Friday, October 19, 2012

Micrometer, its construction, reading ITI fitter, Std 11

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



Construction and Parts
Thus the main parts of a micrometer screw gauge are:

Micrometer Frame



Frame :
Made up of Dropped Forged steel or  Malleable Steel. All other parts joint with frame.





Micrometer Anvil


Anvil :
Most important parts of  Micrometer, which is made up of Alloy steel, hardening, tempering  and finishing is done very accurately. 





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.




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. 








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.


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.




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.



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

More details view video :