Lesson 1/Learning Event 2

Learning Event 2

MEASURE USING THE MICROMETER

SELECTION

Commonly used micrometers are made so that the longest movement possible

between the spindle and the anvil is 1 inch. This movement is called the

range.

Micrometer frames, however, are available in a wide variety of

sizes, from 1 inch up to as large as 24 inches.

The range of a 1-inch

micrometer is from 0 to 1 inch. In other words, it can be used to measure

work that is 1 inch or less. A 2-inch micrometer has a range from 1 to 2

inches. A 6-inch micrometer has a range from 5 to 6 inches, and will only

measure work between 5 and 6 inches.

Mechanics must first find the approximate size of the work to the nearest

inch, and then select the correct micrometer.

For example, to find the

exact diameter of a piece of round stock use a rule and find the approximate

diameter of the stock. If it is between 3 and 4 inches, a micrometer with a

3- to 4-inch gage would be required to measure the exact diameter.

Similarly, when using inside and depth micrometers, rods of suitable lengths

must be fitted into the tool to get the approximate inside dimension within

an inch. After obtaining the approximate dimensions, an exact measurement

is read by turning the thimble.

The size of the micrometer indicates the

size of the largest work it will measure.

READING STANDARD MICROMETERS

Reading a micrometer consists of simply reading the micrometer scale or

counting the revolutions of the thimble and adding to this any fraction of a

revolution. The micrometer screw has 40 threads per inch. This means that

one complete revolution of the micrometer screw moves the spindle away from

or towards the anvil exactly 0.025 inch. The lines on the barrel, Figure 7,

conform to the pitch of the micrometer screw with each line being numbered.

The bevel edge of the thimble is graduated into 25 parts, each part

indicating 0.001 inch or 1/25 of the 0.025 inch covered by a complete

revolution.

Every fifth line on the thimble is numbered to read a

measurement in thousandths of an inch.