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Extrude Along Nurbs Path - Object is Semi Circle, Object ends rotated 90?


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I have a problem with extruding along a nurbs path. The object I'm sweeping is a half of a tube formed by subtracting a smaller solid circle from a larger one and then clipping away half of the resulting ring (I'm trying to model a slide). When I extrude along this 3-D nurbs path which is more or less an arc with a sweep of 123.32? that rises in elevation 116", the object rotates itself along the path so that at the end, the extruded surface is rotated 90 degrees from its original position. Is there away to prevent this rotation so that the orientation of the swept object is the same at the beginning as it is at the end?

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I was wondering, I set my path object based on 3D loci locations. First I tried to use a 3D polygon to draw the path and then I converted it to NURBS, but the program will not let me extrude along the resultant Nurbs Curve, perhaps because the path has vertexes at the loci and is not a smooth curve? (Purely conjecture on my part) So then I draw a native nurbs curve with bezier curves based on the 3D loci points, but this gives me the rotated extrusion. I'd like to post an image, but don't have a website really to mount the image to (there isn't a way to just attach an image?)

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The tube starts of lets say perpendicular to the ground plane and ends up 90 degrees at the end of the rotation...oh and I tried with a fresh nurbs, and still get the same problem...also initially i tried to do a 3 d polygon first which I converted to nurbs but it wouldn't let me complete the extrude along bath even though both the path (nurbs) and path object (2 D polygon) were valid objects.

Here is an image:

http://img101.imageshack.us/img101/5536/tubeextrude2fa9.jpg

Edited by jss273nyc
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I think the extrude along a path command causes the profile to be always normal to the path. The normal wanders in 3d on a path such as yours. You might need to model this in several sections (usually these are built from several sections), or as a 2 rail loft. Rails follow top edges of trough, with a semi circular profile curve. I will see if I can get one to work, but not today.

-B

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