# Sheet metal bending formula

Discussion in 'Maintenance Bay' started by Let'sgoflying!, Dec 7, 2018 at 9:48 PM.

1. ### Let'sgoflying!Touchdown! Greaser!

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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.
T=material thickness

So I want to verify the formula is correct, that I am reading it right.

2. ### CraigPre-takeoff checklist

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Look up bend allowance formula. K is missing from yours.

3. ### Tom-DTaxi to Parking

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It is all in the AC 43.13

4. ### Cap'n JackFinal Approach

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5. ### Bell206Line Up and Wait

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6. ### Let'sgoflying!Touchdown! Greaser!

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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. )
$[​IMG]$

7. ### Let'sgoflying!Touchdown! Greaser!

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8. ### Let'sgoflying!Touchdown! Greaser!

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Please show me that formula above, I cannot locate it. Thank you.

9. ### Let'sgoflying!Touchdown! Greaser!

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K is only for non-90° bends, Craig.

10. ### Bell206Line Up and Wait

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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:

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

Last edited: Dec 8, 2018 at 9:51 AM
11. ### JAWSLine Up and Wait

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12. ### Tom-DTaxi to Parking

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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)

13. ### airdalePattern Altitude

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14. ### weirdjimEn-Route

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15. ### Cap'n JackFinal Approach

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My apologies- I'm pointing to the same table as @Tom-D
Although useful information, it certainly isn't what you were asking for- an equation.

16. ### Tom-DTaxi to Parking

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17. ### Let'sgoflying!Touchdown! Greaser!

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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.

18. ### Tom-DTaxi to Parking

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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.

Last edited: Dec 8, 2018 at 10:58 PM
19. ### Tom-DTaxi to Parking

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Yeah, I didn't understand what she wanted too.
hope my last link solved that.

20. ### Bell206Line Up and Wait

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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.

21. ### JAWSLine Up and Wait

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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.

22. ### Jim_CAKPre-takeoff checklist

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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.

23. ### Tom-DTaxi to Parking

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My biggest problem is getting a joggle to lay right. almost impossible with my little 3 way machine.