Sheet metal bending formula

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Dave Taylor
This is strictly hypothetical; has nothing to do with any specific project or shop work at all.
I am looking at the bend allowance formula, and wonder if anyone has a good reference to it.
copy and pasted: Bend Allowance = 2(pie) (R + 1/2T) ÷ 4

in the format I am viewing it, the 1/2T could be read as "one over two-T" but I doubt that is intended.

So, (2xπ) x(R+(1/2xT))...the product of which is divided by 4.
R=desired bend radius
T=material thickness

So I want to verify the formula is correct, that I am reading it right.
 
Look up bend allowance formula. K is missing from yours.
 
It is all in the AC 43.13
 
If you want a bend calculator there are several out there. Here is one:
http://sheetmetal.me/formulas-and-functions/bend-allowance/

I like the website, thanks.
I also did googling before posting but did not find a formula the same as the one I posted.
On the .me site you link for example it offers:
(This is a lot different from the one I was reading and I don't know why. )
latex.php
 
(This is a lot different from the one I was reading and I don't know why. )
It's an engineering calculator but gives the same result. However, I mainly used the references/formulas found in the Standard Aircraft Handbook for bend allowance--Chapter 6 in my 4th Edition copy.

Other formula references:
AC65-15A (old handbook version) Chapter 5:
http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_65-15A.pdf

FAA-H-8083-31A, Vol 1 & 2 (new handbook version) Chapter 4:
https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/amt_airframe_hb_vol_1.pdf
https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/media/amt_airframe_hb_vol_2.pdf
 
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Please show me that formula above, I cannot locate it. Thank you.
AC 43.13-1B Table 4-6
or
FAA-H-8083-31 Vol. 1 Figure 4-125, go up for hardness and thickness

The subject starts at Para 4-56 in the -1b page 4-13. (setback)
 
Funny you should say that Jack; because 43.13-1B is exactly the first place I looked and unless I am losing my near vision, no where on paras 4-55. to 4-56. do I see a similar formula. (pages 4-13 and 4-14, aka pdf pages 153, 154)
Mystery.
My apologies- I'm pointing to the same table as @Tom-D :redface:
Although useful information, it certainly isn't what you were asking for- an equation.
 
Thanks- I am learning tons about bending sheet aluminum.
I have to wonder if those doing it regularly know generally what the setback and radius should be or if they do calculations every time. Also, does the calculated setback result in a perfect lengths or do they add a smidge to cover losses that normally occur in practical bending.
 
Thanks- I am learning tons about bending sheet aluminum.
I have to wonder if those doing it regularly know generally what the setback and radius should be or if they do calculations every time. Also, does the calculated setback result in a perfect lengths or do they add a smidge to cover losses that normally occur in practical bending.
Me?? It's a TLAR
In normal Cessna it is seldom used, bend it, trim it to fit, rivet in placed, paint to match.
That was a joke, (before some one thought it was a real way)

actually there are very few panels on a single engine Cessna that needs to be bent. (made at the scene.)
You need a spar? get one from salvage. It's better than taking responsibility for building parts.

It's not that we can't, we simply don't.
 
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if they do calculations every time.
Most fulltime metal benders I've seen have shortcuts vs using the formula. But they will calculate on certain jobs. Some general tips:
--Pre-fab SSteel nose radius strips that slip over the brake fingers and provide a repeatable bend.
--Use same thickness metal as gauge to adjust fingers back from brake table edge.
--Never bend with the metal grain; perpendicular best. If bending on 4 edges layout lines 45 degrees to grain so no edge is parallel to grain
--Break (bevel) all metal edges before bending. The smoother the better.
--Practice, practice, practice.
 
--Practice, practice, practice.

Good advice there. I don't do a lot of sheetmetal work consistently, it seems to come in spurts. I like to stop and review my hand book before I start.
 
Thanks- I am learning tons about bending sheet aluminum.
I have to wonder if those doing it regularly know generally what the setback and radius should be or if they do calculations every time. Also, does the calculated setback result in a perfect lengths or do they add a smidge to cover losses that normally occur in practical bending.
Many years ago I wrote CNC programs for a CNC turret punch press for sheet metal components. All of the holes and cutouts were programmed with the sheet in the flat - before bending. I had a bend allowance chart developed for common bend angles for each gage of sheet metal. There might be 3 or four bends across a panel but the subtracted bend allowance was very accurate. As mentioned - practice is the key.
 
My biggest problem is getting a joggle to lay right. almost impossible with my little 3 way machine.
 
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