Where should the focus sit for stainless steel and mild steel?
Updated:
Short answer
The rule of thumb: cutting with nitrogen, the focus goes below the surface — roughly a third to two thirds of the thickness down. Cutting mild steel with oxygen, the focus sits at the surface or slightly above it. Thin sheet under 2 mm is close to zero in both cases.
Why the direction differs
| Symptom | Likely cause | First action |
|---|---|---|
| The melt must be pushed out of a deep kerf without oxidation. | Nitrogen cutting (stainless, aluminium) | Focus below the surface widens the lower kerf so gas can expel melt — go deeper as thickness grows. |
| The exothermic reaction supplies much of the energy at the top. | Oxygen cutting (mild steel) | Focus at or above the surface keeps the reaction stable and the kerf narrow enough to stay hot. |
Focus, speed and pressure per material and thickness — inside the app. Open SBM Laser Help →
Or take it with you:
How to run a focus test
- Cut a short line at a series of focus values, stepping 0.5 mm, and mark each line on the sheet.
- Break the coupon and look at the underside: the cleanest, narrowest kerf with least burr marks the working focus.
- Repeat the sweep with a 0.2 mm step around that value for thick material.
- Record the number per material and thickness — focus is the parameter that quietly drifts after service work.
Focus position by thickness — starting values
| Thickness | Stainless / nitrogen | Mild steel / oxygen |
|---|---|---|
| 1 mm | −0.3 to −0.5 mm | 0 mm |
| 3 mm | −1 to −1.5 mm | 0 to +0.5 mm |
| 6 mm | −2.5 to −3 mm | +1 mm |
| 10 mm | −4 to −5 mm | +2 mm |
| 16 mm | — | +3 mm |
| 20 mm | — | +4 mm |
Negative means below the sheet surface. Verify after every lens or protective-glass change.
Related questions
- Does an autofocus head make focus testing unnecessary?
- It removes the manual adjustment but not the verification. The head knows its own zero, not whether the zero still matches reality after a lens change or a crash.
- How does focus interact with nozzle standoff?
- They are independent settings that both affect the gas and beam geometry. Set standoff first (typically 0.7–1.0 mm), then find focus at that standoff.