Preventing Conical Bend Cracks

[lsissom]

I'm wondering if by slightly enlarging the relief hole .... say 1/8" ... If I will lessen the likelyhood of the F-770 developing a crack while bending? I think I read about this somewhere but not sure. I deburred the area per Mike Bullocks posts, but his cracked anyway and I was hoping to be more sucessfull. Thanks

 

[bruceh]

One side on mine cracked, the other didn't. I would say it is probably more technique than anything. Be gentle and don't force anything!

 

[AlphaCharlieBravo]

I opened mine up to 5/32 which is the diameter of a chain saw file I bought in the Aviation Dept of HD. Also put my skin on the outside and I am pleased with the results. Bend it once. Do NOT bend and then unbend and try again.I clamped my back riveting plate down onto the skin on my shop table.
You want the forward 90 degree bend to be perfectly centered with the relief hole. Your rivet holes will then line up perfectly. Hope this helps you. Have fun.

 

[roee]

Yes, a larger relief hole will reduce the stress concentration, will be more forgiving of slight misalignment of the bend, and should lessen the likelihood of cracking.

Also make sure the relief hole is nice and clean, no burrs, no tool marks.

And do your best to do the bend accurately, and in one shot. Don't work it back and forth, as that would cause the aluminum to work-harden, which would increase the likelihood of cracking.

P.S. Both sides on mine already had cracks as delivered from the factory (quickbuild). But thankfully the cracks were still very small. So rather than stop-drilling them, I just enlarged the relief holes enough to completely eliminate the existing cracks. Hoping that'll also prevent new cracks from forming.