Question on die sizing for rectangular tubing bend

[Mike C5]

I am a hobbyist and am hoping someone can help with a question on tube bending.

This is a Harbor Freight bender and I'm making up dies for the radius of bend I need. Although it bends fine, I'm finding that when bending (steel) at room temp, The radius I end up with is larger than the die (due to spring back effects). The tubing is 1/2 x 1", .062" wall and I'm bending in the 'easy' direction.

The die OD is 5" which I was hoping would give me a 2.5" ID radius on the 90 degree bend. Instead, I'm getting around 3" radius.


My question is, how much smaller does the die diameter need to be so that I'll end up with 2.5" ID radius on the tube? I've looked around on-line and can't find any free calculators that address this and I'm not going to invest in a fancy calculator. Anyone have a rule of thumb here that might give me some idea what diameter to machine the die to? I made some smaller radius bends where I heated the tube up to orange hot before bending and did not have this problem. I tried to do that with the larger bend radius but could not heat the tube uniformly enough over the length and bad bends resulted.

 

[Scruffy887]

Welcome to metal bending. Part science, part VooDoo. Trial and error will work. Take notes. Understand that material formed today may not match material purchased next month. Will 4.5 dia get you what you want? Or kink the tube? Do the math with notes because different metals will have different spring back amounts.

 

[memphisjed]

If I am feeling on point .97 is my starting point for draw or closed die near coining bend- .9 for rough quick and dirty straight from plasma table dies. .83 like what you might be just what your set up gives. There is more voodoo than people want to admit. After a few different bending setups you can get a feel for your bender(s) and methods and become better at first guess sizing.
Changing counter die angle and pressure changes bend a lot, once you find the sweet spot for bend quality you want to stay within that area for almost all your materials. There is no just press play button when you enter the world of bending, respect the material.

 

[Peter from Holland]

Annealing might help to get better repeatability

Peter

 

[fciron]

If the product is coming off your current die 20% oversized then you could try making your new die ~20% undersized. (Technically it should be 83% off the desired size if I’m doing the math right.) I tend to err on the small side since it’s easier to open up than to tighten the curve.

There’s no rule of thumb because the amount of spring back varies across materials and cross sections and also can vary between batches of the same material.

 

[Mike C5]

Thanks guys. I came to the conclusion that I would need to 'burn' some material and experiment. I machined the die down to 2.4 R and that did achieve the desired effect; close enough at least (I had to take small steps to avoid having to buy more material to make another die; no going back if I trimmed it down too much). By the way, I'm making a seat frame for a 1925 Dodge Brother touring car. You can't just buy a new one like you can for Ford Model T and A. There is one place that will make the assembly to your drawing but it ain't cheap. Here's a photo of one welded up and ready for coil springs.

 

[Graham Gott]

Your bends look really good. I wondered what bending rectangular tube the easy way would look like. I've bent a lot of square but never rectangle.

I'm surprised you had that much springback. Generally bending square or round you have a larger bend radius than the die, but what you ended up with seems like a bigger difference than what I'm accustomed to.

 

[Scruffy887]

Thanks guys. I came to the conclusion that I would need to 'burn' some material and experiment. I machined the die down to 2.4 R and that did achieve the desired effect; close enough at least (I had to take small steps to avoid having to buy more material to make another die; no going back if I trimmed it down too much). By the way, I'm making a seat frame for a 1925 Dodge Brother touring car. You can't just buy a new one like you can for Ford Model T and A. There is one place that will make the assembly to your drawing but it ain't cheap. Here's a photo of one welded up and ready for coil springs.

You would be surprised how effective it is to wrap your punch or die with a piece of thin sheet metal for "in between" sizes.