Essential Tip:
Micrometer accuracy is derived from several factors including:
- Thread accuracy
- Flatness of measuring faces
- Parallelism between the faces
- Rigidity of the frame which holds the anvil and spindle
Outside Micrometers are used for measuring the thickness or outside diameter of small parts. They are industry standard measuring tools because of their high accuracy/resolution and ease of use. OD micrometers are available in a variety of different spindle and anvil configurations to allow the operator to measure difficult-to-reach features on a workpiece.
Diagram of a Micrometer
Essential Tip:
To ensure an accurate measurement, the measuring faces of a micrometer must be both flat and parallel. Micrometer measuring faces (anvil and spindle) are normally faced with carbide to reduce wear caused by repeated use. Hardened steel is sometimes used as the measuring face to reduce manufacturing cost.
Outside Micrometer Types:
Electronic Outside Micrometers measure 0-0.25 millimeters (0-1 inches) at a time. They feature an easy-to-read digital (LCD) display that is switchable from inch to metric and can be set to zero at any position.
Electronic Outside Micrometers Sets Available.
Mechanical Outside Micrometers, also known as micrometer, mics or screw gages, measure ranges are from 0" to 20" engineered in 1" steps (lengths). They are graduated in 0 .001" with a 0.0001" vernier and available in metric sizes and graduations.
Mechanical Outside Micrometer Sets Available.
Measuring Range and Accuracy:
Outside micrometers are available in a variety of spindle and anvil configurations to allow the operator to measure difficult-to-reach features on a workpiece. The measuring range on a typical OD micrometer is 1" (25mm). Some special application micrometers have a smaller or greater range than 1" (25mm). Wide range OD micrometers (interchangeable anvil micrometers) are available and reduce the cost of a micrometer set, versus the cost of individual micrometers to cover the same range (typically a 6" range).
Micrometer standards are used to verify (not calibrate) the zero-opening position on micrometers when the micrometer spindle doesn’t contact the micrometer anvil, such as micrometers over 1" (25mm) and some specialty micrometers.
How to Read a Micrometer:
The precision ground thread on the spindle is the basis for a micrometer’s high accuracy and resolution. An English micrometer spindle uses 40 TPI. One revolution of the spindle advances the measuring face by .025". The sleeve is graduated every .025" and the thimble is graduated every .001". In addition, the sleeve may also have a vernier scale that allows the operator to resolve the micrometer reading to .0001".
The micrometer sleeve is divided into forty equal parts. Each division is indicated by a vertical line that represents one-fortieth of an inch or .025". Each fourth line is marked by a longer line and a number which designates one hundred-thousandths. Example: The line on the sleeve marked "1" represents .100", the line marked "2" represents .200" and so forth.
The micrometer thimble is divided into twenty-five equal parts. One complete rotation of the thimble coincides with the smallest division on the sleeve. Thus, the division on the thimble is one-twenty-fifth of .025" or .001".
To read to one ten-thousandth requires an additional scale called the Vernier scale. In the case of a regular micrometer, the vernier consists of ten divisions, marked on the sleeve, which are spaced within nine divisions of the thimble scale. Each division on the vernier, therefore, is one-tenth shorter than that of the thimble's, thus representing .0001".
Micrometer Thimbles:
Several types of thimbles are available on micrometers. The thimble is the mechanism that advances the spindle on the micrometer. The thimble type (friction or ratchet) can help the operator gauge whether a consistent amount of torque is being applied to ensure that deviations are minimal when taking a measure.
Friction thimbles offer a smooth measuring operation and resist shock. They do not offer audible feedback, but are designed to be closer to the operator's hand.
Ratchet Stop thimbles are easy to reach and provide audible and constant measuring pressure on the spindle while measuring. They are considered to be a combination of friction and ratchet stop thimbles.
Ratchet Friction thimbles offer a smooth measuring operation and resist shock. They do not offer audible feedback, but are designed to be closer to the operator's hand.
Sliding refers to a scale with a sensor for digital readings. They are used for quick readings.
Ingress Protection Standard (IP Ratings)
Outside Micrometers are available with IP Ratings which provide a good way to compare the dust and water protection levels of a different tools resistance against particulates and coolant spray on a machine tool. IP ratings features two digits: the first digit indicates the protection level against dust and particulates and the second digit indicates the protection level against coolants and liquids.
Protection | Protection Against Human Touch and Foreign Matter | Protection Against Water | ||
|---|---|---|---|---|
Type | Description | Type | Description | |
4 | — | — | Splash-proof type | Has no harmful effect from water splashes coming from any direction |
5 | Protection against dust | Protection against harmful dust (Penetration of dust not affecting the function is considered OK.) | Jet-proof type | Has no harmful effect from direct water jets coming from any direction |
6 | Dust Sealing | Protection against the penetration of dust and perfect protection against human touch | Water-resistant type | Letting no water in from direct water jets coming from any direction |
7 | — | — | Watertight type | Letting no water in under specified conditions if submerged |