Detailing Cone Cutout? 1

[Darken99]

I am not sure if im in the right forum.

I have a cone drawn in 2d autocad and I am not sure how I determine how many degrees I make each bend if I have a random quantity of bends to make the cone come together seamless.

Any pointers would be great

Thanks

 

[ewh]

I think it would depend on the number of bends.

 

[Darken99]

I have that much figured out but how do I determine what the total bend of the cone to come together seamless?

 

[drawoh]

Darken99,

Are you making a fabrication drawing, or are you helping the sheet metal worker do the flat layout?

If you are doing the fabrication drawing, you should not worry about the flat layout. Specify the height, diameter, angle (or equivalent) and the tolerances. It is possible that the fabricators can roll it, rather then bend it in sections. You should call them and discuss this.

If you are helping the sheet metal worker do layouts, I would hope he knows how to do this, and that he can explain it to you.

One of the very first drafting jobs I had was to make a drawing of a sheet metal Y joint. The layout looked complicated to me, so I simplified it by making three straight, round tubes, with a flat interface plate. About a week later, the boss called me into his office, and showed me the fabricated part. The two upper Y tubes were tapered into the larger bottom tube, and there was no interface part.

Sheet metal workers are skilled. Do not insult their intelligence.

JHG

 

[Darken99]

I am doing a flat layout so that a untrained person can break parts to a certain angle on a break. Unfortunatly there is no one that is skilled enough in the shop to bend it the way I require. At least not over and over.

 

[MikeHalloran]

A brake is not necessarily the best tool for forming cones, especially if you need more than one. Ask the guys in the shop to show you a 'slip roll'.

Now, if you really _do_ need to make a polygonal cone, you do it with a finite number of air bends in the brake, with the punch or the ram set down a little at one end, and the blank is a finite number of Isosceles trapezoids placed leg to leg.

Say there are 8 faces to the cone. The dimension of the smaller base of each trapezoid is 1/8 of the circumference of the cone's small end. The dimension of the larger base of each trapezoid is 1/8 of the circumference of the cone's large end. The legs of each trapezoid are equal to the slant length of the cone. As you increase the number of faces, the trapezoids get narrower, and in the limit the joined collection of trapezoids becomes a shape comprising two similar arcs joined by radial lines. If the arcs happen to be semicircles, i.e. the lines are colinear, the resulting cone will have an included angle of 60 degrees. But you knew that, right?





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Darken99]

All this info is very helpful. I guess the question I should be asking is what is the easiest way to form a 16' wide cone that has a height of 4-5'. They are very large cones and can not be handled as one section. The problem is we don't have a roll large enough to do cones like this. Would it be worth investing in a roll that could do these types of cones?

Thanks

 

[drawoh]

Darken99,

The picture gets clearer.

It is too bad you are not on 3D CAD. I suspect this one would be fairly easy.

It can still be done in 2D. Draw your cone out as a set of 2D orthogonal views, with whatever number of sections you want. Hopefully, the top and bottom of your cone are both perpendicular to the axis, and all the sections are identical.

From a side view, pick an othognal face, and project it. This gives you an accurate length of the face. You can project the widths at the top and bottom from the top view. This is all old fashioned drafting board stuff that works fine in 2D CAD. An old drafting text will explain this far better than anything I can type in a note.

I take it you are going to punch out each face and weld them together?

You should think out fixturing carefully. You should be able to work out a gusset that will support each face at the correct angle for welding.

If your ends are not orthogonal and/or the overall form is not round, this method still works, but you will have to project each face. It will be a lot of work.

JHG

 

[ajack1]

I might be misunderstanding something here, but are you after a cone with a 16 “ diameter with a closed top and a vertical height of 4.5” ?

If so you know the top will all come from a point you know the length of the base Pi X D = 50.26”, you know the true length of one side the square root of 4.5 squared + 8 squared= 9.17.

So you want to draw a circle with a radius of 9.17 but you want the outside length to equal 50.26 which is an inclusive angle of 314.12 degrees. So the part you want would look like a round cheese with a portion cut out.

Like I said I might be misunderstanding what you want and these figures are only in my head so please check them (make a paper mock up) but I think that is what you want.

Hope this helps.

 

[Darken99]

Ok I will try to make this a bit clearer. The cone has a 16' opening at the top and a 24" opening at the bottom. I have a good program in AutoCAD that sets up the flat 2d drawing based on the 2 openings and the height of the cone, which is then plasma cut. We then take the 4 cone sections and break and weld them together. I think we have gotten into more then 1 topic into this forum. I have the actual solving of the breaking angles figured out now. The question I have now is if rolling is a better way of forming the cone. I am having people tell me rolling is faster then breaking but the cone would not be as strong if rolled. Can anyone back up this information?

Thank

 

[MikeHalloran]

Ah. That's 16 FEET in diameter at the top,
and 2 FEET in diameter at the bottom,
and 4.5 FEET deep, right?

I'm getting a blank of 228.167 inches OD and 28.674 inches ID, with an included angle of 302.681 degrees, for 1/4" thick mild steel.

If you divide that into 13 equal pieces, you can get two pieces out of a 4 foot by 10 foot sheet of metal, assuming butt joints.

My math is usually unreliable, so don't take my word for it.

To get the curvature right, i'd make a 3D model and use it to slice a couple of planes, or I'd make a scale model, and fit a couple of templates to the small and large ends of each segment, then scale them up and make plywood templates for use by the press brake operators.








Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Darken99]

So are u saying that breaking it would be a better choice then rolling it?

 

[MikeHalloran]

I'm saying that you can't make the blank in one piece, because you can't buy sheet that large.

It would take a pretty big slip roll, sure. Whether you can afford it depends on how many cones you have to make, and how nice they have to be.

The cone is shallow enough that for some purposes, you could just make it up as 13 or more flat segments, and not bother curving them at all, or maybe just crease them down the middle.

If you do need the pieces curved, and don't want to invest in a roll, you could make a wooden or concrete buck, and form them over that, with pneumatic hammers or by rolling your forklift over them, or something like that.

You didn't tell us the material or thickness. Relatively thin sheet would yield to a light- duty homemade roll.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Darken99]

It is 14 to 12 ga. and the cone blanks right now are divided into 9 to 12 pieces depending on the size of cone. 1 to 3 cones would be required right now a day so it might be worth investing in a 10' roll if one is available to do the job required.

Can anyone confirm that a rolled cone would have less stress consentration then a broke cone?

thanks

 

[MikeHalloran]

Uh, less stress concentration? Does that imply that you're having problems now?



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[Darken99]

No we are not having problems but I am just trying to find out which will give you a stronger cone. Rolled or breaking?

 

[MikeHalloran]

You're using the same raw material (you haven't told us what) and forming it into the same gross geometry in either case, and loading it the same (you haven't told us how i's loaded), so there's not going to be a lot of difference in 'strength', whatever you mean by the word.

There will be a big difference in the amount of labor and time involved in forming.








Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA