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| |
 
_medres.jpg) |
_medres.jpg) |
| MPM Tongs - No Space between Centering Tool
and Specimen (left face) |
Other Manufacture's Tongs - Incorrect
Fabrication results in Space between Tool and Specimen (left face) |
Introduction
Some customers think that
all Charpy tongs are alike. Not so! Precision tongs are an essential part of
the Charpy impact test and incorrectly fabricated tongs can result in bad test
results. ASTM standard E 23 states “Means shall be provided (see Fig. A.1.2) to
locate and support the test specimen against two anvil blocks in such a position
that the center of the notch can be located within 0.25 mm (0.010 of the
midpoint between the anvils”. Research at MPM has shown that when a space
exists between the tool and the test specimen, as in the photograph on the right
above, the ASTM centering tolerance can be exceeded.
MPM Precision Tongs
MPM precision tongs meet the
requirements of ASTM E 23. In order to satisfy the standard, the tongs are
made on a high accuracy computer controlled mill. As shown in the
photograph on the left below, it is essential that the specimen be in contact
with the tool so that the anvil radius properly centers the tool. The
tongs shown in the right photograph below, made by another manufacturer, have
already passed through the anvil radius region by the time the specimen comes
into contact with the anvil support surface. This is a serious problem
which allows excess play in the centering tool which can exceed the ASTM
requirement.
| Click below to enlarge and
enhance photo |
Click below to enlarge and
enhance photo |
 |
 |
MPM Tongs - Tool is Centered
between Anvil Radius when
Specimen is Deposited |
Other Manufacture's Tongs -
Tool is Past Anvil Radius when
Specimen is Deposited |
Another important feature of
the MPM tongs is that the centering of the notch is done very close to the notch
radius region. The centering tool protrudes deep into the notch region to
accurately center the notch. This is important because accurate specimen
centering does not depend on accurate machining of the 45 degree included angle
on the test specimen.
Repeated measurements (30
times) using MPM tongs have shown that the notch is centered on average to
within 0.0016 inches which far exceeds the ASTM E 23 requirement. The standard
deviation for the average is 0.00077 inches. The measurements were made on an
actual test machine.
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