I’m building a titanium frame for Ned right now and it made me think, some might be interested in learning about the infamous “double pass” weld. You hear the term kicked around a lot but what is it, really. Here is the scoop, the double pass (DP) weld is used primarily for titanium although I have seen it employed on some Italian aluminum bikes.  You never see it on steel. You’ll hear all sorts of reasons why people use the DP weld on titanium, most notably because it’s needed in order to produce the penetration necessary to create a strong joint. The biggest reason most builders use the DB is because it produces better looking welds, welds that are out there for the world to see. You don’t get to cover them with paint like you do with steel. The reason you see it done with titanium but not steel is because steel suffers from a reduction in strength through the heating cooling cycles that titanium does not. A person could DP steel but the likelihood of future failure would be very high.

So here is how the double pass works. First you must have very tight fitting joints. The builder will then weld the joints without the filler material that is used in a standard single pass weld. This is called a homogeneous weld or, for simpler minds like mine, the first pass (FP). The result is a very small shiny joint that is not strong enough to be used. Some builders will go around all the tubes like this without any filler but there are all sorts of variations on the theme. For instance I  like to use very small filler wire in all the crotches that require more filler during the second pass in order to avoid undercutting. I also like to grind the transitions in the FP  that exist between the homogeneous sections and the sections that I’ve used a little filler. The gives me a consistent FP to follow and allows me to create second pass also known as the “dress pass” that looks better. Once the first pass is done the builder will then weld around the frame with the second pass using filler just as they would if it was a single pass weld. Since the first pass is done, the frames alignment is fixed and the normal challenges of filler angle, introduction rate, etc don’t exist, the builder can just concentrate on producing a consistent, pretty weld. Once they’re done with the second pass the weld is complete.

There is no single “right” way to to weld. Most builders have their own style and you can see it in the final results. Some use a single pass, others use a double. Some pulse, some don’t. If you pulse, the appearance of the final weld can vary dramatically depending on the pulser settings. Other variables are filler rod diameter and the rate of speed at which the welding is done. Some welds are super pretty and consistent and others…not so much.  The important thing to remember is just because a weld isn’t pretty doesn’t  mean it’s not strong. And conversely just because is pretty doesn’t mean it’s strong, although I will say, if a welder can make a weld pretty, they can probably make it strong. It’s not harder to make a weld pretty than not if you know what your doing.

So there you have it, the “Double Pass” weld. Below are a couple  pictures of Ned’s bike with both a first and second pass. Ned, your frame is just about done and we’ll be assembling it soon.

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