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Z focus on a galvo laser is set by matching the lens’s working distance to the top surface of your material — measured from the lens housing to the workpiece — then verifying with focus ramp tests and shrinking the mark until it’s crispest.
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What Z Focus Actually Means on a Galvo
Unlike a gantry CO₂ or diode machine, a galvo fiber laser steers the beam with mirrors, so the spot only converges to its minimum diameter at one specific distance below the F-theta lens. Every field lens has a rated working distance — that’s your Z target. Common examples:
| Lens (field size) | Typical working distance | Mark area | Spot size at focus |
|---|---|---|---|
| 70 × 70 mm F-theta | ~90–100 mm | Small parts, jewelry | ~20–30 µm |
| 110 × 110 mm F-theta | ~150–160 mm | Plates, tags, tumblers’ flat faces | ~30–40 µm |
| 175 × 175 mm F-theta | ~210–230 mm | Larger panels, batch trays | ~45–55 µm |
| 300 × 300 mm F-theta | ~320–350 mm | Big signage, deep engraving zones | ~70–90 µm |
Find your lens’s exact figure in the machine manual or stamped on the lens cell — do not trust the generic number above, since manufacturers vary by ±10 mm.
Step-by-Step: Setting the Working Distance
1. Measure from the right reference point
Most machines with a manual Z column (common on 20–50 W fiber markers from brands like Cloudray, OMTech, and JPT-based desktop units) specify measurement from the bottom edge of the lens housing or the red-light pointer frame, not the lens glass itself. Use calipers or the ruler built into many columns. If your manual says 190 mm, measure 190 mm from that reference to the top surface of the material, not the bed.
2. Use the red-light coincidence method
Many galvos have dual red-dot pointers designed to overlap into one dot exactly at focus. Lower or raise the Z axis until the two dots merge on the material surface. This gets you within ~1–2 mm — close enough to start, but not final.
3. Run a focus ramp test
Tilt a scrap of the same material (prop it on a block so one edge is ~10 mm higher), or run your software’s built-in Z-axis sweep if it has one. In EZCAD2/EZCAD3, mark a series of identical lines while stepping the Z height in 1 mm increments — e.g., 186, 187, 188…194 mm. The line with the thinnest, darkest, most uniform mark is true focus. This beats the red-dot method because it accounts for your specific power, speed, and material.
4. Lock it and record it
Note the column reading per material thickness you use often — e.g., “2 mm stainless tag = 82.5 on the scale.” A taped reference card on the column saves minutes per job.
Checking Focus Quality — What a Sharp Mark Looks Like
- Edge crispness: under a 10× loupe, focused marks have clean edges; defocused marks halo or feather.
- Mark width: at identical parameters, the narrowest line = best focus. A 1 mm defocus error on a 110 mm lens can widen the spot ~15–20%.
- Ablation feel: on metal, a focused pass leaves a slightly textured groove; a defocused pass is faintly discolored only.
- Sound: a sharp tick-tick on metal vs. a softer hiss out of focus.
Handling Different Material Heights
Field curvature means the edges of the mark area sit slightly farther from the lens than the center. On a 300 mm lens, edge-of-field focus can drift several mm. Practical fixes:
| Situation | Fix | Trade-off |
|---|---|---|
| Parts of mixed height in one batch | Use a jig or shim stock to level all surfaces | Setup time |
| Large field, edge marks look soft | Enable field correction / 9-point calibration in software | Calibration effort |
| Frequent height changes | Motorized Z column or auto-focus sensor (adds ~$150–$400) | Cost |
| Curved parts (rings, cups) | Rotary + 3D/dynamic focus head, or mark only the top ~10–15 mm of arc | Smaller usable zone |
| Deep engraving (multi-pass) | Step Z down ~0.1–0.3 mm per pass to keep focus at the new surface | Longer cycle |
Common Mistakes and Ownership Realities
- Measuring to the bed, not the part. The single most common error — a 3 mm jig under the part puts you 3 mm out of focus.
- Protective window fouling. The flat glass under the lens collects vaporized residue; a dirty window scatters the beam and mimics defocus. Clean weekly with isopropyl and lint-free wipes. It’s the first consumable that wears — replacements are cheap (~$5–15 each); keep spares.
- Loose column. Set-screw creep drifts Z mid-session. Check the reading before long jobs.
- Trusting the ruler sticker. Verify once with a ramp test, then trust your own marks.
- Changing lenses without recalibrating. New working distance AND new field correction file — both required.
FAQ
How close is close enough? Within ±0.5 mm for fine text or annealing marks; ±1 mm for general engraving. Deep engraving tolerates less error as power rises.
Does focus change between materials? The distance doesn’t — the surface does. Only material thickness and topography matter, plus stepped-Z for deep cuts.
Why is the center sharp but edges blurry? Field curvature or a dirty/under-corrected lens. Run the correction routine, or switch to a larger F-theta lens for big-area work.