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Problems with Lofting

Leethal

Member
This is my first attempt at lofting. I'm trying to make a curved cover over this tail section. I only have the two drawings labelled front and rear to attempt to loft between. The rear was only a guess at what it could look like. I'm currently getting the error "Guide curve does not start and stop on the first and last profile respectively"
Can someone tell me if I need to add more drawings for the loft or am I doing something incorrectly
 

Attachments

  • Rear Section.AD_PRT
    637.5 KB · Views: 16

DavidJ

Administrator
Staff member
As the message suggests - guide curves (you'll need more than one in this case) must have ends EXACTLY on the loft profiles.

Consider using either 'project to sketch' to pick up from existing geometry, or place Points in the profiles that you can use to attach ends of guide curves to.

Lofts are probably the trickiest thing to do in Alibre, there are lots of ways they can fail.

Your loft works if done without guide curves - so you need to work on guide curves. I trimmed your guide curve so it ran only between the 2 profiles down one side of the model. Had to extend slightly at the large end and trim back at the small end. That worked. I then added a similar curve on the other side, was better (that using guide curves as 'local') - it would be even better with extra guide curves for the inside.
 

Leethal

Member
As the message suggests - guide curves (you'll need more than one in this case) must have ends EXACTLY on the loft profiles.

Consider using either 'project to sketch' to pick up from existing geometry, or place Points in the profiles that you can use to attach ends of guide curves to.

Lofts are probably the trickiest thing to do in Alibre, there are lots of ways they can fail.

Your loft works if done without guide curves - so you need to work on guide curves. I trimmed your guide curve so it ran only between the 2 profiles down one side of the model. Had to extend slightly at the large end and trim back at the small end. That worked. I then added a similar curve on the other side, was better (that using guide curves as 'local') - it would be even better with extra guide curves for the inside.
Thanks David,

Can you attach what you did so I can have a closer look at where you placed the curves?
Hopefully I can progress from there.

Regards,
Lee.
 

DavidJ

Administrator
Staff member
Here is the part with a single guide curve (edited from your own). All you have to do is add 3 more guide curves (test after each one).
 

Attachments

  • Rear Section1.AD_PRT
    827 KB · Views: 14

Leethal

Member
Here is the part with a single guide curve (edited from your own). All you have to do is add 3 more guide curves (test after each one).
Thanks David. Unfortunately, I have 2019 so I can't open your file. I assume the other guide curves are 3D that go from somewhere around the front sketch to somewhere around the rear sketch? I can't seem to use the lower guides to bring the loft down to the sweep.

I had more luck, just doing the loft without the swept part but it isn't following the bottom curves as they can't be included for some reason.. I'm trying to get it more like the Guide centre curve though, but it doesn't like including it.
 

Attachments

  • Rear Section.AD_PRT
    646 KB · Views: 1
  • Rear Section 1.AD_PRT
    761 KB · Views: 0
  • Rear Section 2.AD_PRT
    397.5 KB · Views: 0
Last edited:

HaroldL

Alibre Super User
The extra guide curves are long the bottom edges of the part, a new sketch on each side.

Start a sketch on the face, select the edges, and use Project to Sketch with Maintain Associativity selected.

1679882833445.png
 

Leethal

Member
Hi Harold,

I've added Sketch 13 and 14 and I still can't get it to loft. I've tried using those 2 curves with 2 other curves as well and no luck.
 

Attachments

  • Rear Section 3.AD_PRT
    681 KB · Views: 1

DavidJ

Administrator
Staff member
Check VERY carefully (zoom right in), sketch 13 and sketch 14 both stop just short of your Front sketch - they'll need extending slightly (easy with 'project to sketch').
 

DavidJ

Administrator
Staff member
I strongly suggest add only ONE guide curve at a time - don't add next one until the previous one works. At least that way you know which curve has the problem.
 

Leethal

Member
I strongly suggest add only ONE guide curve at a time - don't add next one until the previous one works. At least that way you know which curve has the problem.
Thanks David. I did add the small missing segments without any luck. I did finally get something like what I was after by lofting down on to a sketch on the top surface at the rear and then doing a couple of rough 3D splines.

Thank you all for your help. Much appreciated.
 

Attachments

  • Rear Section 3b.AD_PRT
    1.3 MB · Views: 1
Last edited:

HaroldL

Alibre Super User
@Leethal , I took the liberty of examining your Loft model to see what the issues are and recorded my findings in this short video:


I also moved Plane 6 and added a new Guide Curve on it but that needed a new Axis and Point to constrain the guide curve to the Middle profile sketch.

Last update to Loft.jpg
 

Leethal

Member
@Leethal , I took the liberty of examining your Loft model to see what the issues are and recorded my findings in this short video:


I also moved Plane 6 and added a new Guide Curve on it but that needed a new Axis and Point to constrain the guide curve to the Middle profile sketch.

View attachment 38800
Wow Harold, thank you so much. That is the best support I think I have ever received. I wasn't aware that I needed to fully define the sketches and reference curve ends but I have much to learn.
 

JimCad

Senior Member
Thanks VERY much Harold.
I've not done a lot of lofting so I found that very informative. Probably a lot of us did.
 
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