Hi,
I am wanting to make up a frame, with 25mm T section x 3mm thick, aluminium, top and bottom, separated by 25mm SHS x 3mm crossmembers. The tops of the Ts have to face out. So I need to cut 3mm slots in the end of the SHS to accept the uprights of the T sections. Ideally these have square edges. What is the best/professional way of doing this?
Thanks

If it's thin metal like your 3 mm thickness, a fine blade on a circular saw installed so it rotates the wrong way.

This can also work except not for exceptionally thin material.

Carbide tipped circular saw blade.