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Aviation Forum / General / Homebuilt / January 2007Welding; How much undercut?
Some of the welding manuals I have say that a certain amount of undercut is inevitable. Certainly in examining the fuselages of some factory aircraft I've seen some shockingly large errors like this in the finished welds. On many of my practice wleds, particularly on tubing clusters, an otherwise perfect weld has a tiny bit of undercut. I'm talking barely visible here, only just able to catch the end of a rod, for instance. At a guess i'd say no more than .02 inches, maybe a bit more. Is this an acceptable situation? If anyone's interested, maybe I can post some pics of one on a wesite but I don't think the undercut will show up on a pic, it's that little... > Some of the welding manuals I have say that a certain amount of undercut is > inevitable. Certainly in examining the fuselages of some factory aircraft [quoted text clipped - 5 lines] > If anyone's interested, maybe I can post some pics of one on a wesite but I > don't think the undercut will show up on a pic, it's that little... A textbook on welding MAY have excerpts from various welding codes on how much undercut is allowable, here's one such: The AWS D1.5 Code requirement for undercut is: ? Undercut shall be no more than .01 inches deep when the weld is transverse to tensile stress. (Example: If a cross-frame angle is welded into the web of a beam then the allowable undercut along the edge of the weld touching the beam is .01 inch) ? Undercut shall be no more than 1/32 inch deep for all other cases >> Some of the welding manuals I have say that a certain amount of >> undercut [quoted text clipped - 21 lines] > edge of the weld touching the beam is .01 inch) > · Undercut shall be no more than 1/32 inch deep for all other cases Thanks, that's what i needed to know OK. But from a practical standpoint, is it normal to have such an undercut on a weld? I would imagine undercut would be a stress riser, for starters. >> edge of the weld touching the beam is .01 inch) >> · Undercut shall be no more than 1/32 inch deep for all other cases > > Thanks, that's what i needed to know OK. But from a practical > standpoint, is it normal to have such an undercut on a weld? I would > imagine undercut would be a stress riser, for starters. I was hoping someone would come along with some solid data on this one, but if you are talking about an aircraft weld, I hesitate to think ANY undercut should be acceptable. Are you welding tubing or plate? Aluminum or steel? Gas, stick, mig or tig? Usually, undercut is caused by a combination of factors. Weld speed, deposit rate and weld position, are common factors. >>> edge of the weld touching the beam is .01 inch) >>> · Undercut shall be no more than 1/32 inch deep for all other cases [quoted text clipped - 12 lines] > Usually, undercut is caused by a combination of factors. Weld speed, > deposit rate and weld position, are common factors. Yeah, for the most part, I can control the puddle quite well and my welds are tidy enough. It's Gas, I'm talking steel tubing mostly and it's in clusters, mostly in the "armpit" where there's an acute angle between the tubes. I've been practicing in anticipation of finish welding my already tacked fuselage. If the undercutting is a problem, I think it must be in the way I'm viewing the puddle. that is to say I'm consciously getting just a bit too much penetration in those ares where the bead is more concave. I'm afraid if I don't do this I'll end up with a cold weld.I'm feeding rod in at a good rate, I think, and I also think I have the tip size and pressures about right (it's a little difficult to get the bead started in the tighter angles) and the weld runs fairly quicly once I'm out of the tight area. The bead is fine once I'm running up the outside of the longeron.. I've tried varying the angle of the tip in relation to the work, everything from near vertical to angled well towards the progress of the weld, but this only seems to give fine control. Less heat than I'm using makes for a very globby, cold start. Having said all that, I picked up my torch for the first time in a year and a half last month and have made huge progress in welding in that space of time, so maybe I'll get a handle on this through practice. OTOH, i don't want to continue practicing a mistake! In a clusters there is a lot of heat build up necessary because so much metal to draw the heat away. It is a good idea to switch to the next larger diameter rod when welding clusters than when welding single tube to single tube junctions. The larger filler rod helps to suck a little of the heat away from the tube so that the tube doesn't get so hot that undercut forms as you weld. Undercut is a stress raiser, but in a standard truss-frame tube fuselage built with 4130 there is a major overkill situation. The tubes individually just aren't stress enough to lead to cracks where there is minor undercut. That said, I do not think you can allow any undercut in the firewall structure which is holding the weight of the engine, or in the engine mount itself. I would also avoid it in the landing gear structure. If you have the tube up to the red heat level (gets that slight "wet" look) then concentrating the heat on the filler letting the puddle expand outward to incorporate the tube, you will accomplish a solid weld with no cold lap. For the record, as much heat as is put into the base metal when using an oxy/acet torch, cold lap is virtually impossible. Not saying it can't be done, just not likely. > >>> edge of the weld touching the beam is .01 inch) > >>> · Undercut shall be no more than 1/32 inch deep for all other cases [quoted text clipped - 34 lines] > I'll get a handle on this through practice. OTOH, i don't want to > continue practicing a mistake! -- Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL BAFRANK@worldnet.att.net Homebuilt Aircraft Newsletter" | Publishing interesting material| | on all aspects of alternative | | engines and homebuilt aircraft.| *------------------------------**----* \(-o-)/ AIRCRAFT PROJECTS CO. \___/ Manufacturing parts & pieces / \ for homebuilt aircraft, 0 0 TIG welding While trying to find the time to finish mine. > In a clusters there is a lot of heat build up necessary because so > much metal to draw the heat away. It is a good idea to switch to the [quoted text clipped - 18 lines] > base metal when using an oxy/acet torch, cold lap is virtually > impossible. Not saying it can't be done, just not likely. Thank you Bruce! As always, you are the man.. I went out and did a bit more. I had my welds pretty good a few years ago and I went out determined to recapture that. Increasing the feed of filler worked wonders and all signs of undercut are gone.. I think I was afraid of not getting the penetration I needed and was a little too agressive with the puddle as well. At the moment,I'm getting a nice bright glow from the parent edges just before the puddle runs over them so I'm confident there's good penetration (confirmed with the hacksaw) wheras before I was looking for a virtual lava river which was obviously causing the undercut. Welds are pretty good, but I am having one further problem; After a minute or tow, my torch (a meco) often tends to go oxy, even though I had it set right in the first place. Not a big problem once I spot it early, but annoying al the same. It seems to stabilise after a while. Could be that my acetylene tank is just running down, but I thought that the regulators would cover that almost until the gas ran out.. Maybe I need new regulators.. > Thank you Bruce! As always, you are the man.. Yep; it's handy to have a welder man nearby, isn't it? :-)  SignatureJim in NC > Welds are pretty good, but I am having one further problem; After a > minute or tow, my torch (a meco) often tends to go oxy, even though I [quoted text clipped - 3 lines] > the regulators would cover that almost until the gas ran out.. Maybe I > need new regulators.. I had the Kent White set and the Medalist acet regulator would hunt and surge and finally sent it back. Went out and bought another one locally which seems to work well. His video is highly recommended. I find the acet needs a few seconds to stablize any time there is an adjustment on that side. I have a Meco too and it is great, but seems to pop about as much as any other torch. Welding 049 with a N-2 tip I had the gas turned way up and it would still pop. May just be dirty. Kent also includes and recommends E70 rod but it is terrible rod to gas weld with and I switched to coat hanger to practice. Went out and got some RG45 which is wonderful. I can make more or less airworthy welds now, now working on making them look nice. Things got a LOT easier once I learned the importance of using the rod to help control the heat applied to the puddle so now I almost never get burnaways. Still having to do a lot of stopping and "adjusting", but about half the time now I can find that sweet spot where the puddle is just right, the torch angle and heat is just right, and I just have to move it along feeding the rod and the resulting bead looks like a tig bead. John >> Welds are pretty good, but I am having one further problem; After a >> minute or tow, my torch (a meco) often tends to go oxy, even though I had [quoted text clipped - 29 lines] > > John Hi John, If you have a lot of problems with a torch popping, try running a little less OX. From a perfect appearing burn, you should richen the mix just enough to produce a little bit of feather. Maybe a 1/4 to 1/2" will usually make a world of difference. >>> Welds are pretty good, but I am having one further problem; After a >>> minute or tow, my torch (a meco) often tends to go oxy, even though I had [quoted text clipped - 35 lines] > enough to produce a little bit of feather. Maybe a 1/4 to 1/2" will usually > make a world of difference. Thanks I usually set it with a tiny bit of carb showing, just enough to soften the neutral cone edge. Are you saying a 1/4" to 1/2" long carburizing feather is ok? And I think my problem was the N-2 tip was a bit too big because the pieces are small and heat isn't sinking away. I just tried an N-1 tip nearly maxed out and it put out enough heat for a full puddle and no pops. This seems to be one of those borderline conditions where the best tip would be in between N-1 and N-2. I still have to get a 0 tip and a 0 tip drilled to about 0 and a half for doing .028 and .035. Cheers John >>>> Welds are pretty good, but I am having one further problem; After a >>>> minute or tow, my torch (a meco) often tends to go oxy, even though I [quoted text clipped - 41 lines] > the neutral cone edge. Are you saying a 1/4" to 1/2" long carburizing > feather is ok? It shouldn't require an actual 1/2" to eliminate the popping problem, and you shouldn't add more than necessary to stop the torch from popping. But often when popping is a problem, you are running just a bit to lean. It will also cool that N-2 tip down just a bit.
70s-6 is a better choice than 45.
"J.Kahn" wrote: Fortuant1 wrote: --
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