Micrometer – Types , Diagram , Parts , How to Read Micrometer

The accuracy of vernier caliper is 0.02 mm. Most engineering precision works have to be measured to a much greater accuracy than this value in order to
achieve the interchangeability of component parts.
In order to achieve this greater precision measuring equipment of a greater accuracy and sensitivity must be used. Micrometer is one of the most common and
most popular forms of measuring instrument for precise measurement with 0.01 mm accuracy.

However micrometer with 0.001 mm accuracy are also available.

Types Of Micrometers : 

Micrometer may be classified as
(a) Outside micrometer
(b) Inside micrometer
(c) Screw thread micrometer
(d) Depth gauge micrometer

See also: Introduction To Engineering Metrology -Types Of measurements

Principle of micrometer
Micrometers work on the principle of screw and nut. We know that when a screw is turned through nut by one revolution it advances by one pitch distance if the
circumference of the screw is divided into equal parts its rotation through one division will cause the screw to advance through length . thus the minimum length that can by measured by such arrangement will be . so either by reducing the pitch of the screw thread or by increasing the number of division on the circumference of screw, the length value corresponding to one circumferential division can be reduced and consequently the accuracy of measurement can be increased.

Least count of micrometer

Least count of micrometer= pitch of the spindle screw / no of division in the spindle

Outside micrometer
Figure illustrate an outside micrometer. It is used to measure the outside diameter and length of small parts to accuracy of 0.01mm. The main parts of an
outside caliper are:
1. U shaped steel frame
2. anvil & spindle
3. lock nut
4. sleeve or barrel
5. thimble
6. ratchet

micrometer diagram
micrometer diagram

1.U shaped steel frame
The outside micrometer has U shaped or C shaped frame. It holds all the micrometer parts together. The gap of the frame permits the maximum diameter or length of the job to be measured. The frame is generally made of steel, cast iron, malleable cast iron or light alloy. It is desirable that the frame of the micrometer be provided with conveniently placed finger grips of heat insulting materials.

2.Anvil and spindle
The micrometer has a fixed anvil protruding 3mm from the left hand side frame. The diameter of the anvil is the same as the diameter of spindle. Another movable anvil is provided on the front of the spindle. The anvils are accuracy ground and lapped with its measuring faces flat and parallel to the spindle. These are also available with WC faces. The spindle is the movable measuring face with the anvil on the front side. The spindle engages with the nut. It should run freely and smoothly through out the length of its travel. There should be no backlash between the spindle screw and nut. There should be full engagement of nut& screw when the micrometer is at its full reading.

3.Lock nut
A lock nut is provided on the micrometer spindle as shown in fig , to lock it when the micrometer is at its correct reading. The design of the locknut is such that it
effectively locks the spindle without altering the distance between the measuring faces. It thus retains the spindle in perfect alignment.

4.Sleeve or Barrel:
The sleeve is accurately divided and clearly marked in 0.5mm division along its length which serves as a main scale. It is chrome plated and adjustable for zero
setting.

5.Thimble:
The thimble can be moved over the barrel, it has 50 equal divisions around its circumference.

6.Ratchet:
The ratchet is provided at the end of the thimble. It is used to assure accurate measurement and to prevent too much pressure being applied to the micrometer.
When the spindle ratches near the work surface to be measured the operator uses the ratchet screw to tighter the thimble. The ratchet automatically slips when the
correct (uniform) pressure is applied and prevents the application of too much pressure.
The micrometer usually has a maximum opening of 25 mm. They are available in measuring ranges of 0 to 25 mm, 25 to 50 mm, 125 to 150 mm upto 575 to 600 mm.

Procedure to take micrometer reading

The following procedure is followed while measuring the dimension with the help of micrometer.
1. Micrometer is selected with a desired range suitable for w/p
2. checking of zero error.

In case of 0.25 mm micrometer the zero error is checked by contacting the faces of the fixed anvil and the spindle. While using micrometer of 25-50 mm or 125 mm to 150 mm size the zero error is checked by placing a master of 25 mm or 125 mm respectively between the anvil & spindle.

Checking of zero error means the zero error means the zero of the thimble should coincide with zero on main scale. If it does not happen then zero error is present in
the micrometer. A special spanner is usually provided with the micrometer for eliminating the zero error.

3. For measuring the particular dimension the w/p is first held between the faces of anvil and spindle. Then the spindle is moved rotating the thimble
until the anvil and spindle touches the work surface. Fine adjustment is made with the ratchet. Now the reading on the main scale is noted. Let it be 11.00 mm.

4. Subsequently the thimble reading which coincide with the reference line is taken let it be 34.
5. Now total reading= main scale reading+ L.Cx reading on the thimble is 11.00+ 0.01 x 34=11.34 mm

PRECAUTIONS  TO BE TAKEN WHILE USING A MICROMETER
1. First clean the micrometer by wiping of oil, dirt, dust and grit etc.
2. Clean the measuring faces of the anvil a spindle with a clean piece of paper or cloth.
3. Set the zero reading of the instrument before measuring.
4. Hold the part (whose dimension is to be measured) and micrometer properly. Then turn the thimble with the forefinger and thumb till the measuring tip just
touches the part and fine adjustment should be made by ratchet so that uniform measuring pressure is applied.
5. While measuring dimensions of circular parts the micrometer must be moved carefully over representative are so as to note maximum dimension only.

POSSIBLE SOURCES OF ERRORS IN MICROMETER
Some possible sources of errors are :
1. lack of flatness of anvil & spindle surfaces.
2. lack of parallelism of the anvil at some or all parts of the scale.
3. inaccurate setting of zero reading (zero error)
4. inaccurate reading shown by the fractional divisions on the thimble.
5. applying too much pressure on the thimble or not using the ratchet.
6. wear of the anvil surface threads on the spindle due to constant or incorrect use.
7. wear of ratchet stop mechanism locking arrangement etc.

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