depending on the number of threads per inch. When
accuracy. They generally come in matched pairs and
they are laid over the threads in a position that allows
in standard fractional dimensions. Use care in storing
and handling them to prevent damage. If it becomes
between them, the pitch diameter of the threads can be
necessary to regrind the parallel blocks, be sure to
determined. Sets are available that contain all the
change the size stamped on the ends of the blocks.
more common sizes. Detailed information on com-
GAUGE BLOCKS.--Gauge blocks (not shown)
puting and using the wire method for measuring is
are used as master gauges to set and check other
covered in chapter 6.
gauges and instruments. Their accuracy is from
t w o - m i l l i o n t h s (0.000002) of an inch to
eight-millionths (0.000008) of an inch, depending on
CARE AND MAINTENANCE OF
the grade of the set. To visualize this minute amount,
MEASURING INSTRUMENTS
consider that the average thickness of a human hair
divided 1,500 times equals 0.000002 inch. This
The proper care and maintenance of precision
degree of accuracy applies to the thickness of the
instruments is very important to a conscientious MR.
gauge block, the parallelism of the sides, and the
To help you maintain your instruments in the most
flatness of the surfaces. To attain this accuracy, a fine
accurate and reliable condition possible, the Navy has
grade of hardenable alloy steel is ground and then
established a calibration program. It provides
lapped. The gauge blocks are so smooth and flat that
when they are "wrung" or placed one atop the other
and procedures, and a schedule of how often an
you cannot separate them by pulling straight out. A
instrument must be calibrated to be reliable. When an
set of gauge blocks has enough different size blocks
that you can establish any measurement within the
instrument is calibrated, a sticker is affixed to it
showing the date the calibration was done and the date
accuracy and range of the set. As you might expect,
the next calibration is due. Whenever possible, you
anything so accurate requires exceptional care to
prevent damage and to ensure continued accuracy. A
accuracy of your instruments. Some repair jobs, due
dust-free temperature-controlled atmosphere is
preferred. After use, wipe each block clean of all
provided by the program. Information concerning the
marks and fingerprints, and coat it with a thin layer of
procedures that you can use in the shop to check the
white petrolatum to prevent rust.
accuracy of an instrument is contained in the
RING AND PLUG GAUGES.--A ring gauge
following paragraphs.
disk that has a precisely ground bore. Ring gauges
are used to check machined diameters by sliding the
gauge over the surface. Straight, tapered, and
threaded diameters can be checked by using the
appropriate gauge. The ring gauge is also used to set
other measuring instruments to the basic dimension
required for their operation. Normally, ring gauges
are available with a GO and a NOT GO size that
represents the tolerance allowed for the particular size
or job.
for the same types of jobs as a ring gauge. However,
it is a solid shaft-shaped bar that has a precisely
ground diameter used to check inside diameters or
bores.
THREAD MEASURING WIRES.--These
wires provide the most accurate method of measuring
the fit or pitch diameter of threads, without going into
the expensive and sophisticated optical and com-
Figure 1-27.--Ring gauges and plug gauges.
parator equipment. The wires are accurately sized,
1-20