Considering the wide use of screw micrometers, it is desirable to know the degree of confidence that the results of micrometer measurements deserve. The repetitive precision of measurements with a screw micrometer depends on two sets of factors: the inherent accuracy of the measuring instrument , and the combined effect of process errors.
The accuracy of the micrometer will be governed primarily by the following two factors:
The degree of calibration of the spindle movement, which will be affected by the lead errors of the screw; the effect is a usually cumulative , and increases the length of the spindle travel.( Note : The aggregate effect of inaccuracies originating from screw lead errors can be reduced by â€śbalanced calibrationâ€?that is , by adjusting the thimble to produce error-free reading in the middle of the total , or of the most frequently used section of the spindle traverse.)
The linearity of the spindle movement , requiring that any fractional rotation of the screw should result in a proportional advance of the measuring spindle; â€śdrunkenâ€?thread , or stick-and-slip condition of the screw in the nut , will have an adverse effect. Deficient linearity will become particularly harmful when superimposed on major calibration errors.
The instrument accuracy is substantially improved by manufacturing the spindles of well stabilized material , precisely grinding the screw thread after hardening , using lapped nuts and applying in general a high degree of workmanship in the manufacturing process of these instruments.
The calibration process is a dependable means for assessing the accuracy of the micrometer indications . The process consists of the sequential measurement of gage blocks of known size with the micrometer. The blocks to e measured are selected to represent distances over which the spindle travels for a full or a half turn of the screw. In each step the dimension indicated by the micrometer is recorded on a calibration chart. The zero line of the chart indicates the nominal size for each reading , and digressions of the actual indications are plotted in this sense and by the scale of the chart ordinate.
The chart recording will usually have a highest crest and a lowest valley , representing the points of the maximum deviation from the nominal level . As a second over a full screw rotation covering a distance that is symmetrically distributed on both sides of the originally detected peak deviations in either direction , as shown by the two small inserts. This repeat measurement must be made in increments five to ten times smaller than the steps of the original calibration . Intermediate positions explored by the second measurement could reveal that the actual deviation peak is even greater than the originally charted point.
For the evaluation of the micrometer accuracy , the spread of the deviations in the direction of the ordinate axis may be considered the significant dimension. The errors recorded on the calibration chart will comprise the combined effect of all factors that are related to the measuring accuracy of the micrometer. The more important of these additional factors are as follows:
The flatness and parallelism of the measuring surfaces . The precise method for inspecting this condition is by means of an optical flat. As a general rule, the number of visible interference lines under monochromatic light must not exceed the following values: two fringes for flatness( when checking any one of the measuring surfaces) and six fringes for flatness and parallelism combined (making simultaneous contact with both measuring surfaces and using an optical flat whose faces are plane and parallel)
Deflection of the frame , The applied measuring force will cause a deflection of the frame, resulting in the separation of the measuring surfaces. This effect can be reduced by appropriate frame design and by limiting the applied measuring force , with the aid of ratchet or friction screw , to about two pounds. When kept under proper control , potential measuring error caused by frame deflection can be kept within about 50 micrometers for the one-inch size outside micrometers. The amount of deflection will be larger for micrometers of greater frame size or having wider measuring ranges.
Considering the various factors that affect the measuring accuracy of the frame type outside micrometers , a total spread of potential errors not exceeding 0.000150 inch will be indicative of a good quality of micrometer in the one-inch size range. As a guide , the following formula may be considered for assessing the expected measuring accuracy of a precision grade outside micrometer:â–?(150+10L) microinches
â–?=aggregate measuring accuracy over the total measuring range of the micrometer;
L=nominal size( maximum measuring length) of the micrometer , in inches.
The process errors of micrometer measurement can be caused by heat transfer while holding the instrument, reading errors, inadequate alignment or stability in the mutual positioning of 0bject and measuring tool, wear and many other circumstances.
Various design improvements serve to reduce the incidence of these errors in micrometer measurements. Particular design features directed at increasing the dependability of the measuring process by reducing the effect of some of these potential errors, are discussed below . The design features are listed in groups according to the particular source of potential inaccuracy that these improvements primarily control.
Heat transfer can be reduced by employing plastic insulating grips on the frame . With few exceptions , the micrometer frames are made of steel forgings , a material with practically the same coefficient of thermal expansion as most of the parts to be measured. Because of the differential in the rate of thermal expansion , aluminum frames, although light in weight, are seldom used for micrometers.
Reading errors are substantially reduced by such design features as follows:
a. Satin chrome finish to eliminate glare;
b. Distinct graduation lines applied on a beveled thimble surface to facilitate reading with a minimum of parallax error. Some models of precision micrometers have the graduated surface of the thimble and sleeve mutually flush.
c. A particular type of micrometer has window openings on the thimble , where the hundredths and thousandths values of the measured dimension appear in digits , and only the tenths and the ten-thousandths of an inch values must be determined by reading the graduation lines;
d. Large diameter thimbles for direct reading of the ten-thousandths by graduation lines coinciding with a single reference mark, thus eliminating the vernier as a potential source of reading error.
Alignment and holding stability can be improved for measurements of small parts by using a stand that rests on the bench to clamp the hand micrometer instead of holding it in the free hand. For the repetitive measurement of light parts the use of bench micrometers can provide definitive advantages.
The applied measuring force is usually limited by friction screw or ratchet .When precise control of the static measuring force is required , indicator micrometers may be used .
Wear will most commonly occur on the measuring faces because of their direct contact with the workpiece .Carbide-faced anvils and spindle tips greatly reduce the wear on these surface , thus maintaining of the micrometers , as well as the parallelism of the contact surfaces. After prolonged use of the micrometer , wear will occur in the threaded members that can affect the original setting and measuring accuracy of the micrometer. Resetting the thimble position to the clearance of the spindle movement by tightening the nut will usually improve the functioning of the instrument to a level equal or comparable to its original accuracy.