John R. Stoup

Uncertainty and Dimensional Calibrations

The calculation of uncertainty for a measurement is an effort to set reasonable bounds for the measurement result according to standardized rules. Since every measurement produces only an estimate of the answer, the primary requisite of an uncertainty statement is to inform the reader of how sure the writer is that the answer is in a certain range. This report explains how we have implemented these rules for dimensional calibrations of nine different types of gages: gage blocks, gage wires, ring gages, gage balls, roundness standards, optical flats indexing tables, angle blocks, and sieves.

A fiber probe for CMM measurements of small features

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.

Minimizing error sources in gauge block mechanical comparison measurements

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.

Measurements of large silicon spheres using the NIST M48 coordinate measuring machine

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 machines 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.

Area measurement of knife-edge and cylindrical apertures using ultra-low force contact fibre probe on a CMM

Several radiometric and photometric measurements depend on high accuracy area measurement of precision apertures. Some apertures have sharp edges and are generally measured optically. At the Precision Engineering Division of the National Institute of Standards and Technology (NIST), we have developed a contact fibre (fiber) probe for diameter and form measurement of micro-holes (holes of size 100??m or larger). This probe exerts extremely small forces, under 5??N, and can therefore be used on knife-edge apertures without causing edge damage. We have measured the diameter and roundness of three knife-edge and one cylindrical apertures with this probe. The uncertainty in diameter ranges from 0.06??m (k = 1) to 0.17??m (k = 1). The uncertainty contributions from the probing system and machine positioning are together only 35 nm (k = 1). The largest contributors to the diameter uncertainty are the overall form (sampling uncertainty) and surface finish (mechanical filtering due to finite probe size) of the aperture.

Fiber Deflection Probe Uncertainty Analysis for Micro Holes

Abstract: We have recently reported on a new probe, the Fiber Deflection Probe (FDP), for diameter and form measurement of large aspect ratio micro-holes (100 qm nominal diameter, 5 mm deep). In this paper, we briefly review the measurement principle of the FDP. Then, we discuss different error sources and present an uncertainty budget for diameter measurements. Some error sources are specific to our fiber probe such as imaging uncertainty, uncertainty in determining calibration factor, and misalignment of the two optical-axes. There are other sources of error that are common to traditional coordinate metrology such as master ball diameter error, tilt in holes axis, temperature effects etc. Our analysis indicates an expanded uncertainty of only 0.07 qm on diameter.

Uncertainties in aspheric profile measurements with the geometry measuring machine at NIST

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.

Accuracy and versatility of the NIST M48 coordinate measuring machine

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.

Minimizing errors in phase change correction measurements for gauge blocks using a spherical contact technique

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.

Form-Profiling of Optics Using the Geometry Measuring Machine and the M-48 CMM at NIST

We are developing an instrument, the Geometry Measuring Machine (GEMM), to measure the profile errors of aspheric and free form optical surfaces, with measurement uncertainties near 1 nm. Using GEMM, an optical profile is reconstructed from local curvatures of a surface, which are measured at points on the optic’s surface. We will describe a prototype version of GEMM, its repeatability with time, a measurements registry practice, and the calibration practice needed to make nanometer resolution comparisons with other instruments. Over three months, the repeatability of GEMM is 3 nm rms, and is based on the constancy of the measured profile of an elliptical mirror with a radius of curvature of about 83 m. As a demonstration of GEMM’s capabilities for curvature measurement, profiles of that same mirror were measured with GEMM and the NIST Moore M-48 coordinate measuring machine. Although the methods are far different, two reconstructed profiles differ by 22 nm peak-to-valley, or 6 nm rms. This comparability clearly demonstrates that with appropriate calibration, our prototype of the GEMM can measure complex-shaped optics.