This Standard establishes requirements and methods for
specifying and testing the performance of coordinate measuring
machines (CMMs) having three linear axes perpendicular to each
other and up to one rotary axis positioned arbitrarily with respect
to these linear axes. In addition to clarifying the performance
evaluation of CMMs, this Standard seeks to facilitate performance
comparisons among machines by unifying terminology, general machine
classification, and the treatment of environmental effects. This
Standard attempts to define the simplest testing methods capable of
yielding adequate results for the majority of CMMs and is not
intended to replace more complete tests that may be suitable for
special applications. In particular, this Standard is most
applicable to machines used in the point-to point rather than the
contour measurement mode. Although the procedures herein provide
checks for most of the parameters that are relevant to coordinate
measuring machines used in a contouring mode, these procedures do
not actually test contouring accuracy, per se. Additions to include
contouring performance are in process.
This Standard provides definitions of terms applicable to CMMs.
These definitions are separated into two parts. The first part is a
glossary covering technical terms used throughout this Standard.
The second part defines ten common machine classifications.
The actual specification of CMMs is subdivided into four
sections: general machine classification; machine environmental
requirements and responses; machine performance; and machine
subsystem performance. Machine classification includes machine
type, measurement ranges, position resolution, operating mode, and
probing method. Environmental specification includes thermal
response, electrical requirements, vibration sensitivity, and
utility air requirements. Machine performance specification
includes repeatability, linear displacement accuracy, ball bar
measurement performance, diagonal displacement performance (large
machines), offset probe performance, rotary axis performance, and
bidirectional length measurement capability. Subsystem performance
consists of procedures to evaluate probing performance during
point-to-point coordinate acquisition.
One of the most significant features of this Standard is its
treatment of environmental specification and testing. The machine
user is assigned clear responsibility for providing a suitable
performance test environment, either by meeting the supplier's
parameters or by accepting reduced performance. Particular emphasis
has been placed on the performance degradation due to temperature
variation and vibration. The treatment of thermal effects has been
done in conceptual conformance to the provisions of ANSI B89.6.2.
The key feature of this treatment is the relaxation of machine
performance requirements if the thermal environment causes
excessive uncertainty or variation in the CMM performance and does
not meet the supplier's thermal parameters.
Actual machine performance testing is divided into five major
areas: repeatability; linear displacement accuracy; streamlined
artifact testing with a ball bar; rotary axis testing; and
bi-directional length measurement capability. Supplements to the
ball bar testing are provided for large machines. (Note that the
supplemental laser interferometer diagonal displacement
measurements will give numbers that may be different from those
that would be obtained with long ball bars. However, these numbers
also adequately reflect the performance of the machine.) An
important feature of all of these performance tests is the attempt
to use normal operating procedures during the tests. This
emphasizes the importance of measurement procedure details, such as
mode of machine operation and probe type. This also emphasizes the
overall approach of this Standard in considering measurement data
as the results of the complete measuring system, not just the
Subsystem performance, at this time, provides a series of tests
for systematic point-to-point probing errors, such as lobing. Tests
for other subsystems, such as probe changers, software, etc., are
of importance but are not included in this Standard.
Throughout this Standard the concept of range, that is, the
spread between the maximum and minimum values in a set of data, is
used as the measure of machine performance, rather than more common
statistical measures, such as standard deviation. This choice was
made because the dominant errors in coordinate measuring machines
are not random but rather systematic. In such cases, no generally
accepted statistical procedures currently exist.
Repeatability is defined as "a measure of the ability of the
instrument to produce the same indication when sequentially sensing
the same quantity under similar measurement conditions." The
specified testing of repeatability requires a series of
measurements of the center coordinates of a precision ball. This
uses the same testing procedure as the earlier tests to measure the
effect of the thermal environment.
The linear displacement accuracy of the machine is measured
along three mutually perpendicular lines in the work zone, The
tests may be performed using either a step gage or a laser
interferometer. This Standard carefully details the treatment of
these data if any mean temperature in the tests departs from 20°C
(68°F) at which material length standards are defined,
The overall measuring performance of the machine is evaluated
with a ball bar. This provides limited but valuable testing of the
machine throughout most of its working volume. This method has been
chosen due to the speed and simplicity with which a machine can be
evaluated using a ball bar which simulates a real measurement
procedure. For very large machines, diagonal displacement
measurements are used to supplement the ball bar results. Further,
the ball bar is measured in four positions with offset probes to
obtain the offset probing performance.
The performance of the machine's rotary axis, if applicable, is
tested by measuring the locations of two precision balls mounted at
specified positions on the rotary table. Again, this test is
functional and is intended to reflect the values that would be
obtained in actual measurements. The user of this specification is
warned that rotary axes are particularly sensitive to the load
distribution and the moment of inertia of the part being measured.
This Standard does not address this issue at this time.
In order to clarify the use of this Standard, a short guide on
how to use it is included as Appendix A.
This Standard does not currently provide for the measurement of
all parameters relevant to coordinate measuring machine
performance. Working Groups have been formed for creating
extensions to this Standard to include contouring operations, more
complete probe evaluation, software verification, and other types
of artifact standards.
Productivity is an important consideration in the selection of a
coordinate measuring machine. There are numerous factors which
affect relative productivity of measuring systems; these include
variables inherent to both the system and the workpiece. This
Standard does not address methods to specify and evaluate
productivity. Productivity should be evaluated with respect to the
expected use of the system.