Of the 3 weld types I need to get right for the floor (plug weld, lap joint, and inside fillet) in terms of welder settings, I'm 100% there with the plug, about 85%-90% there with the fillet, and about 50% there with the lap weld.
I had 3 techniques to try with the plug weld, and really just a first guess about the voltage and wire speed settings for my welder, which is an Miller Autoset 140 MIG. I used the Miller chart on the inside of the welder cover as my first try for the 16 gauge to 16 gauge plug welds in manual mode on the tunnel. It was pretty good, but not quite there. A few changes and new trials got me there. As to the techniques, one was the traditional method (start in the hole center with vertical filler rod), second was Perry's variation that technique (weave-weld inside) and the 3rd was from the YouTube video referenced a page or two back in this log. They all work, but I found it easy to miss closing the open hole with the traditional approach. Perry's takes a little more welding dexterity than I possess, but using the basic idea of Perry's approach coupled with starting the weld outside the hole, weave-welding around the edge and then finishing in the center does work quite well for me. The hybrid approach gave me more time with the weld, and that is helpful for an amateur. Some one else may prefer another technique.
Here's the basic setup:
16 gauge plate tacked to a halved tunnel RHS:
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Three same size holes in each plate (1/4" or 5/16" or 3/8") and from left to right traditional, Perry's and my hybrid technique done in turn and each same-size hole.
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Here's the flip side with penetrations:
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Penetration was slightly better with the traditional method and Perry's method, but I believe my hybrid approach is still good. You can still see a slight bump in the hole material, so the penetration is complete.
Here are the same 3 with a 3/8" hole:
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For my particular application, I found the 1/4" hole small and unsatisfactory, the 5/16" workable, but the 3/8" hole gave me about the maximum weld area I can get for the 3/4" RHS and the most weld filler/base metal bond, so I'm going to use it.
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I had some old notes about fillet weld settings from doing the chassis rails. They were close and I got some acceptable welds (not pretty, but solid) right away. The one on the right in the photo below is pretty close. I may play with it a little more to see if I can improve on it.
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Fillet penetration was best with my last setting as shown on left below as the order was reversed by flipping the coupon.
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My first lap welds (above) were a disaster. I used the technique from the YouTube Video at right above and a conventional script letter 'e' at left. Long story short, I've decided I don't have the fine motor movement to do the above weld. I'm going to use a straight, continuous weld relying on my voltage setting to get me good penetration, and wire feed speed to get me good weld height, and minimize grinding after welding.
Below are several short welds where I was trying to get the right voltage/wire-speed settings. I like the second from the left in the horizontal sequence, but I'm not fully satisfied yet.
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I'd be interested in input from good welders about my choice above. The penetration for it is second from the right. I think I need a little more penetration, but slightly lower weld bead height. Also, I think a straight weld (no "c" or 'e" pattern for stack of dimes bead) will be strong enough if the penetration is good. Any disagreement?
Cheers,