Grinding down ugly welds and redue

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Another question: If you grind down an ugly weld (Make it flush to the joint) and go on top of it with a new weld are you making the joint any it stronger or wasting wire?Dave ReberWadsworth Ohio
Reply:If the weld is the correct size after it has been ground down, then no, it will not make the joint "stronger."  However, if the weld is ground smooth and blended at the toes, it can help alleviate stress risers.
Reply:Thanks supe I value your thoughtsBy correct size you mean can be ground down to be flush, with not crater or pockets?I will have to look up "toe"Dave ReberWadsworth Ohio
Reply:Reebs:  Typically, a weld size is specified based on the required strength for a given component.  For example, if a 1/4" fillet is specified as being the required weld size, then oversizing to a 3/8" weld is not going to make it any stronger, as the base material will likely fail before the weld does.  However, ignoring some metallurgical factors, the 3/8 weld will not be any "weaker."Where the 3/8" weld may prove to be weaker, is if it has a profile that can create stress risers. i.e, craters, excessive convexity, sharp ridges, peaks or valleys.  The weld "toe" would be where the edge of the weld metal interfaces with the base material.If you look at the stresses in a weld, any abrupt changes in weld profile creates concentrated stresses.  More often than not, a slightly concave weld profile that transitions gradually into the weld metal creates the lowest stress concentrations.  However, because of the tension created at the weld surface upon cooling, this profile is also the most prone to cracking.What it all amounts to, is that oversizing a weld does not necessarily make for a "stronger" weld.  However, starting with a weld that is slightly oversized, and grinding/blending it into a smooth profile/transition can in some cases be more desirable if not taken below the minimum specified weld size.  This is usually a huge waste of time and money however, and is usually only specified if a designed weld will see a particular type of loading/stress concentration.
Reply:I got it.Dave ReberWadsworth Ohio
Reply:Reebs there is a part of the weld called reinforcement. You don't want to grind away that part or you will affect the strength. on the 1/4" fillet above, usually the weld will go up the high side 1/4" and along the bottom piece 1/4". Draw a 45 deg line between the 2 points and thats the minimum material usually for a fillet weld. Often butt welds rely on the "pile" of weld on top for strength, because they may not be full penetration welds. If you grind down this material you may only be left with a very thin layer of weld material. Usually if a weld will be ground flush like this you would V groove the joint to get the most weld penetration and then you could grind the top flat.IF the issue is not just looks, but a bad weld, porosity, slag inclusions, lack of fusion, undercut, cratering, cracks and so on. THEN you want to remove all the "bad" material and replace it if needed. The weld is weak to begin with, the more material you remove, the weaker the weld can become. I think this may be more to  the point of your question..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Thanks again guys I love this place.Supe here is some **** I laid down Sunday. I think I have corrected the problem and generally weld much better than this. But here are some pictureshttp://weldingweb.com/vbb/showthread.php?t=38833Dave ReberWadsworth Ohio
Reply:Dave, the general technique is that each leg of the weld bead should be as long as that face is thick.Weld a lap joint that is 1/8 x 1/4 in., then the 1/8” face will have a 1/8” leg and the 1/4” face will have a 1/4” leg.Over welding is a waste of wire, gas and time, doing nothing to add strength, hiding a defective cold joint.Engineers evaluate the beads strength by the area of the bead, leg height x length = area. The area x the unit strength of weld wire = force. This does not take into account the base metal that has been fused within the joint.
Reply:If the weld looks bad and you want to grind it off and re do it, then go for it.  Just remove all the old weld.  Its the joy of working with metal.  Grinding a poor looking weld to make it look good is a waste all around.  Welds should look good from the start.  If you have to grind it to look good then it wasn't a good weld to start with.If its porosity, you have to goo ALL the way down or the porosity will just return.  Been there done that.To quote welderwomn.  "You learn to be a good grinder before you learn to be a good weldor"Keep at itDavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:TrueYou learn to be a good grinder before you learn to be a good weldor"Dave ReberWadsworth Ohio
Reply:This topic is perfect for my question!! What do you do when trying to grind down a weld and the weld and the metal seems to just blend together? Im afraid if I continue to grind Im just gonna keep removing the original metal. Can I weld over this ground down metal or do I need to find another solution?Hopefully Im not jacking your thread!!Thanks,Slim Attached Images
Reply:You have several options. It looks to me like you have 2 plates butted together and welded at 90 deg. You can V out the old weld with a grinder, carbide burr or air arc and reweld. You can also just grind a bevel from the side and reweld. It's a bit tough to tell exactly whats going on in your pict. To me it looks like the material is 1/4"-5/16" and either method will work. Thicker and I'd V it out, thinner and I'd bevel the edge back into the 2nd plate. Attached Images.No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:Thanks DSW, yes it is butted together. The frame is 1/8" and the plate is 3/16". When you say bevel out the edge am I doing it towards the larger 3/16" and leaving sort of a root or bevel as in creating another space between the two?
Reply:Ok I can see what you have now. I thought the parts might be the same thickness or the vertical one might be thinner. the pict with the 45 deg slash was what I was thinkingI see now the vert is the thicker piece I would grind a V between the 2 and reweld. I'd try to stay in the thicker plate as much as possible.I'll toss in my usual caveat. Vehicle frames are poor choices for welding projects, especially when you are learning. Usually it's much more complicated that just simply running a bead between 2 pieces of metal. Newer vehicles use a variety of different special steels, often they are high tensile strength steels that require special weld procedures like pre and post heat as well as special fillers. Some are heat treated frames, and too much heat can remove the strength in local areas making the "stiffened" area weaker than before anything was done. Frame boxing is something that really requires some advanced knowledge to do right and safely.Unfortunately at this point you don't have a lot of good options if this is an on road vehicle. You may have already weakened the frame. There's no easy way to know unfortunately. Most mfg's will recommend a frame be replaced rather than welded. If this is for off road only use,  it may or may not work. The design idea really isn't right. A heavy vert piece doesn't add all that much strength. It will keep the flanges from buckling, but you get more strength by beefing up the flanges or deepening the frame member than by just adding some heavy plate to the open side. I'll Pm you a link that might make this clearer..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:If in doubt, grind it out!    The best laid schemes ... Gang oft agley ...
Reply:Originally Posted by david rif its porosity, you have to goo all the way down or the porosity will just return.  Been there done that.To quote welderwomn.  "you learn to be a good grinder before you learn to be a good weldor"keep at itdavid