Here is the file i am having the problem with. Also when i click on a spine, the grid moves.
Thanks for the help.
The splines and polylines do not all lie on the same Workplane (which is why the grid moves), AND they do not meet properly at their endpoints. You can sidestep both of these problems by applying The Ol' Hatch Trick, which automatically creates a single extrudeable Polyline outlining the whole shebang. Here's how...
1. First thing to do is modify the splines to reduce the total number of vertices in the drawing to a manageable number: Press Ctrl+T; select "Curve" from the list; click OK. Click the "Properties" icon on the Inspector Bar; click "Curve" in the left hand window of the Properties dialog; change "The number of segments between adjacent control points" to 2
; click OK. (You don't normally want to use such a small value, but I think it's OK here because the splines are not strongly curved.)
2. Press Ctrl+A to select everything in the drawing; press and hold Shift while you click the two centerlines and thus de-select them. Open the Format menu and click "Create Hatch."
3. Go to an Isometric view; activate the Simple Extrude tool; click on the newly created Hatch; extrude it as outlined in my earlier post.