Micrometers are specially designed calipers that work on the principle of a screw with a precisely controlled lead. A given rotation of the screw moves the spindle (movable jaw) a predictable distance such that accurate measurement is possible. Figure 1 shows the principal parts of a micrometer.
The C-shaped frame should be used as the "handle" of this device. The spindle, thimble, and thimble sleeve are integrally connected to the lead screw (not visible in Figure 1). Turning the thimble sleeve turns the lead screw such that the span between the spindle and anvil becomes smaller or larger, depending on direction of rotation. The primary scale is on the barrel sleeve and is graduated in increments of 0.025 inch. The secondary scale is on the thimble, and is graduated in 25 increments with each increment equivalent to 0.001-inch movement along the sleeve scale.
Some micrometers also have a vernier such as that shown in Figure 1. The vernier has ten graduations with each representing 0.0001-inch. The spindle lock serves to hold the spindle at the desired span when locked. The friction nut is a ratchet device that helps to ensure uniform pressure against the boundaries when measurements are made.
Micrometers are made in many sizes with either inch or millimeter units. They are designated by the maximum size that can be spanned by the frame. However, the spindle travel is normally only one inch for any size. For example, while a one-inch micrometer measures from 0" to 1", a 2" micrometer spans two inches but still has one inch of spindle travel. Thus, the 2" micrometer measures from 1?ÃƒÆ’Ã‚Â¯Ãƒâ€šÃ‚Â¿Ãƒâ€šÃ‚Â½?ÃƒÆ’Ã‚Â¯Ãƒâ€šÃ‚Â¿Ãƒâ€šÃ‚Â½?? to 2". Sets of micrometers are available in many sizes; however, the total amount of adjustment will generally be only one inch of spindle travel. Metric micrometers have equivalent ranges in millimeters. On larger micrometers, interchangeable anvil extensions are used to reduce the span in several increments for the sake of economy.
Micrometers have such improvements and special features as friction sleeves, ratchet stops, spindle locks, and slant line graduations and vernier scales that read to one/ten-thousandths (0.0001") of an inch. Probably the most dramatic change to occur in recent micrometer design is the advent of the digital micrometer. Digital micrometers can display a digital readout to 0.00005" in the English system and .001mm in the Metric system. The readout can be read locally or can be transmitted through a cable to a data processor that is capable of statistically analyzing the data.
The operation of a micrometer is based on an accurately machined screw, usually having 40 threads to the inch. With one complete revolution, such a screw will advance 1/40th or 0.025. By putting graduations on the barrel sleeve, which is fixed, and on the thimble, which moves with the screw, it is possible to determine the amount of movement of the screw, and thus to measure.
On inch-reading micrometers, 40 graduations are placed on the barrel to match the number of threads on the screw such that each represents 0.025 inch of movement. To facilitate reading, every fourth line is numbered to show 0.100", 0.200" etc. to 1". (These correspond to the 0, 1, and 2 visible on the sleeve scale of Figure 1.)
The beveled edge of the thimble has 25 divisions around the circumference, numbered from 0 to 25. A beveled edge is used to reduce the parallax with the sleeve scale. When the micrometer is closed, only the zero line on the barrel can be seen next the beveled edge of the thimble, and the zero line on the thimble is aligned with the long axial line on the barrel, called the datum.
The easiest way to read an English Mike is to think that you are making change for a ten-dollar bill.
The numbers on the sleeve represent dollars. (Count them first.)
The vertical lines on the sleeve represent quarters. (Count them second.)
The graduations or divisions on the thimble represent cents. (Count them third.)
Some mikes have the capability of reading to ten-thousandths of an inch. Adding a vernier scale on the sleeve does this. This reading, obtained by matching lines from sleeve to thimble, represents parts of a penny. (Count this last.)
Add up your change and put a decimal point in the proper place instead of a dollar sign.
Each sleeve graduation above the reading line = 1.00mm.
Each sleeve graduation below the reading line = 0.50mm.
Each thimble graduation = 0.01mm.
To obtain reading, add the number of 1.00mm and 0.50 mm sleeve graduations.
Add the number of 0.01mm thimble graduations indicated by the thimble line that comes closest to matching the reading line.
To read a metric micrometer with vernier, add the sleeve and thimble graduations. Then see which vernier graduation coincides with a line on the thimble. If it is the line marked 2, add 0.002 mm; if it is the line marked 4, add 0.004 mm, etc.
The most common micrometer is the outside micrometer shown in Figure 1. It is used for measuring length dimensions where the spindle and anvil can be placed against the boundaries of the dimension. Most accurate measurement occurs when the two boundaries are parallel, flat surfaces. This permits the flats of the spindle and anvil to fit accurately against the boundaries.
Inside micrometers are used for precision measurements of the diameters of holes and width of slots. With the use of supplementary measuring rods and spacing collars, holes from 1" to 12" in diameter may be measured. This instrument is essentially the spindle and sleeve of a conventional micrometer and measuring rods of various lengths and diameters as shown in Figure 2. Various rod lengths permit measuring the diameter of a hole at various depths.
This instrument is a conventional micrometer modified with disk-shaped anvil and spindle end (Figure 3). The extended lips of the spindle and anvil permit measurement of fin-like projections and other outside features where use of a standard micrometer is obstructed.
This modification of the conventional micrometer has a screwdriver shaped anvil and spindle end for measurements at the base of recesses, grooves, and other shallow depths of the outside dimension. The spindle is non-rotating as shown in Figure 4.
In this instrument, interchangeable measuring rods of various lengths replace the conventional spindle; there is no anvil. The rod projects through an accurately finished base that provides a reference surface at right angles to the spindle. The rod travels as the thimble is turned, and depths up to 9" can be accurately measured, as shown in Figure 5. Unlike all other micrometers, depth micrometers measure from a plane to a point.
This instrument permits direct measurement of hole size on a micrometer scale. It has three contact points traveling at right angles to the barrel, spaced 120 degrees apart, which makes contact with the wall of the hole. With extension rods, readings accurate within a few ten-thousandths can be taken in holes up to 15" deep.
This instrument is used for direct readings of the pitch diameters of screw threads. The V-anvil and V-conical spindle tip are shaped to conform to the standard shape of the threads to be measured, as shown in Figure 6.
This instrument uses conical anvil and spindle tips to measure recesses where flat tips could not reach.
Since the thimble graduations on a micrometer represent divisions of the smallest increment on the barrel, it is evident that enlargement of the circumference of the thimble will provide for wider spacing of the graduations and for additional finer divisions. A large thimble would be impractical on an instrument for hand use, but it is provided in the bench micrometer, which permits direct reading in 0.0001" and 0.002mm. The micrometer head is mounted on a rigid base, leaving both hands free for manipulation. Bench micrometers (sometimes called super mics) are used for outside dimensions where greater accuracy and precision are required.
Accuracy and precision of micrometers are generally better than the vernier caliper. Even with precise manufacture of these two types of instruments, the readability of a micrometer is better, and the vernier of the micrometer increases sensitivity by a factor of ten. Features of the micrometer that affect its accuracy and precision include the following as described below.
The accuracy, linearity, and stability of the thread directly affect accuracy and precision of measurements. The threads are precision ground during manufacture, and hardened material is used that has good thermal stability. Wear of the threads during the life of the micrometer gradually reduces accuracy and precision. Wear in localized areas can also be considered as altering linearity.
The manufacturer of the micrometer holds very tight tolerances on flatness and parallelism of the measuring surfaces. Wear of these surfaces during use will degrade accuracy and precision.
Deflection of the frame during measurements reduces accuracy and precision. Such deflection is eliminated by design of rigid frames and controlling the pressure applied across the measuring surfaces. Control of measuring pressure can be improved by use of ratchets or friction sleeves on the spindle movement.
Inadequate alignment and reading errors are the most common operator induced errors. Reading errors are reduced by practice and more practice and by attentiveness during measurement. Alignment means finding the correct span. For small outside micrometers, the best "feel" is achieved normally by manipulation of the micrometer with one hand (see Figure 7).
For large outside micrometers, the weight of the micrometer is supported by one hand on the frame while the spindle is adjusted with the other hand. For inside dimensions, the maximum span is sought. This occurs when the measuring surfaces are at right angles to the face of the bore.
Micrometers, like other measuring devices, require care to provide accurate measurements.
When using a micrometer: