Micrometer (device)

A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw for accurate measurement of the size of components. It is widely used in mechanical engineering, machining, metrology and most mechanical trades, along with other dimensional instruments such as dial, vernier, and digital calipers. Micrometers are usually, but not always, in the form of calipers. The spindle is a very accurately machined screw and the object to be measured is placed between the spindle and the anvil. The spindle is moved by turning the ratchet knob or thimble until the object to be measured is lightly touched by both the spindle and the anvil.

History

The word micrometer is a neoclassical coinage from and. According to the Merriam-Webster Collegiate Dictionary, the word was loaned to English from French, with its first known appearance in English writing being in 1670. Neither the metre nor the micrometre nor the micrometer as we know them today existed at that time. However, the people of that time did have much need for, and interest in, the ability to measure small things and small differences. The word was no doubt coined in reference to this endeavor, even if it did not refer specifically to its present-day senses.
The London Science Museum contains an exhibit "James Watt's end measuring instrument with micrometer screw, 1776" which the science museum claims is probably the first screw micrometer made. 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. However, as the science museum notes, there is a possibility that this instrument was not made c.1776 by Watt, but 1876 when it was placed in that year's Special Loan Exhibition of scientific instruments in South Kensington.
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. In 1844, details of Whitworth's workshop micrometer were published. 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. The ends of the cylinders where they met was of hemispherical shape. One screw was fitted with a wheel graduated to measure to the ten thousandth of an inch. 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 first documented development of handheld micrometer-screw calipers was by Jean Laurent Palmer of Paris in 1848; the device is therefore often called palmer in French, tornillo de Palmer in Spanish, and calibro Palmer in Italian. The micrometer caliper was introduced to the mass market in anglophone countries by Brown & Sharpe in 1867, allowing the penetration of the instrument's use into the average machine shop. Brown & Sharpe were inspired by several earlier devices, one of them being Palmer's design. 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. Beginning in the early 20th century, one could no longer truly master tool and die making, machine tool building, or engineering without some knowledge of the science of metrology, as well as the sciences of chemistry and physics.

Types

Specialized types

Each type of micrometer caliper can be fitted with specialized anvils and spindle tips for particular measuring tasks. For example, the anvil may be shaped in the form of a segment of screw thread, in the form of a v-block, or in the form of a large disc.

Universal micrometer sets come with interchangeable anvils, such as flat, spherical, spline, disk, blade, point, and knife-edge. The term universal micrometer may also refer to a type of micrometer whose frame has modular components, allowing one micrometer to function as outside mic, depth mic, step mic, etc..
Blade micrometers have a matching set of narrow tips. They allow, for example, the measuring of a narrow o-ring groove.
Pitch-diameter micrometers have a matching set of thread-shaped tips for measuring the pitch diameter of screw threads.
Limit mics have two anvils and two spindles, and are used like a snap gauge. The part being checked must pass through the first gap and must stop at the second gap in order to be within specification. The two gaps accurately reflect the top and bottom of the tolerance range.
Bore micrometer, typically a three-anvil head on a micrometer base used to accurately measure inside diameters.
Tube micrometers have a cylindrical anvil positioned perpendicularly to a spindle and is used to measure the thickness of tubes.
Micrometer stops are micrometer heads that are mounted on the table of a manual milling machine, bedways of a lathe, or other machine tool, in place of simple stops. They help the operator to position the table or carriage precisely. Stops can also be used to actuate kickout mechanisms or limit switches to halt an automatic feed system.
Ball micrometers have ball-shaped anvils. They may have one flat and one ball anvil, in which case they are used for measuring tube wall thickness, distance of a hole to an edge, and other distances where one anvil must be placed against a rounded surface. They differ in application from tube micrometers in that they may be used to measure against rounded surfaces which are not tubes, but the ball anvil may also not be able to fit into smaller tubes as easily as a tube micrometer. Ball micrometers with a pair of balls can be used when single-tangential-point contact is desired on both sides. The most common example is in measuring the pitch diameter of screw threads.
Bench micrometers are tools for inspection use whose accuracy and precision are around half a micrometre and whose repeatability is around a quarter micrometre. An example is the Pratt & Whitney Supermicrometer brand.
Digit mics are the type with mechanical digits that roll over.
Digital mics are the type that uses an encoder to detect the distance and displays the result on a digital screen.
V mics are outside mics with a small V-block for an anvil. They are useful for measuring the diameter of a circle from three points evenly spaced around it. An example of when this is necessary is measuring the diameter of 3-flute endmills and twist drills.
Operating principles

Micrometers use the screw to transform small distances into large rotations of the screw that are big enough to read from a scale. The accuracy of a micrometer derives from the accuracy of the thread-forms that are central to the core of its design. In some cases it is a differential screw. The basic operating principles of a micrometer are as follows:

The amount of rotation of an accurately made screw can be directly and precisely correlated to a certain amount of axial movement, through the constant known as the screw's lead. A screw's lead is the distance it moves forward axially with one complete turn.
With an appropriate lead and major diameter of the screw, a given amount of axial movement will be amplified in the resulting circumferential movement.

For example, if the lead of a screw is 1 mm, but the major diameter is 10 mm, then the circumference of the screw is 10π, or about 31.4 mm. Therefore, an axial movement of 1 mm is amplified to a circumferential movement of 31.4 mm. This amplification allows a small difference in the sizes of two similar measured objects to correlate to a larger difference in the position of a micrometer's thimble. 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.
In classic-style analog micrometers, the position of the thimble is read directly from scale markings on the thimble and sleeve. A vernier scale is often included, which allows the position to be read to a fraction of the smallest scale mark. In digital micrometers, an electronic readout displays the length digitally on an LCD on the instrument. There also exist mechanical-digit versions, like the style of car odometers where the numbers "roll over".

Parts

A micrometer is composed of:

Reading

Micrometers are high precision instruments. Proper use of them requires not only understanding their operation itself but also the nature of the object and the dynamic between the instrument and the object as it is being measured. 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.

Customary/Imperial system

The spindle of a micrometer graduated for the Imperial and US customary measurement systems has 40 threads per inch, so that one turn moves the spindle axially 0.025 inch, equal to the distance between adjacent graduations on the sleeve. The 25 graduations on the thimble allow the 0.025 inch to be further divided, so that turning the thimble through one division moves the spindle axially 0.001 inch. Thus, the reading is given by the number of whole divisions that are visible on the scale of the sleeve, multiplied by 25, plus the number of that division on the thimble which coincides with the axial zero line on the sleeve. The result will be the diameter expressed in thousandths of an inch. As the numbers 1, 2, 3, etc., appear below every fourth sub-division on the sleeve, indicating hundreds of thousandths, the reading can easily be taken.
Suppose the thimble were screwed out so that graduation 2, and three additional sub-divisions, were visible on the sleeve, and that graduation 1 on the thimble coincided with the axial line on the sleeve. The reading would then be 0.2000 + 0.075 + 0.001, or 0.276 inch.