[Buckaroo93]

If the two pieces that I want to join are different thicknesses, which material thickness do I set my FCAW machine on? I want good penetration on the thicker one but don't want to blow through the other.

[JayWal]

I’d set it according to the thinner piece. Maybe a little bit hotter, then spend more time on the thicker material when I’m actually welding. You don’t need 100% penetration on the thicker piece anyways, since the thinner material will fail before the weld does. ( if done correctly )

[tweake]

i tend to set it more to the big thickness but it does depend on how much difference between the size.
then run on the thick part and wash it on to the thin part.

if you go cold and try to compensate by going slow on the thick part. you can end up with a really big cold weld that can fail.

the other way is technique/ if you doing O's you start on the thick, go round to the thin then come back onto the thick again. so thick gets heat twice for every one on the thin side.

i think theres also a way of doing it with double pass. a small coldish weld first then once its cooled down go over it with a hotter pass. that way the you build metal up on the thin side which helps it cope with the 2nd hotter pass.

but then again its mig and i don't get much time on that.

[JayWal]

I’ve had some issues with the “washing in” where it didn’t fuse into the thinner metal adequately. ESPECIALLY if I didn’t take off the mill scale first. (Oh come on I know I’m not the only one who cheats occasionally)

If you’re welding, say 16 gauge onto 1/4” thick flat bar it’ll work no problem. I’ve had issues with heavier stuff, say you’re welding 3/16 into 1/2 plate. Then I’d probably use 2 beads or more like mentioned above. Mig can be very deceiving in terms of judging how well you penetrated.

[MinnesotaDave]

I set for the thicker piece and cheat to that side.