go to the page above this one “Tools etc”
Tools – gear tooth calipers
Gear tooth calipers are a tool specially made for measuring gear teeth.
It can be seen, in the photo above, that one caliper points in the 5-11 direction. It is rather like an ordinary caliper with two jaws except the jaws have a slit in each of them. The other caliper points in the 8-2 direction but instead of having any jaws it just has a flat tongue that fits into the slits in the jaws of the other caliper. The tongue is so wide that in the above photo it can be seen protruding below the bottom of the bottom jaw of the other caliper.
It can be seen that this tool essentially consists of a pair of calipers at right angles to each other. The idea of this is that the width of a tooth can be measured at any particular depth on the tooth. In practice we are never sure where the bottom of the tooth is. The only point we can measure the tooth and work out what the width should be is on the pitch circle diameter. It would appear that this cannot tell us the size of the gear or its pressure angle. Really we need to know the size of the teeth or what they are meant to be. We can then measure the teeth to see if they are correct.