What is meant by metrology?

In general terms, the application of scientific principles in the study of measurements is known as metrology. When speaking about measurement, it could be of time, length, microstructures, grain structures, internal crystal discontinuities, surface roughness, measurements of angles, height, width, and so on. While coming to the domain of mechanical engineering, metrology is considered the science of measurements of surface finish, length, surface texture, and angles.

Surface finish

Surface finish is the number of discontinuities or surface irregularities present in the surface. When the irregularities of the surface are high, the surface is referred to be a rough surface, while the surface that has low irregularities is referred to as a smooth surface.

Primary texture and secondary texture of a surface

Primary texture

The primary texture of a surface is also known as the roughness of the surface. Here, the surface roughness parameters are of low wavelength. During machining of the material, if there is a direct application of the cutting tool on the component's surface, high degrees of surface roughness are created. This kind of surface roughness is also produced when the material undergoes fracture due to the force application of the cutting tool during the machining process.

Secondary texture

The secondary texture of a surface is also known as waviness. If the wavelength parameter of the surface roughness has a periodic nature, then such roughness profiles are termed as waviness. This problem of waviness profile arises due to improper set up of cutting tool and due to the presence of machine vibrations.

Terminologies of surface textures

The various terminologies of surface textures are outlined below:

Mean line

An imaginary line that is taken into consideration for the measurement of roughness parameters is known as a mean line.

Peak-to-valley height

It serves as a parameter for deciding the surface roughness parameter of any component. The height of the irregularities are considered within a certain mean line, the irregularities are in the form of crests and troughs. The depth of the largest irregularity is considered to be the value for roughness.

Primary profile

The contour or the domain of any section of the surface is known as primary profiles.

Nominal surface

It is termed as a geometrically perfect surface that has the minimum amount of irregularities/waviness present in the surface profile. The surface texture of such kind of surface is considered to be free from discontinuities.

Measurement of surface texture

Various instruments are used for the measurement of surface texture. Some of the surface texture measurement instruments are outlined below:

Profilometer

The profilometer follows the principle of profilometry. This instrument is used to measure the roughness values of the surface profile in the unit of microns. There are mainly two parts of this instrument, one is a stylus and the other is an amplifier. The stylus remains in contact with the surface being measured. The whole unit is moved over the surface profile, due to the presence of irregularities, there are minor movements in the stylus. The movement of the stylus is amplified by the amplifier and can be viewed and analyzed.

Interferometer

It is based on the principle of interferometry which is the non-contact method for analyzing surface profiles. In interferometry, light waves or other waveforms from the electromagnetic spectrum are used and intentionally caused to form the phenomenon of interference. This interference is later analyzed to extract information about the surface profile.

Surface meter

In the surface meter category, the Tomlinson surface meter is one such example. The stylus of this kind of surface makes use of a diamond probe. This probe is attached to the instrument frame by the use of a leaf spring. The stylus has direct contact with a horizontal cylinder, the cylinder has a long steel arm that acts as a pointer. It is moved over the surface to be measured, the vertical displacements of the stylus are converted to rotatory motion in the cylinder, which in turn, swings the steel arm. The swinging of the arms makes the pointer point to the related roughness values.

Profilograph

This instrument makes use of a stylus that is pivoted to a mirror. The instrument has a light source and a sensitive rotating drum. The stylus is made to follow the surface profile. Due to irregularities, the stylus undergoes vertical movements, which in turn, oscillates the mirror. A light source is made incident on the mirror. The light upon reflection from the mirror falls over the sensitive drum. The drum has a light-sensitive film that captures the light movements.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Technology in Aerospace Engineering 
• Bachelor of Technology in Mechanical Engineering

Practice Problems

Q1) Which of the following instruments uses a diamond probe?

a) Profilometer

b) Surface meter

c) Confocal microscope

d) None of these

Correct option: b

Explanation: The surface meters make use of a stylus, whose tips are made of diamond. This is because diamond is considered to be the hardest substance and it doesn't easily undergo wear and tear upon analyzing hard surfaces.

Q2) Which of the following instruments make use of a stylus that is pivoted on a mirror?

a) Interferometer

b) Profilograph

c) Double microscope

d) Taylor-Hobson-Talysurf

Correct option: b

Explanation: The profilograph makes use of a stylus that is pivoted on a mirror, due to the motion of the stylus, the mirror oscillates and the reflected light from the mirror falls on the light-sensitive drum.

Q3) Which of the following is the baseline taken into considerations for the measurements of surface roughness?

a) Reference line

b) Sampling line

c) Mean line

d) None of these

Correct option: c

Explanation: Mean line serves as the baseline for reference to make analysis for surface irregularities.

Q4) Which of the following represents a geometrically perfect surface?

a) Primary surface

b) Secondary surface

c) Nominal surface

d) All of these

Correct option: c

Explanation: A nominal surface is an ideal surface that is considered to be geometrically perfect.

Q5) Which of the following textures represents waviness?

a) Primary texture

b) Secondary texture

c) Peak-to-valley height

Correct option: b

Explanation: The waviness is the periodic irregularities. It represents the secondary texture of a surface.

Related Concepts

• Kurtosis and skewness
• Skidless profilometer