The nature of quality control measurement is continually changing in response to developments in coordinate measuring machines (CMMs) and other technology-laden metrology instruments.
are prone to misuse; they usually posses ID jaws that come to very sharp
points-thought by some to be ideal for ripping open boxes. This type of misuse
can put burrs on the jaws, maybe too small to see, yet capable of throwing jaws
out of stated accuracy. Source: Mitutoyo America Corp.
nature of quality control measurement is continually changing in response to
developments in coordinate measuring machines (CMMs) and other technology-laden
metrology instruments. Nevertheless, the precision and repeatability of
handheld dimensional measuring tools-calipers, micrometers and gages-are still
heavily relied on throughout most of manufacturing. And as tolerances of
manufactured parts become ever tighter, it is even more important that the
accuracy of handheld measuring tools be maintained, requiring the tools
themselves to be cared for properly.
There are two main categories of maintenance for hand measuring tools. The
first is in response to everyday use and handling. This assumes that the
correct tool is selected in the first place, for example, making sure the IP or
Ingress Protection Rating is suitable.
The second type of maintenance is specified by formal, periodic and documented
inspection and calibration routines. Calibration is most commonly performed
in-house, but many quality programs specify additional calibration at
accredited labs. These labs provide calibration traceable to final standards
such as NIST (National Institute of Standards and Technology). Related to
calibration is use of gage blocks-extremely precise artifacts with a care
regimen all their own.
is caused by foreign matter contamination; some is airborne but most is
deposited from the operator’s hands in combination with oil from the fingers.
Wearing white gloves counters this problem. Source: Mitutoyo America Corp.
Calibration establishes the relationship between the measured value indicated
by a measuring tool and the corresponding value for that same measurement as
set forth by accepted standards. The results of calibration permit adjustment
of the measurement tool so that it performs within a desired limit of accuracy.
In-house calibration may be performed on newly purchased tools, or as a result
of re-adjustment needed for tools that may have been dropped or otherwise
shocked, or in fulfillment of a calibration schedule that may be specified by a
company’s own standards and/or as stipulated by external standards such as ISO.
In-house calibration usually employs gage blocks, precisely manufactured of
steel or ceramic, with dimensional tolerances that fall within known standards.
A tool being calibrated is used to measure a gage block then the obtained value
is compared to the known value of the block.
Gage blocks come in a variety of sizes and in sets ranging from half-a-dozen to
more than a hundred. Different size blocks are stacked together to create the
exact dimension required. When stacked, the blocks’ optically flat surfaces are
rubbed or “wrung” together to eliminate any foreign matter, even air, which may
lie between them.
As a result of their extreme flatness, wrung gage blocks can get so close
together that the molecules from one surface interact with molecules from the
other, essentially welding the blocks together. Rarely, gage blocks left wrung
together for extended periods are impossible to separate.
Care issues for gage blocks include wear, burring and corrosion. Wear is caused
by foreign matter contamination; some is airborne but most is deposited from
the operator’s hands in combination with oil from the fingers. Wearing white
gloves counters this problem.
Burring and scratches are caused when
an edge from one block being wrung slips over and cuts into the other block.
Care should be taken to avoid burring; damaged blocks should be removed and
replaced as soon as possible.
Corrosion can easily result from oil deposited from fingers; again, use of
white gloves provides the solution. Additionally, corrosion can be prevented by
ensuring that gage blocks are always stored in their cases-at room temperature
and humidity-and never stored wrung together.
The correct care and maintenance of handheld measuring tools is well worth the
effort. Properly cared for tools are more accurate, easier to use and offer
extended service life-all factors that impact quality, productivity and the