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5.5 Frame Building Part 2: Brazing




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This article is from the Misc Bicycles FAQs, by various authors.

5.5 Frame Building Part 2: Brazing


I will do my best to describe this brazing process, but I'm still pretty
hazy on how it's actually done. I can tell you some of what Dave told
us, but I could not put all the pieces together to actually do a braze
myself.

To braze the frame once it was taken out of the jig, it was put into a
normal bike workstand where the frame could easily be moved around to
get it into different positions. Normally Dave would do the bottom
bracket area first to secure it and the rest of the frame from moving,
but the bottom bracket is also the easiest to do and I would be able to
do some of that. Instead he started with the head tube area and the two
lugs there, then the junction of the top tube/seat tube and finally the
bottom bracket.

Compared with the brazing of the dropouts and blades/stays, very little
brass is used to braze the tubes together. There is lots of room
between the dropout and the blade/stay that need to be filled, where
with a tube/lug combination there isn't much room between them,
micrometers. The basic process of brazing is the same - gently heat up
the area you want to braze. In this case, concentrate on the lug since
it's thicker and not the tube doesn't get deformed. Once the area is
hot enough, apply more direct heat to the area you want to braze, apply
some brass and then move the brass around to where you want it to go.
(Sounds simple, doesn't it.) Here's a bit more detail.

I'll use the lug and junction between the head tube and top tube. He
started with the frame basically vertically oriented and applied heat to
the top of the lug behind the head tube. Once hot enough (I could never
quite figure out how hot hot enough was except that I know when it
turned orange that was too much.), he applied brass to the back of the
head tube where he had just heated. He applied brass until he could
just see if start to come out the bottom of the lug on the front. You
can tell because the brass is shiny from being molted as compared with
the dullness of the steel. With the brazing goggles on, I wasn't always
able to see this. With enough brass in between the lug and the tube, he
started using the torch to heat the tube where he wanted brass to move
to and then going to where the extra brass was and moving to the area he
had just heated. If you remember that cross-hatching done earlier with
sandpaper, this is where is it was important because that's how the
brass moved through between the tubes. Dave would constantly move the
frame about so that he could use gravity - apply the brass on the top
and watch until it flowed out the bottom and then move the brass around
again. He did this with all the lugs.

I did get to do the bottom bracket though. All the brass was applied
from inside the shell, this made for less cleanup on the outside. I was
able to understand a little of what Dave was talking about by actually
doing it, but I still didn't have it all correct.

In some areas Dave would turn start from the bottom. He did so because
he didn't want too much brass ending up on the other side of the lug.
So he was working against gravity?! But the capillary action caused by
the cross-hatched scratches on the tubing combined with the force of
surface tension of the brass would cause the brass to move upwards
between the lug and tube! And gravity would prevent too much of it from
coming out the top. This is a little bit of applied physics. Gravity
is a very weak force compared with the capillary action and surface
tension, but it was just enough to make this work!

Before leaving each lug, Dave would do as much cleanup as possible
around the lug so that less would have to be done with files once the
brazing was done. Remember that brass that flowed out the `bottom', if
that wasn't smoothed out, it would require files and sandpaper to remove
it. To clean that up, Dave would melt the lumps of the brass with the
torch and move it back under the lug. Additionally he would go around
each edge of each lug, applying heat to the edge of the lug, but not the
tube and smooth it out getting rid of any lumps there. Indeed once the
frame was cooled off, the edges of the lugs remained very defined with
only the thinnest layer of brass showing.

Once the brazing it done, the frame is put into a tank of water. This
is to remove the hardened flux since the flux is water soluble.
The brazing of the fork is done much the same way, except that it is all
done upside down - and that's where Dave was using the upside-down
technique that I described above most. He would apply brass to the seam
between the crown lug and blade and the brass would move upwards. He
also did it this way so that any extra brass would not drip down into
the blade itself. You could possibly end up with a small ball of brass
constantly rattling in your fork. The brazing of the steerer tube to
the crown was done from what would normally be the bottom of the steerer
tube, but since it was upside down, it made it very easy to get brass in
between the crown and steerer tube to braze. One other detail about
brazing the fork - Dave used a different torch head called a rosebud
which produced higher temperatures. The fork crown is much thicker so
needed much more heat.

Brazing vs. Welding

I asked what the difference between brazing and welding was. Here is
it. With brazing you are heated two surfaces and using a third material
with a lower melting point to join them. With welding you are using a
third material with a very similar melting point as the two being
joined. Brass has a much lower melting point that steel and is a easy
material to work with. Silver can also be used for brazing, but
requires more exacting control of temperature since silver isn't as easy
to work with and doesn't have the range of temperatures where it flows
smoothly as brass does. One other thing I should mention about this
brass. The brass is a rod about 4 feet long and about the diameter of
pencil lead. Dave tends to hold it about size inches from the end near
the torch. As the brass melts of the end once in the flame you move the
brass rod in closer. Surprisingly you can get in about as close as
three or four inches without a problem. The heat of the flame is very
concentrated.


 

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