A first plastic part is fixtured in a first fixture, and a second plastic part is fixtured in a second fixture for welding to each other. The first plastic part can be independently moved relative to the second plastic part in each of three directions: 1) linearly in a direction parallel with a z-axis, 2) linearly in a direction parallel with an x-axis, and 3) circularly in a circular arc about an axis parallel with a y-axis.