Trick for accurate conical 2d bends?

[sblack]

I am making ducting for the engine cooling plenum on a homebuilt airplane. Material is 0.032 2024t3, which is quite stiff. I need to make 2d bends i.e. No shrinking or stretching to go from a square section to a radiused section. I ave been doing it by many small bends in a finger brake. It works but it is a bit hit and miss and leaves a faceted surface. Are there any other good tricks out there for doing this?
IMG_1882.jpg

[metal manny]

Hi Scott

I think the solution to your problem is to make up a radius bending tool which fits under the fingers (nose bar) of the brake to soften the bend and eliminate the sharp creases your standard fingers are producing.

With reference to the attached pic, a simple radius tool comprises of a length of round bar, say 10mm (3/8") "a", welded to an equally long length of flat bar "b" which is clamped between the work piece and nose bar.

Some brakes can be adjusted to move the top nose bar clamp back far enough to avoid the radius tool from striking the apron as it rotates upwards on the bend. My brake which has individual removable fingers can't do this, so I have to remove the nose pieces from their holders and fabricate a tool which clamps into the holding slots to get the right positioning of the tool.

Please note that the further back you position the tool from the apron, the gentler the radius you will create with each bend.

Hope this helps

[Dougg]

Not sure what you mean by hit and miss. What your making is really a part of a square to round. This can all be done with triangulation layout. If you use a break the number of bends can be divided by 90. A rounded die would help. Or make the first and last bend and free form over a pipe.

[sblack]

Manny that is just the sort of devious trick I was looking for. I don't have that much adjustment on my brake so I would have to make a custom finger but that should not be a problem. Thanks for that. Doug it is hit and miss because with springback I find it very difficult to bend to a precise angle. Thanks for your reply.

[skintkarter]

Tricky bit to make and end up with a good finish. A radius folder in a regular brake won't work as far as I'm aware (not that I have one...) as it is a cone you are trying to roll. Bit of effort, but could you machine a timber or MDF cone with a slightly smaller diameter to compensate for the springback, mount this somehow with a flat surface leading onto it (so that you could clamp the sheet) and then just pull it around by hand? Maybe if it is pulled around the former and then slapped up a bit to encourage it to retain the shape? Be interested to see your final solution Scott. A lot of effort to get rid of the facets, but I know where you are coming from - it just looks naff on something other than an air con duct.

[BTromblay]

Hi,

I have had good luck making parts with the same problem, radius flowing into a square corner. I use a rubber upper wheel and a narrow delrin lower wheel in my English wheel. I layout with a magic marker the lines of the radius and a few passes it will fold the shape.

Bill

[Oldnek]

Hi Scott,

I had to fabricate one of those exact scoops for an Series 3 E Type Jag, I found the simplistic way was to grab some exhaust pipe and cut to the length you need, then squash one side in a vice. You will now have your forming tool. Then fold over as necessary.
Scoop.jpg

Dont pics of modified pipe as used it for other bits.

[SWT Racing]

I always bump bend that type of part (on a press brake), or multiple bends on a brake.

Since you need a tight radius at one end, but need to soften the bend lines, you can make a simple finger overlay. Take a 2" wide strip of 3003-H14 or 5052-H32 in roughly half the thickness of the radius you need, and bend it with a 0T radius all the way around the fingers. You can tape this overlay onto the fingers, and it will create a slight radius, and soften the bend lines. It will also keep the panel free of marks form the joints in the fingers.

I use guide lines as you have, and divide the number of bends by 90° to get a bend estimate for each line, but I usually just keep shifting it around in between lines to get a nice curve. You want to creep up on it to keep the initial stretch to a minimum.