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A micrometer, sometimes known as a micrometer screw gauge, is a device used widely in mechanical engineering and machining for precisely measuring, along with other metrological instruments such as dial calipers and vernier calipers. Micrometers are often, but not always, in the form of calipers.
Colloquially the word micrometer is often shortened to mike.
Types
Basic types
The image shows three common types of micrometers; the names are based on their application:
- Outside micrometer (aka micrometer caliper), typically used to measure wires, spheres, shafts and blocks.
- Inside micrometer, used to measure the diameter of holes.
- Depth micrometer, measures depths of slots and steps.
- Bore micrometer, typically a three-anvil head on a micrometer base used to accurately measure inside diameters.
- Tube micrometer, used to measure the thickness of tubes.
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; in the form of a large disc; etc.
Universal micrometer sets come with interchangeable anvils: flat, spherical, spline, disk, blade, point, knife-edge, etc. 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 (often known by the brand names Mul-T-Anvil and Uni-Mike).
Blade mics have a matching set of narrow tips (blades). They allow, for example, the measuring of a narrow o-ring groove.
Pitch-diameter mics 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.
Micrometer stops are essentially inside mics that are mounted on the table of a manual milling machine or other machine tool, in place of simple stops. They help the operator to position the table precisely.
Operating principles
The accuracy of a micrometer derives from the accuracy of the threadform of the screw that is at its heart.
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 (and vice versa), through the constant known as the screw's lead (/li:d/). A screw's lead is the distance it moves forward axially with one complete turn (360°). (In most threads [that is, in all single-start threads], lead and pitch refer to essentially the same concept.)
- 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 (here, outer 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 (magnified) 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.
Parts
A micrometer is composed of:
- Frame
- The C-shaped body that holds the anvil and barrel in constant relation to each other. It is thick because it needs to minimize flexion, expansion, and contraction, which would distort the measurement. The frame is heavy and consequently has a high thermal mass, to prevent substantial heating up by the holding hand/fingers. Explanation: if you hold the frame long enough so that it heats up by 10°C, then the increase in length of any 10 cm linear piece of steel is of magnitude 1/100 mm. For micrometers this is their typical accuracy range. Micrometers typically have a temperature specified, at which the measurement is correct.
- Anvil
- The shiny part that the spindle moves toward, and that the sample rests against.
- Sleeve / barrel / stock
- The stationary round part with the linear scale on it. Sometimes vernier markings.
- Lock nut / lock-ring / thimble lock
- The knurled part (or lever) that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement.
- Screw
- (not seen) The heart of the micrometer, as explained under "Operating principles". It is inside the barrel. (No wonder that the usual name for the device in German is Messschraube, literally "measuring screw".)
- Spindle
- The shiny cylindrical part that the thimble causes to move toward the anvil.
- Thimble
- The part that one's thumb turns. Graduated markings.
- Ratchet stop
- (not shown in illustration) Device on end of handle that limits applied pressure by slipping at a calibrated torque.
Reading
Inch system
Micrometer thimble showing 0.276 inch
The spindle of an inch-system micrometer has 40 threads per inch, so that one turn moves the spindle axially 0.025 inch (1 ÷ 40 = 0.025), equal to the distance between two graduations on the frame. 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 (0.025 ÷ 25 = 0.001). Thus, the reading is given by the number of whole divisions that are visible on the scale of the frame, multiplied by 25 (the number of thousandths of an inch that each division represents), plus the number of that division on the thimble which coincides with the axial zero line on the frame. 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 frame, indicating hundreds of thousandths, the reading can easily be taken mentally.
Suppose the thimble were screwed out so that graduation 2, and three additional sub-divisions, were visible (as shown in the image), and that graduation 1 on the thimble coincided with the axial line on the frame. The reading then would be 0.2000 + 0.075 + 0.001, or .276 inch.
Metric system
The spindle of an ordinary metric micrometer has 2 threads per millimetre, and thus one complete revolution moves the spindle through a distance of 0.5 millimetre. The longitudinal line on the frame is graduated with 1 millimetre divisions and 0.5 millimetre subdivisions. The thimble has 50 graduations, each being 0.01 millimetre (one-hundredth of a millimetre). Thus, the reading is given by the number of millimetre divisions visible on the scale of the sleeve plus the particular division on the thimble which coincides with the axial line on the sleeve.
Suppose that the thimble were screwed out so that graduation 5, and one additional 0.5 subdivision were visible (as shown in the image), and that graduation 28 on the thimble coincided with the axial line on the sleeve. The reading then would be 5.00 + 0.5 + 0.28 = 5.78 mm.
Vernier
Some micrometers are provided with a vernier scale on the sleeve in addition to the regular graduations. These permit measurements within 0.001 millimetre to be made on metric micrometers, or 0.0001 inches on inch-system micrometers.
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. The number of this coinciding vernier line represents the additional digit.
Thus, the reading for metric micrometers of this type is the number of whole millimetres (if any) and the number of hundredths of a millimetre, as with an ordinary micrometer, and the number of thousandths of a millimetre given by the coinciding vernier line on the sleeve vernier scale.
For example, a measurement of 5.783 millimetres would be obtained by reading 5.5 millimetres on the sleeve, and then adding 0.28 millimetre as determined by the thimble. The vernier would then be used to read the 0.003 (as shown in the image).
Inch micrometers are read in a similar fashion.
Note: 0.01 millimetre = 0.000393 inch, and 0.002 millimetre = 0.000078 inch (78 millionths) or alternately, 0.0001 inch = 0.00254 millimetres. Therefore, metric micrometers provide smaller measuring increments than comparable inch unit micrometers—the smallest graduation of an ordinary inch reading micrometer is 0.001 inch; the vernier type has graduations down to 0.0001 inch (0.00254 mm). When using either a metric or inch micrometer, without a vernier, smaller readings than those graduated may of course be obtained by visual interpolation between graduations.
Torque repeatability via torque-limiting ratchets or sleeves
An additional feature of many micrometers is the inclusion of a torque-limiting device on the thimble—either a spring-loaded ratchet or a friction sleeve. Normally, one could use the mechanical advantage of the screw to force the micrometer to squeeze the material or tighten the screw threads, giving an inaccurate measurement. However, by attaching a thimble that will ratchet or friction slip at a certain torque, the micrometer will not continue to advance once sufficient resistance is encountered. This results in greater accuracy and repeatability of measurements—most especially for low-skilled or semi-skilled workers, who may not have developed the light, consistent touch of a skilled user.
Testing and calibration
The accuracy of micrometers is checked by using them to measure gauge blocks, rods, or similar standards whose lengths are precisely and accurately known. If the gauge block is known to be 0.7500" (± .00005"), then the micrometer should measure it as 0.7500". If the micrometer measures 0.7516", then it is out of calibration.
The accuracy of the gauge blocks themselves is traceable through a chain of comparisons back to a master standard, such as are maintained in measurement standards laboratories.
History of the device and its name
The word micrometer is a neoclassical coinage from Greek micros, "small", and metron, "measure". Merriam-Webster Collegiate says that English got it from French and that its first known appearance in English writing was in 1670. Neither the metre nor the micrometre nor the micrometer (device) as we know them today existed at that time. However, humans of that time did have much need for, and interest in, the ability to measure small things, and small differences; the word no doubt was coined in reference to this endeavor, even if it did not refer specifically to its present-day senses.
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. Its adaptation for the precise measurement of handheld objects was made by Jean Laurent Palmer of Paris in 1848; the device is therefore often called palmer in French, and tornillo de Palmer ("Palmer screw") in Spanish. (Those languages also use the micrometer cognates: micromètre, micrómetro.) 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 Williams Morley added to the precision of micrometric measurements and proved their accuracy in a complex series of experiments.
