What Causes Undercut In Mig Welding – Stop Making This Common
Undercut is a groove or “trench” melted into the base metal at the toe of the weld that remains unfilled by filler metal. It is primarily caused by excessive welding voltage, moving the torch too quickly, or using an improper torch angle.
To fix it, reduce your voltage, slow down your travel speed to allow the puddle to fill the edges, and ensure your torch is angled correctly to distribute heat evenly.
We have all been there—you lift your welding hood expecting a perfect “stack of dimes,” but instead, you see a nasty groove eaten into the base metal right at the edge of your bead. It is incredibly frustrating when your weld looks like it is sinking into the steel rather than bonding cleanly to the surface.
Understanding what causes undercut in mig welding is the first step toward achieving professional-grade results in your home garage or workshop. This defect is more than just an eyesore; it creates a structural weak point that can lead to catastrophic failure under stress.
In this guide, I will break down the technical settings and hand techniques you need to master to eliminate this common defect for good. Whether you are building a custom trailer or repairing a lawnmower deck, these insights will help you weld with confidence and precision.
Understanding the Basics: What Exactly is Undercut?
Before we dive into the technicalities, we need to define what we are looking at. In the welding world, the “toe” is the point where the face of the weld meets the base metal. Undercut is essentially a void or a recess at this toe that has been melted away by the arc but never filled back in.
Think of it like a trench dug next to a new road. If that trench is deep enough, the road (or your weld) loses its structural support. In structural applications, inspectors will fail a weld if the undercut exceeds a certain depth, usually around 1/32 of an inch.
For the DIYer, undercut is a sign that the heat was not balanced with the amount of filler metal being deposited. It means you are removing metal from the workpiece rather than adding to it. This leads to stress concentrations that can cause the metal to crack over time.
what causes undercut in mig welding
When we look at the core of the issue, what causes undercut in mig welding usually boils down to three main categories: machine settings, operator technique, and material preparation. If any of these are out of sync, the arc will chew into the base metal faster than the wire can fill the gap.
One of the most common culprits is excessive voltage. When the voltage is too high for the thickness of the metal, the arc becomes wide and aggressive. This high-energy arc melts a wide path, but if your wire feed speed isn’t high enough to keep up, you leave a groove behind.
Another major factor is travel speed. If you move the torch across the joint too quickly, the weld puddle does not have enough time to flow into the edges of the path carved by the arc. You are essentially “outrunning” your own filler metal, leaving the melted edges of the base metal exposed.
The Role of Torch Angle and Positioning
Your torch angle plays a massive role in how heat is distributed. If you are doing a fillet weld (a T-joint) and you point the torch too much toward the vertical plate, the arc will “dig” into that plate. Gravity then pulls the molten metal down, leaving an unfilled groove at the top.
Experienced welders know that what causes undercut in mig welding often comes down to the “work angle.” You want to split the angle of the joint evenly. If you lean too far in one direction, you are focusing the intensity of the arc on a single edge rather than the center of the puddle.
Wire Feed Speed (WFS) Imbalance
MIG welding is a balancing act between heat (voltage) and fuel (wire). If your wire feed speed is too low relative to your voltage, the arc length increases. A long arc is unstable and tends to spread out, melting the base metal in a wide, shallow pattern that is difficult to fill.
The Impact of Shielding Gas
While less common than heat or speed issues, your gas choice matters. Using a gas with a high CO2 content increases penetration and arc heat. If you are used to a 75/25 Argon/CO2 mix and switch to 100% CO2 without adjusting your settings, you might find yourself burning away the edges of your joint.
Technical Settings: Balancing Voltage and Wire Feed Speed
To stop undercut, you must master the relationship between your knobs. Most beginner welders tend to crank the voltage up because they want “good penetration,” but they forget that penetration requires volume. Without enough wire, that penetration just becomes a hole or a groove.
Start by checking your welder’s door chart for the thickness of the metal you are using. If you see undercut, try decreasing your voltage by one or two increments while keeping your wire feed speed the same. This narrows the arc and reduces the “digging” force.
Alternatively, you can increase your wire feed speed. By adding more filler metal into the puddle, you give the weld the “bulk” it needs to reach the edges of the melted zone. This is especially helpful if your bead looks flat or concave rather than slightly rounded (convex).
The “Sizzle” Sound Test
A well-tuned MIG welder should sound like bacon frying. If it sounds like a loud, erratic hiss, your voltage is likely too high (causing a long arc). If it sounds like a machine gun with lots of spatter, your wire speed is too high. Aim for that consistent, smooth sizzle to minimize the risk of undercut.
Mastering Your Technique: The Human Factor
Even with perfect settings, your hands can still cause defects. When troubleshooting what causes undercut in mig welding, you should first look at your travel speed. Slow down and watch the “toes” of the weld puddle.
You should see the molten metal flow out and “wet” into the base metal. If the puddle looks like it is struggling to reach the edges, you are moving too fast. Give the puddle a split second to fill the cavity created by the arc before moving forward.
Using a Weave or Oscillation
In some cases, a straight “stringer” bead isn’t enough to fill a wide joint. Using a slight side-to-side weave or a “C” motion can help. When you weave, pause for a tiny fraction of a second at the edges (the toes) of the weld.
This pause allows the filler metal to deposit exactly where the undercut usually forms. Don’t rush across the middle; the edges are where the structural integrity is won or lost. This technique is a “pro secret” for getting clean, full welds on thicker plate steel.
Contact Tip to Work Distance (CTWD)
How far you hold the torch from the metal matters. If you pull the torch too far away (increasing your “stick-out”), the voltage at the arc actually increases, which can lead to a more erratic arc and potential undercut. Keep your contact tip about 3/8″ to 1/2″ away from the work for the most stable arc.
Material and Environmental Factors
Sometimes the problem isn’t you or the machine—it’s the metal. Mill scale, rust, and oil can all interfere with how the weld puddle “wets” out. If the puddle can’t bond to the surface because of contamination, it will pull away from the edges, creating the appearance of undercut.
Always grind your weld area to shiny bright metal. This is non-negotiable for high-quality MIG welding. Removing the layer of oxidation allows the molten metal to flow smoothly to the toes of the joint, filling the area completely.
Welding Out of Position
Welding vertically or overhead is much harder because gravity is working against you. In a vertical-up weld, gravity wants to pull the puddle down and out of the “trench” the arc just dug. This is why vertical welding requires lower voltage and a very specific “shelf-building” technique to prevent undercut.
Step-by-Step Fixes for Cleaner Welds
If you are staring at a piece of metal with undercut right now, follow this checklist to fix your next pass:
- Clean the Metal: Use a flap disc or wire wheel to remove all scale and rust.
- Check Your Settings: Drop your voltage by 1-2 volts or increase your wire speed by 10-15 inches per minute.
- Adjust Your Angle: Ensure your torch is bisecting the joint angle (usually 45 degrees for a fillet).
- Slow Down: Watch the back of the puddle. Ensure it fills the “hole” before you move the torch forward.
- Watch the Arc: Keep a tight stick-out distance to maintain arc stability.
By systematically changing one variable at a time, you can isolate exactly what was causing the issue in your specific setup. Most of the time, it is a combination of moving too fast and running too hot.
Safety First in the Workshop
Before you start practicing these fixes, remember your safety gear. Welding produces intense UV light that will burn your skin and eyes (arc flash) in seconds. Always wear a properly rated auto-darkening hood and flame-resistant clothing.
Ensure your workspace is well-ventilated. MIG welding produces fumes that you should not breathe in. If you are working in a tight garage, use a smoke extractor or at least a high-volume fan to move the air away from your face.
Frequently Asked Questions About MIG Welding Undercut
Can I just weld over undercut to fix it?
Yes, you can “fill” undercut by running another small bead over the affected area. However, you must clean the original weld thoroughly first. If you don’t remove the slag or oxidation, you might trap inclusions inside the fix, making the joint even weaker.
Is undercut always a structural failure?
Not always, but it is always a defect. For non-critical items like a garden trellis, a tiny bit of undercut is mostly a cosmetic issue. For anything that holds weight—like a car frame or a hoist—undercut is a major safety risk and must be corrected.
Does the type of wire cause undercut?
While the wire itself usually isn’t the primary cause, using a wire diameter that is too small for the voltage you are running can contribute. If the wire is too thin, it melts away too quickly, making it harder to fill the groove created by a high-voltage arc.
Why does undercut happen more on thin metal?
Thin metal has less “heatsink” capability. It melts almost instantly. If your settings are even slightly too high, the arc will blow away the edge of the thin sheet before the wire has a chance to create a puddle.
Conclusion: Mastering the Puddle
Eliminating undercut is a rite of passage for every DIY metalworker. It forces you to stop just “gluing metal together” and start actually managing the molten puddle. Once you understand the balance between heat, speed, and filler metal, your welds will transform from amateur to professional.
Don’t get discouraged if your first few attempts at fixing it aren’t perfect. Welding is a muscle-memory skill. Spend some time on scrap metal intentionally creating undercut, then try to fix it by slowing down your travel speed and adjusting your torch angle.
The more time you spend under the hood observing how the metal reacts to your movements, the more intuitive the process becomes. Keep your metal clean, your settings balanced, and your movements steady. You will be laying down flawless, structural beads in no time. Now, get out to the garage and start practicing!
