Friday, March 9, 2012

Fillet Brazed - Welded or Lugged Frame?

 I always wanted to know why some frame makers use one or another technique to put bike tubing together.
Well, as I have found, no individual technique is necessarily better than another. It all depends on the needs.

The frame shown here is brazed. The American Welding Society (AWS ), defines Fillet Brazing as a group of joining processes that produce coalescence of materials by heating them to the brazing temperature and by using a filler metal (solder) having a liquidus above 840°F (450°C), and below the solidus of the base metals.

TIG Welding or Tungsten Inert Gas welding. A form of welding by the use of an electric arc. The area being heated is bathed in an inert gas (argon?) to prevent oxidation. T.I.G. welding is commonly used to build lugless bicycle frames. Most current bicycle frame production is done by T.I.G. welding.
Lugged: A lug is a socket that forms the junction between two or more frame tubes. Traditional bicycle construction uses steel tubes and lugs, joined together by brazing or silver soldering so that the space between the tube and the lug fills up with molten brass or silver alloy. Some aluminum or carbon fiber bicycles also use lugs, with glue instead of the brass or silver.

"Fake" Lugs:  Also called bi-laminate. These "lugs" are created by hand as the bike was fillet brazed. The frame is intentionally given the appearance of a lugged frame, but is produced with by a much more time consuming process. The pictures (left) from Gallus Cycles in Texas was made with bi-laminate, but by wrapping a sleeve around the tubing to then cut out the shapes once the pieces are brazed. It doesn't seem to add any strength, but it certainly looks smooth.
Bi-lam originates from post-war Britain and was pioneered by Claude Butler. It started primarily due to the lack of materials after the war. Originally the “lugs” would start out as a flat sheet of metal with a design cut out of it. After the frame was fillet brazed, the sheet would be wrapped around the tube and brazed in as well. It the gives the impression of a lug.
Now, you might be wondering, how strong are these frames and why would frame builders choose to use one or the other. I have tried to compile some definition of all three techniques to give an idea.
 Basically, you can use any of the methods. It might depend on preference and the look that you are trying to obtain. This last picture shows a forced break weld. Notice how the tube breaks above the joint. Wow...hard core stuff.


Brazing vs. Welding

Welding joins metals by melting and fusing them together, usually with the addition of a welding filler metal. The joints produced are strong, usually as strong as the metals joined or even stronger. In order to fuse the metals, a concentrated heat is applied directly to the joint area. This heat is high temperature. It must be - in order to melt the "base" metals (the metals being joined) and the filler metals as well.
So welding temperatures start at the melting point of the base metals. Because welding heat is intense, it is impractical to apply it uniformly over a broad area. Welding heat is typically localized, pinpointed heat. This has its advantages. For example, if you want to join two small strips of metal at a single point, an electrical resistance welding setup is very practical.

Taipei International Bicycle Show 2011

This is a fast, economical way to make strong, permanent joints by the hundreds and thousands. However, if the joint is linear, rather than pinpointed, problems arise. The localized heat of welding tends to become a disadvantage. For example, suppose you want to butt-weld two pieces of metal - start by beveling the edges of the metal pieces to allow room for the welding filler metal. Then weld, first heating one end of the joint area to melting temperature, then slowly traveling the heat along the joint line, depositing filler metal in synchronization with the heat. This is a typical conventional welding operation. Let's look at its characteristics.
It offers one big plus - strength. Properly made, the welded joint is at least as strong as the metals joined. But there are minuses to consider. The joints are made at high temperatures, high enough to melt both base metals and filler metal. High temperatures can cause problems, such as possible distortion and warping of the base metals or stresses around the weld area. These dangers are minimal when the metals being joined are thick. But they may become problems when the base metals are thin sections. High temperatures are expensive as well since heat is energy, and energy costs money. The more heat you need to make the joint, the more the joint will cost to produce. Now consider the automated process.
What happens when you join not one assembly, but hundreds or thousands of assemblies. Welding, by its nature, presents problems in automation. We know that a resistance weld joint made at a single point is relatively easy to automate. But once the point becomes a line - a linear joint - the line has to be traced. It's possible to automate this tracing operation, moving the joint line, for example, past a heating station and feeding filler wire automatically from big spools. But this is a complex and exacting setup, warranted only when you have large production runs of identical parts.
Of course, welding techniques continually improve. You can weld on a production basis by electron beam, capacitor discharge, friction and other methods. But these sophisticated processes usually call for specialized and expensive equipment and complex, time consuming setups. They're seldom practical for shorter production runs, changes in assembly configuration or - in short - typical day-to-day metal joining requirements.
Welding vs. Brazing considerations
1. Size of assembly?
2. Thickness of base metal sections?
3. Spot or line joint?
4. Metals being joined?
5. Final assembly quantity needed?


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