Micrometer (device)

A micrometer, sometimes known as a micrometer screw gauge (MSG), is a device incorporating a calibrated screw widely used for accurate measurement of components[1] in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and digital calipers.

The first ever micrometric screw was invented by William Gascoigne in the 17th century, as an enhancement of the vernier; it was used in a telescope to measure angular distances between stars and the relative sizes of celestial objects.

This instrument is intended to measure items very accurately by placing them between the two anvils and then advancing one using a fine micrometer screw until both are in contact with the object, the distance between them being precisely recorded on the two dials.

[4] Henry Maudslay built a bench micrometer in the early 19th century that was jocularly nicknamed "the Lord Chancellor" among his staff because it was the final judge on measurement accuracy and precision in the firm's work.

[6] This was described as having a strong frame of cast iron, the opposite ends of which were two highly finished steel cylinders, which traversed longitudinally by action of screws.

His object was to furnish ordinary mechanics with an instrument which, while it afforded very accurate indications, was yet not very liable to be deranged by the rough handling of the workshop.

The micrometer caliper was introduced to the mass market in anglophone countries by Brown & Sharpe in 1867,[8] allowing the penetration of the instrument's use into the average machine shop.

In 1888, Edward W. Morley added to the precision of micrometric measurements and proved their accuracy in a complex series of experiments.

The culture of toolroom accuracy and precision, which started with interchangeability pioneers including Gribeauval, Tousard, North, Hall, Whitney, and Colt, and continued through leaders such as Maudslay, Palmer, Whitworth, Brown, Sharpe, Pratt, Whitney, Leland, Johansson, and others, grew during the Machine Age to become an important part of combining applied science with technology.

Each type of micrometer caliper can be fitted with specialized anvils and spindle tips for particular measuring tasks.

In some micrometers, even greater accuracy is obtained by using a differential screw adjuster to move the thimble in much smaller increments than a single thread would allow.

For simplicity's sake, in the figures and text below issues related to deformation or definition of the length being measured are assumed to be negligible unless otherwise stated.

But note that the nature of the object being measured often requires one should round the result to fewer significant figures than which the instrument is capable.

The additional digit of these micrometers is obtained by finding the line on the sleeve vernier scale which exactly coincides with one on the thimble.

This calibration procedure will cancel a zero error: the problem that the micrometer reads nonzero when its jaws are closed.

[16] The accuracy of micrometers is checked by using them to measure gauge blocks,[17] rods, or similar standards whose lengths are precisely and accurately known.

If the micrometer is in good condition, then they are all so near to zero that the instrument seems to read essentially "-on" all along its range; no noticeable error is seen at any locale.

A micrometer can be calibrated on-site anytime, at least in the most basic and important way (if not comprehensively), by measuring a high-grade gauge block and adjusting to match.

The accuracy of the gauge blocks themselves is traceable through a chain of comparisons back to a master standard such as the international prototype of the meter.

These master standards have extreme-accuracy regional copies (kept in the national laboratories of various countries, such as NIST), and metrological equipment makes the chain of comparisons.

For standard kinds of instruments, in practice it is easier and faster, and often no more expensive, to buy a new one rather than pursue refurbishment.

Modern micrometer with a reading of 1.639 ± 0.005 mm. Assuming no zero error, this is also the measurement. (One may need to enlarge the image to read it.)
Outside, inside, and depth micrometers. The outside micrometer has a unit conversion chart between fractional and decimal inch measurements etched onto the frame
Gascoigne's Micrometer, as drawn by Robert Hooke , c. 1668
Large micrometer caliper, 1908
Another large micrometer in use
Animation of a micrometer in use. The object being measured is in black. The measurement is 4.140 ± 0.005 mm.
Diagram of a micrometer showing a measurement of 7.145 mm ± 0.005 mm
Imperial unit micrometer thimble showing a reading of 0.2760 in. The main scale reads 0.275 in (exact) plus 0.0010 in (estimated) on the secondary scale (the last zero is an estimated tenth). The reading would be 0.2760 ± 0.0005 in, which includes plus/minus half the width of the smallest ruling as the error. Here it has been assumed that there is no zero point error (often untrue in practice).
Micrometer thimble with a reading of 5.779 ± 0.005 mm. (You must enlarge the image to be able to read the scale to its fullest precision.) The reading consists of exactly 5.5 mm from the main scale plus an estimated 0.279 mm from the secondary scale. Assuming no zero error, this is also the measurement.
Vernier micrometer reading 5.783 ± 0.001 mm, comprising 5.5 mm on main screw lead scale, 0.28 mm on screw rotation scale, and 0.003 mm added from vernier.