Measuring with outside micrometers
Reading micrometers correctly - how it works.
Micrometers are used to measure the diameter of shafts, pipes, bolts or similar objects.
Correct reading of the measured value requires a little practice as well as an understanding of the structure and the associated measuring principle of a micrometer.
How does a micrometer work?
A micrometer consists of a fixed and a movable measuring surface that can be adjusted via a feeler ratchet. The distance between the two measuring surfaces is taken/read via the scales on the scale sleeve and the measuring drum.
The measuring principle of micrometers is based on a threaded spindle inside, which can be turned in or out with the aid of the feeler ratchet. The pitch of this thread is decisive for determining the measured values:
As a rule, the pitch of the thread is 0.5 mm. The measuring drum is divided into 50 graduations. If the measuring drum is now rotated by one of the graduation marks, the threaded spindle moves by 0.01 mm (0.5 mm : 50). The micrometer therefore has a reading accuracy of 0.01 mm.
Reading the outside micrometer
The following rules apply to the correct reading of micrometer readings:
- First read off the whole and half millimetres from the scale sleeve.
- Then determine the hundredths of a millimetre using the scale on the measuring drum.
- Finally, add the two values together to obtain the final measured value.
Reading whole and half millimetres
The graduation marks below the horizontal line on the scale sleeve indicate whole millimetres.
The graduation marks above the longitudinal line on the scale sleeve, on the other hand, indicate the displacement of the measuring spindle by 0.5 mm and thus mark half millimetres.
In our example, this results in an intermediate reading of 41.5 mm.
Read off hundredths of a millimetre
Next, determine the hundredths of a millimetre from the scale on the measuring drum. Which graduation mark on the measuring drum lies on or closest to the horizontal line on the scale sleeve? Or between which two graduation marks on the measuring drum is the line on the scale sleeve?
If the horizontal line lies between two graduation marks, the result is a value in the measuring range of less than 1/100 and must be estimated.
In our example, this results in an estimated reading of approx. 0.188 mm. The estimated thousandth value is then rounded up or down to the nearest hundredth.
Then add the two values together to obtain the final measured value.
This results in the following measured value: 41.5 mm + 0.19 mm = 41.69 mm.
Tips & tricks
Observe the following points for an accurate measurement result:
- When measuring, always hold the outside micrometer by the plastic element in order to obtain an exact measurement result and to protect the measuring legs from corrosion.
- The most accurate measurements are achieved at a reference temperature of 20° C.
- Use gauge blocks to calibrate your outside micrometer regularly.
Calibrating the micrometer
Clamp the gauge block with the desired dimension between the measuring surfaces of the micrometer and fix the clamping lever. If the measured value deviates from the value of the gauge block, you can calibrate the micrometer using a hook spanner. To do this, hook the hook spanner into the opening provided on the back of the micrometer and adjust the scale sleeve until the line matches the 0 on the measuring drum.
Micrometer vs. caliper gauge
In general, calipers and micrometers can be used for the same applications. However, micrometers are often preferred for measuring turned parts. Micrometers also generally provide a more precise measurement result. Depending on the application, micrometers are also available in different versions, for example with angled measuring surfaces, tips or drum heads. For example, micrometers with bevelled measuring surfaces can be used to measure multi-bladed tools.
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