Linear Measuring Instruments:
Line graduated measuring instruments incorporate graduation spacings representing
known distances. These are used for direct measurements of specific distances
within their capacity range. The sensitivity of the measurement is dependent
primarily on the instrument's basic design (the least distance between the
individual graduations). The following instruments can be used for linear
measurements.
Steel scale/rule:
It
is a line measuring device. It is the simplest and most common measuring
instrument used in inspection. It works on the basic measuring technique of
comparing an unknown length to the one previously calibrated. It consists of a
strip of hardened steel having line graduations etched or engraved at interval
of fraction of a standard unit of length. Depending upon the interval at which
the graduations are made, the scale can be manufactured in different sizes and
styles. Various possible combinations of scales are found depending upon
whether the scale is graduated either on both sides and both edges or on edges
of only one side. The scale can be either 150 mm long or 300 mm or 600 mm or
1000 mm long. The scale need not be graduated uniformly throughout its length.
In some part e.g. it may have 10 divisions of centimeter, in some portion 20
divisions, so that it can be used for all types of work and the particular
range chosen depending upon the accuracy required.
Steel tape:
A tape measure or measuring tape is a flexible form of ruler.
It consists of a ribbon of cloth, plastic, fiber glass, or metal strip with
linear-measurement markings. It is a common measuring tool. Its flexibility
allows for a measure of great length to be easily carried in pocket or toolkit
and permits one to measure around curves or corners. It can be used for
measuring various structural steel sections in the workshop, in construction work
and s use tape
measures in lengths of over 100 m. For workshop a
3m length tape can be used.
Vernier
caliper: .
The meter scale enables us to
measure the length to the nearest millimeter only. Engineers and
scientists need to measure much smaller distances accurately. For this a
special type of scale called Vernier scale is used. The Vernier Caliper is a
precision instrument that can be used to measure internal and external
distances extremely accurately. The vernier caliper is usually a manual
caliper. Measurements are interpreted from the scale by the user. This is more
difficult than using a digital vernier caliper which has an LCD digital display
on which the reading appears. The manual version has both an imperial and
metric scale. Manually operated vernier calipers are much cheaper than the
digital version.
The
Vernier scale consists of a main scale graduated in centimeters and
millimeters. On the Vernier scale 0.9 cm is divided into ten equal parts. The
least count or the smallest reading which you can get with the instrument can
be calculated as under:
Least
count = one main scale (MS) division - one vernier scale (VS) division.
Suppose
10 division of vernier scale = 9 division of main scale. Therefore one division
of vernier scale = 9/10 = 0.9 mm of main
scale division ( one division of main scale = 1 mm). Therefore the least count
will be
= 1 mm -
0.9 mm
= 0.1 mm
= 0.01
cm
Reading
of vernier caliper:
In the following
example 50 divisions on the vernier scale = 49 division on main scale. The
value of one division on main scale is 1mm. Therefore the least count = 1-49/50
= 0.02 mm
Dial caliper:
Instead of using a vernier mechanism,
which requires some practice to use, the dial caliper reads the final fraction of a millimeter or inch on a
simple dial.
In this instrument, a small, precise gear
rack drives a pointer on a circular dial, allowing
direct reading without the need to read a vernier scale. Typically, the pointer
rotates once every 1 millimeter. This measurement must be added to the coarse
whole centimeters read from the slide. The dial is usually arranged to be
rotatable beneath the pointer, allowing for "differential"
measurements (the measuring of the difference in size between two objects, or
the setting of the dial using a master object and subsequently being able to
read directly the plus-or-minus variance in size of subsequent objects relative
to the master object).
The slide of a dial caliper can usually be locked at a
setting using a small lever or screw; this allows simple go/no-go checks of part sizes.
Digital caliper:
A refinement now popular is the
replacement of the analog dial with an electronic digital display on which the reading is displayed
as a single value. Some digital calipers can be switched between centimeters or
millimeters. All provide for zeroing the display at any point along the slide,
allowing the same sort of differential measurements as with the dial caliper.
Digital calipers may contain some sort of "reading hold" feature,
allowing the reading of dimensions even in awkward locations where the display
cannot be seen.
Ordinary 150-mm digital calipers are made of stainless
steel, have a rated accuracy of 0.02mm and resolution of 0.01mm.
Vernier
height gauge:
This is also a sort of
vernier caliper, equipped with a special base block and other attachments which
make the instrument suitable for height measurements. Along with the sliding jaw
assembly, arrangement is provided to carry a removable clamp. The upper and
lower surfaces of the measuring jaws are parallel to the base, so that it can
be used for measurements over or under a surface. The vernier height gauge is
mainly used in the inspection of parts and layout work. With a scribing
attachment in place of measuring jaw, this can be used to scribe lines at certain distance above surface. However dial indicators can also be attached in the clamp and many
useful measurements made as it exactly gives the indication when the dial tip is just touching the surface. For all these measurements, use of surface plates as datum surface is very essential.
attachment in place of measuring jaw, this can be used to scribe lines at certain distance above surface. However dial indicators can also be attached in the clamp and many
useful measurements made as it exactly gives the indication when the dial tip is just touching the surface. For all these measurements, use of surface plates as datum surface is very essential.
Similar to
caliper, the height gauges are also available as dial height gauges and digital
height gauges. The principal of working remain same.
Vernier depth
gauge:
For measuring the depth of holes, recesses and distances from a plane surface to a projection, the vernier depth gauge is employed. In vernier depth gauge, the graduated scale can slide through the base and vernier scale remains fixed. For use of vernier depth gauge, its base or anvil is rested on or against a reference surface and the scaled beam or tongue is pushed beyond the base to contact the measured point. Errors are made due to manipulation. It must be ensured that the reference surface on which the depth gauge base is rested is satisfactorily true, flat and square. The gauge, though true and square, can be imperceptibly tipped or canted.
Micrometer: The micrometer is a precision measuring instrument, used by
engineers. Each revolution of the rachet moves the spindle face 0.5mm towards
the anvil face. The 0.5 mm is the pitch of the screw to which spindle is
attached. The object to be measured is placed between the anvil face and the
spindle face. The rachet is turned clockwise until the object is ‘trapped’
between these two surfaces and the rachet makes a ‘clicking’ noise. This means
that the rachet cannot be tightened any more and the measurement can be read.
Least
Count (L. C) = Pitch/No. of divisions on micrometer barrel (thimble) where,
Pitch = distance travelled by thimble on linear scale in one rotation, which is usually 0.5 mm unless stated.
Pitch = distance travelled by thimble on linear scale in one rotation, which is usually 0.5 mm unless stated.
In the
examples below, the number of division on the barrel are 50. Therefore the
least count of the micrometer will be 0.5/50 = 0.01
1.
Read the scale on the sleeve. The example clearly shows12 mm divisions.
2. Still reading
the scale on the sleeve, a further ½ mm (0.5) measurement can be seen
on the
bottom half of the scale. The measurement now reads 12.5mm.
3. Finally, the
thimble scale shows 16 full divisions (16 x 0.01 = 0.16 mm).
The
final measurement is 12.5mm + 0.16mm = 12.66
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The
digital micrometer is shown is the figure. There are many types of micrometers
which depend on the type of anvil and spindle faces such as gear tooth
micrometer, sheet metal micrometer etc..
Micrometer
head can be part of any measuring instrument which makes the instrument known
with prefix as micrometer such as micrometer depth gauge, micrometer bore gauge
etc.
Indirect
measuring instruments
Firm
Joint Calipers:
These are the devices for comparing measurements against known
dimensions. In the case of firm joint calipers, two legs and working ends are
suitably hardened and tempered to a hardness of 400 to 500 HV and measuring
faces are hardened exactly identical in shape with the contact points and
equally distant from the fulcrum, the legs are joined together by a rivet. The
legs are set correctly so that the working ends meet evenly and closely when
brought together. The capacity of the caliper is the maximum dimension which
can be measured by it. The capacity of the caliper should not be less than its
nominal size.The distance between the roller centre and the extreme working end
of one of the legs is known as nominal size and these calipers are available in
the nominal size of 100,150, 200 and 300 mm. Different calipers are shown in
figure.
Spring joint calipers: The spring joint calipers are
shown in the figure. The functions these calipers are similar to firm joint
calipers.