Wednesday, December 23, 2015

Welding a bottom bracket to my bike

The bottom bracket on a bike is essentially what connects the pedals to the frame and allows them to rotate.  I had replaced the bottom bracket on my bike awhile back, and then a few months ago it came loose.  I initially tried just tightening it down, then tried locktite (red); neither worked to hold it in place.  I took it to a bike shop and asked them about cleaning up the threads, they looked at it and said, "What threads?  There's nothing left."  At that point I decided to try to weld it (TIG welding), but it was ultimately not successful - the welds were very messy, the arc was behaving erratically, and although initially I was able to ride the bike, after~ 4 rides to and from work the welds broke and I was back to where I started.  This post describes my second attempt at welding, which I think will ultimately be more successful.

Edit:  I forgot to mention - I did this work at Artisan's Asylum in Somerville.  They are fantastic!

I took a class (at Artisan's Asylum) in TIG (Tungsten Inert Gas) welding, and I really like this bike, so I figured I would use / practice my welding by trying to save the bike.  TIG welding gets its name from the fact that you use a tungsten tip and a high current to melt the metal and create the weld.  The inert gas (argon) is used to keep oxygen away from the arc and from the metal.

The first time I attempted to weld the bottom bracket, it ended up looking like this:

The blackened paint is from the metal heating up during the welding process - it can get very hot (glowing red or even white).  The weld itself is terrible - it is lumpy and not continuous, and although there's another weld of similar terrible quality and length on the opposite side of the joint, as I mentioned above they weren't strong enough to last and broke after a few rides.

I had done some practice welding runs immediately before doing the above, and my practice runs weren't nearly as bad.  (They weren't great by any means!  Just not that bad).  It seemed like when I was welding the bike that the arc was behaving badly: it was not as intense as it should be, and it wandered over the surface rather than taking the shortest path to the surface.  During the above I thought I was just not keeping my hand close and/or steady enough, but when I thought about it more it occurred to me that the problem might have been that the bike wasn't properly electrically grounded.  When you are TIG welding you are generating an intense electrical arc from the tungsten tip to the metal surface, and the all those electrons in that intense arc have to be able to go somewhere.  There is a very heavy duty metal clamp that normally is just attached to the steel table you work on.  I knew enough in my first attempt to connect the clamp to the bike directly, but it occurred to me that I still hadn't made a good enough connection.  If there is any small resitance to those electrons, they'll quickly pile up (like traffic), causing the bike to charge up, and that negative charge will then deflect the electrons in the arc and/or prevent the arc from forming properly.  Therefore, I set out to make sure the bike was properly grounded.  I did that by using a file to remove paint from the lower tube, near where I was going to be welding so I could attach the grounding clamp directly to metal of the bike frame:

You can also see in this picture that I removed the remnants of the previous weld - it came off pretty easily using a standard angle grinder.  That was a first for me, it's a powerful tool and I can see how it would be very versatile.

I did a few practice runs on a scrap piece of square steel tubing, just to get a feel for the intensity.  I also did a practice weld joining two pieces of tube together, they were already held in place by welds in other locations so that made it easy to just practice away (a lucky find in the scrap pile).

After that I got down to business.  I tried to remember to do all the basics:  
  • I positioned the bike so that as I was welding I would be moving the tip directly towards me
  • I made sure I had something to rest / brace my right arm (holding the tip) as much as possible
  • I started from a point where the pieces (bottom bracket and frame) were the closest together so I could more easily create a nice puddle, and then drag that puddle to regions with larger gaps
  • I made sure to let the puddle develop before starting to move, and conversely if things started to get hot / glow, to stop and let things cool down
Because I'm a rank amateur, doing the setup took a good chunk of time, and I had to change the setup fairly drastically to get to the different sides of the bottom bracket, but I think it paid off.  First, a picture of the good:

I like the above because there's a section that if you squint pretty hard and use your imagination almost begins to look like the proverbial stack of dimes:

Alright, maybe not, but it's the closest I got.  Moving on from the good we next look at the bad:

The above weld is really only connected in one spot, and above that spot it looks like I deposited metal on either side but nothing bridging the gap - everywhere except where it needed to be. This is basically happened the last time I tried to weld the bottom bracket, but luckily this time it only occurred in this one section.

This next weld is half-way between bad and ugly.  It's not as long as it should be, but it's long enough to probably actually do some good / provide some strength, but it is also rather ugly - no hint of dime stacking here, and the material has been laid on pretty thick:

Finally we look at the ugly:  I think this weld is structurally sound (as sound as anything I've done), but it just doesn't look good.  The material is thick and the line is wobbly, but I'm somewhat optimistic it will hold:

I'll try out the bike soon and update with the results.  I'll try to log how many miles I ride the bike before / if the welds give out (assuming something else doesn't catastrophically fail first).  Thanks for reading!


  1. Awesome. I wonder if you could grind away the excess on the ugly side of things. Is it ok practice to then reweld? I've wondered. Woo!

    1. Thanks! I think you're absolutely right, I could do that - wait for it to cool, grind away the high spots / ugly parts, and then try again. I mainly didn't do that because of time and I suspect that I have enough good welds that it will hold, but we'll see :)

  2. Replies
    1. It held for a good while, about 3 months - and it never broke completely. I noticed that the crank was moving when I pedaled so I stopped riding it. I think that if I wedge shims in tightly between the frame and the bottom bracket, and then weld the shims in place to the bottom bracket and the frame, that will last longer. As it is, the welds were taking the force of pedaling, which is not the right way to do it.