Micrometer

A micrometer is another useful device for magnifying small measurements. In micrometer, the accurate screw and nut are used for measurement. The micrometers having good accuracy (about 0.01 mm), hence is used for most engineering precision work.

Principle of micrometer

• Micrometers works on the principle of screw and nut. The screw is attached to a concentric cylinder or thimble the circumference of which is divided into a number of equal parts. We know that when a screw is turned through a nut by one revolution, its axial movement is equal to the pitch of the thread of a screw.
• If I am pitch or lead of screw-in mm, each rotation of screw advances it in relation to the internal threads a distance equal to I mm. If the circumference of the concentric cylinder is divided into n equal divisions, movement (rotation) of a cylinder through one division indicates 1/n rotation of the screw or 1/n mm axial advance.
• In millimeter micrometer instruments the screw has a pitch (lead) of 0.5 mm and thimble has 50 divisions so that the least count of the micrometer is equal to 0.5/50 =mm. By reducing the pitch of the screw thread or by increasing the number of divisions on the thimble, the axial advance value per one circumferential division can be reduced and the accuracy of measurement can be increased.

See also: Introduction To Engineering Metrology -Types Of measurements

Least count of micrometer

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

Outside micrometer parts : 

The figure illustrates an outside micrometer. It is used to measure the outside diameter and length of small parts to an 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

1.U shaped steel frame

The outside micrometer has a 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-insulating 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 the 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 throughout the length of its travel. There should be no backlash between the spindle screw and nut. There should be a full engagement of nut& screw when the micrometer is at its full reading.

3.Locknut

• 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 the main scale. It is chrome plated and adjustable for zero settings.

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 from 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.

Range Of Micrometer : 

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 up to 575 to 600 mm.

Procedure to take micrometer reading

Procedure for measuring dimensions with the help of micrometer

1. Select a micrometer with a desired range of measurement.
2. Check the micrometer for zero error. It can be check by contacting the faces of anvil and spindle. The zero on the thimble should coincide with zero on the reference line on the barrel (main scale), as shown in Fig. If this does not happen, then zero error is present in the micrometer which must be accounted for in final readings.

3. Hold workpiece which dimension to be measured between anvil and spindle as shown in Fig.  Then make a final adjustment by rachet and lock it with the help of Jock nut.

4. Now, take the reading of the main scale, suppose main scale reading is 13 mm as shown in Fig. Take the thimble reading which coincides with the reference line on the barrel. The 35th division line is coincident with the reference line.

Total reading = Main scale reading + (L.C. x reading on thimble)
= 13.5 + 0.01 X 35
= 13.850 mm

Precautions to be taken while using a micrometer:

1. The micrometer is available in various sizes and ranges, and the corresponding micrometer should be selected depending upon the size of the workpiece.
2. Micrometer should be cleaned of any dust and spindle should move freely.
3. Check and set the zero reading before measuring.
4. The workpiece whose dimension is to be measured must be held in the left hand and micrometer in the right hand.
5. Make sure that the dimension to be measured in parallel to the axis of the spindle and the anvil.
6. Always use the rachet for final adjustment and locknut for taking readings.
7. In case of measurement of diameter make sure that the anvil and spindle faces touch the maximum dimension only.

Sources of error in micrometers

1. The faces of anvil and spindle may not be truly flat.
2. Lack of parallelism and squareness of anvil or spindle at some or all parts of the scale.
3. The setting of zero reading may be inaccurate.
4. Inaccurate readings have shown by fractional divisions on the thimble.
5. Wear on the faces of anvil & spindle, and wear in the threads of the spindle.
6. Error due to too much pressure on the thimble or not using the ratchet.

Types of Micrometers

There is a number of different micrometers available for specific application and accuracy.
1. Outside micrometer
2. Inside micrometer
3. Vernier micrometer
4. Depth micrometer
5. Bench micrometer
6. Digital micrometer
7. Differential screw micrometer
8. Micrometer with dial gauge
9. Screw thread micrometer

Inside micrometer

• The inside micrometer is used to measure the internal dimensions of the workpiece. Fig. 1.25 shows the inside micrometer, the construction is similar to the outside micrometer. However, the inside micrometer has no U-shape frame and spindle. The measuring tips are constituted by the jaws whose faces are hardened and ground to a radius.
• One one of the jaw is held stationary at the end and the second one moves by the rotation of the thimble. The locking arrangement is provided with a fixed jaw. Fig. 1.26 shows another inside micrometer is used for a larger internal dimension. It consists of two anvils, sleeve, thimble, rachet, stop, and extension rods.

• The range of this micrometer is 50 mm to 210 mm. however, the range can be increased by anyone extension rod provided with it. This micrometer has no frame and spindle. The measuring points are at Extreme ends provided with anvils. The axial movement of endpoints is taken place by thimble rotation about the barrel axis.
• A series of extension rods are provided in order to obtain a wide measuring range. Before taking the measurement, the approximate internal dimension of a workpiece (whose dimension is to be measured by inside micrometer) is measured by a scale.
• The extension rod is then selected to the nearest one and inserted in a micrometer head.
• Then, the micrometer is checked for zero error with the help of a standard-sized specimen whose internal dimension is known. The micrometer is then adjusted at a dimension slightly smaller than the internal (bore) diameter of the workpiece.
• The micrometer head is then held finely against the bore as shown in Fig. 1.27 and other contact surface is adjusted by moving the thimble till the correct feel is sensed. The micrometer is then removed and reading is taken. The lengths of extension rod and collar are added to the micrometer reading.

Vernier micrometer

• In order to increase accuracy, the vernier principle also is applied to an outside micrometer. This type of micrometer can be read by 0.001 mm length.
• The vernier micrometer as shown in Fig.consists of three scales as follows:

1. The main scale is graduated on the barrel with two sets of division marks. The set below the reference line reads in mm and set above the line reads in 1/2 mm.
2. A thimble scale is graduated on the thimble with 50 equal divisions. Each small division of thimble represents 1/50 of a minimum division of the main scale. The main scale minimum division value is 1/2 mm.
3. Vernier scale is marked on the barrel. There are 10 divisions on the barrel and this is equivalent to 9 divisions on the thimble. Hence one division on a vernier scale is equal to9/10thatofthimble. But one division on the thimble is equal to to0.01 mm. Therefore, one division on a vernier scale is equal to

Least count of vernier micrometer

L.C. = Value of smallest division on thimble- Value of smallest division on the vernier scale.
= 0.01 – 0.009
= 0.001 mm
Hence the accuracy of the vernier micrometer is 0.001 mm.

Reading the vernier micrometer:

Main scale reading= 11.5 mm

Thimble reading
= No. of thimble division coinciding with reference line x L.C. of a thimble
= 12 x 0.01
= 0.12 mm
The 4th vernier scale line is coincident with the divisions of a thimble

Hence, vernier reading
=No. of vernier division coinciding with thimble scale x L.C. of vernier
= 6 x 0.001
= 0.006 mm
Total reading= 11.5 + 0.12 + 0.006
= 11.626 mm

If vernier line coincident with the reference line is 0, then no vernier reading added to the final reading.

Depth micrometer

• Depth micrometer (micrometer depth gauge) is used to measure the depth of holes, slots and recessed areas.
• It consists of a base (measuring face) which is fixed on the barrel and measuring spindle which is attached with thimble as shown in Fig. The axial movement of the spindle takes place by rotation of thimble.

• The measurement is made between the end face of the spindle and the measuring face of the base. As spindle moves away from the base, the measurement increases due to scales on the barrel are reversed from the normal.
• The scale indicates zero when spindle flush with the face and maximum when the spindle is fully extended from the base Fig.shows the use of depth micrometer.
• The main scale reading is 17. The 14th division line of the thimble match with the reference line. Hence, thimble reading is
  Total reading= 17 + 0.14 = 17.14 mm
• The depth micrometer is available in ranges 0 -25 mm or0-50 mm. The range can be increased up to 0-90 mm by using extension rods in steps of 25 mm. The extension rod can easily be inserted by removing the spindle cap.

Differential screw micrometer

• Differential screw micrometer uses differential screw principle and hence the accuracy of -this micrometer is increased compared to an ordinary micrometer.
• In this micrometer, the screw has two types of pitches as shown in Fig., one smaller and other larger, instead of one uniform pitch as in ordinary micrometer.

• Both the screws are right-handed and the screws are so arranged that the rotation of thimble, one screw. Moves forward and other moves backward. The anvil is not attached to the thimble, but it slides inside the barrel.
• The smaller screw nut is fixed in the anvil while larger screw nut fixed with a barrel, hence screw rotates with the thimble. In the case of a metric micrometer, the normally employed pitch for the screws is 0.4 mm and 0.5 mm.
•  Therefore, one revolution of the thimble, the measuring anvil will advance by an amount equal to 0.5-0.4 = 0.1 mm. The thimble circumference is graduated in 100 equal divisions. Hence anvil moves in an axial direction by mm corresponding one division of thimble this micrometer has a smaller range due to small total axial movement (differential axial movement) of the spindle.

Digital micrometer

• The mechanical measuring device as micrometer and vernier are suitable for making measurements that are accurate within 0.001 mm. This device is inexpensive, lightweight, compact, and relatively rugged.
• But when greater accuracy of measurement is desired, these mechanical devices are inadequate. In the case of digital or electronic instruments, measuring instruments having an electronic digital readout has become common in the industrial measuring instrument in order to get superior precision and ease of reading provided by the electronic digital readout.

• A digital micrometer consists of the frame, anvil, spindle, locknut, barrel, thimble, ratchet, LCD display, and ON/OFF ZERO key as shown in Fig. It has incorporated a digital readout into the structure of the micrometer’s body.
• The digital readout is integrated with a rotary encoder that is capable of reading the axial displacement of a spindle which rotates as a thimble is rotated.
• The digital micrometers are available in a large number of different sizes, normally 0-25 mm, 25-50 mm, 50- 75 mm, and 75-100 mm. They are used to measure length, diameter, or thickness.

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