Need help with Rotary Draw dies

[Tooly236]

I am building a die to bend 1" square, I have a 8" O.D. x 2" Round die that I have machined a 1.05 inch wide by .5 inch deep groove. *I was going to make the back plate 4" long and machine the same groove into it. *But the truth is I am just guessing, I want to know if anyone could tell me if you machine in exactly 1/2 of the thickness of the material you are going to bend or something I dont know about. * Thanks to anyone that can send me in the right direction. * *I also wanted to know if there was a formula to how long the back plate has to be in relation to the material you are bending and the O.D. of the bending die. *I attached some pics so you can understand my rambling

 

[sfriedberg]

It's a decent rule of thumb that the neutral axis is at 50% of the material thickness for a straight bar with symmetric cross-section. It's quite accurate for elastic (spring) bending, and is still pretty good for plastic (permanent) bending of symmetric cross-sections. Refining that initial estimate may require doing a few test bends using the actual material in question and taking some measurements.

You probably already know that the inner face is going to swell perpendicular to the plane of the bend and the outer face is going to shrink. So don't be surprised if a test bend shows your 0.05" clearance on the inner die isn't adequate, by jamming in the groove. I can't tell you if a 7.5" ID for 1" material will be a problem there or not.

 

[Tooly236]

Thanks, I appreciate the feedback. I thought that it is going to swell to 10 though or 200 though so I might as well stop it when I can. I understand the material has to go somewhere so I might put a .125 stip down the middle of the round die. But, I have no experience with this so I appreicate any feedback. If anyone has dies they could tell me I imagine. I have searched for a formulas with no luck. There is a million on the bend diameter but not the depth of inserting the material and swelling.

 

[Graham Gott]

I have several dies for bending square tubing with my JD squared bender. 0.050" clearance seems pretty loose...I haven't measured but I would guess my dies are less than 0.010". The other thing to keep in mind is the bottom of the groove on main die is not flat. There is a raised area in the center to encourage the inside wall of the square to deform inward slightly. The follow die and the small piece that's there for anti-rotation are flat-bottomed grooves. I would estimate the depth of the grooves to be something like 45% or more of the tube width. They don't touch while the bender is in operation, but they're pretty close together.

The tube does expand some in height during the bending operation. In my experience, it's enough that you can't slide the follow die back over the bent tube once it goes past. Generally it requires some force to remove the bent tube from the main die when the bend is completed because it has expanded to be an interference fit with the die.