A Psychophysical Methodology for Determining Manufacturing Tolerance of Feature Lines on Automotive Outer Panels

This paper presents a methodology for determining manufacturing tolerances of feature lines on automotive outer panels using visual sensory tests. Feature lines—narrow and long curved surfaces on automotive panels—play a critical role in the visual appeal of vehicles. However, achieving precise feature lines in mass production is challenging due to material spring-back during the stamping process. Conventional tolerance determination methods are unsuitable for these esthetic elements. To address this, we employed psychophysical sensory tests to find the visual difference thresholds for feature lines. By creating geometric models and conducting controlled sensory tests, we identified the minimum radius variations perceptible to the human eye. Thirty-four participants were tested using the method of constant stimuli, resulting in psychometric functions for feature lines with radii of 8, 10, and 12 mm. The findings suggest manufacturing tolerances of ±1.2 mm, ±1.3 mm, and ±1.5 mm, respectively. This approach provides a quantitative foundation for setting tolerances that balance visual quality with production feasibility.