I'm new to java3D but came across the same problem. I got around it by using a Transform3D object....
Alpha rotationAlpha = new Alpha(-1, 4000);
Transform3D axis = new Transform3D();
axis.rotZ(Math.PI*0.5f); // rotate the object through 90 degrees / 0.5 PI radians
RotationInterpolator rotator = new RotationInterpolator(rotationAlpha, TGSpin, axis,(float)0, (float)Math.PI*2.0f);
if you want to spin it the other way, just use Math.PI*1.5f instead.
..I'm not sure if this is the correct way to do it, but it does work.