Apologies for metalworking content.....
I'm still futzing with my project to make a LED flashlight that has the features I want but have not found in a commercial offering.
I want the body to house 3 AA cells (any chemistry) side-by-side in a triangular configuration. I wondered if round aluminum tubing could be thus shaped simply by forcing a tool of suitable form into the tube, leaving the other end round for elex, 1-watt LED and optics.
I don't yet have suitably-sized tubing for a triad of AA cells, but I tried the idea with a bit of scrap tubing .900 ID and .053 wall thickness. Sturdy stuff, scrap from a cut up eldercare appliance.
I calculated the diameter of three cylinders such that when arranged side-by-side the envelope periphery would be the same as the internal circumference of the tubing I have. Turned some rodstock to that size, cut off three 1" pieces, welded them in triangular formation, chucked the weldment in the lathe to cut a taper on one end, and jammed the resulting tool down the tube. I started squeezing it with my mill vise, thinking I'd move it to the hydraulic press if it put up much fight, but it was an amazingly easy squeeze even with no lube.
Probably obvious to most readers here, but I thought I'd mention it. It's an easy way to make an odd shape with aluminum tubing.