Micrometers
A micrometer is an instrument designed for linear
measurement using the decimal divisions of the
inch or meter (
Figure 29
). While there are many
types and styles of micrometers, most of the proce-
dures in this manual call for an outside micrometer.
The outside micrometer is used to measure the out-
side diameter of cylindrical forms and the thickness
of materials.
A micrometer’s size indicates the minimum and
maximum size it can measure. The usual sizes
(
Figure 30
) are 0-1 in. (0-25 mm), 1-2 in. (25-50
mm), 2-3 in. (50-75 mm) and 3-4 in. (75-100
mm).
Micrometers that cover a wider range of mea-
surement are available. These use a large frame with
interchangeable anvils of various lengths. This type
of micrometer offers a cost savings; however, its
overall size may make it less convenient.
Reading
When reading a micrometer, read numbers from
different scales and add them together.
For accurate results, properly maintain the mea-
suring surfaces of the micrometer. There must not
be any dirt or burrs between the tool and the mea-
sured object. Never force the micrometer closed
around an object. Close the micrometer around the
highest point so it can be removed with a slight
drag.
The standard metric micrometer (
Figure 31
) is
accurate to one one-hundredth of a millimeter
(0.01-mm). The sleeve line is graduated in millime-
ter and half millimeter increments. The marks on
the upper half of the sleeve line equal 1.00 mm. Ev-
ery fifth mark above the sleeve line is identified
with a number. The number sequence depends on
the size of the micrometer. A 0-25 mm micrometer,
for example, has sleeve marks numbered 0 through
25, in 5 mm increments. This numbering sequence
continues with larger micrometers. On all metric
micrometers, each mark on the lower half of the
sleeve equals 0.50 mm.
The tapered end of the thimble has fifty lines
marked around it. Each mark equals 0.01 mm.
One complete turn of the thimble aligns its 0
mark with the first line on the lower half of the
sleeve line or 0.50 mm.
When reading a metric micrometer, add the num-
ber of millimeters and half-millimeters on the
sleeve line to the hundredths of a millimeter shown
on the thimble. Perform the following steps and re-
fer to
Figure 32
.
16
CHAPTER ONE
DECIMAL PLACE VALUES*
0.1
Indicates 1/10 (one tenth of an inch
or millimeter)
0.010
Indicates 1/100 (one one-hundreth of
an inch or millimeter)
0.001
Indicates 1/1,000 (one one-thousandth
of an inch or millimeter)
*This chart represents the values of figures placed to the right of the decimal point. Use it when
reading decimals from one-tenth to one one-thousandth of an inch or millimeter. It is not a con-
version chart (for example: 0.001 in. is not equal to 0.001 mm).
29
30
