Grinder alignment and RDT

By the time you are chasing single-degree grind adjustments and clean, repeatable shots, the grinder is doing more to shape the cup than the espresso machine is. Two grinder topics separate a good setup from a finicky one: how well the burrs are aligned, and how you stop static from scattering grounds everywhere. Neither is exotic, and both are cheap relative to buying a new grinder. This lesson is about what they actually do and when they are worth your time.

Burr alignment and particle uniformity

A burr set grinds by passing beans between two surfaces with a fixed gap between them. That gap is your grind setting. The whole premise is that the gap is the same everywhere around the circumference, so every particle gets cut to roughly the same size. Misalignment breaks that premise: if one side of the burr sits closer than the other, the narrow side cuts fine while the wide side cuts coarse. You set the dial for “espresso” and instead produce a grind that is partly espresso and partly something else.

The cost is uniformity. A real grind is never a single particle size; it is a distribution. Aligned burrs give you a tighter distribution centered where you want it. Misaligned burrs smear that distribution wider, adding both boulders (oversized chunks that under-extract) and extra fines (dust that over-extracts and chokes flow). You then taste both under- and over-extraction in the same shot, which no dialing can fully resolve because the two errors pull in opposite directions.

How to check: the classic test is a paper or marker test on the burrs. Coat one burr’s outer flat with a marker or stick a single layer of paper shim against it, run the grinder briefly at the closest non-touching setting, and see where the ink or paper rubs off. Even contact all the way around means good alignment. Wear on one arc only means that side sits closer. Many factory grinders, especially affordable flat-burr models, ship a fraction of a degree off because the carrier or mounting face is not perfectly square.

The fix is shimming: thin washers or foil shims (often 0.05 mm to 0.1 mm) placed under one burr’s mounting points to tilt it back to parallel. It is fiddly and you re-test after every change, but it costs almost nothing. Conical burrs are generally more forgiving than flats here, because their geometry tolerates small tilt better, though they are not immune.

When does it matter? For espresso and very fine grinds, a lot: small absolute gaps mean a small tilt is a large percentage error. For coarse immersion brewing like French press, far less; the gap is wide enough that a slight tilt barely moves the distribution. So prioritize alignment if you pull espresso or run a flat-burr grinder near its fine end, and largely ignore it if you only grind coarse.

RDT: killing static and retention

RDT stands for the Ross Droplet Technique, and it is exactly as low-tech as it sounds. Before grinding, you wet the beans with a tiny amount of water (a single fine spritz from a mister, or a drop stirred through the dose) and then grind as normal. The water does not brew anything and does not survive into the cup in any meaningful amount; it discharges static.

Why bother? Dry beans build up a static charge as they fracture and tumble through the burrs, especially in low-humidity rooms and especially with light roasts, which are harder and more brittle. That charge makes grounds cling to the chute, the bin and each other. You get three problems: a messy scatter of grounds around the portafilter, clumping inside the dose that feeds channeling, and retention, where grounds stay trapped in the grinder so today’s dose is partly yesterday’s stale leftovers. Retention also wrecks weight accuracy: you put 18 g of beans in and get 17.4 g out, with the rest held back unpredictably.

A spritz of water collapses all of this. Static drops, grounds fall freely, clumps loosen, and grind-to-grind retention often falls to near zero on single-dosing grinders. The catch with single dosing is that you must purge or bellows the chamber anyway, but RDT makes that far more complete.

How much water: very little. A common rule is one to two sprays of a fine mister per dose, or stir in a drop or two with a wet spoon or paperclip. The bean surface should look barely damp, not wet. Too much water can cause grounds to mud up inside the grinder and corrode burrs over time, so keep it minimal and grind promptly rather than letting wetted beans sit. Many people stir the beans after spritzing so the moisture distributes before grinding.

RDT also makes WDT easier, because grounds that are not statically charged break up cleanly when you stir the bed. The two techniques attack the same enemy, clumps, from opposite ends of the process.

When it actually matters

Both of these are advanced refinements, not beginner fixes. If your shots taste fine, you do not have a problem to solve. Reach for them when:

• You see one-sided or jetting channeling you cannot prep away, despite good distribution and tamping. That points at the grinder, and alignment is worth checking.
• You grind light roasts for espresso and fight static, scatter and clumping every morning. RDT will fix the mess almost completely.
• You single-dose and your output weight does not match your input. RDT plus a proper purge cuts retention.
• You have dialed everything else and still cannot get a clean, repeatable extraction. Uniformity is the lever you have not pulled.

RDT is free and reversible, so try it first; it takes a morning to know if it helps. Alignment is more work but a one-time job that pays off on every shot afterward. For coarse filter and immersion brewing, both matter much less, so do not over-invest if espresso is not your focus.

Next

Check your output weight against your input over a few doses to see your real retention, then try RDT for a week. If you suspect alignment, run the marker test before spending on shims. After that, take it back to dialing in with a grinder you can finally trust.