Beginners question - can I weld to my steel frame?
I have a Trek 930 with a screw stuck in one of the rear rack frame mounts. It's seized and rusted in there, broken off, and all attempts to remove it have failed.
I would like to put a rack on this bike, but this is preventing it. I have a mig welder, and I know how to use it - I've welded up all kinds of armor, bumpers, frame stiffeners, shock towers, etc, etc for my Jeep.
What I would like to do is just cut that bung off the frame, and weld a new one on. It would be really easy, but I'm concerned that I may damage the temper of the frame or something.
Are these frames tempered? Will I weaken it by welding?
Can I just weld a new threaded bung?
Why wouldn't a mig work? It is a steel frame, with a wirefeed welder setup to weld steel, and it even has shielding gas. I get why you wouldn't want to make a frame with it,.. but for a simple tack weld to hold on a mount it should work fine right?
Unless I'm missing something?
I was just worried about making it weaker, or ruining a temper if the frame is tempered,..
I tried drilling it already,.. that screw has to be hardened, it's not easy to drill through, I may have to go that route if welding won't work though.
racks take a lot of load, and a tack weld is likely to crack. The other issue is that the tubing is very thin, and the rack fitting is not. If you manage not to blow a hole in the tube, you might well not get good penetration on the fitting. If I couldn't get the bolt out and I didn't have oxy/acetylene handy I would use p-clamps. Easily available and robust. In fact, I would use p-clamps before messing up my paint.
I'm not a framebuilder, but knowing the processes, MIG would problematic. TIG would be better as it's easier to control the heat. Brazing is more appropriate.
However, I'm with erik on this one. Drilling and retapping is the way to go. It's a heck of a lot less destructive, plus you won't have to worry about repaint. If you do it well, you'll only get rid of the old screw threads with no damage to the frame. Drilling the screw out may also remove the stuck screw and you wouldn't have to retap the threads.
Carbide drill bits are pretty cheap in small sizes. Another option is to get a piece of wire, like a cleanly cut coat hanger, and heat it red hot. maybe get two. Hold these against the cut screw, this will normally draw the temper. You will heat the screw which will loose it's temper, and the frame might heat a little, but since it isn't going to be heat treated, particularly in that area, who cares. Watch out for the paint. You may have to cycle the heat a few times if it looks like it is not heating a lot.
Also if you have an arc welder there is the possibility of welding a stub to the screw, which either works or it fails, but you will sure heat the screw. You can stinger the screw, and that will sometimes break it free. I have never successfully welded something there, and also turned it. But every time I blew it's brains out and it came out the old fashioned way. But this is last resort kind of stuff, the heat is highish (you don't need to use welding heat) so you could end up taking the BO off if it was brazed.
I don't know what the smallest size they make those screw removal kits is. But the basic idea is to drive a screw into a hole in the screw. A small self taping screw might work. Normally the deal is you need everything left hand so you can back the screw out, but since this is an open hole, you could just drive it through which means you could use standard right hand. Get a small allen bolt and drill that size root hole in there. then run a wire through it, and heat that wire. After it cools tap it, install your allen head, drive it through. Throw everything at it. Rust remover, penetrating oil, heat the rack mount at the ends, etc... You want to get that screw out, far better than taping the current hole. Keep in mind, that there is a reason why this thing broke off in there - because 5mm rack mounts are a redundant joke. If you have the beef for it, you want to drill it out for 6mm (5mm hole tapped). So that gives you some room to pay with.
MIG is perfectly fine for welding frames, and after a few years of trying it and burning holes in stuff, you might get good enough to do it, Factory robots do it all the time. But you could install a new Rack mount with a propane torch. The solder is real expensive for a one shot job, probably only worth it if you have hard solder on hand.
Keep in mind that before all this arriviste bike stuff we just used P clamps to install our racks, so if you want to put this all off until you get your tools and consumables up to the task, just install the rack with P-clamps, and hit the toad.
TIG allows you to control the heat independently of the rest of the process, and how easy that is, is another mater.
I just upgraded (I hope) to a whole new TIG. You need a university degree in not reading the instructions to figure this thing out. Funny reading forums, there are so many dials, and doodads, that you get these threads where guys get crossed up cause they set slope to do one thing, low end pulse for another, starting voltage, etc... "hey this thing's broken!". Sorta like those derailleur gears that "don't work" but at three thousand degrees. I get a kick out of those folks who say TIG is done to save money, and because it's easier than pointing a torch in the general direction of a cluster of pipes.