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Going nuts trying to get lofting on spline rails

Message 1 of 10
1543 Views, 9 Replies

Going nuts trying to get lofting on spline rails

I'm quite new to F360, but have been digging through videos and tutorials for quite a while now.  This particular problem I've been fighting for two days now.  I want to loft from a circle to a rectangle, and want the loft to be on four spline rails that I created using the ImportSplineCsv sample script (the curves were calculated using Excel and exported to csv).  I got the splines created in F360 along with the two profiles, but no matte what I do, I get the error "The rails do not intersect both profiles".  The end points of the rails/splines are on the profiles edges (numerically), and I have tried Project, Intersect, changing the order of creation of the various things, attempted to edit the points (there doesn't appear to be a way to snap them to midpoints of the profiles that I an find), etc., but no luck at all.


The display does show little violet circles at the points where the rails are intended to contact the profile edges (I assume that means the points are coincident?  Is there some documentation somewhere that describes what the purple rings, white centered circles, grey centered circles, etc. all mean at point locations?).


Is this something that is even possible in Fusion 360 -- to do a a rail-guided loft following externally fed spline points?


Thanks for any tips anyone may have.



Message 2 of 10
in reply to: Anonymous

It looks like the problem is in the 'throat' sketch.  I recreated the sketch and geometry using a plane through three points.  The loft is successful, but the 'mouth' corners are not quite correct.




Message 3 of 10
in reply to: Anonymous

Judging from the shape you want to create, you just need to make loft tangent from the circle profile and and follow direction from the other ... Because no loft can follow more than 1 rail
Message 4 of 10
in reply to: SaeedHamza

Multiple rails are no problem, they do have to be set up right, though

imported data is the likely to be part of the problem, but there is a bug in there somewhere,


About 15 minutes to sort it out, but can be done with your data, I will expalin


Because you imported the splines, and in your file there were no projected points in the Throat sketch,

I found that the circle in the throat sketch would not take a full set of coincident constraints, without an error,

(I think the splines points are not exactly true to the dimension, could be wrong)


so I deleted the circle, Project >Intersect the spline end points into that sketch, and drew (2) coincident 3 point arcs.  (if there is an error you won't notice it, as the arc was showing 12.7mm rad.)

The splines and Mouth sketch did not produce the non intersection rails error,


so, I have not seen it before, screencast will show it better than I can describe it,

Solid Loft, will accept your inputs, but make a mess in the sharp corners,

knowing that the Patch loft works better in some cases, went to the Patch Loft, and it is all good.


Deleting the circle has caused the yellow warnings, will let you play with that.


My file exported, screencast too.

@jeff_strater, or your Lofting guru, you might be interested

Message 5 of 10
in reply to: Anonymous

This might have no bearing on the current lofting troubles, but you us weak too many points to control the  curvature of your splines/rails. You can like reduce the number of spline control points by 80%.

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Message 6 of 10
in reply to: etfrench

etfrench -- thanks a lot for the quick reply and solution.  I hadn't thought of generating the circle (throat) based on the spline end points.  Can I ask -- how did you figure out that it was the points on the circle (and not the rectangle) that were the problem?  The error messages were of little help in identifying where the problem lay, I kept making blind stabs at both end of the loft.



Message 7 of 10
in reply to: davebYYPCU



Thanks for the good looking solution.  I'm going to have to go over what you did a little more in detail to understand it.  Your final shape, using Patch, looks perfectly like what I was trying to get to.  What told you that the problem was at the circle (rather than the rectangle)?



Message 8 of 10
in reply to: Anonymous

Gooday Bill, 


Experience with Loft, and your clue that the Splines were imported and required.

I have taught myself, to Project > Intersect, the Rail end points into sketches for profiles, first thing, then turn off all rail visibility, create the profile by snapping to the purple points.

@etfrench video making new circle was a big clue as well.


In your case I could not get your circle to connect to the new purple dots, so figured the end points were not close enough to the original circle, to "make concentric connections", so went with the two semi circles.  I found the radius to be correct, so we are talking, very very small discrepancy, Fusion handling the data points, or the original calcs. So first attempts to loft after that, the other ends were working, and there were some purple dots, that end of the rails, as well.


Guys @innovatenate in the Forum had taught me that solids are more finicky than surfaces, and the very sharp point may turn out to be a graphics issue, rather than the geometry.  Did see similar with some of my wing tips early in the piece, with solids over surfaces.


So to progress from there, Patch the top and bottom surfaces, then Modify > Stitch them, see if the corners behave when it turns solid, I didn't go that far, in case you were going to Thicken or Shell the model, we already have clues either may not work, if it is Geometry related.


Happy to help....

Message 9 of 10
in reply to: davebYYPCU

Just a few comments here.  And I'm in no way a Loft expert, but I have learned a couple of things along the way that can help.


  1. Yes, Loft is very picky about loft rails exactly touching the profiles.  You almost always have to project from one sketch to the other to get this to work.  Either create the profiles first, then create the rail sketches, project -> intersect the profiles into the rail sketch, and make sure to attach the rail sketch curves to those projected points, or create the rails first, and create the profiles using the project -> intersect points from the rail curves.  This is often the biggest problem with Loft.  Once you internalize this, it becomes second nature, but it is confusing at first
  2. Solid vs Surface.  Yes, in this case the solid loft will fail because it produces a self-intersecting surface, and solids cannot have that property.  The surface loft will succeed, as @davebYYPCU shows in his screencast.  If what you want is a thickened version of that, you should be fine, but if you want to close off either end of this to make it solid, you will have the same problem.  Adjusting the rail curve is the only way to fix this, for this case.
  3. As @TrippyLighting says, you probably should not use quite so many points in your spline.  In an AU class that he and I did at AU last year, this is one of the points we made.  You can see the handouts for this class, specifically for this issue, here.

Glad you were able to make progress.




Jeff Strater
Engineering Director
Message 10 of 10
in reply to: Anonymous

@Anonymous wrote:

etfrench -- thanks a lot for the quick reply and solution.  I hadn't thought of generating the circle (throat) based on the spline end points.  Can I ask -- how did you figure out that it was the points on the circle (and not the rectangle) that were the problem?  The error messages were of little help in identifying where the problem lay, I kept making blind stabs at both end of the loft.



That's easy, changing the rectangle didn't make the error go away, so that meant that either the circle or both the circle and the rectangle were off.  Unfortunately, Fusion 360 doesn't expose the X, Y, Z coordinates of geometry end points which would have made this very easy to find.



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