Internal Top Tube Cable Guide

I decided to try making the next few frames with internal cable guides for the rear brake.  I decided to use some titanium tubing to guide the brake casing the entire length for ease of insertion etc.  I chose titanium because it's light and has a similar coefficient of thermal expansion to carbon composite.

Here is the rather cobbled fixture I used to cut the 20deg entrance holes.  That's a carbide burr to cut through the tubing cleanly (and not get dull).  Once I have my little Garvin horizontal mill set up this will be much easier and less nerve-wracking.

I ended up bending the 1/4" titanium tubing around my 6" rotary table diameter by hand.  A parallel filled the gap nicely and I bent the tube with some cable casing inside.  There was no collapsing of the walls at this bend radius.

It's hard to see but the tube fit quite nicely down the center.  Next time I'll cut the holes 1/2" closer to the ends of the tubes so I can actually reach in with my fingers.  Fishing it through was interesting but running casing through helped guide the tube into place.

When ground and sanded flush the casing should exit at a good angle for the rear brake.

The tube was quite snug as is but a little 3M DP 420 on the outside and some on the inside should hold everything in place. 

I'll let this dry, finish it smooth with the OD of the top tube then everything is ready to bond in the jig.