Final report on inter-RMO Key Comparison EUROMET.L-K5.2004: Calibration of a step gauge

Abstract

The results of the inter-RMO key comparison EUROMET.L-K5.2004 on the calibration of a step gauge are reported. Eighteen National Metrology Institutes and one Designated Institute from four different metrological regions all over the world participated in this comparison which lasted three years, from December 2004 to December 2007.A lack of stability was observed through the shifting of some of the inserted gauges. In order to save the comparison and get valuable and useful conclusions, it was agreed to exclude four gauges from calculation and assume that only seven gauges were reasonably stable so as to get the corresponding reference values. It was also agreed to divide the participants into two groups, analyze separately their results and, taking the pilot as the linking laboratory, refer the results to common reference values.The inverse-variance weighted mean was taken as reference value. Due to the significant instability of the step it was also considered an artefact uncertainty. The reported uncertainties ranged from 0.045 µm to 1.2 µm (k = 1). The uncertainty of the artefact ranged from 0.018 µm (for the 20 mm face) to 0.176 µm (for the 400 mm face).The compatibility of all participants for measuring step gauges was demonstrated with the only exception of a participant showing very high systematic (both positive and negative) errors. Five participants communicated higher uncertainties than the corresponding approved CMCs. A set of Recommendations and Actions were agreed therefore.Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA).

A review of state-of-the-art 1D length scale calibration instruments

“…Specific examples of this layout include a modified SIP F1A length measuring machine used for step gauge measurement at LNE [30] and the Laser automatic interferometric comparator (AIC) developed at NIM [31]. This layout has the same limitation as the use of a mechanical linkage in that it requires a length in free space between the reflector and the interferometer at least as long as the length of the maximum artefact to be measured.…”

“…A third approach is to accept an Abbe error but quantify it and apply appropriate corrections. The NPL Step Gauge Rig 1 [30] is designed with the laser beam directly above the centre of the measurement line, offset from it by a known distance as shown in figure 4. A single plane mirror is mounted in a custom CMM probe mount.…”

A New Computational Model of Step Gauge Calibration Based on the Synthesis Technology of Multi-Path Laser Interferometers

“…Moreover, the International Bureau of Metrology (BIPM) has also listed the step gauge as a key length comparison, making it an important component in measuring the metrological capacity of a national geometric measurement calibration laboratory [1][2][3][4]. Many ambitious countries have established their own step gauge calibration devices [5][6][7].…”

High accuracy step gauge interferometer

“…Several National Metrology Institutes (NMIs) and accredited calibration laboratories offer step gauge calibration with expanded uncertainties (k = 2), starting from slightly less than 0.1 µm as a constant uncertainty term (KRISS, CEM, NPL), but having a length-dependent term with the best expanded uncertainties for a 1 m step gauge being slightly over 300 nm (KRISS, VTT MIKES, NPL) [5]. Most of the step gauge interferometers (SGI) at some NMIs are based either on a length-measuring machine or a CMM and are equipped with a laser interferometer for improved uncertainty [6]. A CMMbased solution is limited to the accuracy of the probing head, and as CMMs are large machines the environmental conditions are not necessarily the best for interferometry.…”

“…A CMMbased solution is limited to the accuracy of the probing head, and as CMMs are large machines the environmental conditions are not necessarily the best for interferometry. There are some reported instruments designed purposely for step gauge calibrations at KRISS [7], NMIJ [8], CEM [6] and NMISA [9], which typically use a linear guide system with a four-pass plane mirror interferometer set-up and a selected probe. The limitation of custom-built systems is that several measurements are required if deviation from parallelism of the faces is to be measured.…”