Part Three
Brazing : Finishing the Front Triangle : Assembling the Rear Triangle : Adding the Braze-Ons

Proceed to Part 4 - Final Machining and Alignment

With this installment we finally get into what many folks might consider to be “framebuilding.” Up to this point all the work has been preparation for the big moment when all the mitered tubes, assembled chain and seat stays, and shaped lugs are assembled into something that for the first time looks like a bike frame. For this reason this set of steps is my favorite.

When many folks think of framebuilding they think of brazing and while brazing is an important step it’s just one step in the process. It’s surprising to many people that brazing isn’t the most time consuming part of the process. An average build may take something like 15 – 20 hours of hands-on time at the bench but brazing only accounts for about 3 hours of this time in total. The rest of the time is spent doing the little things preparing for the brazing or to clean up after the brazing. So I feel it’s safe to say that being a framebuilder is all about the little things and there are countless little things that need to be attended to so that I can be sure the big brazing step goes as it should.

Preparation for Brazing

Now onto describing all that happens here. With the jig set to the proper lengths and angles for this particular frame the tubes and lugs are loaded into it dry to be sure that everything fits as it should. With the fit checked I move onto cleaning the tubes and lugs. The cleaning of the lugs is simple enough and involves soaking them in acetone. After a few minutes they are set aside and I move on to the tubes. The tubes also get cleaned in acetone and then they get sanded lightly with scotchbrite to get them as clean as possible and to give the silver brazing material more of a tooth to bite into.

With everything clean the tubes and lugs are covered with a white paste brazing flux. The flux is an acid and when heated cleans the tube very well allowing the silver to bond properly to it, and it also completely coats the lugs and tubes to be sure that they are not exposed to oxygen during the brazing process. In addition the flux also acts as a temperature indicator and when the flux it heated to the right temperature for the silver to flow it gets very thin, clear and glassy.

With the fluxed tubes and lugs loaded into the jig it’s time to tack them. Tacking is simply a small amount of braze that is applied to the lug points to hold the entire front triangle together so it can be removed from the jig. Each lug will be tacked in at least four places before the triangle is removed from the jig.

Lugs and tubes refit to the jig, covered with flux and ready to tack

The seat lug after being tacked. Tacking is a small amount of braze that is used the hold the joint together to allow final brazing outside the jig. You can see the silver at the tips of the lug.

Bottom bracket after tacking

Brazing

Next I reapply flux (no such thing as too much flux) and break out the torch for the final brazing of the frame. I braze the frame outside the jig so that it is free to move as heat is applied. This keeps the tubes from undergoing the huge amounts of stress they would see if heated and not allowed to expand.

The brazing of each lug is done in an order that allows the frame to move around as it needs to but in the end, when cool, end up straight. It took a very long time to develop my routine and I don’t share it with anyone. The photos show the lugs brazed and covered with dried flux. After the frame is cool to the touch the flux is soaked off the frame in very hot water.

Finishing the Front Triangle

After the flux is soaked off the finish work begins. It’s important to me that the bike looks good and clean before it goes to the paint shop because the easier I make it for the painter the better it will look in the end. So I spend a good bit of time doing finish work. I have a few particular techniques that I’ve developed over the years that allow me to get things clean and smooth and these are techniques I don’t share. One of the things I do not do at this stage is sandblast the frame. Many builders will shorten the amount of time they need to do the finish work by sandblasting the lugs until they are clean. It is a very fast and easy way to do it but it can also thin and weaken the tube and when we are talking about tubes that are only .020” thick sandblasting is not a good way to go.

Once the lugs are clean it’s time to do some machining and the most important machining is the facing of the bottom bracket shell. Having perfectly parallel and concentric bottom bracket faces is the only way to get a good alignment of the frame since the faces are used as a reference for the alignment. I machine them so that they are within .001” of parallel to each other. Once the faces are parallel the front triangle is bolted onto the ground flat surface plate to check its alignment. Measurements are taken using a height gage from the frame to the plate to be sure that the frame’s centerline is parallel to the plate. Only after this is done can I be sure that when the rear end of the frame is added that the whole thing will come out straight.

Assembling the Rear Triangle

With the front end of the frame cleaned, machined and aligned I can move onto the tricky and critical step of the assembling the rear triangle. The alignment of the front triangle is simple in comparison to the rear since the rear alignment controls the position of the rear wheel in all three dimensions. With this in mind extra care is taken to be absolutely sure that everything fits perfectly into the jig. This requires precise mitering of the chain and seat stays and takes a good bit of time to get just right. There is also an aesthetic concern at this point because the full wrap seat stay caps need to be symmetrical and positioned exactly so to be a mirror image of each other. I do this by eye, taking my time so that things line up just so.

Once the stays are all cut and positioned, they are fluxed and prepared for tacking just like the front triangle was. After tacking the rear triangle to the front, the frame is removed from the jig to be brazed freely to let things move as they need to. Before brazing the rear seat stay bridge (or brake bridge on a standard road bike) is mitered and brazed into place. This completes the triangle and stabilizes the rear end during the brazing process so that nothing can shift causing a rear wheel misalignment.

Checking the fit of the seat stay caps, making sure they wrap symmetrically around the seat lug

Adding the Braze-Ons

The last step in this installment is the adding of the various braze-ons. The braze-ons are added at this point so the flux can be soaked off at the same time that the flux is removed from the rear end brazing.

Spilt cable guide freshly brazed to underside of seat stay.
The magnet is used to hold the braze-on in place while brazing

Cable adjuster brazed into brake stud