32 Auto Brakes Copyright by Goodheart-Willcox Co., Inc. Using Micrometers To use a micrometer correctly, you must fi rst know how it is constructed. Th e following sections cover the design of both inch and metric micrometers. Inch Micrometers Th e barrel, or sleeve, of an inch micrometer is graduated in two ways. Th e numbers represent multiples of 0.100″. Th e lines between the numbers represent 0.025″. Th e thimble is also graduated in two ways. Each number represents a multiple of 0.005″. Th e lines between the numbers represent 0.001″. Refer to Figure 3-11. Metric Micrometers Th e numbers on the barrel of a metric micrometer are read directly in millimeters. Th e lines above the centerline and between the numbers each represent one millimeter. Note the lines below the centerline are staggered by one- half interval. Th is allows the technician to measure half millimeters. Th e numbers on the thimble are graduated in hundredths of a millimeter. Figure 3-12 illustrates a metric micrometer’s scale. Reading Micrometers Place the contact points against the part to be mea sured. Th en turn the thimble until the anvils lightly contact the part. Do not tighten the micrometer as this will cause a false reading and damage the micrometer. To read an inch micrometer, start by reading the num- bers and lines on the barrel. Remember that each number is a multiple of 0.100″ and that each line represents 0.025″. If necessary, write down the reading, being sure to include the decimal point in the proper place. Th en, read the thimble numbers, remembering that each number is a multiple of 0.005″ and each line represents 0.001″. To read a metric micrometer, read the numbers and lines on the barrel. Note that each line is one millimeter, and the spaces between upper and lower lines is 0.5 millimeter. Write down this reading. Next, read the thimble numbers. Each number represents a multiple of 0.05 millimeter, and each line equals 0.01 millimeter. 0.0500 B A 0.3000 0.0120 C Goodheart-Willcox Publisher Figure 3-11. Four steps in reading an inch micrometer. A—The first reading is 0.300″. B—The second reading is 0.050″. The third reading is 0.012″. This totals 0.362″, which is added to the size of the micrometer being used to get the final reading. A B C Lock- nut Sleeve long line Thimble Sleeve Frame 0.5 mm 0.01 mm TRW, Deere & Co. Figure 3-12. A—Scale of an outside metric micrometer. B—This micrometer requires two full revolutions of the thimble to equal 1 mm. C—It is read in a similar manner to an inch micrometer.