I used a spare piece of extrusion as a long spacer and used it to set the distance between the upper frame members that take the XY linear guides, and the lower frame members to which the bed lead screws are attached. So if there is any bowing, it'll be obvious and a smack in the right place with a soft mallet will fix it. The advantage is that you get a huge square with sides that are longer than the extrusion.

Actually calbrated is the wrong term - it's more about knowing the error if one exists, and compensating for it. If they cut all the parts at once, then even if the actual length is off by 0.1mm, multiple parts of the same length should be pretty much exactly the same as each other. They guarantee cuts to be within 0.1mm in length and square. Firstly I got my extrusion cut to length from a reputable company (Ooznest in my case here in the UK). Using extrusions and connecting parts can force non-square assembly because the mating surfaces aren't perfect There are always little nudges needed here and there and it's a butterfly effect. I understand the process of comparing diagonals but I don't have a jig table, so I don't know how to hold and adjust the parts. I don't know how to actually, properly square in 3 dimensions.

Said in Want input on frame that's the rub.

T slot frame 3d printer