How to Stack Tetras in Inventor
 

Here's a simple way to stack tetras in Inventor. I used the tetra assembly on our website for this. If you use your own model, you may need to use different axes for the constraints, but the general method should work if you built your models right. Use the axes and planes in the Origin folders of the tetra assemblies - not those of the Origin of the overall assembly.

1. Create an assembly model and place two (or more) tetras in it. The first one placed will be the "reference" tetra - let's say it's on the floor - and the other(s) will stack on top of it. This first one should be "fixed" (has a pushpin icon in the model browser)
2. Constrain the vertical origin axis of the stacked tetra to the vertical origin axis of the reference tetra with a Mate constraint. I used the Z axes in my models.
3. Constrain one of the horizontally-oriented origin axes of the stacked tetra to the corresponding axis of the reference tetra with an Angle constraint of 0 degrees. I used the X axes.
(The tetras may appear stacked at this point, or they may be cattywampus)
4. Constrain the horizontal plane of the stacked tetra with the corresponding plane of the reference tetra with a Flush constraint. I used the XY plane.
(The tetras should occupy the same space now - appearing to be just one tetra)
5. Enter a value in the Offset field for the Flush constraint to separate the tetras - I used -10 in. to start with, which has the stack tetra hovering part-way above the reference one.
6. Right-click the Flush constraint in the Model Browser and select Drive Constraint.
7. Enter values in the Start and End fields - I used 5.5 in. and 5 in., expand the dialog and check Collision Detection and set the Increment to a small value (0.05 or so) if you want to see where they nest.
8. Run the constraint and, if you have the right limits set, it will move the stacking tetra down onto the reference one until they collide. This is about where they will stack. It came out to be -5.35 in. offset in my model.

You can add more tetras to the stack using this procedure, of course. Be aware that this is just a simulation - the real stack height of real tetras will vary! So, do a reality check before you start cutting metal if you use this model for designing your robot.