Evaluation of contact and optical profilometry in the control of the roughness of curved surfaces of machine parts

Abstract

Introduction. Roughness measurement of curved machine parts is a critical metrology task, determining the performance characteristics of the components. The contact method introduces systematic error due to the straight-line trajectory of the probe and the plastic deformation of microroughness. Non-contact optical methods eliminate mechanical stress, but their accuracy decreases in areas with a large local inclination angle due to the shadowing effect. The lack of a unified approach to comparing results for curved surfaces limits the ability to tighten tolerances. The purpose of the work is to conduct a comprehensive comparative analysis of contact and optical profilometry in measuring the roughness of curved surfaces and to develop mathematical models for correcting systematic errors. Materials and methods. Experiments were conducted on samples of 0.45% C steel and GG20 cast iron with curvature radii of 2–25 mm and roughness parameters Ra = 0.1–12.5 μm. Contact measurements were performed using a TIME TR200 profilometer, while optical measurements were performed using a Veeco Wyko NT9100 in vertical scanning interferometry and phase shift interferometry modes. Statistical processing included Student's t-test, correlation, and regression analysis. Results and discussion. The error of the contact method at a radius of 2 mm reaches +27.6%, at R = 25 mm it is no more than +1.4%. The optical method is resistant to curvature, but in areas with high curvature leads to an underestimation of Ra by 10–15% due to the shadowing effect. For flat surfaces (Ra = 0.8–6.3 μm), there are no significant differences between the methods (r = 0.98). Mathematical models have been developed to correct the influence of temperature, contamination, roughness orientation, vibration, and contact force. Conclusions. An algorithm for selecting a measurement method is proposed: for radii of curvature less than 10 mm, the optical method with geometric correction is recommended; the contact method is not applicable for radii less than 5 mm. The obtained models can be integrated into the profilometer software for automatic error compensation.