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bent stock


Columbo™
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Drawing on center will get you close, but if you need exact you need to know the k-factor.  Most materials will not stretch and compress equally on both sides of the centerline; many will stretch easier than they compress, so the bend line is off center.

 

https://www.javelin-tech.com/blog/2017/06/calculating-bend-allowance/

 

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I've been a press brake operator for a decade and a half and constantly need to explain how complex bend calculations are...

 

If you want to dive into bending calculations there is an excellent series of articles starting with: https://www.thefabricator.com/thefabricator/article/bending/a-grand-unifying-theory-of-bending-on-the-press-brake-part-i

 

When you say "bent stock" is this something that is bent because of damage or is it a formed part that you're trying to get a flat pattern for?

 

How tight are the bends and what angles are they?  The larger the inside radius and the closer to flat(more obtuse bend) the closer your neutral axis will be to the center and the better the 'center line' method will work.

 

If you're dealing with all 90deg bends that are closer to sharp than a radius you may get a more accurate flat pattern measuring your inside dimensions because like Matthew said most materials will stretch more than they compress which moves your neutral axis towards the inside of the bend. 

 

If you have access to the machine that made the bends or can ask for a sample for bend calculations have a 4" square bent at 90deg in half, measure the out side of each leg and subtract 4 from the sum of the two sides to get your bend deduction for one bend.

 

Using the tooling that I use for .25" material 90 degree bend deductions are :

 

Steel - .431

Stainless - .445

Aluminum - .402

 

So if you have a 2" x 2" outside dimension angle your flat will be:

 

Steel - 3.569

Stainless - 3.555

Aluminum - 3.598

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3 hours ago, Steelab said:

I've been a press brake operator for a decade and a half and constantly need to explain how complex bend calculations are...

 

If you want to dive into bending calculations there is an excellent series of articles starting with: https://www.thefabricator.com/thefabricator/article/bending/a-grand-unifying-theory-of-bending-on-the-press-brake-part-i

 

When you say "bent stock" is this something that is bent because of damage or is it a formed part that you're trying to get a flat pattern for?

 

How tight are the bends and what angles are they?  The larger the inside radius and the closer to flat(more obtuse bend) the closer your neutral axis will be to the center and the better the 'center line' method will work.

 

If you're dealing with all 90deg bends that are closer to sharp than a radius you may get a more accurate flat pattern measuring your inside dimensions because like Matthew said most materials will stretch more than they compress which moves your neutral axis towards the inside of the bend. 

 

If you have access to the machine that made the bends or can ask for a sample for bend calculations have a 4" square bent at 90deg in half, measure the out side of each leg and subtract 4 from the sum of the two sides to get your bend deduction for one bend.

 

Using the tooling that I use for .25" material 90 degree bend deductions are :

 

Steel - .431

Stainless - .445

Aluminum - .402

 

So if you have a 2" x 2" outside dimension angle your flat will be:

 

Steel - 3.569

Stainless - 3.555

Aluminum - 3.598

Great topic, i love reading responses from the experts that do this type of work on a regular. Sometimes things that we think are so easy can be much more complex than meets the eye.

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