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.
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