First layer calibration, properly

If you fix one thing about your printer, fix this. A correct first layer eliminates most adhesion problems, most warping, and a good share of the failures people blame on filament quality. It costs nothing but twenty minutes.

What a good first layer looks like

Print a large flat square, one layer thick, and look at it closely:

• Correct: lines slightly flattened, touching their neighbours with no gaps, a smooth surface, and the part is firmly stuck down. Slight translucence is fine.
• Too high: you can see gaps between round-looking lines. The part comes loose. This is the common failure.
• Too low: a rippled, semi-transparent surface with ridges where plastic has been forced sideways, and possibly nothing extruding at all in places. This also damages your build surface over time.

Level before you offset

Levelling and Z offset are two different things and doing them out of order wastes an evening. Levelling makes the bed parallel to the nozzle’s travel. Z offset sets how far the nozzle sits above it. Get the bed flat first — whether by manual screws or by running the automatic probe — then set the offset.

On manual beds, the paper method still works: heat the bed and nozzle to printing temperature first (metal expands), then adjust each corner until a sheet of paper drags with slight friction. Do the circuit twice; adjusting one corner moves the others.

Setting Z offset live

The reliable method is baby-stepping during a print. Start a large first-layer test, and adjust Z offset in small increments while it prints, watching the lines behind the nozzle. Move in 0.01–0.02 mm steps — the useful range is much smaller than people expect. When the lines stop showing gaps and the surface goes smooth, stop and save.

Do this at the temperature and on the surface you actually print with. Z offset is not transferable between build plates: a textured PEI sheet and a smooth one are different thicknesses, and swapping plates without re-checking is a classic cause of a suddenly terrible first layer.

Things that quietly ruin it

• A dirty bed. Grease from fingers defeats a perfect offset. Wash with detergent and warm water.
• A blob on the nozzle. A crust of old plastic on the nozzle tip drags through the layer. Wipe it while hot, carefully.
• A warped bed. Some beds are not flat and no amount of corner levelling fixes a dip in the middle. Mesh bed levelling compensates; a glass plate is the mechanical answer.
• Printing too fast. The first layer should be slow — typically 20–25 mm/s. Most profiles already do this; check yours has not been overridden.

Re-check it periodically

Z offset drifts. Belts settle, springs relax, probes shift after a knock, and build surfaces are not identical when replaced. If prints have started lifting after months of reliability, re-run this before buying adhesive — see bed adhesion.