LO 5-3 Identify and describe common instruments and techniques to make linear precision measurements

1.12. Reading Micrometers Graduated in Millimeters

Reading Micrometers Graduated in Millimeters

Because the distance between threads of the metric spindle screw is one-half millimeter (0.5 mm), one complete rotation of the thimble moves the spindle 0.5 mm distance. The reading line on the sleeve is graduated in divisions of 1.0 mm. This means two rotations of the thimble are required to move the spindle 1.0 mm. Every fifth millimeter graduation on the reading or index line is a major division numbered from 0–25. Smaller divisions between each major are each typically 0.025 mm.

The bevelled edge of the thimble is graduated in 50 divisions, with every fifth line numbered as a major division from 0–50. Because one revolution of the thimble moves the spindle 0.5 mm, each thimble graduation equals 1/50 of 0.5 mm or 0.01 mm.

To take a measurement, the numbers on the sleeve and the thimble are added together. This means first observing the number of millimeters and half millimeters uncovered by the sleeve. Next, add the sleeve numbers to the number of hundredths of a millimeter represented by the thimble graduations (which align with the measuring line on the sleeve). See FIGURE 19 to calculate this measurement.

FIGURE 05-19: How to read and calculate a linear measurement made on a metric-type micrometer.

FIGURE 05-20: An outside micrometer is used to make precise measurements of many precision-machined engine components, such as the diameter of this piston.

When a vernier scale is added to a micrometer, an additional fourth significant digit can be added to the measurement’s resolution. The vernier scale has a range of 0.01 mm and is graduated in 10 increments, each of which represents 0.001 mm vernier scale divisions, and are typically numbered every 0.002 mm. To calculate this number, FIGURE 20 shows several micrometers where the number on the thimble lines up with a number on the vernier. Essentially, the numbers on the sleeve are added to the thimble division numbers, which are in turn added to the line designated by the vernier scale aligning with the thimble.

Inside and Depth Micrometers
The inside micrometer is read the same way as an outside micrometer. A depth micrometer, however, reverses the order of the numbers on the sleeve. As the spindle extends from the micrometer, the numbers uncovered by the thimble will increase.

Adjusting and Caring for Micrometers
Looseness or free play in a micrometer spindle is corrected by tightening an adjusting nut inside the barrel or sleeve. A small purpose-made wrench, supplied with the micrometer, is placed around the nut located at the end of the sleeve. The nut is tightened only enough to remove looseness in threads between the nut and the spindle. To adjust the zero reading, the micrometer measuring surfaces are cleaned, and the micrometer is gently closed with the ratchet torque-limiting stop, if equipped. A screwdriver is used to turn either the anvil or sometimes the sleeve until its zero-graduation line aligns with the zero line on the thimble. Precision instrument oil can be applied to the spindle threads and the spindle to help maintain free-running performance. Micrometers should be wiped clean and stored in a case when not in use.