bending 7/8" / 22mm Ti chainstays

[johncaletti]

I've been making attempts at putting bends in 7/8" x 0.035" Ti tubing/chainstays (from Nova), and I have not been successful.
I machined a form using a 7/8" ball end mill that is about a 1/3 of a circle and probably around a 7" radius. I tried using it with an arbor press with the bottom of the tubes supported by blocks machined to 7/8, with the bottom of the blocks filed a bit so they can rock towards the bend as it happens. This method produced slight undulations in the tubing on the inside of the bend, even at a very slight bend.

I then tried setting up that form to bend the tube over, using a roller (7/8" rounded channel in the roller wheel) to roll the tube over the form - still wanted to put kinks in the tube.

I thought a larger radius would be better, as it would strain the tube less, and set up a new piece with a 9" radius, on my 8" vertical rotary table on the mill to cut in the 7/8" channel. Too much chatter and load, even with very shallow (0.005") depth of cut, and it has seemingly damaged the rotary table's main pivot, as it is sticky and hard to rotate. I have taken the rotary table apart to work on it, it was clean and oiled, so it seems that one of the main pivot parts is a little deformed.
I'm considering attacking this again once I get the rotary table back in order but with a piece of hard wood scrap rather than aluminum so it will be easier to machine. I've used delrin/plastic on my seat stay benders, but have no more.

I'd like to be able to get a mellow banana shape on road chainstays, and put S bends into mountain/cx chainstays.

If anyone has suggestions for me, I'd LOVE to hear them!
Thanks!

here's a couple pics - including the mangled chainstay put in place to illustrate how I was setting it up.
IMG_1302.jpgIMG_1303.jpg

[steve garro]

you will get some flattening with that rig. I have done a few hundred pairs with a similar rig. your "rockers" need to pivot on a round bit so not to fidget. of course there are "fillers" - sand, coil springs, elastomer stacks....... - Garro.

[EnginCycles]

DiAcro!!!!!!!!!!!!!!!!!!!!!

[Dave Kirk]

What level of heat treatment do the tubes have?

Dave

[VertigoCycles]

It used to be that Nova only sold annealed tubing.

John, can you post some pictures of your roller die setup so we might be able to offer ways to make it work mo betta?

One thing I know for certain is that, at the point where the bend is taking place, the tube needs to be fully encapsulated. If it's free to deform at all, it'll flatten out a little bit and the inside radius will ripple. That's what's happening with your arbor rig and my assumption is that it's happening with your roller rig too, possibly because the roller is ahead of the curve.

[johncaletti]

The tubing is annealed, from Nova.
I have not tried filling the tube yet. I don't want to use cerro bend - yuck - but I might have a spring for that purpose I could try.
The roller has a small gap - so not fully enclosed as you stated Sean. The ones I made for seat stays have a bit of a gap as well and they work pretty well, but the tubing is smaller OD and the radius is much larger.
I don't have pictures of the roller setup. I used the same form, you can see the tube clamp on it. I had it mounted to a steel base where the pivot was located, and a Delrin roller that went just above the form, long handle.

[johncaletti]

Drew- the DiAcro is definately nice. It's out of my budget though. I'd like to make something, but my machining skills and equipment don't seem quite up to the task - specifically creating a form with a large radius. I did get my rotary table smoothed out and back on the machine.

[VertigoCycles]

John,

the first time I made dies the vibration ate my rotary table too. Frozen solid. Good on your for fixing yours. FWIW, any time I do anything that requires heavy cuts or a lot of time on the rotary table, I fill it up, give it about ten full revolutions and keep one of the locks partially engaged so it's not as free to vibrate itself to death.

How deep is the groove on your die? It's impossible to tell from the pic, but it appears to be deeper than the radius of the tube. If that's the case, it'll be next to impossible to fully enclose the tube to keep it wrinkle free.

[Rody]

John,

If you want to bend thin ti, you've got to have two things going for ya...

an encapsulated tube and internal support.

Increase the diameter of your die (I've got 12" diameter), create lower supports that are directly below your center point with pivots lateral to the center so the middle "falls away" as pressure is applied, and get to know cerro bend. Yes it's nasty, Yes it sucks to work with, Yes it makes custom bending worth it.

Call me if you need more info,

rody

[bellman]

John, don't go the cerro bend route, I have and it's horrible stuff and it is not a sustainable process. You simply cannot clean the tubes well enough after being emptied. Plus, it has a life span.
if you over heat it during the melt, it will harden too hard the next time you use it and break inside the tube when you bend. About the only thing I've had luck with as a filler is soapy water, more specificly, like 4 parts water, 1 part soap. Mostly i've come to the conclusion that fillers are just a huge time sink.
What Sean said about the gap is going to be the key here for you, I've found that reducing the 'moment' to almost zero is the only way I can bend tubing wrinkle free
Thats a really nice piece of machine work you have there in that die, I can appreciate the time it must of took, I think you can build from that and make it into a roller set up that will work great.

[VertigoCycles]

Chiming in again to add more perspective

Cerrobend does exactly what it's supposed to do and can help you make some beautiful bends with no buckling, collapsing etc but that stuff is bad news bears. You could buy it used but you won't know what other crap is in it. It's expensive new and if you account for the time spent heating it, pouring it, letting it cool, melting it back out, cleaning up the mess, and then making sure your tubes are super effing clean, it won't take long before you realize that there are better methods available even if they have a higher up front cost.

[Archibald]

Imho:

[johncaletti]

First, thanks everyone for the ideas and info.
Due to the nastiness I am choosing not to use cerro bend.
Sean is right- the channel in the form I'm using is too deep and although not pictured, the roller was too high.
I'm pretty sure a bigger radius would be a move in the right direction, but I don't have the capacity to machine one.
I'm not sure I understood everything Rody had to say and so I'll follow up with him.
Don's Anvil bender pictured is probably the best way to go. The Anvil tooling I have works fantastically and is a work of art. Don gave me good advice when I made my seatstays benders.
So maybe if the cost of the tool seems high right now, I'll sink some more time and wreck some expensive tubing until I decide to buy one. :)
Don- are those benders available yet?
Thanks!