One thing to keep in mind when designing aluminum extruded tubes is the eccentricity of the part. This is of particular importance when you have tapped holes coming in from the sides or are milling groves in the inner wall to accept other parts. I’ve seen bollard lighting fixtures with 1/2″ thick walls that had an eccentricity of 1/8″ an inch. Screws up the mating threads on the bottom and the light fixture portion at the top has a lip on one side and an overhang on the other. With eccentricity, it varies per the diameter of your part and to some extent with each extruder. CHECK FIRST! If you don’t get involved with an extruder early on you’re risking a costly redesign.
The technical definition of eccentricity as it pertains to extrusion is: “Deviation from a common center, as, for example, the inner and outer walls of a round tube.”
This is not to be confused with circularity which we’ll discuss later.