[serepton]

I am pulling my hair out here!

I have a cycle bottom bracket shell which has a thickness of 3mm. I am trying to weld the rear chain stays to it and they have a thickness of .9mm.

I can't get a puddle going without blowing through the chain stays. If I wait until there is just enough heat to start the puddle then as soon as I add filler rod to the puddle it cools it too much and the puddle solidifies.

Is there a technique to doing this so it wont blow through?

Thank you

Sheridan

[RedIron881]

Sheridan,

I would say if possible to pre heat the thicker piece before putting it together then favor the thicker side while welding. Also, what size rod are you using? Try to find some .020 (~.6mm). I'd use a Gas lens, #6-7 cup, 1/16th Tungsten sharpened to a sharp point for pinpoint accuracy. I'm sure you're having problems with the arc wondering on you. Arc travels to the path of least resistance. Just let the puddle wash up onto the thinner piece and move fast. Could also try the lay wire technic with pulse.

Hope this might have giving you a few ideas. Let us know how it works out.

[serepton]

Thank you for your reply, I shall let you know how I get on tomorrow!

[Otto Nobedder]

You can also try clamping some Al, Cu, or brass to the thin material behind the weld. It will serve as a heat sink as well as physical support for the back side of the puddle. I realize the chain stays are small and odd shaped, but it doesn't take a big piece of metal to make a huge difference.

I'll use a back-up like this when I need to fill holes in thin sections. Aluminum is the easiest to find in useful shapes, it's an excellent heat conductor, and the oxide layer (don't clean it) has a much higher melting point than steel.

Good luck,

Steve

[serepton]

Steve,

Thank you for your reply. I had wondered about using a heatsink of some sort.

I have an update for you, I have had some practice attempts this morning, I started out welding the thinner tube to some section that is actually slightly thinner than the bottom bracket shell but still much thicker than the chain stay tube.

Here was my first attempt;



As you can see, it did blow a hole but I found an instant improvement in the actual starting and moving the puddle, you can see on the left of the tube there is almost what you could call a fillet!

This was my second attempt;



Getting better!

I then switched to a spare bottom bracket shell which is actually slightly thicker than the shells that I will be using, this is why it is scrap.









I am really happy with how it appears to be going, it obviously needs much practice but last night I was pulling my hair out but I can see that with much practice there is light at the end of the tunnel!

One thing that I have noticed is that the shell appears to be getting very hot with the colour changing to a very dark blue. Is this just because the parts are quite small so there is not much material to disperse the heat?

Once again thank you for all of your help, I shall keep you updated!

[weldin mike 27]

Hi there,

I have a mate who makes bikes for a living, some out of ultra thin cromoly tubing. I haven seen him do it but i have seen one of bikes. The welds seem very large for such thin stock, That suggests to me lots of filler. More than you might expect. I think he even said about using 2.4mm filler rods so as to soak up some of the heat.

Mick

[Landyman]

I'd braze or silver solder it. You'd have more control over the heat.
Or, try aiming the arc at the thicker piece and let the pool melt the thinner piece.
I was having to weld M5 flat washers onto the end of 8mm diameter bar to give the powder coaters something to hang them up by. I didn't have to use filler, just melt the two together. After blowing the washers from a circle to a C shape, i found that if i lay the washer on a piece of copper plate about 6mm thick, i could melt the end of the rod and run the pool into the edge of the washer, which was zinc plated too.
I was down to about 25amps.