A Micrometer is one among the precision measuring instruments used to measure dimensions of objects especially very small whom accurate measurement cannot be achieved by using other common measuring tool or scales .The measurement in a micrometer is actually carried out through an integrated screw that is why such meters are commonly referred as micrometer screw gauges as well, this article describes the working principle of a screw gauge, its construction and operation. Working Principle In a micrometer screw gauge measurements are done actually through a precisely made integrated screw with a pitch of usually but not necessarily 02 threads per millimeter, which means that on completing exactly one revolution the displacement achieved is half of a mm i.e. 0.5 millimeter. Least Count. The least count of screw gauge represents minimum value of a dimension which measurable hence it actually shows the degree of accuracy to which an instrument can work. The least count of a screw gauge is represented as: Least Count = Pitch of the screw / No. Of divisions on the circular scale If we are working with the screw whose pitch is ½ mm and the number of divisions on the circular scale is 50 then, using the formula stated above least count for our micrometer is going to be Least Count = 0.5mm / 50 = 0.01mm This means that we can measure up to the 100th part of a mm. Construction Micrometer screw gauge construction is not very complex in nature; let us discuss the most common parts which remain same in different designs and ranges. Frame The body used to hold the anvil and barrel firmly in their place is called frame , in micro meter screw gauges , thick C shaped frames are used . Their mass helps in minimization of expansion or contraction due to temperature some manufacturers also deliver gauges with insulated frames to serve the above mentioned purpose. Anvil It is the fixed part mounted at one end of frame exactly parallel to the moving spindle which move towards it , Anvil face which comes directly in contact to the object being measured is machined extremely fine so to achieve highest degree of precision. Spindle The cylindrical part which displaces by rotation of thimble decreasing the clearance between itself and anvil until the object being measured become stable between the two of them , in modern micrometer screw gauges , anvil and spindle`s open face are tipped with carbide. Sleeve/Barrel the stationary part with having linear scale onto it, called the main scale, it also covers the screw mechanism of screw gauge , now screw gauges are available with adjustable sleeves which makes it easy to eliminate the zero error. Screw It is the most important part of instrument because all measurement is done through it in fact. Very fine stainless steel screws are used for this purpose with a definite pitch. Thimble It is the part through which measuring screw is rotated, this screwing results in the displacement of spindle and thimble itself. Ratchet; Ratchet is itself a small device which is used to provide a limited applied force , It is installed at the right end of screw gauge , ratchet acts as a safety device for instruments and also adds more precision in measurement . Final adjustment is made by a making three turns of ratchet. Locking Device It is actually a nut whose operation is facilitated by means of a lever, usually used to hold tight the spindle at its place so that current reading of that time could be maintained up to a desired length of time. Scales Micrometer screw gauge comes with two scales, one rotating scale which can be found on its rotating cylindrical part it is also called circular scale and the other one can be found on its stationary sleeve which is called main scale or sleeve scale , some designs of instrument can also have a vernier scale as well.
