Power, Speed, Focus and Assist Gas
Line energy, test grids, focus and depth of field, pulse settings, air assist and gases, and finding settings that work.
Line energy
Of all the settings, two matter most: the laser power P and the travel speed v. Their ratio is the line energy, the energy delivered to each millimeter of the cut:
E = P ÷ v (watts ÷ mm/s = J/mm)
Worked example: line energy
- A 40 W beam moving at 20 mm/s delivers 40 ÷ 20 = 2.0 J per mm. At half the speed it is 4.0 J/mm: halving the speed doubles the energy.
- Doubling the power at the same speed does the same. So 25 W at 12.5 mm/s and 50 W at 25 mm/s give the same line energy of 2.0 J/mm, but the faster, stronger setting has a hotter, narrower heat zone and usually cuts better, with less charring.
- There is a window (see the figure below): too little energy and the beam does not cut through; too much and the kerf widens and the edge burns. The width of the window depends on the material and thickness. Within it, prefer the highest power that gives a clean edge: a faster cut loses less heat sideways.
Finding settings: the test grid
Settings in a table from another machine will not transfer: tube age, optics, lens, focus, material batch and air all differ. Find your own with a test grid: a set of small squares or lines cut with a range of powers and speeds on a scrap of the same sheet, then pick the best.
Worked example: a 5 × 6 grid
- 5 powers (20, 30, 40, 50, 60 W) × 6 speeds (10, 15, 20, 30, 40, 60 mm/s) = 30 test cuts, each a 10 mm square (40 mm of cut) with 4 mm between squares: the grid is about 84 × 70 mm.
- The run takes about 1.3 minutes (58 s of cutting plus 0.6 s per square for moves).
- In this grid 14 squares cut cleanly, 7 do not cut through and 9 are burnt. The fastest clean cut is 60 W at 60 mm/s (1.00 J/mm): the best production setting. Choose one a little inside the window rather than on its edge, so that small changes in the sheet do not spoil the cut.
- Then refine: a second, finer grid around the best square, and check the cut-through at the corners and the start of the line.
- Record the result (material, thickness, supplier, power, speed, lens, focus, air, date) in a table for the shop: the next job starts from it.
Focus
The beam is narrowest at the focal point; away from it the spot grows and the power density falls (see How Laser Cutting Works and Laser Types). Set the distance from the lens to the surface with the focus gauge, an autofocus or a test (a diagonal line on a tilted scrap: the thinnest line shows the focus). For thick material, focus into the sheet.
Worked example: focus and flatness with a 2 in lens
- The 2 in lens (spot 94 µm) has a depth of focus of 1.2 mm; a 4 in lens (spot 189 µm) 4.8 mm.
- Thick material: in 6 mm sheet the beam must stay narrow through all of the thickness. With the focus at the middle, the spot at the top and the bottom surfaces is 482 µm for the 2 in lens (5.1 times the waist) and 302 µm for the 4 in lens (1.60 times). The longer lens holds its focus through the sheet far better. A thicker sheet calls for a longer lens.
- Where to put the focus: for cutting, a little below the top surface, about a third of the way into the sheet (here 2.0 mm). For engraving, exactly on the surface.
- Flatness: a bow of 1.0 mm moves the surface a whole depth of focus from the waist of the 2 in lens: the spot there is 184 µm, 1.9 times the waist, and the power density falls to 26 %. Hold the sheet flat: honeycomb, magnets, pins or a vacuum bed.
Power: what the setting really means
The “power %” on a CO₂ cutter is a duty cycle or a drive level, not a measure of watts. The relation to the light is not linear, a tube ages, and mirrors and the lens absorb a share. Measure the power with a power meter at the nozzle, at least once, and record it.
Worked example: an 80 W tube measured at the nozzle
| Setting | Measured power | Watts per 1 % of setting |
|---|---|---|
| 20 % | 7 W | 0.35 W |
| 40 % | 29 W | 0.73 W |
| 60 % | 51 W | 0.85 W |
| 80 % | 67 W | 0.84 W |
| 100 % | 74 W | 0.74 W |
- The output saturates: from 80 % to 100 % the measured power only rises from 67 to 74 W. To get 45 W by interpolation the setting must be about 55 %. Settings at the top of the range wear the tube without adding useful power.
- Optics losses. Each clean mirror reflects about 98 % and a lens transmits about 98.5 %. With 3 mirrors and a lens, 80 W from the tube becomes 80 × 0.927 = 74 W at the work. If the mirrors are dirty (reflecting 90 % each) and the lens is smeared (93 %) it is 80 × 0.678 = 54 W: 27 % less, and the heat sits on the optics. A weak cut is often dirty optics, not a tired tube (see Running a Laser Shop: Costs, Maintenance and Troubleshooting).
Speed, acceleration and corners
The head cannot change speed instantly: it accelerates at a limited rate (a few hundred to a few thousand mm/s²), so short segments and tight corners never reach the set speed. For cutting this is a minor loss, but for engraving and for small detailed shapes it is the main limit.
Worked example: a 50 mm segment
| Set speed | Time at constant speed | Time with 1 000 mm/s² acceleration and deceleration | Average speed |
|---|---|---|---|
| 40 mm/s | 1.25 s | 1.29 s | 39 mm/s |
| 150 mm/s | 0.33 s | 0.48 s | 103 mm/s |
| 300 mm/s | 0.17 s | 0.47 s | 107 mm/s |
A 50 mm segment at 300 mm/s should take 0.17 s, but starting and stopping with 1 000 mm/s² it takes 0.47 s, 2.8 times longer. (Moves are only partly stopped at a corner in practice, so the true penalty is smaller, but it grows with speed.) The machine’s acceleration setting limits the shake and the corner accuracy: do not raise it beyond what the belts and the frame can hold.
- Corners burn because the head slows while the beam stays on at full power; lower the power at corners with “power at corner” or acceleration compensation, or use a lead-in and cut corners more slowly.
- Start and end of a closed cut are hot spots: a small overlap and a lead-in outside the part keep the mark off the finished edge.
- Cut small features first (holes, then outlines) while the sheet is still held flat.
Pulses, frequency and resolution
A CO₂ tube is pulsed at a frequency set in the controller (pulses per inch for some machines). The pulses must overlap along the path: if the spacing v ÷ f is larger than the spot, the line becomes a row of dots.
Worked example: pulse spacing in engraving
| Pulse frequency | Spacing at 400 mm/s | Overlap with a 94 µm spot | Line |
|---|---|---|---|
| 5 kHz | 80 µm | 15 % | dotted line |
| 10 kHz | 40 µm | 58 % | continuous |
| 20 kHz | 20 µm | 79 % | continuous |
At 5 kHz the pulses are 80 µm apart, a bit less than the spot, so there is barely any overlap and the line looks dotted; at 20 kHz they are 20 µm apart and overlap heavily. Raise the frequency until the line is smooth. For cutting, a high frequency (5 to 20 kHz) gives a smooth kerf; for fine marking and thin material, a low frequency with short pulses puts less heat in.
One pass or several
Worked example: three passes
To deliver 2.4 J/mm with a 60 W beam you need a speed of 25 mm/s in one pass. Three passes of 0.80 J/mm each means a speed of 75 mm/s per pass (the same total energy and the same total time) but each pass removes only a third of the thickness and the heat has time to leave between passes. Several passes suit thick wood (less char), engraving to depth and cuts that flare; refocus between passes where the cut gets deep. One pass suits thin sheet and where alignment between passes is poor.
Assist gas and air
A stream of gas through the nozzle, coaxial with the beam, protects the lens from smoke and spatter, blows the molten or vaporized material out of the kerf, and cools the work. In a hobby or small CO₂ machine it is compressed air; in industrial cutting it is chosen to react with or protect the cut (see Industrial Laser Cutting: Metals, Gas and Process).
| Gas | Use | Effect |
|---|---|---|
| Compressed air (clean and dry) | Wood, acrylic, leather, paper, thin metal | Cheap; clears smoke and reduces flare; with wood, oxygen in the air adds a little heat; fine for most work |
| Nitrogen | Stainless, aluminum, galvanized steel; clean edges | Inert: no oxide, bright edge; needs high pressure (10 to 20 bar) in metal cutting |
| Oxygen | Mild steel, thick | Burns the iron: adds heat and speed; an oxide layer on the edge |
| None | Not recommended | Smoke deposits on the lens and the work can flare |
Worked example: the air flow of a small nozzle
A 3 mm nozzle at 20 kPa (0.2 bar) gauge: the jet speed is v = √(2 ΔP ÷ ρ) = √(2 × 20 000 ÷ 1.2) = 183 m/s. The flow through an orifice of area 7.07 mm² with a discharge coefficient of 0.8 is Q = Cd A v = 62 liters per minute. A small aquarium pump is not enough; a compressor or a decent air pump is needed, with a dryer and a filter, because oil and water mist contaminate the lens.
- Use clean, dry, oil-free air: a regulator, a water trap and a filter.
- Higher air pressure is not always better: too much blows the flame and cools the cut, and in fine engraving it blows away the fumes but also pushes smoke into the kerf.
- Keep the nozzle clean and concentric with the beam, and at the right distance from the work: a clogged nozzle makes a ragged cut.
Judging the cut
| Look at | Good | Bad, and what to change |
|---|---|---|
| The underside | A thin even line; a clean cut | Heavy burn marks or a flare: too much energy or poor air; no mark at all: not through |
| The edge | Straight, little taper, light color (acrylic: clear) | A wide, black or ragged edge: too slow, out of focus, dirty lens; stripes across the edge: vibration or corners |
| Kerf width | Narrow and constant | Wider where the head slows or the focus is wrong |
| Corners and starts | Sharp, no burn marks | Dwell burns: slow-down compensation, lead-ins |
| The face | Clean, with masking tape removed | Scorch marks and smoke stains: air, masking, speed |
Related
- Laser Cutting: the overview of this guide.
- Materials, Thickness and What a Laser Can Cut: the energy model behind these settings.
- Kerf, Accuracy and Designing Parts: how the settings change the kerf and taper.
- Engraving, Marking and Raster Work: resolution, speed and pulses for raster work.
- Running a Laser Shop: Costs, Maintenance and Troubleshooting: cleaning and power checks that keep the settings valid.
FAQ
How do I find the right power and speed?
Cut a test grid on a scrap of the same sheet across a range of powers and speeds, and choose the fastest setting that cuts cleanly through, a little inside the window. Record it for next time.
Where should I focus for thick material?
About a third of the way into the sheet for cutting; exactly on the surface for engraving. A longer focal-length lens keeps the beam narrow through a thick sheet.
Why does my laser cut worse than it used to?
Most often dirty mirrors and lens (a few percent of loss each adds up), a misaligned beam, a clogged nozzle, bad air or a tired tube, in that order. Clean and check before blaming the tube.
Is more power always better?
More power allows a faster cut, which usually gives a cleaner edge, but at a given thickness there is a window of line energy; above it the edge burns. Prefer the highest power that stays inside it.
References
- Steen, W. M. and Mazumder, J., Laser Material Processing, Springer (cutting parameters and gas jets).
- Machine makers’ settings guides and application notes (Epilog, Trotec, Universal, Glowforge, LightBurn documentation) for starting values.
- ISO 11146 (beam properties), ISO 9013 (thermal cutting: quality classification of cut surfaces).
Last reviewed: October 2026. The window values and measured powers are illustrative: test on your own machine and material.