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| Micrometer (Screw Gauge) |
Introduction
Micrometer
screw gauge is a form of calipers used for measuring small dimensions. Screw
gauge in extensively used in the engineering field for obtaining precision
measurements. The article describes the principle and main parts of a basic
micrometer screw gauge.
Micrometer
screw gauge (or micrometer caliper) is an instrument or device for measuring
the length of an object which is more precise than a ruler and vernier caliper.
It because a micrometer screw gauge has the smallest scale of 0.01 mm. The
device is widely used in mechanical engineering for measuring small diameter,
thickness, or angles to a high degree of accuracy.
More details video :
Micrometer (Screw Gauge) its construction and How to take reading.
or paste this link : http://youtu.be/6KI7cI0Xb6c
or paste this link : http://youtu.be/6KI7cI0Xb6c
Construction and Parts
Thus
the main parts of a micrometer screw gauge are:
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| Micrometer Frame |
Frame :
Made up of Dropped
Forged steel or Malleable Steel. All
other parts joint with frame.
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| Micrometer Anvil |
Anvil :
Most
important parts of Micrometer, which is
made up of Alloy steel, hardening, tempering
and finishing is done very accurately.
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| Micrometer Spindle |
Spindle :
Is made
up of Nickel Chrome metal, which is also hardened, Grind and accurately made.
Which is move to and fro as we rotate thimble. The maximum displacement of
spindle is 25 mm.
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| Micrometer Measuring faces |
Measuring faces :
Both
the end of Anvil and Spindle, Measuring face is fixed, which is made from Alloy
steel, as well as hardened, grind and accurately finished.
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| Micrometer Barrel or Sleeve |
Sleeve :
The
Barrel or sleeve is connects the frame to the cylindrical tube. It is a
non-movable part of the screw gauge and has a scale inscribed over it which is
the main scale of the device. Moreover, it also carries the most important part
of the micrometer- the screw.
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| Micrometer Thimble |
Thimble :
The
thimble or head is the end of the cylindrical tube and is turned to move and
adjust the spindle. The thimble carries the Micrometer or secondary scale.
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| Micrometer Ratchet |
Ratchet :
There
is one more part called the ratchet which is provided at the end of the tube.
The ratchet is kind of limiting device which applies a pressure by slipping at
a predetermined torque and thus prevents the spindle from moving further.
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| Micrometer Spindle lock nut |
Screw Lock :
screw
gauges also consist of locking devices which holds the scales at a particular
position for prevent any kind of error while taking readings.
A
screw gauge consists of a “U” shaped metallic structure, which is attached to a
hollow cylindrical tube on one end. The hollow tube has a uniformly threaded
nut inside it. A long stud with a plane face is fitted into this nut. Exactly
on the opposite side of this nut and on the other end of the U shaped frame, a
smaller stud with a plane face is also attached. Faces of both the studs are
exactly parallel to each other.
This
U shaped metallic structure is known as the frame
of the micrometer screw gauge. The smaller stud is known as the anvil and the longer one is known as the spindle. The anvil is
the fixed part of the device, whereas the spindle moves as and when the head is moved. The frame carries both the anvil
and barrel, and is also heavier than the rest of the parts. The object to be measured is held
between the anvil and the spindle.
The
Barrel or sleeve is connects the frame to the
cylindrical tube. It is a non-movable part of the screw gauge and has a scale inscribed over it which is the main scale of the device. Moreover, it also carries
the most important part of the micrometer- the screw.
The
screw is the heart of the micrometer and is located inside the barrel. The
screw converts small dimensions into measurable distance using a scale. The
thimble or head is the end of the cylindrical tube and is turned to move and
adjust the spindle. The thimble carries the vernier or secondary scale. There
is one more part called the ratchet which is provided at the end of the tube.
The ratchet is kind of limiting device which applies a pressure by slipping at
a predetermined torque and thus prevents the spindle from moving further. Some
screw gauges also consist of locking devices which holds the scales at a
particular position for prevent any kind of error while taking readings.
A
micrometer screw gauge also uses two scales – main and
secondary scales. The secondary scale is provided on the thimble and is
the measurement of the pitch of the screw. This means that the reading on the
secondary scale measures the distance moved by the thimble per rotation. The
scale on thimble is divided into 100 equal parts and measures hundredths of a
millimeter. The thimble scale rotates over the spindle or the main scale. The
main scale is a millimeter scale subdivided into equal parts with half a
millimeter distance. When the object is to be measured, it is placed in between
the anvil and the spindle. Readings from both the scales are taken into account
for arriving at the final measurement.
Micrometer
screw gauge is a delicate device and thus special care should be taken while
handling it. Moreover, it is also important that the micrometer is well
calibrated to prevent any kind of error in the final reading.
Precaution
Steps
The
spindle and anvil are cleaned with a tissue or cloth, so that any dirt present
will not be measured.
The
thimble must be tightened until the first click is heard.
The
zero error is recorded.
Reading
= Reading of main scale + Reading of thimble scale.
While
taking a reading, the thimble is turned until the wire is held firmly between
the anvil and the spindle.
The
least count of the micrometer screw can be calculated using the formula given
below:
Use the
given formula:
Least
Count (L. C) = Pitch/no. of divisions on micrometer barrel(thimble)
where,
Pitch =
distance travelled by thimble on linear scale in one rotation.
Least
count = 0.01 mm
Types
of error in micrometer screw gauge reading
Every
micrometer prior to its use should be thoroughly checked for backlash error or
zero error.
Backlash error:
Sometimes
due to wear and tear of the screw threads, it is observed that reversing the
direction of rotation of the thimble, the tip of the screw does not start
moving in the opposite direction immediately, but remains stationary for a part
of rotation. This is called back lash error.
Zero error:
If on
bringing the flat end of the screw in contact with the stud, the zero mark of
the circular scale coincides with the zero mark on base line of the main scale,
the instrument is said to be free from zero error. Otherwise an error is said
to be there. This can be both positive and negative zero error.
Calculating
micrometer screw gauge reading:
Total
observed reading = main scale reading + (circular scale division coinciding the
base line of main scale) x least count
True
diameter = observed diameter – zero error
Example,
main scale reading = 2mm or 0.2cm
Circular
scale reading = 56, so 56 x 0.001 = 0.056cm
So
observed reading = 0.2 + 0.056 = 0.256cm
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