Saturday, April 24, 2010

Fixed Top Tube, Seat Tube to Bottom Bracket

In a brazen display of bravado, I decided today to braze my first silver joint in two years—the very important seat tube/bottom bracket—without having first practiced. Such was my confidence in my brazing ("brazen'") skills. Alas, it didn't turn out all that well. But first, the top tube miter.

My TT/ST miter was off my precisely this much. I marked the TubeNotcher+ cutout (note to fellow users of this program, always add about 0.2mm to the tube diameter to make up for the width of the paper—I used 25.6 rather than 25.4 for this 1" TT and it worked well), roughed it in to that outline, and then did a visual inspection for light gaps.

It now more or less fits. There are tiny gaps, and pressing gently on any of the tube is sufficient to change the appearance of things dramatically, but I think it's now absolutely within the margin of error. (Remember Richard Sachs's motto: Imperfection is Perfection!)

The above lug is now very impatient to be brazed.

Speaking of impatience and brazing: here is my effort to braze my seat tube to the bottom bracket. (I asked Olivier to help me out rather than photograph things, so there are no pictures of the process.)

Let's begin with the good stuff. That shoreline is pretty nice! The little blob of silver on the tube and the shell will be very easy to file/sand away.

The shoreline is nice and the penetration is excellent here at the front side of the socket. (The little blob is a leftover from the tack; similarly easy to file off).

But now the bad stuff:

No matter how I tried, I couldn't draw the silver down between the chainstay sockets. Three quarters of the way around, the penetration is exemplary. But at the back here, not so. I did add a lot of silver, so it must be almost up to that lip—but it sure would be nice to know for sure. This is an important spot to get right—this page from Little Fish Bicycles immediately sprang to mind. I'm pretty much sure the penetration is sufficient. But that "pretty much" is certainly on my mind...

In this picture of the fluxed joint pre-brazing, you can see that not a lot of flux appears to have made it through to the "affected area." It was certainly clean. And I think it had the proper sloppy fit. So likely a flux issue...

Quoi faire?!

Update: The answer to the above question is, take it apart and see what happened! Doug thinks likely the only problem was that the very bottom of the tube got cooked, and that I didn't put flux there so I couldn't get the silver to flow out. But I may as well take it apart and see for myself... Should be fun!


ford said...

it could be that the tube/shell got a little cooked while you were trying to draw the silver down.

one thing that stuck with me in doug's class (as i'm sure we all had different 'take home' experiences) is that once the flux gets burnt, there's no more to be done to the joint until you add more flux, or stop and clean up the spot. i found myself inclined to really focus on spots where the silver wasn't flowing, but that would inevitably cook the flux and make all efforts to resolve the situation completely irrelevant. as soon as you've done a pass or two with the torch (or if the steel starts getting blackened/crusty), add more flux or else nothing will move!

anyway, really exciting progress! i got the original marinoni alignment table a few weeks ago, and am now starting to build a workshop for framebuilding in my backyard! yeehaw!


AH said...

It could certainly be that! I may have cooked the flux even before it had a chance to flow, since my main intention in brazing the BB was to get silver in precisely that spot. I might have added too much heat there and cooked things a bit... I did try to add some flux from the inside, without spectactular results.

Anyway, I've asked Doug himself to have a look and comment!

Wow, the original Marinoni alignment table! A backyard workshop! Exciting progress is being made throughout the great province of Ontario!