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Old 05-21-2016, 03:12 PM
billfunk29 billfunk29 is offline
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Default Joint fit

I have read a lot about O/A fusion welding. Lots of different methods but all say to have a tight fit on the entire seam. Anyone have some tips on how to do that. On big things like a roof, that can't be overlapped and scribed, I always struggle.
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Old 05-21-2016, 07:35 PM
crystallographic crystallographic is offline
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Quote:
Originally Posted by billfunk29 View Post
I have read a lot about O/A fusion welding. Lots of different methods but all say to have a tight fit on the entire seam. Anyone have some tips on how to do that. On big things like a roof, that can't be overlapped and scribed, I always struggle.
"Fusion" weld means no added filler, same as "autogenous" or "Scot" or ... etc. welding.
I'm no expert on this, though I've made some long welds (3-4ft - 1-1.5m) on thin sheet for a while now.
Since I have never been able to get molten metal of any alloy to jump across open space in the gaps between sheets, I always assume the edges have to be touching each other for fusion welds to happen.

Scribing off the "master" panel does not always get you a dead accurate fit. However, marking a line that is generous and then belt sanding, or filing back to the line to make the fit, with repeated fitups to check seems about the only Murphy-proof method I have seen or used.

Also,
Well-placed tacks can help pull the gap closed, as can the weld itself.

However, I don't beat myself up anymore when a gap appears just in time to ruin my plans. Things move - so I get used to it and have fallbacks to compensate:

For some gaps I just hammer one edge to move it out.
On larger gaps I hammer both edges.
On larger+ gaps I thin down the filler rod and weld one edge and hammer that out and then weld.
On larger gaps ++ I weld both edges and hammer them out and weld that.

Where the gaps get +serious+ I use "telephone pole rod" and flow that right in between and then hammer that.

I guess I have enough answers for the various deviations that I don't have to fight so hard to attain the exact perfect fitup, anymore. Either that, or as Jim Younkin used to say, with some exasperation - "For you, welding is a non-event."


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Old 05-21-2016, 10:46 PM
efranzen efranzen is offline
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I just patched my roof. I replaced about 4 inches patch all the way around on a 1956 F100. I patched about 2 feet at a time, as the remaining roof will drop in once cut away from the lower piece.

I layed the patch over the existing roof and marked it with a sharpie. Then I removed everything from about 1/2 inch away from that mark down to the drip rail. This way the patch will now sit in place so I can make the final cut mark using a sharp awl all the while pushing up on the existing roof.

I then tacked with O/A and then started tacking between the tacks until I could finish gas welding.

Note that only about the lower inch was rusted, but one needs to be able to reach the backside to planish the welds to combat the shrinking, so that's why I replaced the 4 inch patch from a donor roof panel.

Yes, it was a huge pain,
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Old 05-22-2016, 07:27 AM
John Buchtenkirch John Buchtenkirch is offline
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Originally Posted by efranzen View Post
I just patched my roof. I replaced about 4 inches patch all the way around on a 1956 F100. I patched about 2 feet at a time, as the remaining roof will drop in once cut away from the lower piece.

I layed the patch over the existing roof and marked it with a sharpie. Then I removed everything from about 1/2 inch away from that mark down to the drip rail. This way the patch will now sit in place so I can make the final cut mark using a sharp awl all the while pushing up on the existing roof.

I then tacked with O/A and then started tacking between the tacks until I could finish gas welding.

Note that only about the lower inch was rusted, but one needs to be able to reach the backside to planish the welds to combat the shrinking, so that's why I replaced the 4 inch patch from a donor roof panel.

Yes, it was a huge pain,
I just have to ask …… if you had a donor roof why didn’t you just change the entire roof skin in one shot ? That would have easier & quicker to my way of thinking. ~ John Buchtenkirch
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Old 05-22-2016, 07:41 AM
efranzen efranzen is offline
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The donor roof was trod upon by 7 angry unicorns, but the 4 inches I needed were still ok.
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Old 05-22-2016, 06:34 PM
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Marty Comstock Marty Comstock is offline
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for the best tight fit I have a number of techniques

File it straight, you can see all the highs and lows on an edge easily.

Sheared panels (the new part will have a guaranteed straight line)

Some people hate this, but square corners. Round corners take too much time to get perfect, youll learn to work the distortion out easier.

Good initial panel shape with a sharp scribe. Lay it down taking into account panel thickness if there is a bend somewhere.

File to fit by hand, no air tools. There is no shortcut to a good fitup. Some lines take time.

Straightedges to guide your initial line by.

Get better at adding filler and dealing with the distortion that follows. Its not that big a deal.

Oh, and +1 on all that Kent said.

Marty

P.S. make that +2, especially on using tacks and weld to bring another part of the panel together.
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Old 05-22-2016, 10:32 PM
Oldnek Oldnek is offline
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Plus 3 on all above and Try not to use a scribe, Because the scribe point is ground tapered, its very hard to follow the edge. I just use a off cut of sheet metal cut to angle.

Using this you can get right up to the edge to be scribed
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Old 05-23-2016, 06:20 AM
billfunk29 billfunk29 is offline
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Many thanks for all the valuable comments. No magic bullet. I guess I need to grab a file and get to work.
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