Tuesday, August 3, 2010

Chad's Ti frame...front triangle

When we last left off, we had just got the seat tube to the point of being ready to weld the insert in place.  I layed down a fusion pass that draws a bit of material from both pieces to join them together, then a second filler pass that was a bit heavier in the center rather than feathering in the edge.  I set it up like this so that I can then skim pass the positive overage leaving a seamless, visually pleasing seat tube.  Here we are with two seat tubes all ready for the next step; the binder bolt.

I've got a pretty good stash of 6/4 Ti rod that I use for fabricating binders.  Yeah, I know, Paragon sells them cheap enough, but heck, I enjoy the task of making them on the lathe.  That's why I do this, right?

After some enjoyable lathe time...

The binder is important for my build process as I use it to orient my seat tube and keep all the processes in phase...keeping the bottom bracket miter square, aligning for the water bottle boss holes, etc...  Tacked on and ready for the next step.

...cutting in the bottom bracket miter.  Here you can see that I use the seat tube binder to square up to the face of the miter system to insure that the bottom bracket miter is 90 degrees to my binder and angled top cut.
and checking it in the fixture for position, final length, and oveall good vibes :)
As involved as the seat tube is, the next step takes a lot of time for a couple of reasons we'll discuss.  The down tube is normally pretty straightforward, but I wanted to utilize a larger downtube and manipulate it to create a stronger, more energy efficient frame for Chad's physical size and power.  I started with a 1.75 x .032 straight gauge round tube, cut it to a rough length, and then swaged it at the head tube vertically and at the bottom tube horizontally.  The trick here is to...

1.)  keep the ovalized sections in phase, 90 degrees to each other
2.)  swage the ti material to the approriate final size (remember the dreaded springback) without creating a peak or a subsequent crack.
3.) get through the now almost 2.5" inch long sections of thin tubing without destroying the tube...yikes!

The downtube took almost 4.5 hours, without taking time for pics. Why so long?  Well, the swaging is not difficult, but does take time to do it right the first time.  To achieve the final width I needed over the 12" tube length, the end of the tube almost closed off under the squeeze pressure. With the tube bi-axially ovalized, I then had to address how to miter it. Unlike Aluminum or steel, Ti will tend to pull and tear rather than cut smoothly, often dragging the side wall of the tube in with the direction of the cutter.  To prevent this, I broke out the jigsaw and the sander and created internal wood plugs that matched the internal shape of the tube.  This wooden plug allows me to miter the tube end in a supported fashion.  It takes more time, but it saves some serious cursing and frustration if the tube gets destroyed by a wandering hole saw tooth.

A nice looong oval to round profile...
The basic bottom bracket miter...still have to cut in the seat tube intersection miter.
Setting up for the long head tube miter...
checking the fit at the headtube...
With the downtube in place, let's move on to the top tube.  We'll start by parting it off to it's rough length...

And cut in both ends to match the angles of the head tube and seat tube.  Because this is just a happy nice round tube, no special accommodations are necessary to cut it, just slow and even pressure.
Looking like a triangle...
Next up, I looked up the specs for the Dura Ace crank, front derailleur, and then used the data to lay out the water bottles to insure I had enough space for all the equipment and that it matched my original numbers.  Once satisfied, I drilled all the water bottle holes to spec, 2.5" apart...

With the bottle boss insert in place...fits well :)
Final step for the main triangle is to drill all the purge holes, placing the holes as close to the superior margin of the internal tube diameter as possible to allow the argon, which is heavier than the ambient air, to push as much oxygen out as possible during purging.  The exception is the seat tube top and top of the head tube, as those areas will be occluded partly by heat sinks.
In the morning, I'll clean everything up nice and tighty then focus on welding it up.

cheers,

rody

2 comments:

inclined said...

Woohoo!!! Now we're talking bike!

Rody said...

Yep Chad, fate has been conspiring against us but we're pushing forward :)

Thanks for the patience

r