Thread Micrometer Caliper (Metrology)

2.43.
This is just like an ordinary micrometer with the difference that it is equipped with a special
anvil and spindle. The anvil has an internal vee which fits over the thread. The anvil in this case
is not fixed but is free to rotate. Thus vee of the anvil can accommodate itself to any rake range of
thread. The spindle on the other hand has a ground conical shape. When the conical spindle is
brought into contact with the vee of anvil, micrometer reads zero. Different set of anvils are provided
for different types of threads and the contact points of the micrometer are so designed that some
allowance for thread clearance is always made.
Thread micrometer is used for the measurement of the pitch diameter but the accuracy is
influenced by the helix angle of the thread.
Screw thread micrometer caliper (Refer Fig. 2.129). It is used for accurate measurement of
pitch diameter of screw threads. The micrometer has a pointed spindle and a double V-anvil, both
correctly shaped to contact the screw thread of the work being gauged. It directly reads in terms of
pitch diameter as the zero reading of the micrometer corresponds to the closed position of anvil and
spindle when both are in perfect match with each other.

Fig. 2.129. Screw Thread Micrometer Caliper.
The angle of the V-anvil and the conical point at the end of the spindle corresponds to the
included angle of the profile of the thread. The V-anvil is allowed to swivel in the micrometer frame
so that it can accommodate itself to the helix angle of the thread. The extreme point of cone is
rounded so that it will not bear on the root diameter at the bottom of the thread, and similarly
clearance is provided at the bottom of the groove in the V-anvil so that it will not bear on the thread
crest. The spindle point of such a micrometer can be applied to the thread of any pitch provided the
form or included angle is always the same. The V-anvil is however limited in its capacity; a number
of different blocks being required to cover a full range.