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Filing weld joints

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KWK

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Is it acceptable practice to file welded joints flush? I seem to recall reading it is not, but I can't find it in my books on aircraft welding. As I recall, the claim was the joint is of weaker metal and should be thicker than the 4130 tubing.

I'm sending a factory welded fuselage off for painting, and a few areas such as some of the joints in the doors and tail surfaces would look nicer if the weld bead were filed down. The factory did so in a couple places on the doors and the fuselage, but the locations seem to be at random. Maybe they were trying to inspect the quality of particularly rough looking joints and the guy with the grinder simply got carried away?
 

oriol

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It is more or less standard practice in welding to grind the welds that have been filled in excess for aesthetical reasons.
That is perhaps the reason why you see in some random places that someone has grinded some of the weldings.

If doing welding for furniture and decoration stuff some people grind all the weldings so that those can not be seen once the structure has been painted. This is not the case in aero structures in which the weldings are left ungrinded to keep their maximum strength.

Oriol
 

KWK

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By "random" I mean a joint on the one side of the airplane was ground flush by the factory, but the same joint on the opposite side (just as visible) was not. This has me scratching my head.

For now, I've settled on filing the worst joints flatter but not flush, mostly to ensure the fabric covering won't have to conform to a rough edge anywhere.
 

FritzW

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Typically you don't file an aircraft weld unless your going to take it to back to the base metal and start over. If you file a small bump that will show up in the fabric, "real" airplane folks will understand and turn a blind eye to your transgression. we've all been there...
 
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TFF

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If it is, it is. If it's so bad that it's not contributing to strength, what the heck. You don't want to cut into anything that might be holding it together or add a stress spot cut in where there was none. You have to be the quality control for your own project. If it's been like it is for a long time , I would probably leave it as is. You might grind in and find a nice bubble of air; now you are welding when although not the best, was ok as it was. The best I would want is maybe a scotchbrite roll on a die grinder to buff the high spots. Not aggressive.
 

KWK

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AC 43.13-1B said:
4-81. PRACTICES TO GUARD AGAINST Do not file or grind welds in an effort to create a smooth appearance, as such treatment causes a loss of strength.
The factory didn't grind many welds. One, the top of the rudder post, was ground so far it is below the level of the outside of the tube. :shock:

I limited my filing to taking off any bumps in the weld much higher than those around it, as well as knocking off spatter and pointy peaks left from the arc welding. I doubt I've violated the AC (much).

Thanks for the advice, folks. Off to the painters tomorrow for epoxy priming.
 

Little Scrapper

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When I went to welding school 30 years ago the classroom joke was crappy welders needed a grinder. A good weld never ever requires grinding.

Grinding is something that's usually done for looks, like art. If your life depends on it there's no way I'd recommend it. Most tubing is .035" which is super thin by most standards. One small touch with a grinder on the tubing and it's thinner than tissue paper.

Another reason to never grind a raw weld is gives a very honest appearance for inspection purposes. Grinding hides problems structural problems. That's never a good thing.
 

bmcj

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Now, if I could only learn to grind in a nice weld bead look into a crappily overbuilt weldment...
 

lr27

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Now, if I could only learn to grind in a nice weld bead look into a crappily overbuilt weldment...
There's probably a way, but it's probably NOT easier or cheaper! Might involve NC EDM or something instead of grinding. Or braze with a mold that looks like a bead. ;-p
 

KWK

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NACA TN No. 1262
The NACA report emphasized concave weld beads were the most destructive in their tests. The factory fuselage we have has mostly that type of bead. Also interesting was that brazed joints easily bested the welded joints. The NACA tests were somewhat specialized, being tube to plate instead of tube to tube. Also, I assume this is stick welding, not gas shielded arc.
 

lr27

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Wondering why, if the brazing was so good, it isn't usually used.
 

Chris Young

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I had read in a lot of places that one should'nt grind the welds to keep the material for structural strength, but in the industry (aero), almost all welds are grinded smooth, because the gain in fatigue strength from a smooth surface is much greater than the loss in static strength.
 

PiperCruisin

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In my business, I care a lot about weld fatigue. It tends to be less of an issue in aircraft since the difference is relatively high between high amplitude/low cycle loads and normal flight/vibration/high cycle loads....however, I did find the NACA report interesting.

That is not the case if your aircraft happens to be pressurized.

In industries I have been in, butt welds (full pen.) might be ground if fatigue life improvement is required. In fillet welds, one might do toe dressing, or toe grinding or post weld heat treatment (stress relieving). This generally improves life about 2X. The grinding would be hard or impossible to do on the thin aircraft tubing. Also, any post weld operations cost money and time so it works best to design such that you don't need it.
 

KWK

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... the gain in fatigue strength from a smooth surface is much greater than the loss in static strength.
I wondered about that, when looking at the bumpy weld surface, and recalling lessons in stress concentration. NACA found that where the weld bump meets the tube is where the joint fails. Having a smooth transition from the weld to the tube greatly improved fatigue life.
 

lr27

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It's a tall order, but if someone invented an inexpensive, flexible way to do double butted tubing, it might help with the weld strength and fatigue issues. I suppose the thinning would be easier if done on the outside.
tubing-butted1.jpg
 

Alan Waters

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Grinding can produce stress risers. [Places where cracks can start] Don't do it. But if you do, polish out all of the grind marks.
 

BJC

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It's a tall order, but if someone invented an inexpensive, flexible way to do double butted tubing, it might help with the weld strength and fatigue issues.
What are the weld strength and fatigue issues?


BJC
 
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