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 Post subject: welding......!!!!!
PostPosted: Mon Sep 06, 2010 10:48 pm 
Newbie

Joined: Sun Sep 05, 2010 4:32 pm
Posts: 8
Location: Newquay., Cornwall
I have a Giant chromoly 4130 frame which has broken on tube,which runs from crank to derailer, just in front of rear wheel nut on the join....

My question is., can this frame be mig welded?????

Cheers
Simon.


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PostPosted: Mon Sep 06, 2010 10:52 pm 
Retro Guru

Joined: Sun Aug 22, 2010 7:07 pm
Posts: 1321
Location: Cotswolds
A good picture might help, but with most bike frames its possible to make a strong enough joint by silver brazing, at a much lower temperature.


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PostPosted: Mon Sep 06, 2010 11:13 pm 
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Joined: Sun Sep 05, 2010 4:32 pm
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Location: Newquay., Cornwall
Here's picture...not to good but look for rusty line. :shock:
Image[/img]


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 Post subject:
PostPosted: Mon Sep 06, 2010 11:18 pm 
Retro Guru

Joined: Sun Dec 14, 2008 2:11 pm
Posts: 303
4130 is fine for MIG welding, however there are a number of reasons why you might not want to MIG weld your frame.

The first is that MIG tends to produce pretty fat weld beads, not exactly neat. OK so you could grind or file it down.

The second is that when you're welding a crack you need accuracy, this can be difficult particularly if the crack does not run straight. MIG, particularly with hobbyist plant, tends to be difficult since it's hard to see the weld puddle because it will be obscured by that big fat gas nozzle.

The third and final reason could be the show stopper. Remember MIG was originally designed for automation. In this situation wire feed and current are pre set for known material thickness and weld speed. When converted to hand held use the same controls still apply. This is great where you're welding known workpiece thickness, particularly since you can practice on scrap steel first to get your settings dead right. You don't have that luxury on your frame. Do you know how thick the tube wall is at the break? How will you know if you've got full penetration? (Ooooerrr!) Failure to get full penetration could lead to the weld failing. OTOH too much current and you could blow a hole in the workpiece.

I would tend to go for TIG since you have so much more control. Some prefer using gas because the plant is cheaper, but TIG gives better heat control for thin material. MIG doesn't do delicate, TIG OTOH could be used to weld a razor blade to an engine block edge on.

The fact you are asking about MIG welding suggests you are looking to do a home repair. Are you sure you can do a good job given my questions above?

You might be better off taking it to a frame builder for assessment. Any reasonable frame builder would give you a free quote, and indeed let you know if it is repairable at all. Depending on the nature of the fracture it may no be worth repair. Have you, for example, cleaned up around the fracture to see if it's a clean break? Is there much corrosion in the area? Like my welding tutor said, "you can't weld rust".

Having said all that I do know a man who once affected a successful frame repair with a basic MMA "buzz box" welder. He found himself about 400 miles from home in Scotland with a chainstay cracked near the BB. Rather than cut his tour short he decided to repair the bike, but he couldn't find a frame builder in the highlands. However he did find a farmer who had an old buzz box he used for repairing his farm machinery, trailers and the like. It wasn't neat or tidy (he was using contact rod) but it did the job. OK so he is a professional, but even so it goes to show it's possible.


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 Post subject:
PostPosted: Mon Sep 06, 2010 11:26 pm 
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Joined: Sun Sep 05, 2010 4:32 pm
Posts: 8
Location: Newquay., Cornwall
The break is nice and straight...and slips back in line with rest of frame...

Didn't see it when i bought bike...only noticed it when i got home and started to strip bike down....think it would weld nicely.....was thinking of doing it myself with the mig but think i might leave it ...

thanks for the advice.....any frame builders in Cornwall???

Simon


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PostPosted: Mon Sep 06, 2010 11:26 pm 
Retro Guru

Joined: Sun Dec 14, 2008 2:11 pm
Posts: 303
Actually your picture has loaded now and I can see the break so it's easier to answer your question. It looks like the original weld has failed. Since it is at an already welded joint the material should be nice and thick, which is good news from the MIG point of view. Another thing which is good news is that the nature of the welded joint will make it easier to follow the joint with a MIG torch.

Clearly from the picture it's not well lined up, so if you don't find some way of jigging it into place the frame could end up being noticeably out of line.

The main concern I have here is, however, the rust. Not so much that that failure may be due to rust, from what I can see it wasn't. What concerns me is how well you can clean out the rust from the fracture. It looks like it's been left in it's broken state for some time so the mating surfaces will have a good coating of rust, which is not conducive to good welding.


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 Post subject:
PostPosted: Tue Sep 07, 2010 9:10 pm 
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Joined: Sun Sep 05, 2010 9:57 pm
Posts: 131
Location: Kent
I've had to do something similar on a 531 frame, but brazing the break worked fine. Don't know what rods you would need for chromolly,
Luke


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 Post subject:
PostPosted: Tue Sep 07, 2010 9:53 pm 
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Joined: Sun Dec 14, 2008 2:11 pm
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lclements wrote:
I've had to do something similar on a 531 frame, but brazing the break worked fine. Don't know what rods you would need for chromolly,
Luke


I know some frame builders do it but if you check the spec sheets brazing is not recommended on 4130.


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 Post subject:
PostPosted: Thu Sep 09, 2010 2:34 pm 
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Joined: Mon Mar 02, 2009 4:22 pm
Posts: 1481
I would MIG weld it, but then I have a MIG welder :D

Clean it right back to shiny metal with a wire wheel. File the break to a vee to ensure you get decent weld penetration, because with MIG you want the weld to go somewhere. Just use normal mild steel wire to be honest.

Use an old wheel or hub to clamp it up to make sure it's square (or cover that wheel to protect it). Tack it once or twice and check it's all in the right place, then finish weld. You can use a series of spots if it's easier. Clean it up with a file if you want to.

The only issue you might have is if it was brazed originally (which I think it probably was) because it'll make the weld spit everywhere.


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 Post subject:
PostPosted: Thu Sep 09, 2010 6:00 pm 
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Joined: Sun Dec 14, 2008 2:11 pm
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Wouldn't expect it to be brazed on an old Giant.


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