Rotating a tetrahdron issues

Hi,
I built a tetrahedron and i am trying to rotate it to join it to other tetrahedrons for exploration. This is proving to be really difficult because the tetrahedrons do not have a square base on any side but 30/ 60 degrees.

Has anyone got any ideas how to make this process easier ?

Interesting problem. After playing around with this, I decided that it’s easier to build new ones on the fly…coming off the existing structure, than to take an existing one and try to align it with another…

Try to follow this.
In the pic below, the grey tetra is the original one…component#1. Those little wings on each side (looking a bit like the guy rope attachments on a tent) mark the normal for each side, with the short length being both perpendicular to the face and in the exact centre.

I have also changed the axes on this component. I’ve moved them to the centre of the base and the red axis is now going up the blue direction with the green running // to one of the bottom edges.

Okay so far?

What you need to try now is to draw a triangle onto any of the existing faces of the structure. Click it so that it’s all selected…both the face and the 3 edges then choose Make Component from the context menu. However…before you click Create on the dialog box, choose Set Component axes. Set them at the base of that little line representing the normal, with the red axis running up the line and the green again running // to any edge of the triangle. (this is why I reset the axes on the original…I just couldn’t get the blue axis to run up that little edge)

When all is set, click Create. The new triangle component will be selected. Go to the component browser, right-click on component#1 on the In Model tab and choose Replace Selected. The triangle will then be replaced with a full tetrahedon…hopefully lined up with the others. If it isn’t then infer off the base it’s sitting on and rotate it around its vertex until one of the base vertices snaps to an existing one.

<a class=“attachment"href=”/uploads/default/original/3X/5/a/5aa9a0fd56b9ceaf812f4d979203eaf0e635ecd6.skp">tetrahedra.skp (49.4 KB)

Edit: Forgot to mention that when you’ve finished, you obviously just delete the normals markers from any one component and they’ll all disappear. You’ll also have to purge all the redundant, replaced components from time to time.

Hi folks.

Make the first tetrahedron so its base is on the ground plane.

Then, copy it down and flip the copy along the blue axis.

Finally, move the copy so its top face joins the original bottom face.

Jean.

Yes Thank you for your well presented solution ,
this is what I ended up doing ,adding a triangle wing but it is still clumsy … imo
It would be nice to ground an axis via 2x vertices and then move the other vertice to the corner required

It’s not too clumsy. All you are doing, in effect, is drawing a new base each time, then telling SU to fill-in the rest of the tetrahedron. It just takes a bit of setting up. :slight_smile:
It would be nice to have some further improvements to the locking of axes…as you say. However, the “improvements” in that area in V2016 are already driving me round the bend. I’m constantly finding that when I lock to an existing plane I then can’t move the Rotate tool beyond the confines of that plane to the intended centre of rotation.

I don’t think I have V16 yet

I assume your triangles bring the base to a 90 degree edge yes?

You mean the small triangles? You don’t actually need the whole thing…just that short edge representing the face normal. The triangles just make it easier to see. As I said, it’s in the geometric centre of the face and is perpendicular to it.

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Thanks Jean,
what if we want the faces to come together without flipping but to place two along each other and bring tips together

Another, and surely simpler way, is to rotate/copy about one edge - to select the rotation axis, select the rotate tool, then drag the mouse pointer along the edge to set the axis of rotation on that edge. Then press Ctrl (Option on Mac) to copy, pick the point opposite the edge, and rotate the copy to align with the desired apex of the original, or rotate by any desired angle…

This works in SU2016, and as far as I remember, in earlier versions too. Though when I tried it just now in 2014 on Mac, i got a BugSplat - probably not from this, though.

I don’t have the skill that @Box has to generate GIF animations, but he might be able to reproduce this visually.

In the meantime, thanks to @john_drivenupthewall, I’ve managed a simple example using the rotate/copy native tool.

To draw the Tetrahedron in the first place, I used my Polyhedra plugin from the SketchUcation Plugin Store.

Or you could use TIG’s Mirror plugin, and mirror a tetrahedron about one face - pick the three corners of the face to define the mirror plane. That’s even quicker than rotate/copy.

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I use Iicecap on my mac, even made a button to launch it from SU…

john

I must try that. I have a much clumsier solution that I used once, but it was too difficult - make a screen recording using QuickTime, then a program to convert that to Animated GIF. I had trouble editing the movie, and ended up duplicating it and not being able to chop it back to a single length.

A similar method of copy’flipping along an axis. The left mouse button is held down while you drag the Rotate tool from one end of the edge to the other. Then select the opposite apex, hit Ctrl for copy and flip the copy over to the adjacent face. It’s a little tricky till you get used to it.

You don’t have to drag from one end of the edge to the other, just far enough for the rotate tool to pick up the axis. In v2016, the edge will highlight magenta, as in AlanF’s animation.

As an aside, you can use the Protractor in a similar way to measure the dihedral angle between two such sides. When you do so on a tetrahedron, SU returns an interior angle of ~70.5. Its actually 70.53

You can set the precision of angle measurement display in Window/Model Info/Units - Angle Units - Precision. You can set up to three significant digits. With this setting SU reports 70.529 degrees.

I’m just not that precise a guy. :wink:

Nor am I a lot of the time! But it sometimes helps… Thanks for your animation by the way. Goes further than mine, and shows some of it better.