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I am just posting here to verify that I am properly using a tool that is new to me, a micrometer. Here is what the precision label specifies, basically that it can measure up to an inch and down to 1/1000" (a notch on the round scale).

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I would like to describe how I was converting the reading on the linear and on the round gauges and you tell me if I did it right.

enter image description here

In the above picture, the linear scale is reading 2/10" and the round scale is reading 15/1000". So the total gauge I am measuring is 0.2" + 0.015 = 0.215". I am particularly curious to verify if the round gauge's each notch is indeed 1/1000" and whether I am using the correct numerator in the fraction.

user1586
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amphibient
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    I believe you are using it correctly, but the details depend on the specific micrometer used. You can verify that the 'round scale' is 1000th's by turning it to 100 and checking that you have gone up by 1/10" on the linear scale. – Chris Mueller Aug 23 '15 at 00:05
  • I did that check exactly and yes, it takes 4 turns 25 notches each (so 100 notches total) to cover 1 notch of 1/10" on the linear scale (if that is 1/10" indeed) – amphibient Aug 23 '15 at 00:08
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    Excellent, then you can check the 1/10" against a ruler as a final calibration. – Chris Mueller Aug 23 '15 at 00:09
  • ahhhh, good idea – amphibient Aug 23 '15 at 00:10
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    On a side note, in the lower picture those horizontal lines on the barrel behind the linear scale are used to get even higher precision through the Vernier effect. You have to check for the line which is most closely matched to the horizontal lines on the rotating part. – Chris Mueller Aug 23 '15 at 00:12
  • yeah, i was wondering what those teeny notches are. i didn't bother using them for measuring my cylinder valve tappets because I think the precision I get from the rotating scale is plenty – amphibient Aug 23 '15 at 00:17

1 Answers1

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These kind of micrometers have two scales, a coarse and a fine. You add them together to get the final reading.

The fine scale is etched around the part that turns. With nothing in the microcometer and it closed all the way, the 0 on this scale should line up with the horizontal line on the fixed part to the left. As you turn the handle to open the gap, you will see that the numbers that line up with the fixed line get larger. These numbers are usually directly in the units the micrometer is intended to measure, which looks like mils (1/1000 inch) from your picture.

This all works fine until the gap is opened so that the handle goes thru a full turn. That's where the second more coarse scale comes in. This scale is revealed as the gap is increased. Usually there is one line per handle turn, and these are usually calibrated to come out to a convenient multiple. For example, if each handle turn opens the gap 100 mils, then there is a line every 100 mils on the fixed coarse scale.

You have to know what the coarse scale multiplier is, but that's easy to see by rotating the handle one turn. Let's say the coarse scale is marked every 100 mils and the handle in mils. Then for example, if you can see 3 lines of the coarse scale revealed and the handle scale is at 37, then the measurement is 337 mils, or 0.337 inch.

Olin Lathrop
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