to the Chordal Addendum(ac), and the horizontal scale is used for
finding the Chordal Thickness(tc). Before calculating the chordal addendum
and performing any measurements, the tooth thickness (t) and addendum (a)
must first be determined. The following formulas must be applied:
(a) The formula for the addendum is