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Homemade press brake cracking material

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  • #16
    Try hot rolled steel. Recently I bent a number of corbels for under counter support of A36, 3/8 x 3" hot rolled.

    3" wide lower die with about a 3/8" radius on the top die. No cracking, just a little dusty flaking on the material surface.

    You might also try steel from another vendor. we're having to deal with some pretty low grade made in USA steel in recent years.

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    • #17
      DR, how wide is the 3/8" radius on your top die?

      Steel from another vendor isn't an option, it is from who we have accounts with at work and have to use. I don't know where it is sourced from but they are a reputable steel supplier.

      Hot rolled would be an option but we mostly keep CRS on hand.

      As it stands right not I remade a different bottom die that has just shy of a 3" opening. It does bend the 1/4" ok with the top die I have. This is probably all I will need this for at work as it will be used to make a few bent brackets every now and then which is better than the heat and beat method in the vise we are doing now.

      I am trying to gain a little more info on the subject as I have plans to build a hydraulic press for the house and would like to add a press brake attachment for it. That one will most likely see a range of different materials.

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      • #18
        The steel cracks for one of two reasons :

        A, It lacks the ductility need for the work. Ductility is a function of the chemical nature AND heat treatment of the steel.. you cannot change it, or at least only the HT ( annealing ) part.

        B. It lacks the Tensile Strength needed for the bend . When the "stress" placed on the steel is greater than the Tensile strength, fracture occurs. Tensile Strength is also a function of the above but you can lower the stress by several ways:
        .
        1. Change the radius of the bend ( Punch) .ie make it bigger and lowers the stress
        2. Reduce the "Drawing stress" ( Die) . This is more easily done by reducing the friction between the steel and the die ( not the punch) The top edges of the die represent a friction point and prevent the steel from moving easily towards the center ( punch area) and this increases the stress that the steel's Tensile must overcome. In deep drawing die work , the friction at this point is critical as it controls the work pieces stress levels. The drawing stress can be reduced two ways here, since it is a friction function. Having the Die ( a V Block in your case ) as wide as possible helps because the steel "locks "(friction) on the outside edges of the "V" , and there is a greater distance to the punch point allowing more metal (outside of the bend) to ductililly move to the bend area. The other method is lubrication which can be liquid or solid and would allow the metal to "slide " towards the punch and therefore lower stress. SO in a nut shell, break the edges of the V Die so it is not sharp (lowers initial friction) and use a low friction material like a plastic sheet between the die and work piece ( not the punch) . For the absolute best of both worlds , do not use a V Die, but mount two rollers and have the punch come down between them . The rollers will freely allow the metal to move

        Rich
        Green Bay, WI

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        • #19
          I have had good luck using a round top die and angle bottom. And plenty of lube. JR

          This one is 1" round bar and I used it for 1/4" mild steel.

          Last edited by JRouche; 03-29-2018, 10:33 PM.

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          • #20
            Originally posted by Rich Carlstedt View Post
            The steel cracks for one of two reasons :
            1. Change the radius of the bend ( Punch) .ie make it bigger and lowers the stress
            1. By changing the radius of the bend I am guessing that means a larger radius on the tip? How wide does the radius need to be, basically 1/4 of a circle of that given radius wide? Fwiw, I was looking at some press brake tooling on Ebay and there was a top and bottom set that was listed for 1/4" material. Top die was 1.250" wide but only had an 1/8" radius on it.

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            • #21
              Originally posted by oxford View Post
              1. By changing the radius of the bend I am guessing that means a larger radius on the tip? How wide does the radius need to be, basically 1/4 of a circle of that given radius wide? Fwiw, I was looking at some press brake tooling on Ebay and there was a top and bottom set that was listed for 1/4" material. Top die was 1.250" wide but only had an 1/8" radius on it.
              Inside bend radius can vary for any given material. Look at what RIch Carlstedt wrote.... the bottom die and the punch work together. There is not just one set that is always used for a given material, unless you do MIL stuff, and then there might be, but that would be customer rules, not some sort of natural law.

              We got parts made with a fairly wide variety of bends, and each used a different punch, at least, and often a different bottom die. Some were minimum radius bends, some were virtually "coined" into the bottom die, some were done with fairly large bend radius. Different bends in different materials might end up using the same bottom die or punch.

              The tighter the bend, the more stretch there is on a small area. A wide bend has less stretch, and it is spread out over more material.
              CNC machines only go through the motions

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