MIG/Root Pass Difficulties

[Irish Welder]

I've been taking MIG classes at a local junior college for about one year, and I'm about to test for my first AWS certification. I'll be welding 3/8" steel plate in the vertical down position.

My question relates to making the root pass. I sometimes have difficulty obtaining full penetration through the plates. The plates are prepared with a 30 degree bevel, then ground with an angle grinder. I'll be welding with a Miller CP-302, a three-phase machine, and using .035 wire with CO2 gas. The plates are first tacked at the top and bottom, with a 1/8" gap at the top and 3/32" at the bottom.

My instructor advises that when the plates are viewed from the back, the root pass should have a bit of a crown. If there's no crown, the weld is not acceptable and won't pass the test.

Does anyone have a suggestion on techniques to achieve a proper root pass in these circumstances?

Thanks in advance for your response.

Irish

[Icarus]

I've been taking MIG classes at a local junior college for about one year, and I'm about to test for my first AWS certification. I'll be welding 3/8" steel plate in the vertical down position.

My question relates to making the root pass. I sometimes have difficulty obtaining full penetration through the plates. The plates are prepared with a 30 degree bevel, then ground with an angle grinder. I'll be welding with a Miller CP-302, a three-phase machine, and using .035 wire with CO2 gas. The plates are first tacked at the top and bottom, with a 1/8" gap at the top and 3/32" at the bottom.

My instructor advises that when the plates are viewed from the back, the root pass should have a bit of a crown. If there's no crown, the weld is not acceptable and won't pass the test.

Does anyone have a suggestion on techniques to achieve a proper root pass in these circumstances?

Thanks in advance for your response.

Irish

It wont penetrate as deep as if your doing it in the flat position but it should come through a bit. Stay on the leading edge of your puddle and maybe try turning the inductance up a bit. That helped me when I was in school. Sorry I couldnt be more help, thats all I got.

[shorerider16]

First of all, I want to say that my experiences are based on schooling in B.C. and Canadian standards, I don't know if there are any big differences between standards and the like. This is also my personal experience, so your's and other's milage may differ.

For vertical down with wire I went with a 3/32 gap top and bottom with no landing, we also used run off tabs so there was no worries about the gap pulling together. I ran my machine at 18.5 volts and 150 inches per minute. I used an apr. 25 to 30 degree gun angle (from perpendicular), staying at the leading edge of the puddle and threw in the slightest (and I do mean slight) weave. That's what worked for me.

I pressume that you are going to cut your test pieces into coupons and bend them? If so, the root only needs to be flush, although slightly convex is preferable. Our instructor essentially told us that as long as the weld was flush or better, with no concavities, it would be good.

Good luck.

[hankj]

Pound a couple pints of Guiness first......

Hank

[MIKEDOGG]

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