Machinery's Handbook, 31st Edition
Sine-Bars 693 Micrometers graduated according to the English system of measurement ordinarily have a table of decimal equivalents stamped on the sides of the frame so that fractions such as sixty-fourths, thirty-seconds, etc., can readily be converted into decimals. Reading a Metric Micrometer.— The spindle of an ordinary metric micrometer has 2 threads per millimeter, and thus one complete revolution moves the spindle through a distance of 0.5 millimeter. The longitudinal line on the frame is graduated with 1 millime- ter divisions and 0.5 millimeter subdivisions. The thimble has 50 graduations, each being 0.01 millimeter (one-hundredth of a millimeter). To read a metric micrometer, note the number of millimeter divisions visible on the scale of the sleeve, and add the total to the particular division on the thimble that coincides with the axial line on the sleeve. Suppose that the thimble were screwed out so that gradu ation 5 and one additional 0.5 subdivision were visible (as shown in Fig. 5), and that grad uation 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.
5 30 25
0
Fig. 5. Metric Micrometer To permit measurements within 0.002 millimeter to be made, some micrometers are provided with a vernier scale on the sleeve in addition to regular graduations. Micrometers of this type are read as follows: First, as with an ordinary micrometer, determine the number of whole millimeters (if any) and the number of hundredths of a millimeter, and then find a line on the sleeve vernier scale that exactly coincides with one on the thimble. The number of this coinciding vernier line represents the number of two-thousandths of a millimeter to be added to the reading already obtained. Thus, for example, a measurement of 2.958 millimeters would be obtained by reading 2.5 millimeters on the sleeve, adding 0.45 millimeter read from the thimble, and then adding 0.008 millimeter as determined by the vernier. Note: 0.01 millimeter = 0.000393 inch, and 0.002 millimeter = 0.000078 inch (78 mil lionths). 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. 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. Sine-Bar Use The sine-bar is used either for very accurate angular measurements or for locating work at a given angle as, for example, in surface grinding templets, gages, etc. The sine-bar is especially useful in measuring or checking angles when the limit of accuracy is 5 min- utes or less. Some bevel protractors are equipped with verniers which read to 5 minutes, but the setting depends upon the alignment of graduations, whereas a sine-bar usually is located by positive contact with precision gage-blocks selected for whatever dimension is required for obtaining a given angle. Types of Sine-Bars.— A sine-bar consists of a hardened, ground and lapped steel bar with very accurate cylindrical plugs of equal diameter attached to or near each end. The form illustrated by Fig. 1 has notched ends for receiving the cylindrical plugs so that they are held firmly against both faces of the notch. The standard center-to-center distance C be - tween the plugs is either 5 or 10 inches. The upper and lower sides of sine-bars are parallel to the center line of the plugs within very close limits. The body of the sine-bar ordinarily
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