Homemade press brake cracking material

[oxford]

I will try and get some pictures up later but for now I built a homemade brake that goes into the hydraulic press. Looks similar to all the other ones on google images, top die is a piece 1”x5” crs with a 45 degree angle cut on each side to make 90. Bottom die is just a piece of angle stood up and cradled in between 2 other pieces of angle. Width of brake is 6”.

Trying to bend 1/4” cold rolled steel 4” wide, the back of the material stress cracks. Cracks start to form around 30 degrees of bend. Top die opening is 2 1/4” wide. Top die was initially a sharp point but has since been blunted and slight radius with no change.

1/8” material bent without any issues. Any ideas on what move to make next?

[reggie_obe]

Bend radius too small?
https://www.protocase.com/resources/bend-radius.php

[BCRider]

Combined with the surface skin work hardness found in cold rolled steel?

Can you heat up a small piece to roughly a blue color and try it again? Quite likely though you'll need to make a second forming blade with a radiused nose

[barracudajoe]

Steel has a grain direction much like wood. If forming with the grain you need to have a larger die opening and radius. Other than "cold rolled" you didn't mention the grade, do you know what it is?

[Bob La Londe]

These guys pretty much answered what I had to add. I've had similar cracking when braking .125 5052. .0625 seems to brake just fine though.

In order to get a good radius on .125 that doesn't break I sectioned out some pieces of pipe that I can slide over the fingers of my box brake as I close it down on the work piece. An alternative with aluminum since it work hardens badly is to bend it to about 45, anneal it with a torch, and then bend it the rest of the way. I use a Tempil stick to make sure I get to annealing temperature without over heating and blowing it out.

A quick and dirty trick (I don't know if this would work with steel) is to throw several layers of solid (not corrugated) cardboard between the fingers and the sheet. This gives a larger radius than the sharp finger edges, and often allows a piece to brake nicely without breaking.

On my finger brake of course I need to adjust the finger set back and clamping height for the trick I am currently using, but on your press brake it should be simpler to implement.

[J Tiers]

Ar they actual cracks going into the material? Or are they just stretch marks?

Bending close, meaning under a metal thickness on the inside bend, it is not unusual to find stretch marks. The metal is being stretched, and does not always stretch evenly. You should not have actual cracks. If you do, you might need to either re-orient the metal (due to grain), or increase the punch size to get a larger inside radius.

The grain goes along the coil length (from the rolling process), so bends across the coil width crack less than bends along the coil width.

[JCByrd24]

I don't have my homemade press brake in front of me so I can't measure my die spacing but looking at some charts online you should be in the ballpark. I've seen 10x thickness recommended so you are just under that. My dies don't come to sharp points either, they started as hex bar but the edges have become rounded. My punch is not sharp , it's 3/8" flat bar and pretty much a 3/16" radius so that may be a difference but I would not expect that to be the cause of your issue.

I've bent 3/8" x 3" HR flat bar to just beyond 90 degrees cold with no cracking. The mill scale will flake off but no cracking. Typically when bending heavy stuff like that though I'll use a MAPP Berzomatic to heat somewhat during the bend, but it definitely won't make a heavy chunk like that change color.

Are you sure you are actually getting cracking and not just scale flaking? I know CR doesn't have much of a scale but still cold be something similar.

Try another piece of material, your setup sounds decent.

[cameron]

If you want to investigate the effect of a larger bend radius without grinding the nose of the die, you could bend a strip of thinner material and attach it to the die when you bend the 1/4" stuff.

Just curious, how close to a 90 degree bend are you getting with the 90 degree die?

[oxford]

To answer a couple of questions,

I don’t know the grade of steel, when I talk to the steel supplier I just ask for cold rolled flat bar.

It is actual cracks in the material, looks like maybe half way though.

I did wonder about the grain and had tried a piece of the same material oriented the opposite direction and the results were the same.


Now some more questions,

I have seen talk about bend radius, what determines the bend radius while press braking? I did try and put a piece of 1/2” rod under the top die and that resulted in the same cracking.

Where I am at now is I tried a different bottom die that has a 2 3/4” opening, it bent the 1/4” fine but still cracked 3/8”. I always though die opening was 8 times material thickness so a little short for 3/8 but getting big for 1/4.

[garyhlucas]

I have done lots of bending. Cold rolled doesn’t bend well except to a large radius on a radius die several times the material thickness. If you need cold rolled it needs to be annealed. Stainless 303 cracks, 316 not so much, 304 bends well. Aluminum 1100, 3000, bend well 6063 fairly well, 6061 and 5053 not so well.

[J Tiers]

.....

I have seen talk about bend radius, what determines the bend radius while press braking? I did try and put a piece of 1/2” rod under the top die and that resulted in the same cracking.

.....

Obviously, you cannot have a bend radius less than what is on the nose of the punch part, unless the material is forced into an odd shape by the bottom die, and does not form around the punch.

The punch nose plus two thicknesses is the space taken up by the material going down into the die... the die has to be big enough to take that without cramping the material so that it does not form on the nose.

We got a lot of cold rolled formed without it cracking....and we had bends going in both directions, material up to 1/4".

[Norman Bain]

Get some Australian steel.

[oxford]

Get some Australian steel.

Wouldn’t it just crack on the inside of the bend instead of the outside?

[oxford]

Sorry, I didn’t end up getting any pictures from work of this but I think everyone is on the same page of what I have and what is happening.

Some more with the bend radius. From what I am gathering is the top die needs to have a radius on it equal to material thickness. How wide does this radius need to be? Equal to 90 degrees of the full circle? Why did I get the same cracking results when using a piece of 1/2” rod on top of the material and the top die pushing down on that? Also increasing bottom die opening stopped the cracking without changing top die.

I know I watched 1/4” material being bent at the last place I worked and the bottom die had no more than a 2” opening, maybe less and the top die did not have a 1/4” radius on it. Granted this was commercial made tooling.

[ulav8r]

Bottom die opening needs to be 8 times the thickness of the material being bent. Top punch radius determines the bend radius. Last time I ran a press brake was about 1990, so I don't remember much more about it.