Micrometer - SKengineers

 

MICROMETER -

A micrometer, sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw widely used for accurate measurement of components in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier, and digital callipers. Micrometers are usually, but not always, in the form of callipers (opposing ends joined by a frame). 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.

Micrometers are also used in telescopes or microscopes to measure the apparent diameter of celestial bodies or microscopic objects. The micrometer used with a telescope was invented about 1638 by William Gascoigne, an English astronomer.

Types -

Specialized types -

Another large micrometer in use -

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. (often known by the brand names Mul-T-Anvil and Uni-Mike).

Blade micrometers –

have a matching set of narrow tips (blades). They allow, for example, the measuring of a narrow o-ring groove.

Pitch-diameter micrometers –

(aka thread 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. 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 (spherical) 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 (which is also done with conical anvils or the 3-wire method, the latter of which uses similar geometry as the pair-of-balls approach).

Bench micrometers –

are tools for inspection use whose accuracy and precision are around half a micrometre (20 millionths of an inch, "a fifth of a tenth" in machinist jargon) and whose repeatability is around a quarter micrometre ("a tenth of a tenth"). 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 (versus the two points of a standard outside micrometer). An example of when this is necessary is measuring the diameter of 3-flute endmills and twist drills.

Parts

The parts of a micrometer caliper. Note the addition of a unit conversion chart etched onto the frame, useful for converting between fractional inch measurements and their decimal equivalents.

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. It is often covered by insulating plastic plates which further reduce heat transference.

Explanation: if one holds 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 specified temperature at which the measurement is correct (often 20 °C [68 °F], which is generally considered "room temperature" in a room with HVAC). Toolrooms are generally kept at 20 °C [68 °F].

Anvil -

The shiny part that the spindle moves toward, and that the sample rests against.

Sleeve, barrel, or stock -

The stationary round component with the linear scale on it, sometimes with vernier markings. In some instruments the scale is marked on a tight-fitting but movable cylindrical sleeve fitting over the internal fixed barrel. This allows zeroing to be done by slightly altering the position of the sleeve.

Lock nut, lock-ring, or thimble lock -

The knurled component (or lever) that one can tighten to hold the spindle stationary, such as when momentarily holding a measurement.

Screw -

(Not visible) The heart of the micrometer, as explained under "Operating principles". It is inside the barrel. This references the fact that the usual name for the device in German is Messschraube, literally "measuring screw".

Spindle -

The shiny cylindrical component that the thimble causes to move toward the anvil.

Thimble -

The component that one's thumb turns. Graduated markings.

Ratchet stop -

(Not illustrated) Device on end of handle that limits applied pressure by slipping at a calibrated torque.

Reading -

Customary/Imperial system

Micrometer thimble showing a reading of 0.2760 ± 0.0005 in.

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 (1 ÷ 40 = 0.025), 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 (0.025 ÷ 25 = 0.001). Thus, the reading is given by the number of whole divisions that are visible on the scale of the sleeve, 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 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 (as shown in the image), 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.

Metric system -

Micrometer thimble with a reading of 5.78 ± 0.005 mm.

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 millimeter. The longitudinal line on the sleeve 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 on the sleeve (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 micrometers –

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.

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 millimeters (if any) and the number of hundredths of a millimeter, as with an ordinary micrometer, and the number of thousandths of a millimeter 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 millimeter = 0.000393 inch, and 0.002 millimeter = 0.000078 inch (78 millionths) or alternatively, 0.0001 inch = 0.00254 millimeters. 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.

Other Types of Micrometer -

Universal Micrometers -

These come complete with interchangeable anvils, which may be flat, spherical, spline, disk, blade, point, or knife edge. You may be expected to use universal micrometers featuring modular components, which allow for outside, mic depth, or alternative functionality.

Pitch Micrometers -

Otherwise known as the thread mic, this tool features a specially designed set of thread-shaped tips for successful identification of the screw thread diameter.

Limit Micrometers -

Particularly well suited to measuring the thickness of tubes, micrometer limit mics come complete with two anvils and two spindles, functioning as effective snap gauges. The gaps correspond directly to the upper and lower tolerance levels.

Bore Micrometers -

Commonly featuring a three-anvil head in combination with a solid base, bore micrometers are ideally suited to the accurate measurement of inside diameters. They are particularly helpful when it comes to the measurement of objects situated around machine fluids and coolants.

Bench Micrometers -

Bench micrometers offer extremely high levels of accuracy and are typically used during workplace inspections. They allow for measurement up to somewhere in the region of 20 millionths of an inch, with a repeatability of around a quarter of a millimetre.

V Micrometers -

Purpose-made for external measurement, V micrometers come complete with small V-blocks for the anvil. They are ideally suited to the measurement of circle diameter, with equidistant separation of three points. This allows for the effective measurement of three-flute end-mills and twist drills.

Micrometer Measurement Conversions -

This section details how to convert your micrometer result into another measurement unit.

Micrometers to mm -

The quickest and easiest way to convert micrometers (um) to millimetres is to divide by 1000. An alternative method is to move the decimal point three units to the left of the original figure.

Micrometers to inches -

There are 25,400 micrometers to every inch, with 1 metre being equivalent to 1,000,000 micrometers. You should also be aware that each micrometer equals 3.9×10e-5 (with e meaning to the power of).

Micrometers to cm -

1um is equivalent to 0.0001cm (otherwise written as10e-4).

How to Calibrate a Micrometer -

To make sure that your micrometer returns accurate data, you will need to ensure that your tool is kept calibrated.

 

How Do I Test if My Micrometer Needs Calibrating?

Standard one-inch micrometers have readout divisions of 0.001 inch and an accuracy level of ±0.0001 inch. However, you must ensure that both the micrometer and the object being measured are at room temperature for this high level of accuracy.

The micrometer reading test will involve the measurement of guide blocks in order to ascertain the desirable accuracy. If such a gauge block is known to be 0.75000 ±0.00005inch then the micrometer should give a reading of 0.7500 inch. If the corresponding measurement is 0.7503 or more, then the micrometer will be deemed to be out of calibration.

If you want to avoid such calibration issues then you must take an exceptional level of care, carefully cleaning, using, and storing the micrometer for continued use. It might be necessary to perform micrometer adjustment and recalibration in some instances. However, adjustment won’t be sufficient when it comes to correcting issues such as the micrometer being misshapen or of the incorrect size. Repair will be necessary for such instances.

Calibrating A Micrometer -

The following steps explain how to calibrate a micrometer and adjust it to zero.

Before proceeding to calibration, it is necessary to ensure that the tool is working effectively and that there aren’t any binding or related issues. You should clean the anvils and make sure that the reading is set to zero

The next step will be to check the micrometer at a range of test points using gauge blocks or alternative standards with high levels of accuracy. A variety of readings should be taken at each test point. The tolerance level of the particular micrometer should be taken into account when writing down the readings

It is highly likely that your micrometer features a small pin spanner. This should allow for the turning of the sleeve in relation to the barrel, ensuring the optimum repositioning of the zero line in relation to the thimble markings

You can expect to find a small hole in the sleeve, designed for acceptance of the spanner’s pin. In following this calibration process, you will have the assurance of avoiding the non-zero error, which may otherwise occur when the jaws are shut. You are advised to repeat the zeroing process a few times for the assurance of micrometer accuracy.

Any Doubt Related To Micrometer, You Can Comment In The Comment Box...