This study investigates how structural alignment between mathematics teaching and vocational welding practice can support student engagement and strengthen the perceived relevance of mathematics. The research examines an interdisciplinary intervention in Danish vocational education, where mathematics and welding teachers collaboratively designed modules linking trigonometric calculations to practical workshop tasks. Using Cultural–Historical Activity Theory (CHAT), the mathematics classroom and the welding workshop are analysed as interacting activity systems. The analysis draws on video observations, field notes, and interviews with teachers and students. The findings show that interdisciplinary collaboration reorganised relations between classroom and workshop practices by aligning artefacts, division of labour, and shared objectives across learning environments. Students reported improved understanding of how mathematics is applied in vocational practice, increased precision in workshop tasks, and more positive attitudes towards mathematics.