ASTM E703-98
(Practice)Standard Practice for Electromagnetic (Eddy-Current) Sorting of Nonferrous Metals
Standard Practice for Electromagnetic (Eddy-Current) Sorting of Nonferrous Metals
SCOPE
1.1 This practice describes a procedure for sorting nonferrous metals using the electromagnetic (eddy-current) method. The procedure is intended for use with instruments using absolute or comparator-type coils for distinguishing variations in mass, shape, conductivity, and other variables such as alloy, heat treatment, or hardness that may be closely correlated with the electrical properties of the material. Selection of samples to evaluate sorting feasibility and to establish calibration standards is also described.
1.2 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: E 703 – 98
Standard Practice for
Electromagnetic (Eddy-Current) Sorting of Nonferrous
Metals
This standard is issued under the fixed designation E 703; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
1. Scope heat treatment temper, or hardness, is read on the display of an
indicator. In the comparative coil method (encircling or probe
1.1 This practice describes a procedure for sorting nonfer-
coils), the test specimen in one coil is compared with a
rous metals using the electromagnetic (eddy-current) method.
reference piece in a second coil to determine whether the test
The procedure is intended for use with instruments using
specimen is within or outside of the required limits.
absolute or comparator-type coils for distinguishing variations
4.1.1 Absolute Coil Method:
in mass, shape, conductivity, and other variables such as alloy,
4.1.1.1 Encircling Coil—Samples of known classification
heat treatment, or hardness that may be closely correlated with
(standards) are inserted consecutively in the test coil, and the
the electrical properties of the material. Selection of samples to
controls of the instrument are adjusted to obtain appropriate
evaluate sorting feasibility and to establish calibration stan-
response. Typically, three samples would be used representing
dards is also described.
the upper, lower, and mid-range for which calibration is
1.2 This standard does not purport to address all of the
required. The test is then conducted by inserting the specimens
safety concerns, if any, associated with its use. It is the
to be sorted into the test coil, and observing the instrument
responsibility of the user of this standard to establish appro-
response.
priate safety and health practices and determine the applica-
4.1.1.2 Probe Coil—The probe coil is placed consecutively
bility of regulatory limitations prior to use.
on the standards of known properties and the controls of the
2. Referenced Documents
instrument are adjusted for appropriate response (see 4.1.1.1).
The test is then conducted by placing the probe on the
2.1 ASTM Standards:
specimens to be sorted and observing the instrument response.
E 105 Practice for Probability Sampling of Materials
4.1.2 Comparative Coil Method:
E 122 Practice for Choice of Sample Size to Estimate a
4.1.2.1 Encircling Coil—Known reference pieces (stan-
Measure of Quality for a Lot or Process
dards) representing the minimum or maximum limits, or both,
E 1316 Terminology for Nondestructive Examinations
of acceptance or sorting category are inserted in the reference
3. Terminology Definitions
and test coil. The instrument controls are adjusted for appro-
priate responses. The test is then conducted by inserting
3.1 Definitions of terms relating to electromagnetic testing
specimens to be sorted in the test coil, leaving the known
are given in Terminology E 1316.
reference in the reference coil and observing the instrument
4. Summary of Practice
response.
4.1.2.2 Probe Coil—Both probe coils are placed on the
4.1 The techniques that are primarily used in electromag-
reference pieces (standard) representing the minimum or maxi-
netic sorting employ the absolute (single-) and comparative
mum limits, or both, of acceptance or sorting category. The
(two-) coil methods using either encircling or probe coils. The
instrument controls are adjusted for appropriate responses. The
decision of whether to use single-coil or two-coil operation is
test is then conducted by placing the test probe on the
usually based on empirical data. In the absolute-coil method
specimens to be sorted (the other probe is left on the reference
(encircling or probe), the equipment is calibrated by placing
standard) and observing the instrument response.
standards of known properties in the test coil. The value of the
4.2 The range of instrument response must be so adjusted in
tested electrical parameter, which may be correlated with alloy,
the initial step that the anticipated deviations will be within the
range of readout.
This practice is under the jurisdiction of ASTM Committee E07 on Nonde-
4.3 Both absolute and comparative methods using encir-
structive Testing and is the direct responsibility of Subcommittee E07.07 on
cling coil(s) require comparing the specimens to be tested with
Electromagnetic Methods.
the reference piece(s). Two or more samples representing the
Current edition approved May 10, 1998. Published July 1998. Originally
published as E 703–79. Last previous edition E 703–79 (92)e1.
limits of acceptance may be required. In the absolute method,
Annual Book of ASTM Standards, Vol 14.02.
Annual Book of ASTM Standards, Vol 03.03.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E703–98
the electrical reference signal from the instrument is adjusted 6.1.7 Conductivity has an unambiguous relationship to
with the standard in the coil. In the comparative method, any hardness for certain alloys. However, when alloys are mixed,
electromagnetic condition, that is not common to the test identical conductivity does not necessarily indicate the same
specimen and the standard will produce an imbalance in the hardness.
system. The comparative method is usually more stable since it 6.1.8 Care must also be exercised in using conductivity to
suppresses most of the interferences. sort overheated parts quenched at a high temperature as the
4.4 The testing process may consist of manual insertion of conductivity reading for acceptable parts may repeat at a large
one specimen after another into the test coil or an automated increase in temperature.
feeding and classifying mechanism may be employed. In 6.1.9 Lift-off can result in a change in the test system output
automated setups, it is sometimes necessary to establish with probe coils. This effect is a change in the magnetic
empirically the time required for the test specimen to remain in coupling between the test specimen and probe coil. Care must
the test coil while the reading is being taken, especially if low be exercised to prevent this effect from interfering with test
frequencies are employed. results; either mechanical or electronic compensation must be
used.
6.1.10 For certain heat-treatable (aluminum) alloys, conduc-
5. Significance and Use
tivity values can also repeat themselves during the aging cycle
5.1 Absolute and comparative methods provide a measure
at a constant temperature. Thus, for such alloys, conductivity is
for sorting large quantities of nonferrous parts or stock with
not unique as a monitor of temper, etc.
regard to composition or condition, or both.
5.2 The comparative or two-coil method is used when
7. Apparatus
high-sensitivity testing is required. The advantage of this
7.1 Electronic Apparatus—The electronic apparatus shall
method is that it almost completely suppresses interferences.
be capable of energizing the test coils with alternating currents
5.3 The ability to accomplish these types of separations
of suitable frequencies and power levels and shall be capable of
satisfactorily is dependent upon the relation of the electric
sensing changes in the electromagnetic response of the coils.
characteristics of the nonferrous parts to their physical condi-
Equipment may include any suitable signal-processing devices
tion.
(phase discriminator, filter circuits, etc.) and the output may be
5.4 These methods may be used for high-speed sorting in a
displayed by meter, oscilloscope, recorder, signaling devices,
fully automated setup where the speed of testing may approach
or any suitable combination required for the particular appli-
many specimens per second depending on their size and shape.
cation.
5.5 Successful sorting of nonferrous material depends
7.2 Test coils may be of the encircling or probe-coil type
mainly on the variables present in the sample and the proper
and shall be capable of inducing an electromagnetic field in the
selection of frequency and fill factor.
test specimen and standard, and sensing changes in the electric
5.6 The accuracy of a sort will be affected greatly by the
or magnetic characteristics of the test specimen.
coupling between the test coil field and the tested part during
7.2.1 When selecting the test coil, the objective should be to
the measuring period.
obtain a coil fill factor as large as possible. This means that the
inside of the test coil should be filled by the test specimen as
6. Interferences
much as possible. This is of primary importance for tests
6.1 The influence of the following variables must be con-
requiring high sensitivity.
sidered for proper interpretation of the results:
7.2.2 For complicated test specimen shapes, a correspond-
6.1.1 The correlation shall be established so that electrical
ing insert shall be provided to ensure that each test specimen
properties of various groups do not overlap and are well
can be placed in the same position within the test coil. These
defined in the calibration procedure used.
inserts, as well as any other accessories, should consist of
6.1.2 The test frequency must be selected to provide a
nonferromagnetic, electrically nonconductive material.
well-defined separation of variables.
7.3 Mechanical Handling Apparatus— A mechanical de-
6.1.3 The temperature of the standard and test specimen
vice for feeding and sorting the test specimens may be used to
shall be controlled within limits that will permit a well-defined
automate a particular application.
range of conductivity or permeability, or both, for which the
8. Sampling
correlation of the group or groups is valid. Cooling of the test
standard when high field strengths are used or allowing test 8.1 Sampling (see Practices E 105 and E 122) is a method to
specimens to cool or heat to an established ambient range, or
obtain assurance that materials are of satisfactory quality.
both, may be required. Instead of 100 % inspection, a portion of the material is
6.1.4 The geometry, mass, and thickness of the standard and
examined to show evidence of the quality of the whole. There
test specimen shall be controlled within limits that will permit are two important needs for this approach: first, in the final
sorting.
inspection or tests made to assure that products delivered are in
6.1.5 Magnetic permeability variations can interfere when conformance with specification requirements; second, to con-
sorting paramagnetic materials.
trol parts and assemblies while they are being processed.
6.1.6 Signal response can result from a change in relative Statistical acceptance sampling tables and statistical process-
motion between the test specimen and the test coil, such as the control sampling tables have been developed to meet these
length of time the specimen is in a test coil (see 4.4). needs.
E703–98
8.2 Acceptance sampling may be conducted on an accept/ position in the coil and adjust the test instrument to get an
reject (or attributes) basis; that is, determining whether or not on-scale meter or oscilloscope reading, or both. Replace the
the units of the sample meet the specification. Examination of acceptable standard with a known unacceptable standard in the
the samples may also be conducted on a measurements (or same exact position and adjust the sensitivity of the instrument
variables) basis; that is, determining actual readings on the to maximize the indicated difference reading without exceed-
units in the sample. The majority of acceptance sampling is ing 90 % of the available scale range.
carried out on a sampling by attributes basis and the usual 10.3 When using the comparative coil method (encircling),
acceptance sampling table is designed for accept/reject. select a reference piece (usually one that falls within the
8.3 Process control sampling may be conducted on material acceptable limits of the specimens being tested), place it in the
during the course of production to prevent large quantities of reference coil and set this coil and piece in a location so that it
defective parts being found in the acceptance tests. Many parts will not be accidentally disturbed during the sorting operation.
and materials are subjected to several successive machining or For this method, when used with a two-way mix, choose two
processing operations before they become finished units. Parts calibrated standards, one of which represents the acceptable
can be most effectively controlled during production by exam- and the other the unacceptable group. Place the acceptable
ining small samples of these parts at frequent regularly calibration standard at a fixed position in the test coil coincid-
scheduled intervals. The object of this process check is to ing with the position of the reference piece in the reference coil
provide a continuous picture of the quality of parts being and balance the instrument. Replace this acceptable calibration
produced. This helps prevent production of defective parts by standard with one representing the unacceptable group and
stopping and correcting the problem as soon as it begins to adjust the test instrument’s phase, sensitivity, and coil current,
appear in the manufacturing process and thereby keeping the to maximize the indicator reading without exceeding 90 % of
process in control. Sampling may be by attributes or by the available scale range. Reinsert the acceptable standard and
variables and process control sampling tables are used. The alternately readjust the instrument controls to retain a null
measurements (variables) control chart is by far the most value for the acceptable standard and maximum indication for
effective process control technique. the unacceptable standard.
8.4 Statistical sampling tables have four definite features: 10.4 For a three-way sort, it is best to have three calibration
(1) specification of sampling data—that is, the size of the standards, two of which represent the high and low limits of
samples to be selected, the conditions under which the samples acceptability for one group or one each of the two unacceptable
are to be selected, and the conditions under which the lot will groups. The third standard represents the acceptable lot of
be accepted or rejected; (2) protection afforded—that is, the material.
element of risk that the sampling schedules in a given table will 10.4.1 A typical case of high and low limits of acceptability
reject good lots or accept bad ones; (3) disposal procedure— standards is in measurements where standards representing
that is, a
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