euspen’s 20 th International Conference & Exhibition, Geneva, CH, June 2020 www.euspen.eu Axial calibration of an on-machine focus variation surface texture and form sensor Subbareddy Darukumalli 1,2 , Teguh Santoso 1 , Wahyudin P. Syam 1 , Franz Helmli 2 and Richard Leach 1 1 Manufacturing Metrology Team, University of Nottingham, UK. 2 Alicona Imaging GmbH, Dr Auner Straße 21a, 8074 Raaba, Austria. E-mail: s.darukumalli@bruker.com Abstract To address the increase in tight tolerance requirements for small parts produced by precision manufacturing, on-machine optical areal surface topography instruments are emerging. To calibrate these instruments and estimate their measurement uncertainty, their metrological characteristics need to be determined according to ISO 25178 part 600. In this paper, the amplification coefficient and linearity deviation metrological characteristics in the vertical axis of a prototype compact on-machine focus variation areal surface texture and form measurement sensor are determined. With a series of experiments in different positions of the vertical axis using calibrated materials measures with heights from 0.2 µm to 1000 µm, we determine the amplification coefficient and linearity deviation for the vertical axis. In addition, with a procedure derived from ISO 10360 part 8, the maximum permissible unidirectional stationary error of the vertical axis is determined. Metrological characteristics, axial calibration, on-machine metrology, focus variation 1. Introduction Advances in precision manufacturing technologies lead to an increase in the tight tolerance requirements of small parts. To measure such tolerances to sub-micrometre accuracy and avoid the measurement instrument’s influence on the measured features, in contrast to the traditional contact measurement instruments, commercial non-contact optical areal topography instruments are emerging. On-machine areal topography instruments are becoming popular because surface texture and form errors can be used as the fingerprint of the manufacturing process [1]. Focus variation microscopy (FVM) areal topography measuring instruments provide measurement data using an optical setup along with a high-precision encoder for measurement axis position measurement [2]. To calibrate a FVM instrument, a series of standardised metrological characteristics must be determined [3], where calibration is defined as “an operation, under specific conditions, in a first step required to establish a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and in a second step, uses this information to establish a relation for obtaining a measurement result from an indication” [4]. In practice, calibration of the instrument refers to a series of operations required to establish the contribution of the metrological characteristics to the measurement uncertainty associated with the instrument measurements [5,6]. In this paper, we address the metrological characteristics associated with the on-machine focus variation sensor measurement axis in section 2, the experimental design and measurement procedure are explained in section 3, measurement data and analysis are presented in section 4 and finally the conclusions and future work are given in section 5. 2. Metrological characteristics The metrological characteristics used for FVM instrument calibration include: amplification coefficient, linearity deviation, residual flatness, measurement noise, lateral period limit and xy mapping error [5,6]. In this paper, we address the amplification coefficient, linearity deviation and maximum permissible unidirectional stationary error associated with the on-machine FVM sensor measurement axis (z-axis) using calibrated material measures. These characteristics are defined as: • Amplification coefficient is the slope of the linear regression curve obtained from the response curve [5,6]. • Linearity deviation is the maximum local difference between the line from which the amplification coefficient is derived and the response function [5,6]. • Maximum permissible unidirectional stationary error  ::, is determined using a method derived from the ISO 10360-8 maximum permissible error determination procedure ( where  indicates the measurement direction,  for stationary, ODS for optical distance sensor and  for maximum permissible error) by the measurement of height steps in a single field of view [7]. 3. Experimental design The compact on-machine FVM sensor used is presented in reference [8] with a magnification 20× (0.4 numerical aperture, working field of view 0.59 mm × 0.59 mm and sampling distance approximately 0.30 µm) objectives lens, together with ring light illumination. The working range of the z-axis is 15 mm. Single step height artefacts were measured in five positions within the working range of the instrument and the measurement positions were chosen to cover the total working range. Each step height artefact was measured at 90% (13.5 mm), 70%