Another Press Brake question... about max tonnage per ft or inch.
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  1. #1
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    Default Another Press Brake question... about max tonnage per ft or inch.

    I read an article about applying to much force to a small area of the press brake bed, So yes, it makes sense, my brake is a 12' x 240 ton brake, you dont want to apply 240 tons of force over a 2' area due to damaging the bed or the ram... ( Not to mention the die set itself )

    The article said if you 150 ton brake that is 10' between centers, the max tonnage per ft you should apply to the bed itself is 25 tons, I did some calculations and come up with the max tonnage per ft that my bed can handle is 39.96 tons per ft.

    So, the minimum die you should use when bending 3/4" plate is 10 times material thickness which would be a 7-1/2" bottom die, but lets say I only have a 7" die that would require 42.2 tons per ft which would exceed my tons per ft., even though I may only be bending a 36" piece which only requires 126.6 tons of force, I may be damaging the bed and ram...

    This is definitely something that I might not have thought of if I didnt run across this artcle.

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    I read that too, but then the books for my press said a max ton per inch on ram which was way higher than the calculation in the article. It is definitely something to consider though. The other thing you can do, assuming cnc control of each cylinder, for your 126 ton example, is place it under one cylinder, pretty much no risk of bending ram in middle but max ton is effectively halved. Cant do it on a torque tube type machine as the other cylinder would try to move too much, but cnc control it controls each ram independently.

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    JP machining, My press brake is not what I would consider CNC, thanks for bringing that to my attention, centering the part on the frame, My press does not have the torque tube design like the accurpress, I'm getting a Betenbender.

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    I bought this book back when I was learning how to use the brake...if you'd like to borrow my copy I can drop it off next week. Granted, it's a digital copy, so you'll either have to read it on the computer or print it, but it is a good book.

    Press Brake Technology
    SME.org

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    Thanks for the offer Snowman... I'm more of a hands on guy, I'll be doing a lot of test bending and making cheat sheets to help me calculate my bends... I have to say, Matt and Ron are human calculators, they could calculate exactly what size piece you needed to start with and where to bend it in about 5 seconds..., Meeee..., I have to get the calculator out.

    I've gotten much better since I got my apron brake and have to figure it out myself.

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    Quote Originally Posted by Portable Welder View Post
    Thanks for the offer Snowman... I'm more of a hands on guy, I'll be doing a lot of test bending and making cheat sheets to help me calculate my bends... I have to say, Matt and Ron are human calculators, they could calculate exactly what size piece you needed to start with and where to bend it in about 5 seconds..., Meeee..., I have to get the calculator out.

    I've gotten much better since I got my apron brake and have to figure it out myself.
    There's nothing wrong with the calculator or the fingers (Eric), as long as the end result is the same. Matt was very good, it drove me nuts. I liked to draw everything out...that drove him nuts.

    The reality is that for a majority of the stuff you'll do, if you just add up the inside faces of the bend, you'll come within a working tolerance. Let your experience guide you and you'll get it really quick. I was NOT able to learn this way...I needed to understand the theory behind it before I could remember the tricks. That's the way I learn though.

    If you start doing precision bending, simply adding inside faces doesn't work. Matt was very good at this work because he was a hell of an experiential learner. He had the feel for it. I struggled with it because on the brake at VBS, it was all by eye, not by precision back gauge...I have always been a person that does best with a stop to push something against, instead of trying to line it up by eye.

    You will love the brake!

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    Ya, they hated using the back stop, when doing stair pans we would use the back stop and they always acted like I was being ridiculous for doing so.

    Like you, I like drawing my stuff out... less mistakes.

    The new brake I'm getting will store up to 5 set ups for the ram and 5 for the back gauge. I bought new tooling for it, a segmented gooseneck punch so I can use it like a finger brake to bend up 4 sided pans, a heavy punch for 3/8" material x 12' and a 4 way bottom die with a 1", 2", 3" and a 4" opening x 12'.
    I would like to get a 7" or 8" die for doing 3/4" plate, a 5" die so I can bend 10' of 1/2" plate... however that would max me out on tonnage capacity, a 5-1/2" or 6" opening might be required due to the higher tensile plate now days.

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    Quote Originally Posted by Portable Welder View Post
    Ya, they hated using the back stop, when doing stair pans we would use the back stop and they always acted like I was being ridiculous for doing so.

    Like you, I like drawing my stuff out... less mistakes.

    The new brake I'm getting will store up to 5 set ups for the ram and 5 for the back gauge. I bought new tooling for it, a segmented gooseneck punch so I can use it like a finger brake to bend up 4 sided pans, a heavy punch for 3/8" material x 12' and a 4 way bottom die with a 1", 2", 3" and a 4" opening x 12'.
    I would like to get a 7" or 8" die for doing 3/4" plate, a 5" die so I can bend 10' of 1/2" plate... however that would max me out on tonnage capacity, a 5-1/2" or 6" opening might be required due to the higher tensile plate now days.
    Do you have enough vertical height to fit a die that big in there?

    That brake in combination with the big shear at vbs could really make some money. There isn't really anyone else around that can bend 1/2" in length other than contractors. Contract can, but doesn't really do job work. Troy can, but they are a good hour away.

    I didn't expect the big shear at vbs to get much work, but it really got quite a bit.


    Sent from my iPhone using Tapatalk

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    Ya, we went 2" over stock, so I have 16" opening, it has 8" of travel, so when fully closed there will be a 8" gap, the reason I went 2" over stock is because I ordered a 5 1/4" four way die with a 2" tall die holder..., I may have to buy a die riser if I get shorter dies.

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    There is 70/30 rule that the sheet metal industry uses. No more than 70% tonnage over 30% of bed length. There are three main risks: permanently distorting the ram or bed (extremely slim), "imprinting" the ram or bed/bolster with die set (they will literally bite-in and upset the face under over-tonnage, and third, grenading the dies. Most guys who end up grenading the dies do it by running the punch into the die at "approach-speed" by fat fingering the speed-change point. Be safe, and make tons of money!

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    EF Thuman, thanks for the advice 70/30 rule, got it!!!!, since I bought a hydraulic brake verses mechanical it will minimize the chance of crashing my dies.

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    A similar "rule of thumb" that has kept me mostly out of trouble:

    Generally, you should never apply full machine tonnage over an area that’s less than 60 percent of the distance between the side frames.

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    I use this app a lot to calculate tonnage. And it is free too. I have the Delem control on my press brake and if I try something that is out of spec for the machine it will tell me so. Things like over loading a given punch or die, or even the machine itself.
    BendGuide | TRUMPF

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    Thanks for the link scruffy.

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    If the products you make require precision bending what I would advice you to do is make samples with just about every thickness you work with and compare them with a K factor table and if the values differentiate a lot then you can make your own table for calculating bend deductions. This of course would work well only with consistent material, e.g. you are buying always from the same place and the properties of the used material are consistent. When designing with solidworks and using K factors the software makes "unfolded" drawings with exact lines of where the punch should hit the material. General bending rules say that the radius of the bend shouldn't be smaller than the material thickness, you should experiment see what works with your punch and die sets, in my experience we have made a whole lot of R = 1mm in stainless 2mm but it requires testing and experience to be done right, another of the old rules of thumb is using die 6 times the material thickness, e.g. for 2mm stainless you should use die V = 12, however if flange lengths don't allow this you can use V = 10 or V = 8 with some extra attention. Not exactly OT but since you are just starting to use a press brake I hope you find something helpful along these lines.

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    Ahnt, all other recommendations have been 8 times material thickness until you get past 1/2 " then it's 10 times material thickness.

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    Quote Originally Posted by Portable Welder View Post
    Ahnt, all other recommendations have been 8 times material thickness until you get past 1/2 " then it's 10 times material thickness.
    Stick with 8-10...splitting dies is expensive.

    You can go tighter when you need to, but it's unlikely you will ever need to.



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