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Dalian University of Technology, China (DLUT) successfully developed class-1 spur gear artefacts. However, the expanded uncertainty U95 of the existing instrument for measuring gear helix deviations is more than 1/3 of the class-1 gear helix tolerances, which cannot meet the measuring requirements of the helix deviations for the class-1 spur gear artefacts. In order to solve these problems, the influences of the mandrel locating error on the gear helix deviations are analysed, and a novel instrument for measuring spur gear helix deviations to submicron accuracy is designed. The instrument is optimized for the measurement of specific gear types (external teeth, medium module, with through-hole, reference diameter ≤126 mm). The expanded uncertainties U95 of the instrument for measuring the total helix deviation Fβ, the helix slope deviation fHβ and the helix form deviation ffβ are 0.23 µm, 0.20 µm and 0.18 µm respectively. This is less than 1/3 of the class-1 gear helix tolerances (the total helix tolerance FβT, the helix slope tolerance fHβT and the helix form tolerance ffβT are 2.4 µm, 1.6 µm and 1.8 µm respectively: ISO 1328-1:2013). Verification measurements with a calibrated measurement standard show the proficiency. [ABSTRACT FROM AUTHOR]