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Measurement (2): English micrometer
Page 45
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THE USE of calipers, described Id the last article, is somewhat linited and the more sophistic ted micrometer is better suited for accurate measure
ents where a direct reading is r quired. Micrometers are made ith both English and metric ales and as both types are still use, both will be described. The diagram shows a 0to 1 inch micrometer. The instruent consists of a steel U
s aped frame. At one end of the U is a fixed anvil face and at the o her a spindle threaded 40 t reads per inch. A timble is a tached to this spindle, and When the thimble is turned one complete rotation, the spindle will move 1/40in forwards or backwards depending on which way it is turned. Now 1/40in is equal to 0.025in or 25 "thou". The thimble has 25 divisions, each of which represents 6001 in or one thou.
The stationary barrel of the micrometer has 1/10in or 0.1 in divisions, and these 1/10 divisiOns are in turn subdivided into fcrur parts. 1/10 divided by four equals 1/40 or 0.025in or 25 thou. So, the large divisions on the barrel are Olin apart and the small ones .025in — see photo 1 When reading a micrometer, the dimension is calculated in the following way: (a the largest visible 1/10 division plus (b) the largest visible 1/40 division plus (c) the thimble division in line with the datum line 2 fortieths 2 thousandths tal =0.300 =0.050 =0.022 0.372 (ie 0.025 x 2) (a direct reading off the thimble).
When using a micrometer it must be appreciated that it is a p-ecision instrument to be treated with great care. When taking measurements the amount of pressure which is used when it is screwed up important. Obviously t reading will vary with t amount of force used. A hea) handed person will obtain smaller reading than one w uses the thimble lightly. So tt: uniform readings are obtain no matter who uses the tool rachet is provided at the end the thimble and this rachet sli when the pressure on the son reaches a predetermin amount. This rachet shou always be used.
A knurled locking nut is al generally provided so that t spindle may be retained at o particular reading. Photo shows the thimble of micrometer with a spring c pointer which can be adjusted indicate a certain reading. I cidentally this micrometer is to read 0.25in, that is 1/4in.
More about micrometer and particularly the metric tyr in the next article.
by Precept(