Low freq shielded room, 6mm aluminium

[hamiltoncork]

Gday from australia, and cheers for having me.

I work for a company that builds faraday cages, we are currently quoting a job for a low frequency shilded room for an electron microscope. Its essential a box made from 6mm aluminium sheet, room size is roughly 7200X6200X5200, quite large, sheet size is going to be 1200X2400 and every join and corner needs to be welded. The sheets are going to be screwed down to fire-rated plywood before welding out so warpage will be less of an issue than the plywood smoking out and interupting the weld.

My main questions are what would be the best process and procedure wor welding this room. From what little knowledge i have with aluminium a spoolgun is reccomended with straight argon or argon/helium.

Will each sheet need to be v-ground for a v-butt weld or can you get away with a slight gap without v-grinding. also is it possible to weld vertically down with mig aluminum?

Bear in mind that the welds are non-structural and purely for fusion of the sheets.

 

[Brad93]

Tbh mate that’s going to be a nightmare to weld if screwed to plywood.

Get a good fabricator with experience in aluminium to make it then screw your plywood sheets on.

Where are you in Australia as there’s a few members on the forum who could probably point you in the right direction.

 

[pedrobedro]

Why does it need welding ? If the corners need sealing use some aluminium angle and rivet it in but it sounds a bit overkill to me as long as the sheets meet.

 

[fizzy]

I would rivet and bond it. Or just rivets - can always be dismantled afterwards.
Use a strip of ali over each joint.

 

[500e]

It will leak like a sieve if not welded

 

[ajlelectronics]

It will leak like a sieve if not welded

Not at LF, as the gaps would need to exceed a quarter wavelength at the working frequency. That is how mesh satellite dishes still work! However, welding it up means that it is a better job and can be used at higher frequencies.

 

[fizzy]

It will leak like a sieve if not welded

Not meant to hold water.

 

[fizzy]

Not at LF, as the gaps would need to exceed a quarter wavelength at the working frequency. That is how mesh satellite dishes still work! However, welding it up means that it is a better job and can be used at higher frequencies.

True but would they need to shield from frequencies higher than satellite dishes?
Remember cosmic rays will still get through lol.

 

[julianf]

Not meant to hold water.

No, but it's supposed to hold out waves... : )

 

[fizzy]

No, but it's supposed to hold out waves... : )

 

[defender dan]

The dishes we have constructed work absolutely fine at the lower frequency range, wich have been basically overlapped with fine square mesh and ali strips to secure all the edges down. We have made the prototypes here for testing before finalising design for the hundreds being constructed at a low cost in south Africa.

Dan.

 

[ajlelectronics]

When you look at C band dishes running 4GHz, they use something that looks like chicken wire! It's all to do with the wavelength. Many amateur dishes are constructed in similar ways. At 1296MHz the wavelength is around 23cm, so a quarter wave is around 2".

 

[knighty]

can't the joints just be overlapped?

6mm 8x4 ali sheets?

put one sheet on the wall, then put a slightly under 8x4 sheet of 6mm ply next to it, then the next sheet of ali next to that

then you fit the next ali sheet on top of that slightly smaller 6mm ply you just put up

should be much easier and quicker to do?

doesen't need any real skill either, have the ali and ply sheets ready... you'll be able to fit them pretty quick


idk the best way to fit them, what's the best practice for shielding... if screws/pop rivets are acceptable to keep everyhting blocked out you can have the sheets punches/drilled/lasered/whatever before you start... if you have all the sheets made the same (say 8 holes evenly spaced along the side 2mm in from the edge).... then the holes one one sheet will line up with the holes of the next... helps live everything up nice and easy?

 

[500e]

Solid cages generally attenuate fields over a broader range of frequencies than mesh cages.
I would think @hamiltoncork has a specification for this job & continuous weld required.
Big old sheets so some form of constraint during welding will be required, perhaps as someone said construct with angle to stabilise then weld.
How is access going to be sealed Mu Metal strips or

 

[hamiltoncork]

@500e spot on mate, unfortunately welding is the only option as its what is specd and yes mu-metal is going over the top of the ally.

The boss has asked to price up any equipment needed to get the job done so if we get the job we will most likely be getting a new welder out of it, including spool gun, water cooler etc.

So going back to my original question, can it be welded vertical down? I assume a backing behind the joins in the sheets will also be required to stop the ply burning through

 

[Brad93]

@500e spot on mate, unfortunately welding is the only option as its what is specd and yes mu-metal is going over the top of the ally.

The boss has asked to price up any equipment needed to get the job done so if we get the job we will most likely be getting a new welder out of it, including spool gun, water cooler etc.

So going back to my original question, can it be welded vertical down? I assume a backing behind the joins in the sheets will also be required to stop the ply burning through

You won’t get decent fusion welding vertical down.

Try to do all the welds flat if possible. Make you’re life a lot easier if you’ve never welded aluminium before.

@Hood @henry Kadzielski

 

[henry Kadzielski]

Good afternoon @hamiltoncork.
I have done a bit of 6mm alli 5083 fuel/ water tanks that sort of thing. As @Brad93 has mentioned vertical down. Can it be done from one side, answer yes, however if possible alli is done on both sides. 1st side (in the case of 6mm )is 'V' prepped, welded, the reverse side is then gouged back to the new weld material and is then capped. Again this is done where possible. That way there is no possible gaps, holes, porosity, etc to deal with. If done from one side you will need some horse power to get through the material.
Ideally you will need a pulse mig with a push pull gun, your spool gun will overheat and cause you grief. If you have plenty of time on your hands it is possible to do it with a spool gun. I have welded a 2000l tank in one day used a spool of 5356 1.2mm wire 7.5kg, however it was only possible with a water cooled torch. What you have here is several times bigger, you will want to slash you wrists before you get half the way through with a spool gun. During the welding there will be a fume issue which you will need to deal with. Again ideally you will need a pressure fed mask for breathing air and you will need to be covered from head to toe as the reflection inside the alli box is extreme, just ask someone who has welded inside alli enclosure. A good feeding reliable machine is what is required min 300 to about 350amps, if you like and if the boss is interested I could come down and do it for you

 

[hamiltoncork]

Good afternoon @hamiltoncork.
I have done a bit of 6mm alli 5083 fuel/ water tanks that sort of thing. As @Brad93 has mentioned vertical down. Can it be done from one side, answer yes, however if possible alli is done on both sides. 1st side (in the case of 6mm )is 'V' prepped, welded, the reverse side is then gouged back to the new weld material and is then capped. Again this is done where possible. That way there is no possible gaps, holes, porosity, etc to deal with. If done from one side you will need some horse power to get through the material.
Ideally you will need a pulse mig with a push pull gun, your spool gun will overheat and cause you grief. If you have plenty of time on your hands it is possible to do it with a spool gun. I have welded a 2000l tank in one day used a spool of 5356 1.2mm wire 7.5kg, however it was only possible with a water cooled torch. What you have here is several times bigger, you will want to slash you wrists before you get half the way through with a spool gun. During the welding there will be a fume issue which you will need to deal with. Again ideally you will need a pressure fed mask for breathing air and you will need to be covered from head to toe as the reflection inside the alli box is extreme, just ask someone who has welded inside alli enclosure. A good feeding reliable machine is what is required min 300 to about 350amps, if you like and if the boss is interested I could come down and do it for you

Cheers henry,

Unfortunately only one side of the weld can be accessed, the sheet is going to be fixed down to fire rated ply.

And yeah as stated its all non structural welding, penetration isnt an issue, the sheets only need to be fused.

Being as its for low frequency shielding im not sure exactly what degree flaws are passable, you may be able to get away with some degree of porosity, shribk holes etc as there is another layer of shielding (mu-metal) over the ally

 

[Hood]

If the contract is paying then get a pulse Mig and a push-pull gun. Just make sure it is a high end make such as EWM, Fronius, Lorch etc. I am not sure how these brands are supported in Australia but I think Henry has a Fronius and was happy with the support. It won't be cheap for such a welder but a Pulse mig will make positional welding a breeze and should help a good bit with the warping. Ply backing is going to be a PITA but probably not much you can do unless you can sink strips of stainless or similar into it at the joints. It should help doing that but probably still wont stop the gasses coming through.

If you go standard Mig with a spool gun it will take a lot of practice before you can do decent vertical welds in Alu.

 

[Brad93]

If the contract is paying then get a pulse Mig and a push-pull gun. Just make sure it is a high end make such as EWM, Fronius, Lorch etc. I am not sure how these brands are supported in Australia but I think Henry has a Fronius and was happy with the support. It won't be cheap for such a welder but a Pulse mig will make positional welding a breeze and should help a good bit with the warping. Ply backing is going to be a PITA but probably not much you can do unless you can sink strips of stainless or similar into it at the joints. It should help doing that but probably still wont stop the gasses coming through.

If you go standard Mig with a spool gun it will take a lot of practice before you can do decent vertical welds in Alu.

EWM non existent in Australia as far as I know.

Fronius is growing.

Lorch have a distributor over there I think I remember seeing something on linkedin.