Burr types, particle distribution, and grind consistency

Every grinder reduces beans to particles using one of two burr geometries. Flat burrs are two parallel ring-shaped discs, usually spinning at higher RPM; they tend to produce a more uniform particle size and are common in high-end espresso grinders. Conical burrs are cone-shaped and nest inside each other, usually running slower and cooler; they tend toward a slightly wider particle spread but often with more retained fines, and are common in both entry-level and some premium grinders alike — geometry alone doesn't determine quality, build quality and burr sharpness matter as much.

No grinder produces perfectly identical particles: every grind has a particle size distribution, a spread around the target size. A wide distribution — too many very fine "fines" alongside too many large "boulders" — extracts unevenly, since fines over-extract quickly while boulders under-extract, muddying the cup even at a correct average grind setting. Retention is the grounds left behind inside the grinder's chute and chamber from a previous dose, which can mix into the next dose, especially problematic when switching between beans or roast levels — WDT (Weiss Distribution Technique, gently stirring the grounds with fine pins before tamping) and single-dose grinders (no standing hopper, one dose at a time) both exist mainly to reduce this cross-contamination and improve distribution.

Recalibration is needed whenever the bag or roast date changes, when humidity shifts noticeably, or when a shot drifts out of its target window despite an unchanged recipe — burrs also dull with use over months to years of heavy service, showing up as a slow drift toward needing a coarser setting to hit the same time, which is the practical signal that it's time to inspect or replace them.