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Thread: How To Calculate Forces In A Sheet Metal Bender?

  1. #1
    Associate Engineer
    Join Date
    Sep 2018

    How To Calculate Forces In A Sheet Metal Bender?

    I want to build myself a sheet metal bender.

    That is an instrument with a hinged flat bed we lay a sheet of metal on, clamp it down near the hinge and then lift the movable side thereby making the metal bend at that point.

    I would like to know the force required and therefore the force experienced in the jaws of the thing and therefore how strong I have to build it.

    The plan is to be able to bend up to 4 foot wide and metal up to 1mm thick. Mild steel.

    My researching has taught me a bit and I've come up with formula for bending by a die forcing metal down into a V and for a die forcing metal down over the corner of a block. They're both pretty similar.

    A constant multiplied by the length of the bend multiplied by the thickness of the metal (squared in one instance) multiplied by the tensile strength of the material and divided by the gap length in the case of the 'V'.

    So I get the idea but I don't get it well enough to be able to figure out the formula for the simple bender I'm thinking of.

    And I seem to have exhausted google's resources... can't find anything more with better information. A bit surprising really because I'd think this kind of bender would be the ubiquitous, most common and basic, etc... But the info I find is much more about big machine shop die presses and stuff..

    Can anyone help ?
    Last edited by arthur_brogard; 09-10-2018 at 04:18 PM.

  2. #2
    Administrator Kelly_Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Determine the yield strength of the material, draw a free body diagram of the forces applied by the bending machine and using statics determine the forces required to achieve yield.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
    Join Date
    Sep 2018
    Thanks for the reply but I'm afraid it doesn't help much. I don't understand 'statics' nor 'free body diagram'.

    But here's a drawing I've just made.


    Seeing the formula I've seen so far are just basically a constant times tensile strength times material specs I'm thinking it is going to boil down essentially to the same thing again: a constant times specs times tensile strength.

    But I'm so ignorant I don't know - is there a division somewhere like in the example of a die forcing the material over an edge? What's the constant in this case? Does the thickness need to be squared in this case?

    The length is set, I've said 4 foot so that's alright. The thickness is set, I've said 1mm. That only leaves the length of the 'leaf' - the hinging portion of the machine. I could set that at an arbitrary 100mm, doesn't matter.

    The whole thing can be seen alternatively as a piece of metal standing up in a vice being forced down to a right angle by the application of a flat piece of metal bending it down.

    All that is happening is right there at the bend, I can see that. Or I think I can. The length of the bending 'lever' just serves to transmit and multiply the force. i.e. if the bending arm of the bender is also 4 foot long then any forces I put on it at the end there- like how strongly I lift it up - gets multiplied by that length and manifests as a force down there at the bend.

    And it is that force down there 'at the bend' that breaks the hinges off a bender, that distorts the clamping bar, that bends the bed etc.

    I don't even know what 'at the bend' is, really. Perhaps that area that will be effected by distortion in the final result?

    Don't know. I'm asking. Anyone know?

    Probably a table exists somewhere even ?

    p.s. I've just thought this case is so simple perhaps it is essentially the tensile strength case? i.e. it's a setup like this they use to ascertain the tensile strengths? so the force required is simply the tensile strength x cross sectional area x length?

    i better look up how they do the tensile strength thing..
    Last edited by arthur_brogard; 09-10-2018 at 05:53 PM.

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