Press brake crowning, really why?
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  1. #1
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    Default Press brake crowning, really why?

    Ok, I know press brakes crown.

    What I would like to know is exactly why. Can't find much on Google.

    I spent the day if front of mine and I find it hard to imagine the ram and the bed flexing. I'm using a mech brake and do 95% air bending.

    Maybe they do?

    I get almost zero crowning, but the dies are new, the die holder has been reground, has a new upper clamp bar and I don't ever bend over capacity.

    Assuming it takes but a few thou's in the middle to make a difference?

    So.......if a manuf put in a center "push" point would the crowning stop?

    Andy

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    There isnt a shop on earth where they dont try to exceed the rating of the brake.
    If it will bend 8' of 16 gage, then of course it will bend 2 feet of 1/4", right?

    This is the main cause of crowning that i have seen.

    A lot of older mechanicals are out there that are woefully small tonnage for the size- seems like Chicago made hundreds of 20 ton 8 foot machines. And nobody, but nobody, believes a machine that big wont bend 1/4" plate.

    Remember, the force required goes up with the square of the increase in thickness.

    so 1/8" is 4 times as hard to bend as 16 gage, and so on.

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    I would guess that it has something to do with how the dies seat...

    Try blueing one, or both of your dies to see how they seat together and work from there

    some other info fwiw:

    http://www.thefabricator.com/article...rake-ram-upset

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    Ries said
    "There isnt a shop on earth where they dont try to exceed the rating of the brake."

    Well I am the exception

    Also neither shear gets anything thicker than plate rating and is de-rated for the stainless.

    Not saying that before I owned them that stuff didn't happen.

    Still... new brakes come with auto crown and crown adjusters.

    If rams and beds are flexing why not make them stronger?

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    because making them stronger:
    a. costs too much in a price driven market far too often
    b. ask the product liability people why you want something to bend visibly before failure..... they figure crown adjusters will get you back usable.

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    Quote Originally Posted by MwTech Inc View Post
    Still... new brakes come with auto crown and crown adjusters.

    If rams and beds are flexing why not make them stronger?
    Throwing more steel at the deflecting bed doesn't make it stop flexing and people generally try to avoid buying equipment that requires a deep pit in their floor for a front apron that goes feet underground. Brakes that have a fixed amount of crown machined into their bed are only compensated for one loading condition. For example Cincinnati uses 2/3 of the rated tonnage for fixed beds but our Auto-Crown bed works well throughout it's range. Some of the latest pulley comp systems should be perfect because they directly counteract the distributed beam loading.

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    Default Crowning

    Just curious for more info. What is the machine particulars, the gauge, the length of the material. The dies, are they 8 to 1 ratio ? Are you stroking it clear through or hitting it in steps ?

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    Is there a chance that when bending a given length of plate there could be more "spring back" in the center of the bent plate vs. the ends?

    Maybe the crowning adjustment addresses that problem rather than deflection in the dies.

    SCOTTIE

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    Having spent 19 years on a pressbrake, many of them older than me when I started at 27 years of age, a lot of machines just have the snot knocked out of them. Until recent times, most setups were done in the middle of the bed. Longer dies of course set between the frame sides will then show hollows in the middle of the bed. Air bending, in the places I worked at was not done. All coining, or set bending. Therefore more pressure to do the bending. Around 1986 or so, one place bought 2 new Cincinnati 350 ton hydraulics with Autocrown system. These also had tonnage limiters which were adjustable so the setup could be set by tonnage or position. Those worked pretty well, but after a while, also needed shimming. Also, picture also a Cincinnati 225 ton mechanical with a stess gage or tonnage meter now inoperable reading over 350 tons. Then, there was a much more limited desire to set up more than two die sets for the same job.

    Last job, all lightweight Amada machines, 120 tons or so max, all air bending all the time. Seldom only a single dieset , sometimes as many as six from one end of the bed to the other. These would sometimes require shimming too. This last job, there was no effort to have premium quality material, meaning you could expect thickness to vary to all over the range specified for the gage standard. I never liked the idea of striking the part twice to achieve the angle of bend needed, as it is generally unreliable, but I know it is done when the machine doesn't have the capacity to do the job with a single stroke.

    Air bending on the Amada showed how tough air bending can be insofar as .0001 in ram position can bring a part in or out of square.

    So, you see a machine with the impressions of the bottom die in the bolster, and impressions of the top die in the ram nose and hold down plates, its been beaten on.
    Resurfacing those areas doesn't always solve all the problems. And as previously notes, tonnage is not the same over the length of the bed, that is the longer the part, tonnage will decrease.

    Thats my story, and I'm sticking with it.

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    I think Smootz has the right answer .
    If you think about the bending dynamics of a sheet of steel , the ends will always want to get to square first simply because there is only more material in one direction .
    the bend line in the middle of the sheet has material on both sides , so it will take more pressure . That is why you want a crown in the center of the die , so it will overbend a bit more than the ends , then spring back to square .
    Of course smarter people than me have figured out how to do this in a predictable way
    given changing parameters : material thickness , strength , and angle of bend .
    Thats my theory anyhow .
    FBBob

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    Well......

    stopped in at a friends shop today, there sits a new Cincinnati Proform brake, 135 ton 14 ft bed.



    After the drooling stopped............I asked the tech who was setting up
    the brake about crowning.

    His thoughts were the beds will deflect. Went on about tonage on small bed areas etc.

    Cincinnati ground a .028 rise in the center of this machines bed to combat crowning.

    Now I'm wandering what will happen if a 5' 14 ga SS sheet was bent, will it overbend the middle?

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    my guess is will not. SS needs greater pressure, but its all going to depend on the nose radius of the top die. Air bending?

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    Crown is to counteract the flexing of the press brake ram and bolster. Not only do they flex, but you can upset a machine by pushing it too hard on too small a die set. Doing so is the equivalent of placing a point load in the center of a beam, as opposed to the same load distributed across it's width.

    This article may help explain it a little better:

    How to prevent press brake ram upset - TheFabricator

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    Ok...with .028 more height in the middle of the die set and you air bend,

    say 14 ga ss with 14 gauge dies..........

    you are bending to 90 deg.............

    the sheet is 8' long............

    seems to me the center would be over 90 deg.

    There is no way this new 135 ton machines ram or bed is going to deflect bending this piece of 14 ga.

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    In the old days press brakes had shim charts for incrementally shimming the dies for a particular bend. Some even had tin snips hanging from the front of the machine to cut the shim stock. Now days its a simple matter to add a active counter pressure ram to the center of the bed or machine in crown. The difference is in the final price of the machine. Beds flex, period. Even if lightly loaded. With machined beds they have to try to average the deflection for the full range on tonnage the machine might be used at.

    For accurate work over long lengths shimming or crowning is necessary. If you just use your square on the end you will never see the full story. Were talking fractions of a degree here. So on a one inch flange 1 degree is about .017 at the end. For most bending this is more than adequate. If you have bent short leg angles that are the length of the bed then you will wish for crowning or be pulling out your shim stock.

    In your air bending example try this thinking example. The crown or shim compensates for tonnage variations not position variations. Think of it as a tonnage compensation feature. Like a fish belly crane beam or a crowned trailer.

    Tom Lipton

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    Default Crowning

    I have found that nobody admits to crowning anymore. The ram and bed do deflect and it doesn't take much. Up acting press brakes don't need to crown because they deflect in parallel. (Amada, Guifil,Promecam) If you are getting a canoe in the middle, just slip some shim stock under the die rail in the middle. It is kindof a trial & error thing. They also make adjustable crowning die rails now but they are very expensive. A ten footer from Power Brake Dies is about $8K. Thats alot of shim stock:-)
    Good Luck, McTool

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    Default press brake crowning

    Quote Originally Posted by MwTech Inc View Post
    Ok, I know press brakes crown.

    What I would like to know is exactly why. Can't find much on Google.

    I spent the day if front of mine and I find it hard to imagine the ram and the bed flexing. I'm using a mech brake and do 95% air bending.

    Maybe they do?

    I get almost zero crowning, but the dies are new, the die holder has been reground, has a new upper clamp bar and I don't ever bend over capacity.

    Assuming it takes but a few thou's in the middle to make a difference?

    So.......if a manuf put in a center "push" point would the crowning stop?

    Andy
    There is mechanical crowning, hydraulic crowning, and electrical crowning. The crowning function should be set when the press brake is making. So now what you can do is buy the mechanical crowning device. Hope below articles which state crowning in detail will help you: The Ultimate Guide to Press Brake (218 Updated) | MachineMfg.com

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    Quote Originally Posted by sheetmetaltools View Post
    There is mechanical crowning, hydraulic crowning, and electrical crowning. The crowning function should be set when the press brake is making. So now what you can do is buy the mechanical crowning device. Hope below articles which state crowning in detail will help you: The Ultimate Guide to Press Brake (218 Updated) | MachineMfg.com
    Spam Spam Spam Spam Spam Some chinaman drags an 8 yo post up to try to make a sale.

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    Quote Originally Posted by sheetmetaltools View Post
    There is mechanical crowning, hydraulic crowning, and electrical crowning. The crowning function should be set when the press brake is making. So now what you can do is buy the mechanical crowning device. Hope below articles which state crowning in detail will help you: The Ultimate Guide to Press Brake (218 Updated) | MachineMfg.com
    This is really one very good article writing about press brake, including crowning.


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