This jig is designed to help to cross drill holes centrally in pipe / tube / round bar stock. It has three 2mm diameter holes in the top and bottom halves. The idea is to clamp the lower half in a machine vise, put a 2mm drill bit in a bench drill, align the drill bit with one of the holes (by moving the vise), lock the vise to the table, insert the tube (optionally add the top half to help locate the tube) and drill on that axis.
I've also used this successfully with a hand drill and without a vise, so a bench drill isn't a necessity. Take it easy, and remove the drill a few times to clear the swarf. After drilling a 2mm pilot hole all the way through, you can incrementally drill up to the final size needed.
This would suit tube from about 6mm to 12mm diameter. Using only the base section, you could go up to 22mm.