Sheet Metal Fabrication Techniques 8

[Dinga17]

Hi all,

I am attempting to make a non-truncated cone with a very shallow taper/slant angle. My knowledge of fabrication methods and processes is not up to par and, thus, I am curious about how this cone as shown in my attached pictures would be fabricated.

Firstly, is it possible to fabricate this without the need for welding? ie. It wont need to have a flat pattern then bent into shape then welded along a seam. Can it be fabricated by some sort of punch or roll process to get the slant without the need for a seam or welding? It is quite a large cone ie. 2.34 m diamater. Is it possible for smaller diameters?

Secondly, in general what are some of the typical fabrication methods that could be utilised for a job like this? If you have any good links or discussions of processes etc I would love to have a look

Lastly, if I wanted to have a flange on the outer edge of the cone, is it possible to do this after the cone shape is formed? If so, what process would it be completed by?

Apologies for my lack of fabrication terminology, I am new to this

Thanks.

 

[robyengIT]

pictures attached are useless without any dimension.
Punching or rolling process needs a mould/shaped equipment to press against : it is worth according to the quantity to be produced

 

[BUGGAR]

I can't figure out your pictures either. This looks like something that you would fabricate by spinning, like a Moon disk. But that is generally done with aluminum. NASSCO, our local shipyard, has rolls that can taper roll two inch thick plate. This would do the trick for you. Don't know about your seam, though.

 

[MikeHalloran]

You might be able to distort a >2.34m dia. circle of metal into a cone.
... but it will probably buckle.
... and it may be hard to find a sheet that big.
... and it will have some grain anyway, so just pushing on it will not produce a nice cone.
... spinning might work, but again you need a really big blank,
and a humongous spinning lathe.

Roofs for upright circular tanks are normally fabricated from pie shape wedges, which can be cut from ordinary sheet stock that you can actually get. ... and are often flanged on every edge or otherwise reinforced so the roof won't collapse under snow.




Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

Dinga 17.
The shape you are describing is sometimes called a " Chinaman's hat." The only way that can be made without a seam is by hand hammering on a sand bag. An English wheel,or by pressing or spinning . Now finding a press that big may be a problem . Spinning may be better, you may have to go to a tank end -forming company to find somebody with a spinning lathe big enough, remember too ,before you can spin the part, you have to make a pattern for it.

What is the reason for not wanting to put a seam in it? You do not say what thickness this is, If it is thin metal you could put a hammered down lockform seam in it. If it is thicker you can weld it.

As far as forming the piece. A three roll sheet metal roller will only give you a rough approximation requiring a lot of hand work.
If you can find a shop with a cone roller they may be able to do it , if they have a machine big enough . The easiest way to make this after you have cut the pattern will be to " Bump" it on a press brake . You can do this with a single seam " V" notch pattern or as you have drawn it, two pattern halves. A flange can be rolled on an edge afterwards with a swageing machine or spinning jenny if the metal is thin enough.

Food for thought. Now come back with what you really want to do.
B.E.

You are judged not by what you know, but by what you can do.

 

[robyengIT]

OP said that it is a non-truncated cone : it can't be done with a three-rolls bending machine

 

[racookpe1978]

The OP wants a large, very shallow, relatively thin, conical "hat" with a flat outer rim. No welded joint.

What would be the practical "minimum limit" on thickness for a conventional cast part with the parting line at the top edge of the flange?
What is the practical limit for a Centrifugal cast part? (Yes, requires a large dia centrifuge station, but not a 3 meter dia press, die, and forming tool.)

 

[SnTMan]

You can get plate in 8 ft (2.44m) and wider widths, but I don't know about sheet. How thick is it?

As others have said it could be formed as one piece by several processes, but they would all involve dedicated tooling. How many are there?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[chicopee]

See the attachment about the development of the cone. You may want to increase the angle alpha if you need an overlap to secure the ends. Descriptive geometry is a reference for such development.

 

[chicopee]

By the way try the development on paper before going with stiffer material. I expect some interference at the apex of the cone with the overlap therefore you'll need to trim the overlap some.

 

[berkshire]

Without more details on what actual thickness of metal is used and a reason why the OP does not want to put a joint in.
We are just speculating.
B.E.

You are judged not by what you know, but by what you can do.

 

[chicopee]

The closest that I have seen to the request of the OP are cymbals used by musicians. There are several processes but one that I am somewhat familiar is a cold forming process by a rolling wheel while the flat plate is spun. If one were to examine such cymbal the cone surface has multiple concentric ridges from the forming work. Tolerance to the slope of the cone could be a problem so I am more inclined to the cone development as mentioned above, then weld and grind smooth the ends and rework the shape.

 

[chicopee]

Depending on the thickness of the material, do dome research on explosion forming.

 

[EngineerTex]

Are you needing one of these? Or are you building 500 of them?

If it were me in my garage and the material were light-gauge steel or aluminum, I would:

1) Cut it into a circle with a diameter that is about 50mm larger than necessary.
2) Clamp/screw the edges of two steel angles where the end corner sits at the center of the circle.
2a) The two steel angles that these two stiff backs will make will be one half of the angle that you would cut out of it if you were going to cut a pie wedge out of it.
2b) The two steel angles will be on opposite sides of the sheet.
3) Hammer a crease up on one side with a rubber mallet or a dead blow hammer over the corner of the steel angle.
4) Hammer a crease down on the other side over the corner of the steel angle.
5) After the two creases are established, manhandle the piece by hand/with the rubber hammer until the two creases have made a 180-degree fold
6) Drill and rivet the three-layer area between the two creases.

For the flange, I would mark a line 25mm from the edge and progressively fold it over the corner of a round pipe with a squared face. I would go around it multiple times to fold the flange.

If you're making hundreds, send them to a fab shop that will do any of the techniques mentioned in the other posts here. What is the reason for not welding?



Engineering is not the science behind building. It is the science behind not building.

 

[berkshire]

Chicopee,
The process you are describing is called Flo-Turning . It is an automated spinning process. It is also the same process for turning domed tank ends from plate.
B.E.

You are judged not by what you know, but by what you can do.

 

[berkshire]

Having made the shape and size of cone described, for a cap, for a ventilation exhaust duct, here is how we did it: We simply cut out the pattern ,added a seam allowance, then simply pulled the two ends together at the perimeter, clamped it, drilled it and put a cleco in, then put it flat down on a work table. Next we tapped around the piece with a rubber hammer to get the bulging seam to close. Then drilled and added clecos working towards the center. As the seam was closed more clecos were added. when the metal would not bend freely, the part was supported over the edge of the work table and a hammer and dolly were used to form the sharper curve at the crown. When everything looked good the clecos were removed one at a time and the part riveted . Next it was Soldered down the seam. A wired edge added to the perimeter, support brackets added at 4 places. the part was put on a truck and shipped.
B.E.

You are judged not by what you know, but by what you can do.

 

[Dinga17]

Hi all,

Apologies for the late reply.

Thank for all you information and posts - I definitely have a lot to think about now. In terms of dimensions I have changed slightly to an OD of 1.65 metres and will be utilizing either 1.6mm or 2mm stainless. Regarding the different methods, I would just prefer to have it made without welding however, I am thinking the best route would be to go with the seam. I just wanted to have an idea of some methods before speaking to a fabrication company - which I do now - so cheers all.

Thanks again.

 

[berkshire]

With a single seam your best bet would be bumping the cone to form on a press brake, a lot would depend upon the surface appearance you require. We used to bump stainless steel column covers with a half inch square top die. Onto a urethane bed bottom die. this would produce a curve with no discernable ripples.

B.E.

You are judged not by what you know, but by what you can do.

 

[hendersdc]

If you could change the material away from SS you could electroform the part.

 

[hpaircraft]

My bet is that unless you special order it, you will not find material in the requisite 1.65m width. You can kill two birds with one stone by:

* Resigning yourself to having it welded or riveted.
* Making it out of two identical halves.
* Joining the halves with: a)joggles and either rivets or spot welding b) butt welding that will almost certainly distort the part
* Accepting that it will take some bit of finish work to make it look nice