How to join lines that touch at apexes


I have a feeling that this may be a stupid question, but…

I have three lines. One of the lines is highlighted in the picture below:

If I select all three lines and try and join them, all that happens is that the two lines at the top of the picture join each other but they do not join the highlighted one below. Even trying to run the join command a second time does not join all the lines into a single polyline.

The lines definitely touch, because I used the select end points when drawing them.

The reason I want to do this is that I want to run the Sweep rail command on a similar group of lines.


You can’t join together a set of curves like that because Rhino calls that “illegal” geometry. I have an explanation here,


Non-manifold structures - things like star or T-intersections - are not allowed for either surfaces or curves in rhino. Only two curves can be joined by their endpoints, once a curve end is joined to another, nothing else may be joined there.


(Brian Gillespie) #4

Why do you need these curves joined together as one?


Well the issue is that I have drawn a sculpture using just a bunch of 2D lines in 3D space. I now want to add body to it using the Sweep Rail command. If the above lines aren’t connected then I have to run the Sweep Rail command twice and then I have problems where the two sections join.

For the sections that I have drawn in one step of lines, all is fine, but the problem occurs where I have drawn lines in a second step and try and join them to the first grouping.

(The above example is for illustrative purposes only. The actual object is different, though that shouldn’t make a material difference to this issue.)


Thanks Jim and Helvetosaur for your help.

Following from my last post, here’s a better explanation. I’ve done the Sweep Rail command along one curve as you can see in the following picture. I have another line that I’d like to have the same cross-section and which joins the rest of the structure. The cross-section is irregular and so it is hard to line it up with the rest of the structure to perform a second Sweep command. The line is not in any of the same planes as any part of the rest of the structure either. Can anyone recommend the best way for me to proceed please?

(David Cockey) #7

I assume the cross-section is a planar curve. To create the cross-section perpendicular to the curve:

  1. Pick a viewport and set the CPlane perpendicular to the desired end of the curve.
  2. Set Camera in that viewport to CPlane Top
  3. Create the desired cross-section on the Cplane in that viewport.
  4. Reset the CPlane and View

(David Cockey) #8

If the desired cross-section already exists OrientOnCrv with the Perpendicular option can be used to place the cross-section at one end of the curve.


Thanks David but your suggestion wouldn’t work as the cross-section that I apply to the line in the picture I posted may not line up with that of the portion of the object that has already been thickened.

This is what I’ve now found to work:

  • duplicate the edge of a branch of the object that has already been thickened with the Sweep Rail command;

  • make a new rail that overlaps the abovementioned thickened branch;

  • run Sweep rail on this new branch. There will now be two surfaces, one of which overlaps part of the first;

  • explode the first surface and delete the overlapping portion.

I will then have to somehow deal with issues relating to the join.