Stick Micrometers (Metrology)

Stick micrometers are designed for the measurement of longer internal lengths. These
comprise of the following parts :
(a) A 150 mm or 300 mm micrometer unit fitted with a micrometer of 25 mm range and
having rounded terminal faces.
(b) A series of extension rods which together with the micrometer unit, permit of a continuous
range of measurements upto the maximum length required.
How the stick micrometer will look like is shown Fig. 2.126. Its accuracy is of the order of
± 0.005 mm throughout the range. It is essential that the radius of curvature of the terminal faces
Stick micrometer
Fig. 2.126. Stick micrometer.
of the micrometer unit be slightly less than one-half the smallest measuring range of the micrometer
unit, and abutment faces should have minimum radial width of 2 mm and properly lapped ; these
should be parallel to each other and normal to the axis of the micrometer unit to within 0.005 mm
across their diameters.
It may be noted that screwed joints are used for joining the end piece, extension rod and the
measuring unit. The screw units generally have threads of 0.5 mm pitch. The extension rod is
generally hollow and has a minimum external diameter of 14 mm. In the design of extension rod
shown in Fig. 2.126, it will be noted that the contact faces come together by screwing and the
pressure between the contact faces depends upon the heavy or light tightening of the screw. In order
to overcome this difficulty, the extension rods of type shown in Fig. 2.127, are generally employed.
This type consists of length standards which are mounted in protective bushing and when these
extension rods are combined, the contact faces of the length standards come together under constant
pressure and thus the measurement is not affected by heavy tightening of the screw. The accuracy
of this instrument is of the order of ± 0.005 mm, i.e. the micrometer unit with terminal piece in
position should record correctly throughout its range to within ± 0.005 mm. In this type of
micrometer, there should be sufficient play between the external and internal threads of the joint
to permit the abutment faces of the various parts of the micrometer to butt together solidly.
Improved extension rods of stick micrometer
Fig. 2.127. Improved extension rods of stick micrometer.

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