Stick Micrometer
Stick
Micrometer
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.
(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. 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 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., 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., 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.