What's inside
As an Amazon Associate I earn from qualifying purchases. This post may contain affiliate links at no extra cost to you.
A lap joint puts one piece of metal over another, with the weld usually placed along the edge of the upper piece. It is common in brackets, sheet-metal repairs, ductwork, equipment frames and automotive fabrication. The joint is easy to assemble, but it can be surprisingly easy to undercut the top edge, burn through the lower sheet or trap an unwelded gap.
MIG settings for a lap joint depend on material thickness, wire diameter, shielding gas and joint fit-up. Start with the machine’s chart, then tune the arc on scrap from the same material. A sound weld should tie into both pieces with a smooth transition, without excessive buildup or a deep groove beside the bead.
Choose the Wire and Shielding Gas
For clean mild steel, solid ER70S-6 wire with an argon-rich shielding gas is the most forgiving choice. A 75% argon/25% carbon dioxide mix produces less spatter and a smoother arc than straight carbon dioxide. Straight CO2 is cheaper and gives good penetration, but it runs hotter, spatters more and can make thin lap joints harder to control.
| Material or job | Practical MIG setup | Main trade-off |
|---|---|---|
| Thin mild steel, about 18–14 gauge | 0.023-inch solid wire, 75/25 gas | Low heat and clean control, but less tolerant of dirt |
| Medium steel, about 1/8–3/16 inch | 0.030-inch solid wire, 75/25 gas | Good general-purpose balance |
| Thicker steel, 1/4 inch and above | 0.035-inch wire, higher amperage and often multiple passes | More deposition, but greater heat input and distortion |
| Dirty or lightly rusty steel | Self-shielded flux-core wire | More tolerant of contamination, with slag and more cleanup |
For occasional home repairs, a MIG welding wire spool and 75/25 gas are usually cheaper to run than buying specialty wire. Use stainless or aluminum wire only with the correct liner, contact tip, drive rolls and gas; changing only the spool is not enough.
Starting Voltage and Wire Speed
Voltage controls arc length and strongly affects bead width. Wire-feed speed controls how much wire enters the arc and, on a conventional MIG machine, is closely related to amperage. More wire speed generally means more current. Do not treat the voltage and wire-speed controls as independent “heat” knobs: they must be balanced.
Use these as starting points for solid wire with 75/25 gas. The exact settings vary by machine, polarity and manufacturer chart:
| Steel thickness | Wire diameter | Starting voltage | Starting wire speed |
|---|---|---|---|
| 18 gauge, about 0.048 inch | 0.023 inch | 15.5–17 volts | 180–260 inches per minute |
| 16–14 gauge, about 0.060–0.075 inch | 0.023 or 0.030 inch | 17–18.5 volts | 220–330 inches per minute |
| 1/8 inch | 0.030 inch | 18–19.5 volts | 280–380 inches per minute |
| 3/16 inch | 0.030 or 0.035 inch | 19–21 volts | 350–500 inches per minute |
These figures are not a substitute for a test weld. If the machine provides an inductance or arc-control setting, leave it near the middle initially. Higher inductance usually gives a softer, wetter arc; lower inductance makes the arc more forceful and can help with control, but may increase spatter.
Prepare the Lap Joint
Clean mill scale, paint, oil and rust at least 1 inch on both sides of the weld line. A flap disc or wire wheel is useful, but do not grind the upper sheet so thin that the edge disappears. Clamp the overlap tightly. A gap between the sheets acts as a heat sink in some areas and a burn-through trap in others.
For thin steel, an overlap of roughly 1/2 to 1 inch is practical. Tack at both ends and in the middle before making the final weld. Check that the parts have not pulled out of alignment. If the joint is long, alternate short welds rather than laying one continuous bead that dumps heat into one corner.
Use a MIG welding clamps or locking pliers to hold the sheets firmly. Basic clamps are fine for occasional work; expensive positioning fixtures become worthwhile when repeatability matters more than cost.
Control Torch Angle and Travel
Keep the contact-tip-to-work distance around 3/8 inch with solid wire. A longer stickout makes the arc unstable and reduces effective current; a very short stickout increases the chance of the tip touching the puddle. Hold the gun about 10–15 degrees from perpendicular and use a slight push angle when welding mild steel with solid wire.
A lap joint needs the arc to reach both sheets. Aim near the upper sheet’s edge, not at the center of the overlap. For a small fillet, pause just long enough at each edge to wash the puddle into both pieces, then move steadily across the joint. Excessive weaving adds heat without reliably improving fusion. A stringer bead is normally easier to control.
Travel speed is one of the most important adjustments. If the bead is tall and narrow, slow slightly or increase voltage within the machine’s recommended range. If it is wide, flat and heavily spattered, travel may be too slow or the voltage and wire speed may be mismatched. On thin sheet, short stitch welds can prevent burn-through, but leave enough overlap between stitches to avoid weak unwelded sections.
Recognize Common Problems
Burn-through: The puddle falls through the lower sheet, often because of excessive wire speed, slow travel, a large gap or too much dwell at the upper edge. Reduce wire speed and voltage together if necessary, move faster, and use short stitches. A copper backing bar can absorb heat when access allows.
Undercut: A groove beside the upper sheet means the arc has melted the edge without filling it. Reduce voltage slightly, slow the travel, or pause briefly at the edge. Undercut is especially dangerous on thin structural brackets because it reduces the parent metal’s thickness.
Lack of fusion: The bead may look acceptable while failing to bond to the lower sheet. This is caused by aiming too high, low wire speed, fast travel, contamination or insufficient heat. Make a test weld, cut through it, and inspect the cross-section when the joint carries a meaningful load.
Porosity: Pinholes or bubbles usually indicate poor gas coverage, drafts, moisture, oil or a blocked nozzle. Set gas flow around 20–30 cubic feet per hour indoors, protect the arc from wind, and keep the nozzle free of spatter. More gas is not always better; excessive flow can create turbulence and draw air into the shielding zone.
Use the Right Equipment
Wear a properly rated auto-darkening welding helmet, welding gloves, cotton or leather clothing and closed leather footwear. Keep the work area free of solvents, aerosols and flammable dust. Welding galvanized or painted steel can produce hazardous fumes; remove coatings where possible and use local exhaust or suitable respiratory protection.
A compact 120-volt MIG welder is sufficient for many 18-gauge through 1/8-inch lap joints. A 240-volt machine is the better buy if you regularly weld 3/16-inch material, need long continuous beads or want a wider duty cycle. Do not buy a larger welder merely for its maximum amperage if your work is mostly thin sheet; controllable low-end output matters more.