On 9 March 2026 at 21:37 Andy_C Said:

What I can’t seem to fathom is where is the actual bend line?

It does not need fathoming. It needs doing. Even with the most advanced calculations, bending is a black art. For example, rolled material behaves differently if bent parallel with the rolling direction or perpendicular to the rolling direction.

Every commercial item that is folded has a couple of prototypes made to verify what the calculations say.

Take a spare piece of the same material. Mark a line on it a known distance from an edge. Bend it. Measure the result. Adjust line position if result is unsatisfactory.

The free version of F360 places a line in the middle of the bend which is where you would place the middle of the Vee knife on a press break. It will work out all the various allowances etc for whatever bend radii you want.

The U shape below with one bend using the material thickness (2mm) as the radius for one bend and the other at 6mm. It is 50 x 30 overall but as you can see the material is not 110mm. If I wanted I could place a dimension at the start of the 6mm radius bend which would be better if bending over a former. So position is really determined by how you intend to bend the part.

How thick is the material and what is the tank for?

Not very helpfuil Jason as it could be for a stationary engine aero engine etc whic would use different materials and methods

As an example the rectangular water hopper sitting ontop of this hit & miss engine cylinder had the top, sides and bottom bent from a piece of 20swg CZ108 brass, joint centrally at the bottom. End plates were from 3/16″ CZ121 rebated to form a spigot to hold them in place during silver soldering. Top ring and lower neck soft soldered.

I don’t have a 3in1 type folder so started my bends at teh edge of the allowance using two pieces of angle iron, one having the required internal radius filed on teh corner. Pushed down with a block of wood rather than hammered.

Ah, I was not allowing for so much thread drift. Silly me looking at the title of “Box” and then reading the sizes did not bring aero to mind. The stated 2mm thickness of the tank bottom also tends to eliminate the OP’s need for lightness.

Assumed height but something like this is what I understand the OP wants. Probably a top plate too.

So a bit shorter than I drew and with protruding top & bottom plates, the tank in question for anyone not familiar with it.

https://www.ridders.nu/Webpaginas/pagina_happer_inw_schuif/inw_schuif_frameset.htm

If you have spare material than a thin strip will let you practice the setups and bends, I think it is unlikely you will get access to all three of teh right angle bends before the box closes up too much and stops you using the 3in1 machine.

If but joining the top & bottom then I would suggest cutting the material a bit oversize to allow for any movement when soldering. Then you can file or mill back to an even overlap all round. An Alternative is to cut a rebate around the edge of the plates which will locate the sides more accurately during soldering. Something like this, the corners don’t have to have the bend radius, a straight 45deg cut will do for location.