The word ‘micrometer’ is known by two different meanings. The first is as a unit of measure, being one thousandth of a millimeter. The second meaning is a hand-held measuring instrument using a screw-based mechanism. The word micrometer is believed to have originated in Greece, the Greek meaning for this word being small.
The first ever micrometer screw was invented by William Gascoigne of Yorkshire, England, in the 17th century and was used in telescopes to measure angular distances between stars. The commercial version of the micrometer was released by the Browne & Sharpe Company in the year 1867. Obviously, micrometer as an instrument has a long and cherished history in metrological applications.
There have been many variants of the instrument, and modern industry makes use of highly sophisticated micrometers, such as digital micrometers and laser scan micrometers. A micrometer can provide better least counts and accuracy than a vernier calliper. Better accuracy results because of the fact that the line of measurement is in line with the axis of the instrument, unlike the vernier calliper that does not conform to this condition. This fact is best explained by Abbe’s principle, which states that ‘maximum accuracy may be obtained only when the standard is in line with the axis of the part being measured’. Figure 2 illustrates the relevance of Abbe’s law for micrometers and vernier callipers.
Fig.2 Abbe’s law (a) Micrometer (b) Vernier calliper
In case of a micrometer, the axis of the job being measured is in line with the line of measurement of the instrument, as illustrated in Fig. 2 (a). In case of a vernier calliper, for the reading to be accurate, the beam would have to be perfectly straight and the two jaws perfectly at 90° to it. However, this is rarely the case. There is always some lack of straightness of the beam, and the jaws may not be perfectly square with the beam. With continuous usage and wear and tear, the jaws will develop more and more play (Play refers to uncontrolled movements due to slip of one part over the other.) because of repeated sliding movements.
Therefore, a certain amount of angular error, marked as x in Fig. 2 (b), will always be present. This angular error also depends on how far the line of measurement is from the axis of the instrument. The higher the value of this separation “h”, the greater will be the angular error. We can therefore conclude that the degree to which an instrument conforms to Abbe’s law determines its inherent accuracy.
Figure 3 illustrates the details of an outside micrometer. It consists of a C-shaped frame with a stationary anvil and a movable spindle. The spindle movement is controlled by a precision ground screw. The spindle moves as it is rotated in a stationary spindle nut. A graduated scale is engraved on the stationary sleeve and the rotating thimble. The zeroth mark on the thimble will coincide with the zeroth division on the sleeve when the anvil and spindle faces are brought together.
The movement of the screw conforms to the sets of graduations. The locknut enables the locking of the spindle while taking a reading. The ratchet ensures a ‘feel’ while taking a reading and prevents application of excessive force on the job. The ranges of micrometers are normally 0–25, 25–50, or 0–50 mm. The maximum range of micrometers is limited to 500 mm.
Fig.3 Outside micrometer
A micrometer is made of steel or cast steel. The measuring faces are hardened to about 60– 65 HRC since they are in constant touch with metallic jobs being measured. If warranted, the faces are also tipped with tungsten carbide or a similar material to prevent rapid wear. The anvil is ground and lapped to a high degree of accuracy.
The material used for thimble and ratchet should be wear-resistant steel. Micrometers with metric scales are prevalent in India. The graduations on the sleeve are in millimetres and can be referred to as the main scale. If the smallest division on this scale reads 0.5 mm, each revolution of the thimble advances the spindle face by 0.5 mm. The thimble, in turn, will have a number of divisions. Suppose the number of divisions on the thimble is 50, then the least count of the micrometer is 0.5/50, that is, 0.01 mm.
Figure 4 illustrates how the micrometer scale is read when a job is held between the anvil face and the spindle face. In this example, the main scale reading is 8.5 mm, which is the division immediately preceding the position of the thimble on the main scale. As already pointed out, let us assume the least count of the instrument to be 0.01 mm. The 22nd division on the thimble is coinciding with the reference line of the main scale. Therefore, the reading is as follows:
8.5 + 22 (0.01) mm = 8.72 mm
Thus, a micrometer is a simple instrument to use. However, there are two precautions to be observed while reading a micrometer. The thimble must be read in the correct direction. The other precaution concerns the zero position on the thimble. When passing the index line on the main scale, there is a chance to read an extra 0.5 mm. This is caused by the fact that the next main scale graduation has begun to show but has not yet fully appeared. This is avoided by being careful to read only full divisions on the barrel. Assuming that these simple precautions are adhered to, a micrometer has many advantages over other linear measurement instruments. It has better readability than a vernier scale and there is no parallax error. It is small, lightweight, and portable. It retains accuracy over a longer period than a vernier calliper and is less expensive. On the flip side, it has a shorter measuring range and can only be used for end measurement.
Types of Micrometers
A micrometer is a versatile measuring instrument and can be used for various applications by simply changing the anvil and the spindle face. For example, the anvil may be shaped in the form of a V-block or a large disk. Figure 5 shows a few variants, namely the disk micrometer, screw thread micrometer, dial micrometer, and blade micrometer.
Fig.5 (a) Disk type (b) Screw thread type (c) Dial type (d) Blade type
The following paragraphs briefly highlight the use of each type of micrometer in metrology applications:
It is used for measuring the distance between two features with curvature. A tooth span micrometer is one such device that is used for measuring the span between the two teeth of a gear. Although it provides a convenient means for linear measurement, it is prone to error in measurement when the curvature of the feature does not closely match the curvature of the disk.
Fig.6 Disk Micrometer
Screw Thread Micrometer:
It measures pitch diameters directly. The anvil has an internal ‘vee’, which fits over the thread. Since the anvil is free to rotate, it can accommodate any rake range of thread. However, interchangeable anvils need to be used to cover a wide range of thread pitches. The spindle has a conical shape and is ground to a precise dimension.
Fig.7 Screw Thread Micrometer
The dial indicator fixed to the frame indicates the linear displacement of a movable anvil with a high degree of precision. It is especially useful as a comparator for GO/NO-GO judgement in mass production. The dial micrometer normally has an accuracy of 1 μm and repeatability of 0.5 μm. Instruments are available up to 50 mm measuring distance, with a maximum measuring force of 10 N. The dial tip is provided with a carbide face for a longer life.
The anvil and spindle faces are in the form of narrow blades and useful for measuring narrow grooves, slots, keyways, and recesses. The blade thickness is around 0.75–1 mm. The spindle does not rotate when the movable blade is moving along the measuring axis. Due to the slender nature of the instrument and non-turning spindle working against a rotating screw, it is vulnerable to rapid wear and tear and needs careful use and maintenance.
It has interchangeable anvils such as flat, spherical, spline, disk, or knife edge. It is called universal because of its modular design. The micrometer fitted with the required accessories can function as an outside micrometer, a depth micrometer, a step micrometer, etc.