i see no problem with starting and ending at the same point. i have tried creating loops in 3d and it works fine. when you enter the make datum curve menu, there is an option called Use Equation. It will ask you to choose a coordinate system as well as a coordinate system type. ( i generally stick to cylindrical type.) the equations you have must be rewritten in terms of the variable t. t's range is from 0 to 1, so if you want this changed, multiply all your t values by the new limit needed. hope this helps.