Why Do I Have Burrs on Bottom of Cut? (Speed vs Power Solved)

Introduction

In the metal fabrication business, there is a sound that hurts a shop owner’s ears more than any alarm. It is the sound of an angle grinder running in the finishing department.

Every minute your team spends grinding parts is a minute of lost profit. The promise of a fiber laser cutting machine is a “finished part”—a component that comes off the slats ready for welding, bending, or shipping. But the reality is often different. You pick up a part, run your gloved finger along the edge, and feel it: the sharp, jagged, or bubbly residue known as dross, slag, or burrs.

When you ask yourself, “Why do I have burrs on bottom of cut?”, you are essentially asking a physics question. You are asking why the molten metal failed to eject from the kerf before it re-solidified.

It is rarely just “bad luck.” It is a thermal imbalance.

Many operators immediately reach for the speed dial, turning it down to be “safe.” Others crank the power up to “blast through.” Often, both instincts are wrong. The relationship between Laser Power and Cutting Speed is a delicate dance, and getting it wrong leads to two very different types of burrs.

In this guide, we are going to put down the grinder and pick up the parameter sheet. We will diagnose the specific type of burr you are seeing, explain the thermodynamics behind it, and provide a step-by-step tuning protocol to eliminate it.

Diagnosing the Dross – Reading the “Crime Scene”

To fix burrs on bottom of cut, you must first identify what kind of burr it is. The burr tells a story. It tells you if the cut was too hot, too cold, too fast, or too slow.

In the industry, we generally categorize dross into two main types based on how hard they are to remove.

Type 1: The “Low Speed” Dross (Over-Burn)

• Appearance: Heavy, globular, bead-like droplets hanging off the bottom edge. The cut face might look rough or washed out.
• Feel: Very hard. It feels like it is welded to the part. You cannot knock it off with a simple scraper; you need a grinder or a hammer.
• The Cause: Too Much Heat. You are putting more energy into the sheet than is required to melt the metal. The excess heat melts the surrounding material, widening the kerf. The gas pressure drops because the hole is too big, and the heavy molten pool just drips down and freezes.

Type 2: The “High Speed” Dross (Drag)

• Appearance: Sharp, thin, jagged icicles. Often accompanied by “Trailing Drag Lines” (striations) on the cut edge that curve backwards opposite the direction of travel.
• Feel: Sticky but sometimes brittle. On stainless steel, it is very sharp.
• The Cause: Insufficient Heat (or Time). The laser beam is moving faster than the melt can fully liquify and eject. The beam “lags” behind. The bottom of the cut isn’t receiving enough energy to stay fluid, so it gets “dragged” along and freezes before the gas can push it out.

The Speed Variable – The Accelerator Pedal

Speed is the primary control for heat input. When diagnosing burrs on bottom of cut, speed is usually the first parameter we adjust.

The “Sweet Spot” Logic

Imagine you are cutting 2mm Stainless Steel with Nitrogen.

• Speed 100% (Optimal): The laser melts the steel, and the nitrogen blows it out instantly. The edge is clean.
• Speed 80% (Too Slow): The laser stays in one spot too long. The metal overheats. The gas becomes turbulent. Result: Type 1 Dross.
• Speed 120% (Too Fast): The laser rushes past. The bottom 0.5mm of the sheet doesn’t get enough energy. It stays semi-solid and sticky. Result: Type 2 Dross.

How to Tune Speed

Don’t guess. Run a “Speed Ladder” test.

1. Draw a simple square with 5 straight lines.
2. Assign a different speed to each line: 80%, 90%, 100%, 110%, 120% of your current setting.
3. Cut it.
4. Inspect the bottom.
   • If the slow lines have heavy dross and the fast lines have sharp dross, your “Sweet Spot” is in the middle.
   • If all lines have dross, your problem isn’t speed—it’s Focus or Nozzle (see Part 4).

The Power Variable – The Heat Engine

Laser Power (Duty Cycle) works in tandem with speed. You cannot simply “max out” the power and expect a good cut, especially on medium-thickness Carbon Steel.

The Duty Cycle Trap

When laser cutting steel (Carbon Steel) with Oxygen, we rely on an exothermic reaction. The iron burns itself.

• If you use 100% Power on corners or intricate details, you trigger a “Thermal Runaway.” The reaction gets too hot, the metal boils, and you get massive, crusty burrs on the bottom.
• The Fix: You must lower the Duty Cycle or use Pulse Modulation in corners. By pulsing the laser (turning it on/off rapidly), you reduce the average heat input, allowing the dross to clear before it fuses to the edge.

Viscosity and Temperature

For Nitrogen cutting (Stainless/Aluminum), high power is generally good because it lowers the viscosity of the molten metal.

• Hotter metal = Runnier liquid = Easier to blow away.
• If you have burrs on bottom of cut with Nitrogen, try increasing the power (if you aren’t already at 100%). A hotter melt pool flows faster, reducing the chance of it sticking to the bottom edge.

The Hidden Culprit – Focus Position

You can have the perfect speed and power, but if your Focus Position is wrong, you will never get a clean edge. This is the most common technical reason for persistent burrs.

Understanding the “Waist”

The laser beam converges to a tiny spot (the waist) and then diverges (spreads out).

• Focus Zero: On the surface.
• Focus Negative: Inside the material.
• Focus Positive: Above the material.

The Dross Connection

1. Focus Too High (Positive): The beam spot inside the material is too wide. The energy density drops. The cut gets wide, but the bottom of the cut is “cold.” The metal doesn’t melt fully at the bottom exit.
   • Result: Heavy dross that is hard to remove.
2. Focus Too Low (Negative): The beam is tight at the bottom, but the kerf (cut width) at the top is very narrow.
   • Result: The narrow opening chokes the gas flow. You have plenty of heat to melt the metal, but not enough gas volume to push it out. The dross gets trapped inside.

The Adjustment Strategy:

If you have dross, adjust the focus in 0.5mm increments.

• For Carbon Steel (Oxygen): Try raising the focus (more positive) to open the kerf for better gas flow, but not so high that you lose melting power.
• For Stainless Steel (Nitrogen): Try lowering the focus (more negative) to ensure the maximum power density is closer to the bottom of the sheet, helping to vaporize the exit burr.

The Ejection System – Gas Pressure and Nozzle

The laser melts it; the gas moves it. If your gas system is weak, the melt stays put.

1. Nozzle Centering

This is critical. If your laser nozzle is off-center by even 0.5mm, the gas flow hits one side of the kerf wall.

• Symptom: You will have burrs on bottom of cut on one side of the part, but the other side is clean.
• Fix: Perform a “Tape Shot” and center the nozzle perfectly.

2. Gas Pressure (The Force)

• Nitrogen Cutting: Requires high kinetic energy. If you are cutting 3mm stainless with 10 Bar, you will get burrs. Crank it up to 18-20 Bar. You need the brute force of the gas to shear the molten metal off the edge.
• Oxygen Cutting: Requires low pressure (0.6 – 1.0 Bar). Too much pressure creates turbulence, which cools the chemical reaction and causes chaotic dross. If you have dross on mild steel, try lowering the pressure slightly to smooth out the flow.

3. Nozzle Condition

A damaged nozzle tip creates turbulence. If the round orifice has a nick or a piece of slag stuck to it, the gas stream will flare out instead of shooting straight down. This lost pressure results in immediate dross formation. Never cut with a damaged nozzle.

Material Specific Troubleshooting Guide

Cutting Mild Steel with Oxygen

The Problem: Heavy, crusty burrs on the bottom.

The Diagnosis:

• Is the cut face smooth? If yes, but there is dross, your speed is likely too slow. Increase speed by 10%.
• Is the cut face rough (gouged)? Your pressure is too high, or your power is too high. Reduce pressure to 0.8 Bar.
• Check the Steel: Is it rusty? Rust (Fe₂O₃) interferes with the chemical reaction. Clean the rust or slow down significantly to allow the heat to penetrate.

Cutting Stainless Steel with Nitrogen

The Problem: Sharp, fine micro-burrs (sandpaper feel) on the bottom edge.

The Diagnosis:

• Gas Pressure: Is it high enough? Increase to 20 Bar.
• Focus: Is it deep enough? Lower the focus by 0.5mm.
• Nozzle: Are you using a single-layer nozzle? Double-layer nozzles create turbulence for high-pressure Nitrogen cutting and should be avoided here. Use a large diameter single-layer nozzle (1.5mm – 2.0mm) to allow maximum gas volume.

Cutting Aluminum

The Problem: Sticky, large dross.

The Diagnosis: Aluminum has high thermal conductivity (it sucks heat away) and low viscosity (it’s sticky).

• Speed: You are likely going too slow. Aluminum needs to be cut fast to prevent the heat from spreading.
• Focus: Focus needs to be deeper (Negative) to keep the bottom hot.

The “Wear and Tear” Checklist

Sometimes, the parameters are perfect, but the machine is tired. If you suddenly develop burrs on bottom of cut without changing settings, check these hardware issues:

1. Protective Lens: Is it dirty? A dirty lens absorbs power. You think you are cutting with 3000W, but only 2500W is reaching the plate. The reduced power fails to melt the bottom completely.
2. Focus Lens: Has thermal lensing occurred? If the lens is old, the focal point shifts as it heats up. Your “perfect” focus setting drifts by 2mm after 30 minutes of cutting.
3. Gas Purity: Are you using liquid nitrogen or a generator? If the purity drops below 99.9%, the oxygen impurity causes an oxidation reaction on stainless steel, creating a hard, black burr that is impossible to tune out.

Conclusion:

Eliminating burrs on bottom of cut is not about finding a magic number. It is about finding the thermal equilibrium for that specific sheet of metal on that specific day.

It requires a systematic approach:

1. Check the Hardware: Is the nozzle centered and the lens clean?
2. Identify the Burr: Is it low-speed (round) or high-speed (sharp)?
3. Adjust Speed First: Find the limit where the cut fails, then back off.
4. Adjust Focus Second: Move the heat where it’s needed.

Every minute you spend tuning your parameters on a scrap piece saves you an hour of grinding on the production run.