Making a rectangular box shape

[Andy_CParticipant @andy_c]

I need to make a rectangular box sides out of a strip of brass.  The box sides are 41mm x 26mm with three corners folded with minimal radius and one corner having a radius of 6mm.  I have allowed for a 3mm flange to solder to one of the folded corners.  This will then be mounted on 2mm brass and a lid fitted to create a tank for fuel.  I have used bend deduction and bend allowance calculations to ensure finished dimensions are as stated and therefore the flat brass strip is shorter than merely adding the dimensions.  What I can’t seem to fathom is where is the actual bend line?

I have used the below calculations

Bend Allowance
Bend Allowance is the arc length of the bend as measured along the neutral axis of the material.

Bend Allowance is calculated as BA = π / 180 x ‘B<’ (IR + K x MT)

Bend Deduction
Bend Deduction is the difference between the sum of the flange lengths (from edge to the apex) and the initial flat length.

Do not be intimidated by the length of the formula for Bend Deduction. It is long because it solves for the Out Side Set Back (OSSB) as well as the Bend Allowance: BD = 2 x (tan (‘B<’ / 2) * (IR + MT)) – BA. If you already know the Out Side Set Back, it’s simply: BD = OSSB – BA

is the fold line in the middle of the bend deduction for each fold and for the radius corner the radius bend starts at the OSSB measurement?

I could just use the measurements as described but wanted to learn a bit of sheet metal practice.

many thanks

 

[Speedy Builder5Participant @speedybuilder5]

I just use the ZEUS handbook.

Bob

[Andy_C]

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.

[Bo’sunParticipant @bosun58570]

Good morning Andy.  Have you considered making a hardwood former to the inside dimensions, then folding/hammer forming the annealed metal around it.  Cut the strip oversize and trim accordingly.  I’ve used this method successfully for quite a few fabrications.

[JasonBModerator @jasonb]

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.

[JasonBModerator @jasonb]

One more to illustrate position depending on how you are bending. For the 6mm if you had a suitable radius on your press you would then use the ctr line not the start of the bend

So to answer the question:

The ctr line is used if bending with a press as you will then get an equal amount of radius either side

The start is used if bending over a former as you will then get all the radius to the side of where you start

[Andy_CParticipant @andy_c]

Very many thanks for the responses.  Much appreciated – now to turn theory into practice.  I have a 3-1 for it which will do three of the folds and then over a former to create the 6mm radius bend.  Thanks again – at least I now have a starting position to go from.

[DC31kParticipant @dc31k]

Another thing to assess is the consequences of being either too short or too long.

If the box side ends up too long, a quick trip to the belt sander can rectify the situation.

If the flange is a tiny bit short, the solder joint is slightly less.

[bernard towersParticipant @bernardtowers37738]

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

[JasonBModerator @jasonb]

Says it is for a fuel tank in the opening post.

[bernard towersParticipant @bernardtowers37738]

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

[JasonBModerator @jasonb]

Why different materials, the OP has said he wants to use brass, soft or silver soldered or even JB Weld will not affect how the part is bent. I’m not even sure how stationary or aero would make much difference if any?

What I will say is to choose an easy to bend brass which is one with a higher copper content such as CZ108 for the part that needs to be bent, not so critical for the 2mm bottom or the assumed flat top.

[JasonBModerator @jasonb]

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.

[bernard towersParticipant @bernardtowers37738]

Not trying to teach someone to suck eggs but if it was an aero tank then it would traditionally made of tin plate for lightness or if more modern blow moulded plastic. I’ve made my point so I’ll stop now.

[JasonBModerator @jasonb]

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.

[Andy_CParticipant @andy_c]

Jason your drawing is spot on.  It is for a stationary engine, Jan Ridders Flame Licker and whilst there are easier methods to create a fuel tank that would serve the purpose, I wished to try and challenge my skills and learn something new.  The brass strip I have is 0.64mm or 0.81mm thick (can’t check at the moment).  And will check type of brass – I have a number so at least some practice material.

[JasonBModerator @jasonb]

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.

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