Weld seam after machining

[gunsmith]

This does not cause a problem except that it is unsightly to me. When I build up a portion of a shaft and machine it down I am always left with the problem of the starting point of the weld or slight undercut. I have faked it by cutting a small groove at this point but only when it did not interfere with the work. I know spray welding solves this problem but a spray welder is not in my shop. Is there another solution??

[CCWKen]

I'm not sure what you mean. I've built up a couple of axles with a mig and always build up extra. At the point where the weld ends on the rest of the shaft, I go back with a slow wire speed and put a fillet. It gives a good blend point and doesn't undercut the shaft.

When this is cut or ground, you can't tell where the weld ends. If you cut a grove at this point, you just induce a fracture line.

[wierdscience]

Thats a tough one,you always are gonna see something there even if it's just a discoloration between the weld metal and the base.
I try to use MIG or TIG for build ups because it's easier to eliminate undercut.I have built up a shaft and then heated and hammered the bead down to just above flush,but this is only works on large shafts.

Remembering to cut the heat down once the first bead has been laid helps greatly too.

[torker]

The solution is your technique. There should never be any undercut. You didn't mention the process used. Weird is correct...excess heat can cause this. If using mig...preheat first so you don't have to weld so hot. A "pushed" mig weld can easily have undercut. If you are weaving you may not be holding on the sides long enough to allow the undercut to fill in. If you have too long of an arc length...this can also cause it. Or the wrong electrode angle. You should maybe look a little further back on your puddle to make sure the undercut is filled. We have to pass proceedure tests all the time and any kind of undercut is an immediate failure. The coupons(pipe or flat bar) have to be ground and polished flat...any undercut will show. You can't see the weld joint until after the coupon is bent. Your welds should look the same.
Russ

[Smokedaddy]

Gunsmith,

Listen to Russ, there shouldn’t be any under cut in the first place, especially at the start of a weld, if I understood your question. You must be doing something wrong like Russ mentioned … rod angle perhaps too. If that’s true, then you obviously aren’t waiting for the puddle to fill in the crater. The only reason I’m mentioning this is the start is usually the place where most welders have “too much” build up, not under cut. Under cut usually (notice the usually) takes place because you’re simply going to fast and not letting the molten metal fill the crater you’ve created with the arc. It usually happens on the sides of a fillet weld, when welding pipe, or when you pull out of the puddle at the finish. As was mentioned, running to hot can cause under cutting too, but you’d have to be extremely hot. You can nearly always fill in the under cut if you simply slow down and let it fill the crater. Long arcing simply raises the heat as well and depending on the rod will cause prosity. Just remember, the molten puddle is a liquid and liquid runs down hill and follows the heat.

http://www.pbase.com/smokedaddy/tig&page=all

-SD:

[This message has been edited by Smokedaddy (edited 01-24-2005).]

[tattoomike68]

the guys are right on technique.

undercut is from going too fast, pushing the weld and too much heat.

sometimes I turn a little undercut .030 if the shaft has melted bearing on it or massive work hardening, it also gives a clean place to weld and the turned weld will be the weld metal and not a mix or old workhardend metal and weld.

If done right a peice of crap shaft will have one nice place on it.

[G.A. Ewen]

Well shoot, there is nothing left for me to say.

[torker]

Now GA...We expect, in the future...that you constantly warn us that....mild steel can shatter like glass
Russ

[torker]

Oh ya...gunsmith...are you rolling these shafts on a roller as you weld them or how are you doing it. That can be part of the problem.
Russ

[cliff69]

Smokedaddy, that house looks familiar, where is it exactly, Somewhere around northern and 12th?