Friday, January 23, 2009

Working with Ti...a trick to make bending easier.

As I've been working on Luvs this week, I thought I'd post some insight on working with Ti for the folks out there.

Often designs force materials to bend and move in ways that are not in the best interest of the material. Ti especially, does not like to be manipulated into sharper oval shapes or bends. Often, you will see fracture failures at the peak of the oval, most commonly experienced at the top or bottom of chainstays where significant swaging has taken place to allow for tire clearance.

Gradual, swoopy bends, often seen in bikes like James's Black sheep or Jeff's Space frame designs, place less stress on the material and are easier to control.

So, what's the trick to making these higher stress bends successful? I did a little demo to show ya...

In the case of the Luvs, I have to reduce the od of the center section (1.00" tubing) to about .785" to meet the .875" grip section, providing efficient surface area without overlapping.
As we mentioned, the swaging process causes a lot of stress on the peaks of the ovalization, which can cause fracture failures in the material.

Here's a piece set up in the swaging fixture...
Using only mechanical force, about 2 out of 10 pieces will have a crack develope from the edge of the piece. Below is a pic... standard 1.00" piece in the center, cracked swaged piece on the left, proper piece on the right.

A close up of the fractured piece...can you see the crack developing?

In order to achieve 100% process success, we use a little trick shared by Sandvik back in the day. Using a standard propane torch, gently heat the piece until a gentle straw color develops (about 300 degrees) and then apply the mechanical force. Here's a shot of a heated and swaged piece below. Note the barely perceptible yellowing on the tube.
This process will not affect the purity or durability of the Ti if the heat control is focused. The reason I choose to use the propane torch is to avoid the potential for accidental over heating, possible with a standard oxy set up. Always better to err on the side of safety if possible.
So, cheap propane is your friend ;)
This process works equally well on stays or any other manipulated Ti piece.
Remember, working with Ti has a steep learning curve and you need to be willing to waste some time and money on material while you learn the tricks of the trade. Hopefully, this tip will save you some of that.

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