Backgouging, or backgrinding, is necessary to ensure that the weld is full-penetration. The root pass is the one most susceptible to inclusions, voids or incomplete fusion as a result of insufficient shielding, poor temperature control (rapid cooling), contamination (inclusions from dirt or moisture, etc.) or pool flow into the gap you are bridging and rapid cooling of the bead without actual fusion to the parent material. Ideally, a full-penetration weld between two cylinders would be a double-V butt joint, in order to assure complete fusion with a minimum amount of rod fill. In your example, a 1/2" wall would entail a 0.20" - 0.25 chamfer" on both sides, such that you either have a perfect V or a small parallel land in the center of the wall. In either case, the root pass fills the gap and joins the two cylinders at the wall centerplane (center-cylinder?), and then you begin to lay passes on one side. Given that the root pass could have poor fusion due to the cooling / bead flow problem, you grind it away from the side you haven't welded yet, to the extent that you are 100% assured of having ground back into material which is high quality, strong weld that is fully fused to both sides of the assembly - essentially removing the weld toes of the root pass on both sides and exposing the fusion interface to assure its quality. Then, you start laying passes into the ground-out root to guarantee 100% fusion.
Granted, the double V is a lot of work to prepare, and if you can't or won't do that, you can effect a full-penetration weld on a strict square-ended butt joint, but you have to ensure that if you lay a root pass on one side, your electrode can reach fully down into it from the opposite side to achieve 100% fusion - here, you have conflicting goals, because the larger the gap between the two cylinders, the easier access you have for the electrode (essentially achieving the effect that the V preparation is intended for), but in increasing this gap you make it much more difficult to lay a quality root pass to begin with, necessitating deeper grinding when you backgouge it to ensure that you expose perfect weld before covering it. You may end up having to grind out this pass entirely, exposing your first covering passes from the other side, and then covering those. I would recommend considering the weld-preparation, as it may very well save you the same amount of effort in backgouging and recovering.
As for your covering passes, you should be able to maintain high quality 100% fusion welds as you proceed, but if you notice a mistake, or are at all suspect about the extent of fusion of any given bead (or any other quality issue), just backgouge it until you have it back to a perfect weld, and proceed from there.
Once you have a high-strength, high-quality weld, you can further improve its fatigue life by profile grinding (where you remove geometric discontinuities / stress concentrations at the exposed weld surface) and stress-relieving with heat.
-Sean
Jon Wallace wrote:
>
>
> Thanks for that Frank. What about 1/4inch rod? Is it logical to > think that 1/4 rod would produce results twice as fast as 1/8 rod > (assuming all other things equal), or does it just not work like > that? I'm sure 1/4 rod requires more amperage to cause an arc which > is one drawback, but if you have the power available, does 1/4 rod > make for faster work?
>
> Also, I hate to sound dumb, but somebody has to take that role so it > might as well be me. Can someone explain "backgrinding" and why it is > necessary? Let's say I've got the rolled cylinder for my main hull. > There's an open seam running longitudinally which essentially is a > "butt" joint, correct? So I put down a root pass which I hope is > going to fill the void between the two sides of the seam at least on > the inner side of the cylinder. I likely won't fill the entire seam > on the outside of the cylinder depending on the thickness of the > material (if it's 1/6 thickness I will, but if its 3/4 thickness I > won't). So let's assume I'm working with 1/2 inch material and need > multiple passes because I'm using 1/8 rod. What am I backgrinding on > the root pass? The inner side, or the outer side? And assuming my > root pass is perfect, is "backgrinding" necessary on the build-up > passes that go over the root pass?
>
> I'm going to take a welding course, but these questions have bugged me > for some time and I can't wait for the class. :)
>
> Jon
>
>
>
ShellyDalg@aol.com wrote:
>> Miller makes good machines but definitely you want a DC machine. AC >> "buzz boxes" are OK for some small farm quality welding, but not much >> use beyond that. ( like fences or gates.)
>> A good DC machine ( and they also have an AC switch on them) can be >> used for TIG as well. A cheap TIG torch runs about $80 and a flow >> meter about $75. Those two items with a bottle of argon mix and >> you're all set to TIG just about anything. As for magnetizing the >> metal, that's not a problem. A 250 amp machine is enough for what >> we're doing. The "duty cycle" relates to how long you can burn rod at >> the maximum setting, based on a 10 minute cycle. 50% duty cycle means >> you can burn rod on maximum for 5 minutes, then let the machine cool >> down. It's pretty hard to "smoke" a stick weld machine. The older or >> better machines have copper windings on the transformer, the newer or >> cheaper machines have aluminum wire on the transformer. Get a copper >> wound one if you can. You won't be welding at max power anyway. 3/32 >> and 1/8 size rod is the most common. Typically running at 90 to 125 >> amps.
>> Make sure you keep the rod DRY or use new rod. I buy my welding rod >> in small batches ( 5 pound box ) so it's always new. Frank D.. >>
>> ------------------------------------------------------------------------
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