How to avoid undercut?

[oldfornew]

My name's Colin and I have a problem with undercuts... Having worked up the courage to recognise this, wondering if any of you chaps can help me resolve the problem?

To explain, I am pretty newbie. The stuff I'm doing is small/light MS repair and fab in sheet/small gauge tube. And my scourge is small areas of undercuts. Take this example...

This is 3mm sheet MS, which is at the upper end of the material I am working with. I have drilled some 8.5mm holes in the wrong place (another story) and am remediating these. I have parted off some 'buttons' from a piece of bms round bar that push tightly into the holes as little plugs. So it's a very neat, tight fitup. I then run the torch around the periphery and over the top of these small plugs, fusing them in without using any rod. This is at 120A with a 2.4mm gold tip tungsten.

The result is just fine, except for dashes of undercut. The photos show the finished weld, and then after passing a grinder over it to flatten. Before you suck too much air at the apparent scale of the problem, remember that the weld area here is probably 15mm in diameter (it's an 8mm button) and that undercut some fraction of a mm deep. But it's really annoying when you're trying to generate flattish surfaces...

If it helps, I am right-handed and these undercuts usually appear at the back of a weld, or the left of the area I am circling in this case. Is torch angle playing in here? You can imagine on that top photo, that I am pushing the torch anti-clockwise over the top, and dragging it around the bottom.

All help and advice welcome.

Cheers, Colin

 

[Shox Dr]

You need to add rod, as metal will burn/gas off during the welding process. Welding also gases off other properties you need to add back in with rod

 

[hotponyshoes]

As shoxs says.
Also, even with a tight cut-up you still have some sort of gap there and the metal has to come from somewhere to fill it.

You could try cutting a longer slug so it sits proud and melt it from the middle pulling it outwards to use as filler but that might just end up causing more contamination.
Put some filler rod in there or cut some thin strips of the metal you are working with if you dont have any filler rods available.

If its structural rather than just cosmetic filling in holes there are other advantages to using filler rods that would make it worth getting some.

 

[zeppelin builder]

bear in mind the slug will get realy hot before the sheet pile the wire in to bridge the seam and leave an allowance for dressing .
btw a foot pedal would help to control the heat

 

[Richard.]

fusing them in without using any rod.

I’ll echo the posts above but I can’t understand how you thought it was going to go any other way.
If your just fusing metal together with an arc by melting it you are essentially cutting into it because your using part of its own mass as a filler to bridge the joint.
What your getting isn’t undercut as such it’s underfill and it’s inevitable if your welding with just an arc.

 

[morssdn]

When I'm doing this to fill a hole before re-cutting a bigger or smaller one, and using a holesawed insert, I just use MIG to go round the gap.