ASME B89.4.1 1997 Edition, June 16, 1997
METHODS FOR PERFORMANCE EVALUATION OF COORDINATE MEASURING MACHINES
Includes all amendments and changes through Addendum B , 2001
Additional Comments: W/D S/S BY ASME B89.4.10360.2
Published By:ASME International (ASME)
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 mode rather than the contour measurement mode. Although this Standard provides checks for most of the parameters relevant to coordinate measuring machines used in a contouring mode, the checks do not actually test contouring accuracy, per se. Additions to this Standard to include contouring performance are in process.
This Standard provides definitions of terms applicable to CMMs. These definitions are separated into two parts: first, a glossary covering technical terms used throughout this Standard, and second, an explanation of twelve 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, offset probe performance, diagonal displacement performance (large machines), duplex performance (machines used in the duplex mode), rotary axis performance, performance under loaded conditions, and bidirectional length measurement capability. Subsystem performance consists of procedures to evaluate probing performance during point-to-point coordinate acquisition with single and multiple tips.
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 is placed on the performance degradation caused by temperature variation and vibration. The treatment of thermal effects in this Standard is in conceptual conformance to the provisions of ASME 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 recommendations regarding 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 bidirectional length measurement capability. Supplements to the ball bar testing are provided for large machines and for machines used in the duplex mode. (Note that the supplemental laser interferometer diagonal displacement measurements will give numbers that may be different from those obtained with long ball bars. However, these numbers also adequately reflect the performance of the machine.) Performance tests for machines under loaded conditions are also included. An important feature 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. In addition, the use of normal operating procedures during the tests serves to emphasize 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 are also provided for machines with multiple-tip probing. This includes the use of probe changers and probe indexing capabilities. Tests for other subsystems, such as software, 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. This choice was made in favor of more common statistical measures, such as standard deviation, and because the dominant errors in coordinate measuring machines are systematic as opposed to being random. In such cases, no generally accepted statistical procedures currently exist.
Repeatability is defined as the "ability of a measuring instrument to provide closely similar indications for repeated applications of the same measurand under the same conditions of measurement." The specified testing of repeatability requires a series of measurements of the center coordinates of a precision ball, using the same testing procedure as the 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, providing limited but valuable testing of the machine. This method has been chosen due to the speed and simplicity with which a machine can be evaluated using a ball bar to simulate a real measurement procedure. For very large machines, diagonal displacement measurements are used to supplement the ball bar results. For machines used in the duplex mode, measurements of a fixed ball in various positions are performed by both machines as a supplement to ball bar measurements by each machine. 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 from 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. A separate section is included that allows for performance testing of coordinate measuring systems under loaded conditions.
In order to clarify the use of this Standard, a short guide is included as Appendix A. To assist the user in tracing possible environmental problems, appendices are also provided for thermal environment testing (Appendix B), vibration analysis (Appendix C), electrical power analysis (Appendix D), and utility air analysis (Appendix E). Appendices on hysteresis testing (Appendix F), ball bar test equipment (Appendix G), straightedge tests for ram axis roll (Appendix H), and interim testing of CMM systems (Appendix I), also provide the user with important subsidiary information.
Productivity is an important consideration in the selection of a coordinate measuring machine. There are numerous factors that affect relative productivity of measuring systems, including variables inherent to both the system and the workpiece. This Standard does not address methods to specify and evaluate productivity; rather, productivity should be evaluated with respect to the expected use of the system.
Contents and Specification Form
Any specification described as complying with this Standard shall include at least the following items.
(a) Machine classification (see para. 2.2). If no machine classification is applicable, the actual configuration shall be described in equivalent detail.
(b) Principal mode of operation (free-floating manual, driven manual, or direct computer control). If desired, repeatability, linear displacement accuracy, volumetric performance, bidirectional length measurement capability, point-to-point probing performance, and multiple-tip probing performance may be specified for more than one mode of operation.
(c) Principal probe type (passive, switching, proportional, or nulling). If desired, repeatability, linear displacement accuracy, volumetric performance, bidirectional length measurement capability, point-to-point probing performance, and multiple-tip probing performance may be specified for more than one probe type.
(d) Probe approach rate, probe approach distance, and settling time(s) for the principal probe type(s) specified.
(e) Nominal voltage, frequency, and power requirement.
(f) Utility air pressure, pressure variation, flow, temperature, dew point, and particulate content.
(g) Permissible environment vibration amplitude as a function of frequency. The amplitude must be specified at the interface between the equipment supplied by the user and that supplied by the CMM supplier.
(h) Statement of availability of data required for foundation design and machine installation.
(i) Statement of the significant mean temperature change, if available, safe operating temperature range, nominal location for the temperature variation error test, and the availability of other thermal response data for the machine.
(j) Statement of nominal coefficients of thermal expansion of the machine scales, by axis.
(k) Parameters describing a recommended machine thermal environment.
(m) Linear displacement accuracy defined by measurement with a laser interferometer or a mechanical master. The choice shall be clearly specified.
(n) Volumetric performance including ball bar performance, offset probe performance, volumetric tests for machines with large work zones, tests for duplex machines, rotary axis testing, and tests for machines under loaded conditions.
(o) Bidirectional length measurement capability.
(p) Point-to-point probing performance.
(q) Multiple-tip probing performance.
(r) A sample machine specification form. This form is illustrated in Fig. 1 for a typical machine. It is divided into three sections: General (Fig. 1A), Environmental (Fig. 1B), and Performance (Fig. 1C). The General section is intended to characterize the machine by configuration, size, operation mode, and probe type. The Environmental section is intended to describe environmental requirements for the machine. The Performance section illustrates the parameters used to specify performance within the context of this Standard. In the case that more than one operating mode/probe type combination is specified, performance shall be specified for each combination. This form cannot be effectively used outside the context of this Standard as the Environmental and Performance sections are closely connected through working tolerance derating procedures described in Sections 4 and 5.
This Standard allows parts of the environmental tests section to be deferred or bypassed and only the performance tests to be carried out. This alternative is acceptable only if it is acceptable to both the user and the supplier and if deferred as specified in Section 4.1.