I used the "curves by equasion" to produce a helix wrapped around a cylinder. Then with Variable Section Sweep, I extruded a square section along the curve. I have made a simple, square, flat "truck" with a square hole in the middle and the square truck is to travel along the square, helical rail. Using Pre defined constraints I find that I can not choose any of the surfaces on the helical rail. This restricts me to using the SLOT option which only recognizes points on curves and edges. Three datum points on the truck correspond to the positions where the curve and edges would be. If I attach the the center point to the curve and the other two points to the corresponding edges on the square helix, the truck will NOT move with the "Drag Components" hand. If I disable ANY one of the constraints, the truck moves along famously. Except that it tumbles along using the two points as an axis, instead of remaining oriented to the VSS. I was wondering if reducing the accuracy will help, or is there some setting that I can use to make it recognize the surface? It is probably because the surfaces are constantly changing in all directions along the square helix. But strangly enough, using the SLIDER option allows me to choose the surface of the helix for the ROTATION. However, it still needs an AXIS and there isn't one to use. So it won't complete the slider option. Peter
