Two different shapes moving coffee through in two different ways, which is the actual mechanical reason behind a flavor difference people talk about constantly but rarely explain.
A conical burr set uses a cone-shaped inner burr spinning inside a ring-shaped outer burr. Beans get pulled into the gap between them and fall through by gravity as they're cut down in size, with the cone shape guiding that downward path.
A flat burr set uses two parallel, ring-shaped burrs facing each other, one usually spinning against a stationary one. Beans enter at the center and get pushed outward toward the edge by centrifugal force as they're ground, rather than falling straight down.
The conical design lets gravity do a lot of the work, which is part of why conical grinders often run at a genuinely lower RPM, the Niche Zero, for example, runs at about 330 RPM. Slower rotation generally means less heat generated during grinding and often a quieter grind overall.
The flat design relies more on rotational force to push beans outward, which typically calls for a higher RPM to grind effectively, the DF64 runs at roughly 1400 RPM, well over four times faster. This is a tendency, not an absolute rule, since specific engineering can offset it in either direction.
The flavor difference people describe between flat and conical burrs largely comes down to particle size distribution, how uniform or varied the ground coffee particles actually are. Flat burrs tend to produce a narrower, more consistent range of particle sizes. Conical burrs tend to produce a wider mix, more fine particles and larger particles alongside the main size, rather than one tightly clustered size.
That difference in distribution is generally associated with the flavor tendencies people describe: flat burrs leaning toward brighter clarity and clearer separation between flavor notes, conical burrs leaning toward fuller body and rounder, less sharply defined sweetness. Neither is more correct, they're just different outcomes from a genuinely different grinding mechanism.
It's tempting to assume burr shape also decides how much retention a grinder has, but that's more about the specific grinder's internal design than the burr shape itself. The Niche Zero, with conical burrs, has near-zero retention by design, while some other conical grinders retain considerably more. The DF64, with flat burrs, also keeps retention relatively low through its own single-dose engineering. Retention is a design choice within either burr category, not something the shape alone locks in.
A conical burr set uses a cone-shaped inner burr spinning inside a ring-shaped outer burr, with beans falling through the gap by gravity. A flat burr set uses two parallel, ring-shaped burrs facing each other, with beans pushed outward between them by centrifugal force.
It largely comes down to particle size distribution. Flat burrs tend to produce a narrower, more uniform range of particle sizes, which is associated with brighter clarity and more distinct separation between flavors. Conical burrs tend to produce a wider mix of particle sizes, which is associated with fuller body and rounder, less sharply defined sweetness.
Not on its own. Retention depends more on the specific grinder's internal geometry and engineering than on whether its burrs are flat or conical. Some conical grinders have excellent retention and some flat burr grinders do too, it's a design choice within either category, not a fixed rule tied to burr shape.
It's a common tendency, since flat burr grinders often run at higher RPM, but it's not an absolute rule. Some flat burr grinders are specifically engineered for quiet operation, so the tendency can be designed around.