This is one of the most accurate devices for measuring available in most home workshops.
The micrometer by itself is not as useful when milling as it is when turning. When turning it is often necessary to measure a diameter to 0.001mm. This can only be measured reliably with a micrometer. When milling it is only really possible to machine to an accuracy of at the most about 0.01mm which makes such accuracy unnecessary. It also happens that an analogue or digital calliper can measure to this sort of accuracy.
This is partly because it fulfills the Abbe criteria of measuring along the line of the points being measured.
A conventional micrometer will only measure, directly, an outside length. Micrometers for measuring inside lengths also exist. There are also a large variety of micrometers for specialised purposes
In practice the limit on accuracy is how it is determined that the piece being measured is touching both faces of the micrometer. It is easy to see that when using one there is a point where the moving anvil just touches. But it is easy to just turn the thimble a bit more. It all depends upon the pressure. One way of solving this is to use a micrometer with a built in ratchet mechanism. When tightening the micrometer up the ratchet is turned till it clicks.
It is said that a skilled worker can measure more consistently by feel rather than by using a ratchet.
All micrometers are right handed, ie, they are arranged assuming the user is right handed.
Analogue micrometers are either metric or imperial. If they are metric then they measure in multiples of 25mm. That is, they go from 0-25mm, 25-50mm etc. If they are imperial they measure in multiples of 1 inch.
Analogue micrometers are often part of a set. Easily the most useful is the first in the set, that is, 0-25mm or 0-1 inch. Sometimes the next is useful but the usefulness of the others rapidly diminishes as they get bigger.
Digital micrometers are only commonly available for 0-25mm and/or 0-1 inch. If the micrometer is digital then it will have a scale that is imperial but the display can display this in either metric or imperial.
Some analogue micrometers have verniers on their scales so that they can read to a tenth of the smallest division, ie 0.001mm or 0.0001inches.
Digital micrometers always read to 0.001mm directly.
Before any testing can be done it is essential that there is no play in the screw mechanism in the micrometer. If the thimble is removed from the micrometer it will be found that the “nut” that holds the screw is slit down its sides. The nut also has a thread on its outside. This thread is tapered and fitted with another nut.
fig the screw thread inside the thimble, thread on the outside and nut for adjusting
This nut can be used to adjust the fit of the screw and the “nut” holding it. This is adjusted till there is no play but not so much as to stop the screw turning freely.
If there is no play and the zero has been set the next step is to test the reading when the micrometer is fully open.
When testing some sort of measuring device it is usually necessary that the reference is far more accurate than the device being tested.
Firstly the micrometer must read zero when the two anvils are touching. If it does not it can always be adjusted to do so by rotating the sleeve till it does. It will be noticed that the sleeve has a small hole in it. This is made so the spanner provided with the micrometer when new can be used to rotate the sleeve.
fig spanner for adjusting the sleeve.
Once this has been done the next simplest test is to check that when fully open the gap is actually 25.000mm or 1 inch etc. This can be done in two ways.
A slip gauge of the right size can be used as the reference.
fig testing a micrometer with a slip gauge
Firstly ordinary slip gauges can be used These can be combined so as to test the micrometer at several points over its range.
Alternatively it can be done with a setting bar
fig setting bar
Further tests are to test the reading at certain points over the range the micrometer covers. It is possible to buy special sets of slip gauges to do this but these are not worth the space in the home workshop.
fig set of slip gauges for testing a micrometer.
It is also possible to test that the anvils are both at right angles to the axis of the micrometer. This is done with a special set of optical parallels.
This is covered elsewhere
See MEW 97 p16