Micrometer Types and Usage Guide
Introduction
Over a past number of years more sensitive measuring instruments have been developed due to the need of satisfying requirements for higher accuracy. These newer types of instruments however, are of the comparator type and require a setting master. Also, the measurement range can be rather small. Micrometers have an advantage over these gauges as they can measure absolute lengths over a much larger range. Smaller micrometers are widely used due to their versatility, although the larger micrometer types have been replaced for applications with tighter tolerances. In comparison to other measuring instruments, micrometers are relatively cheap and their operators require very little training. The digital micrometers, rather than the analogue, are much more reliable as they eliminate parallax error and are easier to read. Internal micrometers use the same scale and mechanism but are used to measure large bores.
Types of Micrometer
Environmental Requirements
- The frame of the micrometer can be warmed by the hands which can result in significant measurement errors on the larger types of micrometer. Therefore, gloves or insulation on the frame may be used to minimise the heat transferred from the hands to the frame.
- The micrometer should always be used in an environment with adequate lighting. A minimum of 700 lux is appropriate.
Usage Guidance
- The user of the micrometer should be conscious of any changes that could indicate possible errors with the gauge.
- The thimble should run evenly and not stick.
- The ratchet should turn smoothly.
- The zero line on the spindle should align with the index line that is on the micrometer sleeve.
- The end of the thimble should be aligned with a graduation line on the sleeve and not cover it.
- If the micrometer has been subject to any kind of damage, then the checks mentioned above should always be carried out. The parallelism between measuring faces and flatness of the micrometer can also be checked if an optical parallel is available. Do not use a micrometer if there are any doubts in regards to its condition.
- Before using an analogue micrometer, all the scales should be checked for clearly marked graduations.
- Different types of anvils can be used to measure different types of features. Some examples of common anvils are pin, ball, or disc anvils.
- Internal micrometers (also known as inside micrometers) can be used on large diameters when feature tolerance does not justify using a more accurate comparative gauge.
- Micrometers should be periodically checked for zero error. For micrometers that do not start at 0, (e.g. 25-50mm or 75-100mm), a calibrated length bar should be used to zero the scale.
- The accuracy of the measurement in analogue micrometers relies on the lead of the screw. The error in the screw is cumulative, which means the error increases with the length of travel. For this reason a micrometer with the closest possible range to the nominal dimension should be used.
- The zero point of micrometers should be checked regularly. It is possible on a digital micrometer to re-zero the scale. Continued re-zeroing however, may mask any significant changes in the scale, and therefore should be avoided.
Micrometer Capability Charts
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