Laser systems are often presented as precision tools governed by simple rules: set the focus, choose a power level, add air assist, and run the job. In practice, many of the rules repeated in manuals, forums, and videos are incomplete or misunderstood.
This article addresses some of the most persistent laser cutting myths, using real-world testing and practical physics to explain why many commonly repeated rules fail in practice. Throughout this article, CO₂ laser behaviour is examined using measurement and demonstration, with CO₂ laser physics explained in practical terms rather than simplified rules of thumb.
This page consolidates evidence-led explanations of common laser cutting myths and misunderstandings as well as laser engraving myths, based on practical demonstrations and measurement-driven analysis. The purpose is not to contradict manufacturers, but to explain what is actually happening at the beam, lens, and material interface, so results become predictable rather than experimental.
Laser Cutting Myths | Focus and lenses: optical focus vs intensity focus
A persistent misunderstanding is that focus is a single, fixed point defined by the lens manufacturer. Many laser lens focal length myths arise from assuming that a longer focal length automatically delivers better cutting performance, without considering beam quality, intensity distribution, or cutting speed.
Manufacturers specify an optical focus — the geometric point where light converges to form a sharp image. Laser cutting and engraving, however, depend on intensity focus: the location where energy density is high enough to vaporise material. These two points are not always the same.
In real machines, intensity focus shifts with:
- Cutting or engraving speed
- Delivered power
- Material type and thickness
- The internal energy distribution of the beam
This explains why a job can improve when focus is moved slightly above or below the nominal focal distance.
A related misconception is that CO₂ lenses must always be installed flat side down. While that orientation suits image projection, practical testing shows that reversing a lens can, in some cases, reduce surface scorching by filtering low-power peripheral rays created by spherical aberration.
However, this orientation change comes with a significant performance trade-off. In controlled testing, reversing the lens reduced cutting speed by approximately 40 % compared with the conventional orientation used as a benchmark. It is therefore a selective optimisation, not a general improvement.
Pro Tip
Reversing a lens can improve cut cleanliness in specific cases, but it does so at the expense of speed. Treat it as a problem-solving technique, not a default setup. Consistent, well-specified CO₂ lenses make these trade-offs easier to evaluate repeatably.
Laser Cutting Myths | Beam behaviour and cutting physics
Laser beams are commonly described as perfectly Gaussian and neatly confined to the diameter quoted in specifications. In practice, this description hides several important effects.
The quoted beam diameter usually refers to an international standard that represents only a small fraction of peak intensity. Significant power exists beyond that boundary. In real systems, the effective beam diameter can be much larger, which explains why mirror mounts and frames may heat up despite apparently correct alignment.
Beam divergence is another frequently ignored factor. On flying-optics CO₂ systems, the beam grows in diameter as it travels. As a result, intensity at the lens — and therefore cutting performance — changes depending on where the head is positioned on the bed.
Curved cuts or so-called “beam drag” in thick acrylic are often blamed on light reflecting off the kerf walls. Light does not bend in this way. What is observed is heat-induced cutting, where hot gases trailing the beam continue to remove material after the beam has passed.
Similarly, striations on acrylic edges are usually a consequence of the material’s intermittent vaporisation process, not a mechanical fault or airflow problem.
Pro Tip
If cuts vary across the bed or mirror mounts show unexpected heating, the cause is often off-axis beam energy rather than alignment error. Mirror substrate quality and mounting stability become increasingly important as power increases.
Laser Beam Alignment Myths and mirror setting
Alignment is frequently treated as a single adjustment exercise: tweak the screws until the burn looks centred. This approach overlooks the mechanical reality of most laser systems.
Accurate alignment consists of two separate steps:
- Adjusting mirrors so the beam travels parallel to the machine axes
- Mechanically positioning the mirror or head so it sits on the beam path
Attempting to achieve both through mirror screws alone leads to repeated iteration without convergence.
Another common belief is that the laser tube must be perfectly parallel to the gantry. In practice, the first turning mirror can compensate for tube angle, provided the remainder of the system is adjusted correctly.
Visible red-dot alignment tools and reverse-alignment methods can assist with initial setup, but they are fundamentally limited. Most assume a fixed laser head and an adjustable tube — the opposite of how many hobby and mid-range machines are constructed. Ultimately, only the laser beam itself reveals its true path through the system.
Pro Tip
Use red-dot tools only as rough indicators. Final alignment must always be performed with the laser beam itself, as visible pointers cannot account for divergence, beam profile, or mirror interaction.
Air Assist Laser Myths: pressure, flow, and purpose
Air assist laser myths usually begin with the pressure gauge.
The pressure shown on a compressor regulator is static pressure, not the pressure delivered at the nozzle. Once air is flowing freely, actual nozzle pressure is often only a few PSI. This is why increasing regulator pressure does not always improve cutting.
High-pressure air is also not universally beneficial. A fast cut with slanted kerf walls can block airflow entirely, whereas slowing the cut slightly can produce parallel walls that allow even low-pressure air to clear debris effectively.
For engraving, the primary purpose of air assist is lens protection, not smoke removal. Excessive airflow can blow resin-laden smoke back onto the surface, permanently staining wood and some plastics.
Pro Tip
Before increasing air pressure, test whether a small speed reduction improves kerf geometry. Improved kerf shape often reduces airflow requirements more effectively than higher pressure.
Materials: acrylic, wood, and wavelength reality
A large number of acrylic laser cutting myths stem from misunderstanding how acrylic responds to heat, phase change, and gas flow rather than from limitations in laser power. Acrylic cutting is often described as a chemical process similar to burning wood. It is not. Acrylic cutting is a physical phase change: the material melts, boils, and evaporates. The resulting vapour temporarily shields the cut, producing the characteristic striations seen on edges.
This also explains why clear acrylic cannot be processed by standard blue diode lasers. At those wavelengths, clear acrylic absorbs almost no energy; the light passes straight through. CO₂ lasers operate at a wavelength that acrylic strongly absorbs, forcing energy into the surface.
It is sometimes claimed that thick acrylic requires very high power and long focal length lenses. Practical testing shows that 20 mm acrylic can be cut with a 70 W system and a 1-inch lens, but only under extreme operating conditions — approximately 0.5 mm/s cutting speed at full power. This is a special-case technique, not a general cutting strategy.
Wood introduces a different challenge altogether. It is not a uniform material. Variations in cellulose and sap content mean energy absorption changes across the grain, regardless of software settings.
Pro Tip
When diagnosing inconsistent cutting or engraving depth, measuring output stability with a calibrated power meter is often more effective than repeated trial-and-error parameter changes.
Cooling, chillers, and power stability
A common belief is that laser output drops by roughly one watt for every degree Celsius increase in cooling water temperature. Practical measurements show that total output power remains largely stable across normal operating ranges.
What does change is beam quality. Rapid temperature cycling — particularly from refrigerated chillers switching on and off — can cause cold water to strike the rear face of the first internal mirror inside the laser tube. This induces temporary curvature in the mirror, distorting the Gaussian intensity distribution and reducing effective cutting or engraving performance until thermal equilibrium is restored. In photo engraving, this appears as banding.
Laser tubes are made from borosilicate glass with very low thermal expansion. Operating temperatures in the 30–40 °C range do not inherently cause cracking. Stability and gradual temperature change are far more important than chasing the coldest possible water.
Pro Tip
If engraving banding appears after fitting a chiller, prioritise temperature stability over lower setpoints. Measure output consistency before assuming insufficient cooling.
Photo engraving, greyscale, and “3D” misunderstandings
Humans do not see in true 3D. We perceive depth by interpreting 2D images using shading, contrast, and prior experience. Many “3D engravings” rely on this illusion rather than actual dimensional control.
Greyscale engraving is commonly used to simulate tonal variation by varying power. In practice, this often creates accidental depth, rough surfaces, and loss of detail. True photo replication depends on one-dot-to-one-pixel control, using dithering to vary dot density rather than digging deeper into the material.
A critical mechanical condition applies: the laser’s burnt dot size must be equal to or smaller than the image pixel size. If the dot is larger, dots overlap regardless of dithering algorithm, resulting in excessive heat build-up and “charcoal” images with no tonal control.
Pro Tip
If photo engraving looks muddy or over-burnt, measure the real burnt dot size before changing image settings. Most photo issues originate in optics or mechanics, not software.
Where these findings apply — and where they don’t
The explanations on this page apply most directly to:
- CO₂ glass-tube systems with flying optics
- RF CO₂ systems using PWM power control
- Diode systems, with important wavelength-specific limitations
While fibre lasers share optical principles, their wavelengths and material interactions differ and are outside the scope of most examples discussed here.
Frequently Asked Questions
What is the difference between optical focus and intensity focus?
Optical focus is the geometric focal point for image projection. Intensity focus is where energy density is highest and material vaporisation actually occurs.
Why can a 1-inch lens cut thick acrylic?
Only under extreme conditions. Demonstrated cuts required approximately 0.5 mm/s speed at full 70 W power, making this a special-case technique.
Why does clear acrylic not cut with diode lasers?
Clear acrylic absorbs almost no energy at common blue diode wavelengths, so the beam passes through without heating the material.
Do higher air assist pressures always improve cutting?
No. Kerf geometry and airflow direction matter more than pressure. Slower cuts often allow better airflow with lower pressure.
Why do acrylic cuts show vertical striations?
They are caused by the material’s intermittent vaporisation process, not by mechanical vibration or air pulsing.
Does laser power drop significantly as cooling water warms?
Total power remains largely stable. Rapid temperature cycling affects beam quality, not wattage.
Additional Recommended Videos (Reference Watch List)
The following videos provide deeper technical demonstrations and extended explanations that support the topics covered in this article. They are recommended for readers who want to explore specific areas in more detail.
All videos are by Russ Sadler and are best viewed alongside the relevant section above.
Focus, Lenses, and Beam Behaviour
- Laser Beams and Lenses — Part 1
- Laser Beams and Lenses — Part 2
- Cutting Matrix: Why Test Files Matter
- Beam Divergence and Real Beam Diameter
Beam Alignment and Mirror Setting
- Beam Setting and Alignment Myths
- Up the Bottom Beam Alignment — Part 1
- Up the Bottom Beam Alignment — Part 2
Air Assist and Cutting Physics
Materials: Acrylic and Wood
- Acrylic Cutting Myths and Misunderstandings
- Cast vs Extruded Acrylic — What Really Changes
- Multi-Colour Engraving on Wood
Photo Engraving and Image Processing
- Photo Engraving — Part 1 (Dot Size and Dithering)
- Photo Engraving — Part 2 (Exposure, Burnt Dot, and Banding)
- 3D Engraving Myths and Visual Perception
Cooling, Chillers, and Power Stability
- Mini Chillers and Thermal Cycling Effects
- CW-5200 Chiller Specification Explained
- Cooling a Laser Tube — Part 2
Recommended Reading and Practical Resources
Understanding laser cutting myths is only part of improving real-world performance. The next step is to connect the physics on this page with specific laser platforms, upgrade paths, and the optics you actually run in your machine.
You may find the following resources useful:
- Diode Lasers Explained – Under the Hood Guide
A deep-dive into how diode lasers behave in practice, covering spot shape, modulation, thermal limits, and why headline wattage figures often mislead. - K40 Xtreeem Laser Cutter Upgrade Series
A step-by-step upgrade path for low-cost CO₂ systems, showing how changes in mechanics, optics, air assist, and control electronics translate into real cutting and engraving performance. - Compatible CO₂ Laser Lenses and Mirrors
If this article has highlighted issues with your current optics, you can review compatible laser lenses, mirrors, and accessories for a wide range of CO₂ machines – including K40, Chinese gantry systems, and OEM platforms such as Trotec, GCC, Epilog, and ULS.
By combining evidence-led laser physics with the right hardware and optics, you can move away from rule-of-thumb settings and towards predictable, repeatable cutting and engraving.