On the "Iron Horse" heavy-duty truck produced by our factory, there are a number of T-shaped tee fittings of different sizes, as shown in Figure 1. When turning the vertical end of the workpiece on the lathe, if the special fixture is not used, the four-claw single-action chuck is generally used to clamp the workpiece. The adjustment and adjustment are time-consuming and laborious, which affects the work efficiency. To this end, we have designed a simple jacket that directly grips such workpieces with a three-jaw self-centering chuck, as shown in Figure 2.