distance between two points in a straight line. Linear measurement is applicable to all external and
internal measurements such as distance, length and height difference, thickness, straightness,
squareness, taper, etc.
The instruments used in length metrology are generally classified into two types:• Non – precision measuring instruments, e.g., steel rule.
• Precision measuring instruments, e.g., vernier calipers, micrometer.
A caliper is an end – standard measuring instrument to measure the distance between two points.
Calipers typically use a precise slide movement for inside, outside, depth or step measurements.
Types of Calipers
• Inside Calipers
• Outside Calipers
• Spring Calipers
• Centre Measuring Caliper
• Vernier caliper is a measuring tool used for finding or transferring measurements (internal or
external). Internal calipers are used to check the inside diameters of pipes. External calipers
are used to determine the diameters of a round pipe or a turned spindle.
• A vernier caliper is a combination of inside and outside calipers and has two sets of jaws;
one jaw (with a depth gauge) slides along a rule.
• Vernier calipers are based on principle which states that “the difference between two scales
or divisions which are near, but not alike are required for obtaining a small difference. It
enhances the accuracy of a measurement.
• The vernier caliper essentially consists of two steel rules.
• A solid L – shaped beam is engraved with the main scale. This is also called true scale, as
each millimeter marking is exactly 1 millimeter apart.• On the movable measuring jaw, the vernier scale is engraved which slides on the beam. The
function of vernier scale is to subdivide minor divisions on the beam scale into smallest
increment that the vernier instrument is capable of measuring.
Instructions on use:
• Close the jaws lightly on the object to be measured.
• While measuring a round cross section make sure the axis of object is perpendicular to the
caliper.
• Ignore the top scale, which is calibrated in inches and use the bottom scale, which is in
metric units.
• The boldface numbers on the fixed scale are in centimeters, the tick marks on the fixed scale
between the boldface numbers are in millimeters.
Instructions on use:
• Examine the vernier scale to determine which of its divisions coincide or are most coincident
with a division on the main scale. The number of these divisions is added to the main scale
reading.
This is one of the most useful and versatile instruments used in linear metrology for measuring,
inspection and transferring the height dimension over plane, step and curved surfaces.
• It follows the principle of a vernier calliper and also follows the same procedure for linear
measurement.
It consists of the following parts:
• Base: It is made quite robust to ensure rigidity and stability of the instrument.
• Beam
• Measuring jaw and scriber
• Graduations
• Slider
The vernier height gauge consists of a vertical beam on which main scale is engraved. The
vernier scale can move up and down over the beam.
The bracket carries vernier scale which slides vertically to match the main scale.
The whole arrangement is designed and assembled in such a way that when the tip of the
scriber blade rests on the surface plate, the zero of the main scale and vernier scale
coincides.
The scriber blade can be inverted with its face pointing upwards and which enables
determination of heights at inverted faces.
A vernier depth gauge is used to measure depth, distance from plane surface to a projection,
recess, slots and steps.
The basic parts of a vernier depth gauge are base or anvil on which the vernier scale is
calibrated along with the fine adjustment screw.
To make accurate measurements, the reference surface must be flat and free from dust and
burrs.
When the beam is brought in contact with the surface being measured, the base is held
firmly against the reference surface.
• Micrometers have greater accuracy than vernier calipers and are used in most of the
engineering precision work.
• Micrometers with an accuracy of 0.001 mm are also available.
• Micrometers are used to measure small or fine measurements of length, width, thickness
and diameter of a job.
Principle of Micrometer:
• A micrometer is based on the principle of screw and nut. When a screw is turned through
one revolution, the nut advances by one pitch distance, i.e., one rotation of the screw
corresponds to a linear movement of the distance equal to the pitch of the thread.
• If the circumference of the screw is divided into n equal parts then its rotation of one
division will cause the nut to advance through pitch/n length. The minimum length that can
be used to measure in such case will be pitch/n.
• If the screw has a pitch of 0.5 mm then after every rotation, the spindle travels axially by 0.5
mm and if the conical end of the thimble is divided by 50 divisions, the rotation of the
thimble of one division on the micrometer scale will cause the axial movement of screw
equal to 0.5/50 mm = 0.01 mm i.e. Least Count of the micrometer.
The main parts of outside micrometers are the following:
1) U-shaped or C-shaped Frame: It holds all parts of the micrometer together. The gap of the
frame decides the maximum diameter or length of the job to be measured.
2) Carbide – Tipped measuring faces – Anvil and Spindle: Anvil is fixed and located at 3.5 mm
from left hand side of the frame. The carbide tipped anvil guarantees extreme precision and
long life of the instrument. The spindle is the movable measuring face with the anvil on the
front side and it is engaged with the nut. When the spindle face is touched with the anvil
face, the zero of the micrometer must match with the reference line on the main scale and
the thimble is required to be set at zero division on the main scale.
3) Locking Device: It is provided on a micrometer spindle to lock it in exact position. This
enables correct reading without altering the distance between two measuring faces.
4) Barrel: It has fixed engraved graduation marks on it. The graduations are above and below
the reference line. The upper graduation marks are of 1 mm interval and are generally
numbered in multiples of five as 0,5,10,15. The lower graduations are also at 1mm interval
but are placed at the middle of two successive upper graduations to enable reading of
0.5mm.
5) Thimble: When the thimble is rotated, the spindle moves in a forward or reverse axial
direction, depending upon the direction of rotation. The conical edge of the spindle is
divided into 50 equal parts. The multiples of 5 and 10 numbers are engraved on it and the
thickness of graduations is between 0.15 to 0.20 mm.
6) Ratchet: It is provided at the end of the thimble. It controls the pressure applied on the
workpiece. The ratchet gives a clicking sound when the workpiece is correctly held.
Errores
When the anvils are brought in contact without applying undue pressure, the zero of the
circular thimble scale should coincide with the reference axial on the barrel. If it is not so,
then the meter has a zero error. It is determined by noting the number of circular thimble
scale divisions by which the zero of the circular scale is left behind the reference line.
Measuring instruments operating on the principle of screw work satisfactory if the screw
moves in an accurate cut nut without any play. However, there is always a little play
between the two and this increase with use. This give rise to shake and backlash i.e. the
screw does not moves backward or forward for a little while even when the head is turned
Dial gauges indicators perform mechanical amplification of length or displacement and
translate it into rotational motion of a pointer over a circular scale. The object of
amplification and translation is achieved by a rack and pinion arrangement. Dial gauges are
adaptable to many linear measurements where easy readability and moderate precision are
required.
• Depth micrometers are used for measurements of depths, groove spacing and groove
widths.
• The measurement is made between the end face of a measuring rod and a measuring face.
Because the measurement increases as the measuring rod extends from the face, the
readings on the barrel are reversed from the normal; the start at a maximum and finish at
zero.• Dial gauges or dial test indicators are used for checking flatness of surfaces and parallelism
of bars and rods. They can also be used for measurement of linear dimensions of jobs which
require easy readability and moderate precision;
• The dial is divided into 100 equal divisions, each division represents a spindle movement of
0.01 mm. For 1 mm movement, the bigger arm turns through one complete revolution.
• Interferometer: Is a tool which use source of monochromatic (one color or wavelength)
light. It is an optical instrument either processes light waves to enhance an image for
viewing, or analyzes light waves (or photons) to determine one of a number of characteristic
properties.• What is an autocollimator?
An autocollimator is an optical instrument that is used to measure small angles with very
high sensitivity. As such, the autocollimator has a wide variety of applications including
precision alignment, detection of angular movement, verification of angle standards, and
angular monitoring over long periods.
Principles of operation
the autocollimator projects a beam of collimated light. An external reflector reflects all or
part of the beam back into the instrument where the beam is focused and detected by a
photodetector. The autocollimator measures the deviation between the emitted beam
and the reflected beam. Because the autocollimator uses light to measure angles, it never
comes into contact with the test surface.
• An autocollimator is used to detect and measure small angular tilts of a reflecting surface
placed in front of the objective lens of the autocollimator.• An autocollimator is based on the principle that a collimating lens can project and receive a
parallel beam of light and that the reflected beam of light will change its direction by
changing the angle of the surface reflecting the light.
If a parallel beam of light is projected from the collimating lens and if a plane reflector R is
set up normal to the direction of the beam, the light will be deflected back along its own
path and will be brought to a focus exactly at the position of the light source. If the reflector
is tilted through a small angle Ɵ, the parallel beam will be deflected through twice the angle,
and will be brought to a focus in the same plane as the light source but one side of it.
• The image will not coincide but there will be a distance = X = 2fƟ between them, where f is
the focal length of the lens
