Bend Radius

Discussion created by Seth Castleberry on Jan 24, 2007
Latest reply on Feb 3, 2007 by Mauricio Martinez-Saez

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Hello,

I am new the whole sheet metal world. Thecompany I work for is starting a fab shop and im trying to build asmany parts in SW as i can before we start production. I know thatthere will be alot of trial and error thing that will happen, and iwill learn alot from this as well. I am looking for advice orsome general rules that people go by when assigning or determininga bend radius. All our parts will be from 12-18 ga mostcommonly. Thanks for the advise.

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Martin Cox Jan 24, 2007 2:58 PM
Your Sheet metal parameters will be dependent on the tooling and to a lesser extent the material that you willl be using.
Internal bend radius is usually dependent on the size of the Vee that is used.
The vee that you use depends on the minimum radius the material will support without splitting and the maximum pressure that the press can deliver.(bigger radius = less pressure)
You may have some data that you can use that is supplied from your press brake supplier.
If not you may need to find out some bend deductions for your material by either pressing some material to 90 deg or entering the parameters into your press brake controller.
You can then use the thicknesses, bend deductions, and internal radius dimensions to extrapolate the K factor for your materials.
K factors have the advantage of being independent of the angle of bend used.
Finally you could then tabulate all the results into a bend table
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- Jim Hilgenberg @ Martin Cox on Jan 27, 2007 3:19 PM
  Hi Seth...
  I'm with Martin and all that he suggested.
  
  Another consideration is what tolerances restrain the end product. If you're working to ±1/16" then an exact bend adjustment is just overkill--a simple, general empirical or K-factor formula will work fine. But if you're required to hold ±.010 per bend then you need to dial things in and even possibly setup a bend table based on bend samples from the shop (noting the material type, thickness, bend radius, and tool sizes used)--this is what I use for my customer, generally. And also, aluminum forms differently than cold rolled steel as does stainless steel and even hot-rolled steel, etc. etc. Each type of material requires it's own K-factor, for example.
  
  There's also air-bending -vs- bottom-bending (sometimes called coining and usually applied to 90° bends only, and takes more tonnage or pressure). The method of bending affects the bend adjustment.
  
  Also, I reiterate what Martin stated, the size of your v-die opening (bottom die) is what generally determines the radius of your bend (if you're not coining). For example, 12ga steel bent with a 1/2" die will produce a different inside radius than, say, a 3/4" die even though the the radius on the punch (top die) is 1/32" in both instances. The material wants to form to a natural bend radius based on the width of the bottom die.
  
  I"ll leave you with a basic rule-of-thumb that I use for choosing a minimum bend radius for cold rolled steel-- not less 1/2 the material thickness (with a variance tolerance of 1/64") for the guages you listed...and use a K-factor of 0.430. You can sometimes cheat on this if conditions support it.
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J. Mather Jan 27, 2007 6:09 PM
I assume that you will be hiring (or already have) at least one knowledgeable machine operator who has significant sheet metal experience. I came from the shop floor and it always aggravated me when new engineers would overlook the value of the hands-on experience a few steps from their doors. Frankly I got the impression they were afraid they would make fools of themselves if they walked out there and asked a few questions. Take it from me - if you don't come out there and ask questions you will make a fool out of yourself.
The guys out on the shop floor love to show off what they have learned. One way or another they will show you. Go talk to them.
I mean this as a general statement of good practice.
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- John Layne @ J. Mather on Jan 27, 2007 8:49 PM
  Strongly agree with what the others have said, take note of what JD Mather says talk to the Operator and get a good understanding of the processes involved.
  
  A couple of Sheet Metal links
  
  Sheetmetaldesign.com A great resource for SolidWorks/Sheet Metal.
  
  BendWorks A free bend deduction calculator with a reliable (at least for my work so far) "rule of thumb" K-factors. Also on the same page a PDF document "The fine-art of Sheet Metal Bending" is well worth reading.
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  - Mauricio Martinez-Saez @ John Layne on Feb 3, 2007 5:14 PM
    Start with 0.6 of thickness for Air Bending.... them if you need abetter number go to the floor talk to machine operators and do sometest with some scrap... take measurements.... nothing better...since actual values depend on the characteristics of the material,geometry of the tools, the machine (and this will change with"age"), the operators, the weather, and the last speechof Mr. G. Bush.... Bending Radius on sheet metal is like cooking agood BBQ... how long you keep the beef over the fire?
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