Abstract: For a certain series of automotive flanges produced by the company, the coaxiality requirement between the internal spline and the spigot is ≤0.05 mm. After finish-turning of the inner hole and the spigot, surface electrophoresis treatment is required. Due to the narrow width of the spigot, local protection of the flange spigot cannot be realized during the electrophoresis process, so finish machining of the spigot has to be carried out after electrophoresis. The design and manufacture of the finish-turning fixture for the spigot, together with the selection of positioning datum, have long been challenging problems in production. To solve this problem, a floating self-adjusting expansion-sleeve chuck is designed, and the mandrel, as the core component of the fixture, is optimally designed. Practical machining verification shows that with this chuck, the pass rate of flange spigot machining reaches 100%, which satisfies the mass-production requirements of the product and offers a reference for the machining of similar products.