READING ON MICRO METER

READING ON MICRO METER

It works on the fine assembly of nut and bolt principle where pitch of both nut and bolt plays a big role. The graduation on the barrel of micrometer is in two parts, namely one aboveย theย reference line and the otherย below.ย The higher line graduation above the reference line is graduated in 1 mm intervals. The first and every fifth are long and numbered 0, 5, 10,ย 15,ย 20ย andย 25.ย Theย lowerย orย smallย graduationsย areย graduatedย inย mmย intervalsย butย eachย graduation shall be placed at the middle of the two successive upper graduations to be read 0.5 mm.ย Theย micrometer screw has a pitch of 0.5 mm, while the thimble has a scale of 50 divisions round its circumference.ย 

Thus, on making or rotating through one complete turn, the thimbleย  moves forward or backward by one thread pitch of 0.5 mm, and one division of its scaleย is,ย therefore, equivalent to a longitudinal movement of 0.5 ร— 1/50 mm = 0.01 mm. It is the value of one division on the thimble, which is the least that can be correctly read with the help ofย ย ย  a micrometer and is known as the least count. For measurement, the job is kept betweenย ย ย  the end of the spindle and the fixed anvil, which is fitted to the frame. When the micrometer is closed, the line marked 0 (zero) on the thimble coincides with the line marked 0 (zero) on theย graduatedย sleeve.ย Inย metricย outsideย micrometer,ย theย pitchย ofย theย spindleย screwย isย 0.5ย mmย and the graduations provided on the spindle of the micrometer are in millimeters and subdivided intoย 0.5 mm. Now in one turn of the thimble ofย theย micrometer, owing to the 0.5 mm. pitch ofย theย spindleย screw,ย the spindle will move throughย 0.5ย mmย andย therefore,ย theย correspondingย openingย betweenย theย facesย ofย theย fixedย anvilย andย theย spindle will be 0.5 mm. This opening will go on increasing by the same distance 0.5 mm for each further rotation of the thimble. The beveled edge ofย theย thimble carries 50 equal divisions on its periphery in which every 5th division is marked. It is seen above that for one complete turn of the thimbleย ย the spindle moves through 0.5 mm. Now letย theย thimble is rotated one small division on its beveledย edge i.e. 1/50ย ofย theย turn.ย Theย correspondingย displacement of the spindle will then be 0.5 ร— 1 / 50 =ย 0.01mm.

Depthย micrometerย isย used forย measuring depth ofย holes and is shown in Fig.1. Screw threadย micrometer (Fig.2)ย isย usedย toย measureย theย pitch diameterย ofย theย threadย to anย accuracyย ofย 0.01mmย andย 0.001 inches.ย Itย comprisesย ofย similar parts asย thatย ofย outside micrometer acceptย theย shapesย ofย fixedย andย moveable anvils.ย Theย fixedย andย moveable anvils possessย theย thread profilesย forย threadย  adjustmentย forย measurementย ofย theย pitchย diameter.



A depth micrometer
Fig. 1: A depth micrometer


A screw thread micrometer
Fig.2: screw threadย micrometer

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