A "simple" lofting problem

pridbor · Dec 11, 2022

I have virtually never used Lofting, so when I did this weekend, and failed, I decided to look in the new "Design Book" we just got in an email.

There's an example (Tutorial 2, page 422) showing how to make a simple Loft, and I make it fine enough, except when I add the center guideline, then I get a message which I don't get.

"SKIN_PATH_PROFILE_NO_PERP: Profile plane and tangent of the path are not perpendicular."

Your guidance is much appreciated.

Preben

HaroldL · Dec 11, 2022

Splines do not work for centerline lofts since you can't get them to be perpendicular to the end point planes. There may be step I'm missing with splines or haven't tried yet.

Step 6 states that the centerline sketch is a line and a tangent arc, which does work.

kcoffield · Dec 18, 2022

I had the same problem, and Harold very generously took a look for me. Even though he made short segments attached to the end of the spline that were constrained to be perpendicular to the loft faces, it still failed for the same reason. His short video was very instructive and helpful.

Centerline Lofting along 3D Spline

Hi all, In the attached file I was attempting to loft (Loft 7) the filleted rectangle (Port Window) to the circular sketch (Carb Opening) along the 3D Spline (Runner Centerline). It fails and I get the error "SKIN_PATH_PROFILE_NO_PERP: Profile plane and tangent of the path are not...

www.alibre.com

HaroldL said:
Splines do not work for centerline lofts since you can't get them to be perpendicular to the end point planes. There may be step I'm missing with splines or haven't tried yet.

Step 6 states that the centerline sketch is a line and a tangent arc, which does work.

I was able to use the simple autogenerated loft in the example linked above, but in the future I will not be able to do so. I need to be able to drive the loft on centerlines along curved surfaces and defined 3-space locations for features that recur in a very high percentage of my projects. This capability was on my must have list and in part why I selected Ailibre Pro (over Atom or others), so if anyone does know of a method to drive the loft along a centerline that was at least originally derived from a spline, I'd greatly appreciate any advice you could offer. Absent that I see myself trying to patch together arcs in 3-space that approximate the spline, but what a nightmare, and it may fail as well.

Best Regards,
Kelly

MilesH · Dec 18, 2022

HaroldL said:
Splines do not work for centerline lofts since you can't get them to be perpendicular to the end point planes. There may be step I'm missing with splines or haven't tried yet.

Is there something I'm missing here? You can certainly add planes perpendicular to 3D splines.

kcoffield · Dec 18, 2022

MilesH said:
Is there something I'm missing here? You can certainly add planes perpendicular to 3D splines.

Give it a try on a simple example and see if you get the perpendicularity error, but do make sure to use a 3-space spline guideline, not just an arc with tangency. I tried three different methods and confident the portion of the spline was perpendicualr to the lofted faces with straight segments/extensions but all returned the same error. I can see how a spline with sharp radii could not be lofted for some faces if the face width caused the radius of the inside loft curvature along the spline to become less than zero, but this was not the case in my attempts.

Best,
Kelly

HaroldL · Dec 18, 2022

MilesH said:
You can certainly add planes perpendicular to 3D splines.

Good point, Miles. Maybe the missing step is to create the planes perpendicular to the spline, then create the profiles on those planes. In Kelly's case the planes and profiles were already defined and the 3D spline sketch was created for the path and couldn't be constrained perpendicular to the profile planes. Something to try would be to create the 3D path sketch and create planes at both ends of it. Then adjust the planes to their final orientation, one parallel to one of the default planes and the other end plane at the required angle from the first one. Something tells me that it won't work but when I have a few minutes I'll give it a shot.

MilesH · Dec 19, 2022

Yes. I always start with the Guidecurve and then add the profiles. I haven't yet had a case where there was no alternative but to work the other way around. I can see how that would be problematic though.

Oldbelt · Dec 19, 2022

You are right you cant adjust planes angles or position, thy are defined of the guide curve, what ether its a 3D or a 2D curve.

kcoffield · Dec 19, 2022

MilesH said:
Yes. I always start with the Guidecurve and then add the profiles. I haven't yet had a case where there was no alternative but to work the other way around. I can see how that would be problematic though.

Oldbelt said:
You are right you cant adjust planes angles or position, thy are defined of the guide curve, what ether its a 3D or a 2D curve.

So just to be clear, you fellas are saying it's the order the spline and face features are created that makes the difference? Spline first then faces perpendicular to spline?? But reverse the order and it fails? Seems strange but if it works, I wont reason why. I see you attached a file in the other lofting post. I'll have a look at it when I can. I'll link for future reader's reference.