Thread Micrometer
Thread Micrometer
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. 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.
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.