how to fabricate an air tight compartment!??

[picksbendy]

hi!

i am currently working on an air tight compartment by using 2mm thk mild steel. Say i have 5 pieces of 2mm mild steel being cut to size to build up the perimeter and base of the box, anyone have any ideas on how to join them together into a seamless air tight box??
been checking out on existing machines where they manage to join metals together and from outside it looks exactly as if it is bended instead of metals joined together, while from inside it is obvious metals are put together.
really urgently need solution to this,
hope that i have asked my questions comprehensively.

Best regards!

 

[desertfox]

hi picksbendy

Well you could weld them using oxyacetalene welding torch, however as with any welding you might get unwanted distortion, but its the only way I know to get it airtight.
How will you test for air tighness?

desertfox

 

[Ron]

Agree with desertfox, with a couple of additions...

Use GTAW (TIG) welding process. Lap the corners rather than trying to butt them...fitup is easier. Then fillet weld all sheet edges. Lapping will cut down some of the distortion, but you'll probably still have some.

 

[rb1957]

but then lapping to corners will also need bending the edge (so that it aligns with the adjacent side).
you could use separate splice pieces, bent as needed, welded or bolted in place. an advantage being you could use two pieces on each corner (one inside, one outside, the corner) to help minimise distorsion if you're welding. of course no distorsion if you're bolting them together, but you'll need need a seal of some sort ...

plenty of ways to skin a cat ...

 

[GregLocock]

Those who are lazy and unskilled would pop rivet the box together and rely on RTV.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[MintJulep]

How airtight does it need to be?

Why are you using 5 pieces when you could use only 2?

Brazing or soldering.

 

[racookpe1978]

Lettuce assume (and this might be a bad assumption!) that you need an "air-tight" enclosure, but one that is NOT "pressure tight", with "right-=angle" corners and sides - not the usual pressure-vessel round walls and welded elliptical or curved heads ...

Even steel cans are "air-tight" for food protection...

If so, I agree: Start with a rectangular four-sided bent shape of your thin-walled sheetmetal. Your vertical seam should overlap both ends. Weld this vertical seam. (Sheetmetal fab shops have automatic folding machines to make seams like this for ducts and vents all the time, but 2 mm thick steel is a bit thick for most duct bending rollers and benders. The duct builders use a "Z" type fold, two doubled "U" bends would also work. A single "U" with the second sheet going into the "U" and being welded on both sides is an option.)

Fold the end tab's of the four walls at the bottom up at 90 degrees to make four "seats" or flanges that will overlap your bottom sheet.

place the bottom sheet onto the seat, weld all seams around the bottom.

fold the upper walls of your walls also into a seat and weld it up. Add your inlet/outlet/opening/access/lid ...

Optional: Fold the walls as above, but then fold a second rectgular shape one well thickness smaller. Nest the second into the first to eliminate the bottom joints - you'll still need to weld, but there are fewer seams but a heavier (more expensive for material, less expensive to weld ?) assembly might be acceptable.

What is this for? What are your design limits? What goes inside? What must be kept outside?

 

[MikeHalloran]

A butt joint in this case means touching the inside edges of adjacent sheets, and laying a fillet in the resulting V-shaped channel, with a TIG torch. It requires a fair amount of filler wire to build up the weld enough to grind it down to a nice radius.

A lap joint means touching the outside edge of one sheet to the inside edge of the adjacent sheet, then running a TIG torch along the exposed sheared face to melt the sheets together. For mild steel, the weld can be mostly autogenous, i.e. with a minimum of filler metal added. The weld would require minimal grinding to look like a bend on the outside.

Accounting for real shearing tolerances, you are mostly working with a half-lap joint, where you have to add some filler metal, but not an entire sheet thickness' worth.

I've seen it done in production for aircraft battery boxes, in 16 and 18 gage mild steel, assembled over a frame made of aluminum rectangular bar, which kept the steel in alignment and served as a nonconsumable backer and heat sink. Without the frame, you'd have to weld an inch at a time and work in alternation on all the welds at once to distribute the heat and distortion.






Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

Note that even then, this is not necessarily a walk in the park if you're worried about leakage. We had some 6000 psi brazed tanks that took about 6 months before all the welding was up to handling the pressure without loss.

TTFN

FAQ731-376

 

[picksbendy]

Thanks everyone, this is really the best forum ever.

racookpe1978, your assumption is absolutely correct, the compartment is actually the filtration compartment for a dust collector, and i'm confused between the lap joint and butt joint, i think my current planning is a butt joint if not mistaken. with attachment is a full drawing and a corner section detail, please advise.

 

[berkshire]

picksbendy (Mechanical)
To get the effect of a radiused corner after welding, most sheet metal shops will do what is called a half butt corner.
They will place the metal one half thickness over the other at the corner, then do an autogenous weld with a TIG torch. If the operator is skilled enough this will result in a smooth weld requiring little or no cleanup afterwards.
Unless your box exceeds standard sheet sizes, common manufacturing practice would be to miter the top and bottom flanges and bend at least two of the corners to avoid welding. If the finish does not have to look that good, a MIG welder may be used instead.
B.E.

 

[NomLaser]

You will need to define 'air tight' first. Pulling a vacuum? Pressurized? You get the idea.

As for the design, those above have given you some good tips for the welding the seams. I would suggest talking with your sheet metal fabricator as to what their capabilities are. Often times you just need to give them the dimensions of the final component and define the material and they will use their discretion and your input on the best way to fabricate the box.

Personally, I would see if their break press has the clearances to make this out of one piece so there is only one weld seam. Then they can weld the miters on the corners. This will get you the best chance of holes lining up post assembly and a better seal along the top face. As for the miters, give some room between where the two faces come together. Typically I would give 0.025-0.050" so that when they bend everything, there is room for tolerances.