Request advice: Bending CroMo tubing

[MattyCiii]

Hello, I have two questions for those experienced in bending metal tube:

1) I have 1" x 0.5" rectangle tubing, wall thickness 0.035" on the way to use as a custom built rear triangle. I'd like to make the chain stay as a single piece, bent into a "U" shape rather than cut and weld. That would mean a rather aggressive 3" inside diameter bend to just under 180 degrees. Is this possible - or should I just re-design to cut and weld a squarish "U" shape?

2) If it's possible to do, any recommendations on how to do it? I've heard of people stopping into their local muffler shop and getting bends done, but if the equipment at such a place is fundamentally incompatible with what I'm trying to achieve, better to hear it here than to waste my time driving down the road...

Any advice appreciated, thanks.
Matt

[Drunkskunk]

If you bought it from Aircraft spruce, then you're trying the same thing with the same size tube I just tried.

And the results were not good.

It shattered.

someone else may know more, but from my experiance You have to mandrel bend square tube, and get it cherry red hot or it wants to collaps on it's self. So what I tried was a series of relief cuts along the inside and tried to bend it cold. I was using a vice and a form to try the first bend, and it shattered like glass.

[chisixer6]

look at Warren's bobber build... He got a tube bender from harbor freight that bends cromo pretty good. he's got avideo on it's use too.

viewtopic.php?f=3&t=33004

he has the tube bending a couple pages in, really neat

[MattyCiii]

Thanks!

[PaulD]

That's a tough bend to make. My race bike chainstays have a 3" radius bend that is only about 20 degrees. I made my own bending die for my bender (pretty much the same as the harbor freight model) which is fairly simple, just a 6" diameter, 1" tall cylinder with 7" diameter plates sandwiching it. You'll have to get the tube dull red in the bend area (big rosebud tip on an oxy acet torch) to prevent cracking (i'm really surprised drunkskunk had his shatter, normalized tubing would probably just crack on the outside).
The key thing that took a while to figure out, was that the bending die needed a feature on it right in the middle to push on the the inner wall of the tube to collapse it inwards prevent it from buckling. I'll take a picture next time i'm at my shop.

[MattyCiii]

Thanks Paul,
Sounds like it's something a bit over my head.

My original design was to put that "U" bend into the steel, then weld on two "legs" that then get welded to a piece of round tubing that is the pivot:


But I don't want to do anything that would weaken the steel, and I don't want my design to be in the "too hard to do" category and never get done/not get done right.
My current thinking is to put "S" curves into two lengths of steel like so, and connect the two together with a single welded piece.


As shown, I still use a 3" inside diameter radius on the curve, but I'm sure I can use something less aggressive like 4" or 5". Any thoughts on this design?

[Harold in CR]

Make the bent piece out of solid stock, then, weld it to the Cro-moly tubes, maybe

[MattyCiii]

Make the bent piece out of solid stock, then, weld it to the Cro-moly tubes, maybe

That's a great idea, thanks. Although I'd probably just cut and weld square cuts to the tubing at that point:


5" inside diameter "S" curves come out pretty good. Note the cross piece is also bent in this design:

[Thud]

Bending tight radius's in rectangluar or square tubing is a tough propsition.

When I need a radius that tight
I actualy slit the tube from an end to just past where you need the radius, heat it untill soft & pull the radius...then its stitch the tube back together on the slits. Its a little time consumong but it elliminates the whack deformation on the top & bottom sides of the square.
sounds like more work but the results are fantastic.

Hint: make the bends as smaller parts of the assembly & weld them together at assebly

[John in CR]

I like the S a lot better than relying on welds at the U for strength, and to me it looks better too. I think I'll copy it if you don't mind.

John

[amberwolf]

I was going to do something very similar, a U-shape of rectangular tubing, but realized that any method I could manage here would probably destroy the tubing at the bend, unless I were to cut triangular sections out of it's inner radius, and then reweld it together after the bend.

I decided to make it an H-design instead, as it is much simpler for me to build.

Your S-design would also be easy enough; I didn't think of trying that, but it should work much easier than a U-bend.

[MattyCiii]

Thanks guys.
I re-ran the numbers on the last design - the "S" curves - and it'll work well with 6" or 7" inside diameter curves too (Probably 8" ID curves but I have not checked that yet). Hopefully the inner weld where the cross piece is mounted is not too tight for my welder friend to accomplish.

The curves are 31deg one way then 28 reverse, not much in the grand scheme of things, so hopefully the shallower curve and the larger inner radius make this possible to do with my limited tools/skills.

[Harold in CR]

I hope you thought about putting the solid piece inside the tube a ways before welding. There are other good ideas, also.