Micrometers for Outside Measurements

The most common application for micrometers is the measurement of length dimensions between two parallel end surfaces on the outer side of an object or feature. This is currently known as external or outside measurement.
The micrometer is a convenient instrument for outside measurements because 1) it supplies promptly the desired size information; 2) it has a relatively wide range of measurement ( for example, 10,000 times the least increment of its scale ); 3) its use does not require specialized skill; and 4) the measuring tool is easily adaptable to diverse object forms. These and many other advantages account for the wide application of micrometers in production and inspection.
It is a logical consequence of this extended field of potential applications that outside micrometers are required and made in many varieties of sizes, forms and accuracy grades.
The size of a micrometer refers to the limits of its measuring range . The most common size is the one-inch size, which permits measurements over a range from zero to one inch in length. The same range of one inch can also be applied to other limit dimensions by mounting the basic screw mechanism on other frame sizes . Accordingly , outside micrometers are also made in sizes of 1 to2 inches , from 2 to 3 inches, and so on to a 24-inch maximum limit, exceptionally even larger . Because micrometers whose lower range limit is greater than zero cannot have direct contact between spindle face and anvil, setting masters must be used for calibration purposes. These setting gages, also known as standards, must be a size equal to the lower limit of the micrometer’s measuring range, and are usually procured together with the micrometer.
The larger size micrometers are available in sets, complete with standards , generally in cases with individual compartments , permitting micrometer measurements over a wide range of lengths , from zero up to the maximum size of the largest instrument in the set.
Although the basic form of the outside micrometer frame is a bow, with the spindle at one end and the anvil at the other end, variations in design are frequent, particularly in the larger sizes . The common micrometers have fixed anvils , limiting the useful range of measurements to the models with adjustable or interchangeable anvils, resulting in a widely extended measuring range.
While on most standard micrometers the contact surfaces on the spindle and on the anvil are the flat faces of these basically cylindrical members, micrometers are also made for special measurements where the contact tips have particular forms. Examples are the following:
a. The disc type micrometer to measure measurements on features that provide a narrow clearance only for the penetration of the gage contact elements.
b. The blade type micrometer to measure diameters and the bottom of narrow grooves.
c. The screw thread micrometer with vee-formed anvil and conical spindle tip, to measure pitch diameters;
d. The pointed contact micrometer , with conical tips having a very small flat land on the spindle and on the anvil , to measure inside recesses where regular tips could not penetrate ; and
e. The ball contact micrometer for measuring the wall thickness of tubes and other cylindrical bodies. Usually the ball anvil is supplied as an attachment for standard outside micrometers. The attachment can easily be snapped on the end of either the anvil or the spindle . The diameter of the ball, commonly 0.200 inch, must be subtracted from the value of the micrometer reading.
It is customary to assign the grade or quality level of micrometers on the basis of the least increment of the indicated dimension . Although most micrometers have screws with 40 threads per inch, resulting in a 0.025-inch advance per turn of the screw, the graduations of the thimble can vary , providing different subdivisions of that advance for each compete turn. A plain 25 graduation on the thimble circumference observed by a single line mark on the sleeve will permit the reading of each 0.001-inch spindle advance, this type of micrometer being commonly designated the “thousandths�?grade. Substituting a vernier scale for the single reference mark on the sleeve will permit the subdivision of each thimble graduation interval into ten parts, resulting in a “ten-thousandths�?micrometer .Similar results can be accomplished by retaining a single line reference mark, but subdividing each of the 25 basic thimble graduations into 10 intermediate increments . This arrangement will allow the direct reading of the ten-thousandths , thereby eliminating the potential errors connected with the reading of the vernier . For practical reasons , however , the direct subdivision of the basic thimble graduations is only feasible for oversize thimbles , with 2-inch or larger diameter.
Although the least graduation is not equivalent to accuracy , it is a common practice with the manufactures of micrometers to assign instruments with the curacy , supported by suitable design and appropriate workmanship.