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Old 08-08-08, 05:26 PM   #1
tkehler
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Remove everything before welding - why?

I'm getting a disc tab welded on my titanium bike. (Something that's a bit trickier than just approaching a pal or neighbour who welds steel, believe me.)

When I spoke to the shop, they said that I should be sure to remove everything on the bike.

Me and a friend got to thinking about this. Why would that be a stipulation? First we thought it was because the heat of the welding would affect the attached components.

Then we realized that this is absurd. Okay, it is one thing to weld something onto the chainstays with the rear wheel attached but there should be no problem with leaving the front derailleur on, or the headset, or the front wheel. Even the crank, I should think. Heat doesn't transfer very far along a tube during welding.

Then we thought: it's because the shop doesn't want to be responsible. And the bike shop wants convenience. I.e., it's just easier to work on a frame that doesn't have anything attached to it.

Anyone have an answer?
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Old 08-08-08, 05:57 PM   #2
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They will most likely want to clean the entire frame because contaminants can cause problems with welding ti, not to mention getting their shop or equipment contaminated.
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Old 08-08-08, 06:02 PM   #3
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Why not ask the shop?
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Old 08-08-08, 06:02 PM   #4
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You are right - - heat is not the issue. I presume this is going to be done with a MIG or TIG welder. These are electric welders that pass electrical current through the frame. Electricty always takes the shortest path to ground. If that happens to be through something that is attached to the bike (bearing, water bottle cage, bolt) that does not have full ground contact, there, I suppose, is a chance that there could be a current arc across the non-contact void. This could, in effect, 'spot-weld' that piece to the frame. Best give 'em a clean frame.
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Old 08-09-08, 01:08 PM   #5
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They'll also have to pump an inert gas into the frame tubes to prevent contamination of the welded zone. Removing everything from it will make it easier to securely plug all the openings and jig the frame for welding.
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Old 08-09-08, 05:14 PM   #6
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As a mechanical engineer and crew chief for a FSAE (race car) team, I can speak authoritatively on this. It is the bearings the welder is concerned about. While theoretically the electricity should take the shortest path to the welder's ground, in reality it is best practice to remove electronics (obviously) and bearings to make sure no current passes through them. Bearings are high precision pieces and even even a basically microscopic imperfection (spot of corrosion where a current passed from ball to race) can spell disater for the bearing.

The need for bearing races to be perfect is why bearings are never shipped or stored vertically, but always flat. The vibration from even a few hundred miles of vertical transportation on a truck can significantly reduce a bearing's life.

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Old 08-09-08, 06:16 PM   #7
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sounds like voodoo to me.
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