The National Institute of Standards and Technology as the U.S. National Metrology Institute has the fundamental
responsibility to continuously push the limits of measurement science (metrology) to promote U.S. innovation and
industrial competitiveness. In 2004, NIST finished construction of a $235 million, 49 843 m<sup>2</sup> Advanced Measurement
Laboratory to enhance its measurement capabilities in response to the fast-growing metrology needs of the scientific and
The Geometry Measuring Machine (GEMM) of the National Institute of Standards and Technology (NIST) is a profilometer for free-form surfaces. A profile is reconstructed from local curvature of a test part surface, measured at several locations along a line. For profile measurements of free-form surfaces, methods based on local part curvature sensing have strong appeal. Unlike full-aperture interferometry they do not require customized null optics. The uncertainty of a reconstructed profile is critically dependent upon the uncertainty of the curvature measurement and on curvature sensor positioning. For an instrument of the GEMM type, we evaluate the measurement uncertainties for a curvature sensor based on a small aperture interferometer and then estimate the uncertainty in the reconstructed profile that can be achieved. In addition, profile
measurements of a free-form mirror, made with GEMM, are compared with measurements using a long-trace profiler, a coordinate measuring machine, and subaperture-stitching interferometry.
We report on performance of a new form of fiber probe, which can be used in conjunction with a coordinate measuring machine (CMM) for microfeature measurement. The probe stylus is a glass fiber with a small ball (≈75 μm diameter) glued to the end. When the ball is brought into contact with a surface, the fiber bends, and this bending is measured optically. The fiber acts as a cylindrical lens, focusing transmitted light into a narrow stripe that can be magnified by a microscope and detected by a camera, providing position resolution under 10 nm. In addition to the high resolution, the
primary advantage of this technique is the large aspect ratio attainable. (Measurements 5 mm deep inside a 100 μm
diameter hole are practical.) Another potential advantage of the probe is that it exerts exceptionally low forces, ranging
from a few micronewtons down to hundreds of nanonewtons. Furthermore, the probe is relatively robust, capable of surviving more than 1-mm over-travel, and the probe stylus should be inexpensive to replace if it is broken. To demonstrate the utility of the probe, we have used it to measure the internal geometry of a small glass hole and a fiber
ferrule. Although the intrinsic resolution of the probe is better than 10 nm, there are many potential sources of error that could cause larger errors, and many of these errors are discussed in this paper. Our practical measurement capabilities for the hole geometry are currently limited to about 70 nm uncertainty. Hole measurements only require a twodimensional probe, but we have now extended the use of the probe from 2-d to 3-d measurements. Measurements of the
z-height of a surface can be carried out by detecting buckling of the stylus when it is brought down into a surface.
The NIST M48 coordinate measuring machine (CMM) was used to measure the average diameter of two precision, silicon spheres of nominal diameter near 93.6mm. A measurement technique was devised that took advantage of the specific strengths of the machine and the artifacts while restricting the influences derived from the machine's few weaknesses. This effort resulted in measurements with unprecedented accuracy and uncertainty levels for CMM style instruments. The results were confirmed through a blind comparison with another national measurement institute (NMI) that used special apparatus specifically designed for the measurement of these silicon spheres and employed very different measurement techniques. The standard uncertainty of the average diameter measurements was less than 20 nanometers. This paper will describe the measurement techniques along with the decision-making processes used to develop these specific methods. The measurement uncertainty of the measurements will also be rigorously examined.
The NIST Is continuing to develop the ability to perform accurate, traceable measurements on a wide range of artifacts using a very precise, error-mapped coordinate measuring machine (CMM). The NIST M48 CMM has promised accuracy and versatility for many ears. Recently, these promises have been realized in a reliable, reproducible way for many types of 1D, 2D, and 3D engineering metrology artifacts. The versatility of the machine has permitted state-of-the-art, accurate measurements of one meter step gages and precision ball plates as well as 500 micrometer holes and small precision parts made of aluminum or glass. To accomplish this wide range of measurements the CMM has required extensive assessment of machine positioning and straightness errors, probe response, machine motion control and speed, environmental stability, and measurement procedures. The CMM has been used as an absolute instrument and as a very complicated comparator. The data collection techniques have been designed to acquire statistical information on the machine and probe performance and to evaluate and remove any potential thermal drift in the machine coordinate system during operation. This paper will present the data collection and measurement techniques used by NIST to achieve excellent measurement results for gage blocks, long end standards, step gages, ring and plug gages, small holes, ball plates, and angular artifacts. Comparison data with existing independent primary measuring instruments will also be presented to show agreement and correlation with those historical methods. Current plans for incorporating the CMM into existing measurement services, such as plain ring gages, large plug gages, and long end standards, will be presented along with other proposed development of this CMM.
The current definition of the length of a gage block is a very clever attempt to evade the systematic errors associated with the wringing layer thickness and optical phase corrections. In practice, most laboratories wring to quartz or fused silica reference plates, and in addition there are very large systematic operator and surface effects. We present quantitative data on these effects and how that the current definition of gage block length is a primary source of measurement uncertainty.
Error sources in gage block mechanical comparisons can range from classical textbook examples to a completely counter- intuitive example of diamond probe tip wear at low applied force. Fortunately, there are methods available to metrologists that can successfully be applied to minimize these and other effects. Techniques such as statistical process control, use of check standards, thermal drift eliminating measurement algorithms, improved sensor calibration, and well-tested deformation modeling are used at the National Institute of Standards and Technology to minimize errors. These same methods can be applied by anyone making mechanical comparison gage block measurements.
One of the most elusive measurement elements in gage block interferometry is the correction for the phase change on reflection. Techniques used to quantify this correction have improved over the year, but the measurement uncertainty has remained relatively constant because some error sources have proven historically difficult to reduce. The precision engineering division at the National Institute of Standards and Technology has recently developed a measurement technique that can quantify the phase change on reflection correction directly for individual gage blocks and eliminates some of the fundamental problems with historical measurement methods. Since only the top surface of the gage block is used in the measurement, wringing film inconsistencies are eliminated with this technique thereby drastically reducing the measurement uncertainty for the correction. However, block geometry and thermal issues still exist. This paper will describe the methods used to minimize the measurement uncertainty of the phase change on reflection evaluation using a spherical contact technique. The work focuses on gage block surface topography and drift eliminating algorithms for the data collection. The extrapolation of the data to an undeformed condition and the failure of these curves to follow theoretical estimates are also discussed. The wavelength dependence of the correction was directly measured for different gage block materials and manufacturers and the data will be presented.
In this paper we report measurements of the dimensional stability of samples of brass,
beryllium copper, and tellurium copper taken over an 18 month time span. Of the
materials, brass was the most stable, decreasing slightly in length at the rate of
1 part per million per year (ppm/y) with an uncertainty (3a) of about 1 ppm/y.
Tellurium copper shrank at an average rate of 2.Li ppm/y and beryllium copper, the
least stable, at the rate of 5.8 ppm/y. To measure the instrumental uncertainty 4
samples of each material were measured, and the measurement scheme was designed to
detect and correct for thermal drift ,during measurements. The experiment design
problems associated with these measurements and the associated uncertainties are