Capillary Action
Mon, May 16, 2022 
David R. Ball photo
Capillary Action is one of those laws of physics that most people know exists but don’t think about too much. It is the reason a paper towel or a sponge will soak up water. It is the reason a wick in an oil lamp draws the oil upwards, defying gravity, to the flame where it burns.
However, it is not just soft fibrous materials that have these properties, a clay flower pot or a clay brick will soak up moisture because it is porous. It occurs in any situation where there are tiny gaps or fissures between otherwise solid materials, any liquid will be naturally drawn through that gap.
When a framebuilder is brazing a lugged joint on a bicycle frame, it is capillary action that draws the liquid molten brass through the gap between the tube and the lug. If the tubes are mitered correctly, in other words the end of one tube is cut to precisely fit the curvature of the tube it butts up against, it too leaves another tiny gap between the two separate pieces of metal.
The molten brass will also be drawn between this gap to actually braze one tube to the other inside the lug. The final result is a strong joint, but one that spreads the stresses over an area.
When joining metal, in this case steel tubing, there is a need for the tubing and the finished joint to be of somewhat equal strength. If the joint is much stronger than the tube, the tube may fail adjacent to the joint. Conversely, if the joint is weaker than the tubes, the joint will fail.
The above picture is me brazing the main triangle of a frame together. In particular, I am brazing the head lugs. I am using an oxy-acetylene torch as my heat source. I used a fairy small but very hot flame, which allowed me to pin-point the heat where it was needed.
Metal when heated becomes red hot. A dark cherry red first. This is the temperature silver solder melts. I used brass for all brazing of the main joints so this melted at an orange red color. Temperature was controlled by constantly moving the torch on and off the part I was working on, and the color of the hot metal was my temperature guide.
In my right hand is the brazing torch, and a small hammer. I am not actually using the hammer, although at first glance it might appear that I am. It is just there in readiness should I need it. In my left hand is the brass filler rod. I am heating the top head lug and the top tube, and when it reaches the desired temperature, I feed in the brass.
As I feed in the brass, I watch for it to flow through the lug to appear on the head tube. You will notice the head tube extends beyond the head lugs by an inch or so. When the lug is full of brass, (I know it is because I saw the brass flow from one side to the other.) I flow out all the lug edges and any surplus brass is flowed out on the head tube where it extends beyond the lug.
This will later be cut off as scrap. Working in this fashion there is very little excess brass to clean up after. There is a similar situation at the seat lug where the seat tube is left sticking through the seat lug, to be cut off later. Again, excess brass is flowed out onto the scrap portion.
Brazing different thicknesses of steel together can create a problem. For example, the front and rear drop outs. These are of course much thicker than the tube it is slotted into, and if you come in with the flame at the point you need to braze, the tube will almost immediately glow red hot, whereas the drop out itself is still relatively cold.
The trick is to heat the dropout away from the tube, wait for it to turn red, then move towards the tube, which will quickly glow red to match the pre-heated drop out. When the first drop of brass melts and forms a bridge across the two separate parts, it magically becomes one piece of metal and all glows evenly at the desired orange-red color.
One cannot see how much brass is flowing inside the tube where the tang of the dropout reaches beyond the slot, unlike the lugged joint where the framebuilder can see the brass flow from one edge of a lug to the other. However, the builder gets a feel for how much filler rod is going into the joint to know whether it is full or not.
Finally, what is that little hammer for? Well, lugs usually come in standard angles of 73 degrees. But not every frame I built was those exact same angles. So when I assembled the frame and pulled the lugs to the desired angle it left a little gap on one side.
As soon as the lug was heated it relieved any stress, but there was still a little gap to contend with. A quick switch of hands, moving the brazing torch to my left hand, keeping the joint heated and a quick tap-tap with the little hammer in my right hand closed the gap in the lug. Then switching back to as I was, I continued brazing. No loss of heat or time while I searched for my little hammer.
A little refined blacksmithing if you like.
Reader Comments (3)
Had a chance years ago to braze a copper pipe system for ultra-pure gases.
Brazing Cu was a pain because it conducts the heat away so fast.
Large rosette touch tips and a lot of motion.
We actually measured the filler out and placed a ring of it at the joint so that we got even fill.
Only ever brazed a frame once, doing some repairs.
I have gotten watch a few builders working.
Real technique, and each a bit different.
Respect
Dave, you didn't mention the flux.
Love the details! I was wondering how angles were accommodated!
Not sure what "flow out" results look like. Very thin residue on the tube?
For clarity, the brass is added at one side of the lug, head tube, and it shows up on the other side, down tube or top tube? That would be a challenge to keeping the whole lug and tubes at the same temp. How fast does that happen?