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

1.10. Reading Micrometers Graduated in Inches

Reading Micrometers Graduated in Inches

Objects and work pieces measurable by a sliding caliper are measured more accurately using the precision screw adjustment in a micrometer. Micrometers, measuring in inches, operate with a spindle thread pitch of 40 threads per inch and move the spindle 1/40 of an inch, or 0.025", with each turn. A micrometer measuring in inches will require turning the thimble 40 times to move an inch. In contrast, metric micrometers have threads spaced 0.5 mm apart for each rotation. This means it will take 50 turns to move the spindle a normal distance of 25 mm.

Two graduated scales and a vernier scale are used to calculate linear measurements of the most precise micrometers. The first scale is the reading line, also called the index line , on the micrometer barrel or sleeve that has 40 equal divisions designated by vertical lines that correspond to the number of threads on the spindle. This means each vertical line division represents 1/40 or 0.025 of an inch of spindle travel and every longer fourth line designates tenths of an inch, also expressed as hundreds of thousandths. For example, the major line marked “1” represents 0.100", the next major line marked “2” represents 0.200", etc. Smaller lines (between the major lines) represent 0.01", 0.02", etc. On the rotating thimble is the second graduated scale. Its bevelled edge has 25 divisions, with each consecutive graduation line designating 0.001" movement of the spindle. Each mark indicates spindle movement equal to 1/25 of 0.025". One complete rotation represents 0.025 of an inch of spindle movement. A vernier-like scale is used as an option on micrometers providing finer resolution.

To read a non-vernier micrometer in thousandths, first note the largest major division uncovered by the thimble on the index or reading line. Next, multiply the number of smaller vertical divisions observed after the larger major division visible on the sleeve by 0.025". Add these previous two numbers to the smaller line graduations of thousandths of an inch shown by the thimble on the sleeve. The sleeves index line will align with the thimble’s divisions FIGURE 16. The “2” line on the sleeve is visible, representing 0.200" (hundreds of thousandths or one-tenth). The number of lines between “2” and the thimble edge, each representing 0.025", is 1 × 0.025" = 0.025". The number of lines on the thimble with each line representing 0.001" is 16 × 0.001" = 0.016"; therefore, the micrometer reading is 0.241". An alternate method of calculating dimension is to think of the various measuring units as change returned after making a purchase with a large-denomination bill. Count the numbered, large measuring units on the sleeve as dollars, the smaller vertical lines on the sleeve as quarters, and the divisions on the thimble as pennies. Add up the change and place a decimal point instead of a dollar symbol in front of the number. In the above number, the change is 2 dollars and 41 cents. Examine the measurements in FIGURE 17 to further practice calculating measurements.

FIGURE 05-16: Steps to calculate a precision measurement of an outside micrometer measuring in inches.

FIGURE 05-17: Examples of micrometer readings and calculation of measurement in inches.

FIGURE 05-18: How to read a sliding-type vernier micrometer.

A vernier-type micrometer uses a third scale around the sleeve providing 1/10,000" (0.0001") resolution. A typical vernier inch scale has 11 parallel lines that are the same length as 10 graduation lines on the thimble. This means the difference between the lines on the sleeve and the thimble is one-tenth of a thousandth of an inch. To read the vernier scale, first lock the micrometer after measuring the object and remove the micrometer from the work piece and observe the thousandths reading. Next, determine which of the lines on the vernier scale align with a line on the thimble. To calculate the reading, examine FIGURE 18.