# How to bridge edge loops with a twist of .5?

I'm trying to make a 3D model of a twist-cut torus, like this one:

https://youtu.be/3_VydFQmtZ8?t=3m39s

The current version of the model costs about \$50 in Shapeways. I'm trying to get that down by cutting both of the half-tori into two hollow pieces, so 4 pieces total. Then I would make them tight fitting so I can assemble it later.

The problem is that the 'Bridge Edge Loops' -tool pairs the vertices up wrong,

and I can't cut the tori straight:

How would I go about making this possible?

• The bridge tool has a twist option in the redo panel (F6 right after running the operator) which offsets the vertex pairings. Is that what you are looking for? – gandalf3 Jun 6 '15 at 19:51
• – gandalf3 Jun 6 '15 at 19:59
• I was using 12 minor segments, and it didn't help if I twisted the bridge. But now using 10 segments it seems to work with no twists. Quite strange. – Lupilum Jun 6 '15 at 21:44

It just so happens I modeled a Mobius strip a little while back, and I'm sure the technique I used can address your case as well.

• Model a plane, or some variation of a plane. Best to keep it simple at first.

• Place an Empty at the end of your object. Actually, it should be a little further from the end of your object, but we'll adjust it later.

• Select the Empty, then the mesh object.

• In Edit Mode, with the end vertices selected, Hook those verts to the Empty Ctrl + H (Hook to Selected Object).

• Add an Array Modifier to your plane/mesh object. Choose Object Offset and point it to the Empty. Set the Count to 3 or 4 iterations so you can see the effect.

• Now in Object Mode rotate that Empty and you'll see that mesh twist like a DNA double-helix.

• This will probably need some fine-tuning, so in Edit Mode with those same end verts selected you can un-hook the verts (also Ctrl + H]). Then move the Empty and re-hook the verts. You can play with it until the positioning is right. It's hard to describe this part, but if you try it you'll get it.

• Finally, you should have a strip that you can twist and stretch using the Empty.

• Once you've twisted the strip to the degree suitable for your design, save your file, then apply the modifiers.

• Place your 3D cursor where the center of your ring will be and align your view accordingly, then use the classic warp tool - Mesh > Transform > Warp to wrap your mesh into a ring shape. Remove Doubles.

Another way to to this would be to use the Curve Modifier, but it won't give you an evenly incremented twist like this technique does.

For anyone creating a (Mobius strip like) twist where the normals oppose each other where they meet, there is the additional challenge of how to merge these, which I will briefly address. Here, the vertices at the edge where the normals are opposite have been ripped with pressing V.

Halving the Solidify Modifier's Thickness value, then duplicating the object and making the value negative on the duplicate is one way to fix this. Then you can apply the Modifiers, join the two objects, and merge the vertices at the seam. After that, additional clean-up of of the mesh, such as the deletion of internal faces may also be necessary.

Once the two objects have been joined, and vertices welded, here is the final result:

Two .blend files for reference:

http://www.pasteall.org/blend/36552

http://www.pasteall.org/blend/36553

I did this shape other way. Hope it would be interesting at least for practice.

1. Create section from two semi-circles (for example: delete half of circle, connect ends of arc, move a bit from center, duplicate and place appropriately);
2. Extrude up and use Cut Ring with proper number of cuts, like 3*n-1;
3. Create Bezier Circle about 3-4 times larger first circle (using scale);
4. Put Curve modifier on mesh, switch in modifier axis to Z (if you created shape in XY plane by default);
5. Switch to top view and scale bezier circle slowly until you get your mesh exact 3/4 of turn
6. First vertex in bezier circle is where mesh starts, direction is pretty understandable, so select second vertex and assign tilt to 900 (Transform panel, at right side of 3D View), third to 1800, and fourth to 2700;
7. Apply Curve modifier;
8. In Edit Mode delete first (of 3) quarter of mesh, then select all mesh and duplicate it rotationg around global Z and cursor (hope you are doing everything around cursor and 0,0,0), and remove duplicate vertices;
9. PROFIT.

The trick with 3/4 was done just because I didn't find how to divide bezier loop in vertex without loosing shape.