Dismountable Connections for 50x25x2 Closed Steel Profiles: Hidden, Secure & Easy to Assemble

Hello,
- I have a structure to be made of closed steel 50x25x2 profiles. The problem with the implementation of dismountable connections that occur in the corners and when connecting transversely - see sketch. The connection must not only be disassembled, but not loose and, in addition, hidden inside the profiles. I tried to combine with some carpentry fasteners, but did not find a solution.
Oh, the joined ends are open, and the profiles can be drilled at will.

Thanks in advance for your help
greetings

koczis_ws wrote:

Do you allow any connecting elements to be welded to the profiles?

I am not saying absolutely no. Something could be welded on, but in this view as in the sketch it should be invisible.

olador wrote:

Ie. - mounting on the inside, what dimensions of the structure?

It's hard to say, because I create a broken line in the profiles, but the longest element is less than 2 meters. If these were not steel profiles, but wooden square timbers, I would probably connect them with pins or 10 other sophisticated furniture connectors.

krzysiek_krm wrote:

Yes, from a different barrel, have you not thought about building your structure from ready-made aluminum profiles?
There are some ready-made systems in which, apart from the aforementioned profiles, there are a lot of accessories: fasteners, screws, covers, plugs and what else.

Yes, I thought, until I looked at the prices. Astronomical! Aluminum systems, and there are several dozen of them, actually have an innumerable number of connectors. They can be connected at almost any angle in the way that would suit me
There are also ready-made connectors for closed steel profiles, but they are not available up to the size of 50x25x2 and, in addition, they are too expensive - about 30 to 50 PLN per piece.
I glue for posterity one joint for the aluminum system and a four-side connector for closed profiles 30x30x3


Added after 14 [minutes]:

koczis_ws wrote:

You cover the fronts of the profiles with threaded plates or weld nuts to the plates and screw them with screws, preferably allen screws.

This is an almost good solution, except for this welded plate with holes. Ie. with the difference in the thickness of the profile (2mm) and the threaded plug (I assume 6mm), I somehow can't imagine how to weld it well to get a solid weld and at the same time a straight contact surface with the screwed profile. I guess you need to push this sheet in from 3 millimeters and weld from the inside?

Added after 4 [minutes]:

koczis_ws wrote:

Optionally, without welding, using a bolt connection (pin with crosswise tapped holes, such as so-called hammer bolts in woodwork), but then it will be visible from the front.

Could you draw it?

101pawel wrote:

somehow I can't imagine how to weld it well to get a solid weld and at the same time a straight contact surface with a screwed profile.

You chamfer the edge of the insert a few millimeters and fill it with a joint. How to maintain the perfect right angle, you plan on the milling machine after welding.

Radiokiller wrote:

Blind assembly insert.

Note that two profiles are to be twisted perpendicular .
greetings

Added after 8 [minutes]:

koczis_ws wrote:

Something like that:

This solution with a welded plate is better. These pins had to be made very precisely, and secondly, the walls of the profiles are only 2 mm. With greater force acting on the connection, this pin can distort the profile in which it rests against the poor 2mm and fall inside, and then the connection will fall apart
greetings

wojtek1234321 wrote:

But these pins could, in a way, be installed "permanently". Drill the holes, insert the bolts and weld on the outside. Then the connection could be stable.

OK, but the pin for e.g. an M6 bolt would have to be close to 20 mm in diameter (nooo, minimum 16). The drilled hole 20 should be made maximally perpendicular to the section, and with this diameter it is not so simple. Besides, while in the solution with an M6 screw plate, a 6 mm thick plate is enough, the 6 mm thread can be made perpendicular to the plate without any problem, but making two parallel 20 mm long holes in a round rod is a bit of gymnastics. Just making such long threads is also a bit of fun. In a word - the board is better.
greetings