Burr types, particle distribution, and grind consistency

Burr geometry, particle spread, retention and recalibration

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.