What is Micrometers

The micrometers are commonly employed for measuring small dimensions with extreme accuracy of 0.01 mm. They may be of the
three kinds 

(a)  External micrometer for measuring external dimensions,

(b)  Internal micrometer for measuring internal dimensions, and

(c)  Depth micrometer for measuring depths.

For measuring a dimension in external micrometer, the work piece is held between the fixed anvil face and the spindle face
of the micrometer. The spindle of the micrometer is allowed to move linearly towards the work by rotating thimble.

When the spindle will touch the work piece properly, the ratchet will give its sound. The small locking lever is then  rotated to clamp the spindle so that reading can be taken more accurately. Outside micrometers are used for measuring the outside dimensions of jobs, such as diameter of a bar, rod and thickness of plate. Generally, until and unless they are provided with the vernier attachment, the former can read up to 1/1000 or 0.001 inch and the latter up to 0.01 mm.

The former are known as inches micrometers and the latter metric micrometers, which are gradually replacing the former due to the introduction or adopting of metric system. Inside micrometers are commonly used for measuring inside dimensions of the objects, such as inside dia. of a hole, width of a slot or cavity, etc.

The outside micrometers are the most extensively used in industrial applications.
All the micrometers, irrespective of the fact as to whether they carry graduations in inches or millimeters, are similar in construction. An out side micrometer is discussed as under.


Fig. shows an outside micrometer. It consists of the following main parts.

1. Metallic frame

2. Axial graduated sleeve

3. Circumferential screwed spindle

4. Hardened steel anvil

5. Thimble

6. Ratchet stop screw

7. Lock nut

Outside micrometer
Outside micrometer

Micrometer works commonly on the principle of nut and bolt assembly. The sleeve carries inside threads at the end, which forms the nut, and the screwed part of the spindle passes through it. The spindle and the thimble are secured to each other such that by rotating the thimble the spindle rotates. With the result, when the thimble is revolved, it advances towards or retards away from the fixed anvil, together with the spindle of the micrometer. 

The sleeve carries the graduations, which, in conjunction with the beveled and graduated part of the thimble, give the measure of
the opening between the end faces of the anvil and
the spindle. The ratchet arrangement provided at the end of the thimble prevents the spindle from pressing further against the surface of the piece being measured after the required feel has been attained, thus
facilitating a uniform reading and preventing the instrument from being damaged. Lock nut or locking lever is used for locking the micrometer for a desired amount
of time after taking or setting the reading. The construction of the outside micrometer is discussed as under.


(1)  Frame. 

The U frame of micrometer is made of steel, cast steel, malleable cast iron  or light alloy.

(2)  Hardened anvil. 

It protrudes from the frame for a distance of at least 3 mm for holding and supporting the jobs for measurement.

(3)  Screwed

It does the actual measuring and possesses threads of 0.5 mm pitch.

(4)  Barrel or

It has datum or fiducially line and fixed graduations.

(5) Thimble. 

This is a tubular cover fastened with the spindle and moves with the spindle. The beveled edge of the thimble is divided into 50 equal parts, every fifth being numbered.

(6) Ratchet. 

This part is commonly recognized as friction stop  of  the  micrometer, which acts as a precautionary measure also. It is a small extension to the thimble in which the ratchet slips when the pressure on the screw exceeds a certain amount. This produces uniform reading and prevents any damage or distortion of the instrument.

(7)  Spindle

It is used to lock the instrument at any desired setting or at any particular reading .


It works on the fine assembly of nut and bolt principle where pitch of both nut and bolt plays a big role. The graduation on the barrel of micrometer is in two parts, namely one above the reference line and the other below. The higher line graduation above the reference line is graduated in 1 mm intervals.

The first and every fifth are long and numbered 0, 5, 10, 15, 20 and 25. The lower or small graduations are graduated in mm intervals but each graduation shall be placed at the middle of the two successive upper graduations to be read 0.5 mm. The micrometer screw has a pitch of 0.5 mm, while the thimble has a scale of 50 divisions round its circumference. 

Thus, on making or rotating through one complete turn, the thimble  moves forward or backward by one thread pitch of 0.5 mm, and one division of its scale is, therefore, equivalent to a longitudinal movement of 0.5 × 1/50 mm = 0.01 mm. It is the value of one division on the thimble, which is the least that can be correctly read with the help of    a micrometer and is known as the least count. For measurement, the job is kept between    the end of the spindle and the fixed anvil, which is fitted to the frame. When the micrometer is closed, the line marked 0 (zero) on the thimble coincides with the line marked 0 (zero) on the graduated sleeve.

In metric outside micrometer, the pitch of the spindle screw is 0.5 mm and the graduations provided on the spindle of the micrometer are in millimeters and subdivided into 0.5 mm. Now in one turn of the thimble of the micrometer, owing to the 0.5 mm. pitch of the spindle screw, the spindle will move through 0.5 mm and therefore, the corresponding opening between the faces of the fixed anvil and the spindle will be 0.5 mm. This opening will go on increasing by the same distance 0.5 mm for each further rotation of the thimble. The beveled edge of the thimble carries 50 equal divisions on its periphery in which every 5th division is marked. It is seen above that for one complete turn of the thimble  the spindle moves through 0.5 mm. Now let the thimble is rotated one small division on its beveled edge i.e. 1/50 of the turn. The corresponding displacement of the spindle will then be 0.5 × 1 / 50 = 0.01mm.

Depth micrometer is used for measuring depth of holes and is shown in Fig.1. Screw thread micrometer (Fig.2) is used to measure the
pitch diameter of the thread to an accuracy of 0.01mm and 0.001 inches. It comprises of similar parts as that of outside micrometer accept the shapes of fixed and moveable anvils. The fixed and moveable anvils possess the thread profiles for thread  adjustment for measurement of the pitch diameter.

A depth micrometer
Fig. 1: A depth micrometer


A screw thread micrometer
Fig.2: screw thread micrometer



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