Big 40 Amps

[weirmeister]

I just purchased a 1970's big 40 portable welder. I'm using 7018 5/32 for 3/4 and 1 inch plate, flat welding only. I'm having problems with slag pockets. I have been trying different amperage settings and still having problems. The amperage I'm using is 175over 100 and 55 on the fine amperage setting. I have to say I don't get the relationship between the main setting and the fine setting. Any ideas will be appreciated.

Thanks Doug

[Portable Welder]

The big 40 puts out 400 amps max if I remember right without looking in a book, I do know you should be able to run a 1/4" 7024 because I use to on my 1989 Miller big 40 G.

The big dial at its lowest setting will let you go from say 0 to 90 lets say, you then use the fine dial to give you amperage between zero and 90, when you move the big dial to the next notch up, it takes you to a range of 90 to 180 and you then use the fine tune knob to adjust your amperage between 90 and 180 amps.

There is a little overlap typically.

The numbers I gave are not actual for that machine, I was just using them as an example of how it works.

[gnforge]

Big 40 Amps

If your course setting is 100-175 amps. Your fine setting is in percent of that range. As long as your fine side goes from 0 to 100 as my 1988 model year. That means at 0 % you would be at 100 amps.
At 100 % on fine you would be at 175 amps.
So at 55% on fine side of 100-175 amps. That would be welding around 140-amps.
As for the 7018 5/32 Rod. Miller recommends 150-220 amps. So you might be running a little low.
I would try same course setting and 75% to 100% on fine side and see if that helps.
Next you could turn course setting to next higher setting and try 0 to 55% on fine setting.
Another thing to check is you should be welding on DCEP for 7018 rods that is Elec. Pos.
I bought my 1988 Big 40D used and aft welding on both settings it welded as if they were reversed. After welding a little with it I was almost sure it was reversed. I had my LWS rep. stop by and check with meter and sure enough it was reversed. Someone must hv had it apart and reversed wires when put together. So make sure your welding on Elec. Pos with 7018 rods.
That welder should hv plenty of amps for that Rod. Hope it helps. Greg

[M J Mauer]

Doug welcome to the forum.

[Portable Welder]

Gnforge, Well said.