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 CMM.
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.