I want to have a hole such that the end mill axis always points toward the tube axis and a triangular hole is cut. Basically, I want to take a big tube, and cut truss holes in it to save weight.

If you draw a triangle on a plane, and extrude cut, you get an endmill always "straight" type of cut (no 4th axis).

I've tried:

1. Solid sweep with a 2D sketch of a triangle. Aside from weird things that happen at the radiused corners (this is not at all like CAM software), you get the same as above; that is, the end mill is always straight.

2. A lofted cut, between a sketch on a plane outside the cylinder and the "projection" of that sketch onto a plane inside the cylinder, projected such that all projection lines intersect the cylinder axis. This is very tricky, particularly because circles do not project as circles so I doubt I'd get the geometry correct even if I tried to get the sketch to work.

3. Solid sweep onto a triangle "wrapped" around the cylinder wall. There are two challenges here:

A. How do you create the triangle, with curved corners, on the surface of the cylinder? I took a flat 2D skech, extruded-cut through the cylinder, then created a 3D sketch, selected the loop of the cut, and converted entities. I have tried both leaving the original extrude-cut hole in the tube wall and also deleting it (but first removing relations from the 3D sketch so that the "projected" shape wouldn't get deleted also).

B. Getting the sweep to actually work. No matter what I do, I just get the generic "sweep failed" error message.

Even if someone gets this working, there's got to be a better way to provide the path sketch than what I did in A... right?