Capillary Action
Tue, September 19, 2017 
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.
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Reader Comments (6)
Not Maxwell's Silver Hammer, rather Moulton's Brass Hammer :-)
You make a difficult process appear simple. Your ability to produce good results came as a result of hundreds of hours of practice. In one of my earliest jobs, I was required to make thousands of soldered connections of telephone wiring. The process is similar to brazing but at a much lower temperature. If done well, you use a minimal amount of joining material, but the trick is to bring both parts to the same temperature at the same time. With what I was doing, it was a small wire and a larger terminal. For you it was a heavy lug and a lightweight tube. The process is the same. You downplay the skill involved
I often wondered how a 74 head angle (or a 74 or 72 seat tube angle) was done.
Here to learn, thanks for this. On a similar topic, when I was an apprentice HGV mechanic, the mechanics would carry out pre-MOT inspections and use a small hammer to see if the bolts were loose. I asked "how do you know?" and they would tease me and say "can't you hear the difference? Listen to this, now this". I never could. Actually they were looking to see if the bolt or nut moved when they hit it. Eventually they told me.
Peter,
That reminds me of when I was an Apprentice Engineer, a coworker sent me to the company tool store to ask for a Long Weight.
I was told to stand off to one side, and after about 30 minutes I asked, "Where's my Long Weight.?"
The guy looked at his watch and said, "You just had one." I realized I'd had a Long Wait." When I got back everyone was smiling and asking of I got my long weight."
I later got back at the guy who sent me on this errand. That evening after he had left I arc welded the exposed steel toecaps on his work boots to his work bench.
Dave
A fascinating and insightful post. Thank you Dave.
It's worth mentioning that the phenomenon of capillary action is also crucial to the transport of water and nutrients within plants.