Burr alignment

A grinder does its work between two metal cutting surfaces called burrs. One is held still; the other spins. Beans fall into the gap and are crushed down to whatever size that gap allows. Alignment is the question of whether those two surfaces are actually facing each other squarely — parallel to one another, and centred on the axis the motor spins around.

If they are not, the gap is wider on one side of the grinder than the other. Coffee going through the narrow side gets crushed harder; coffee going through the wide side barely gets touched. You end up with fines (dust-like particles) and boulders (oversized chunks) coming out of the same grinder at the same setting.¹ Coffee Navigated puts the consequence bluntly: “The more alike coffee particles are in size, the more clear taste and aroma. The more difference in particle size, the more both over- and under-extracted coffee (mix of bitterness, bad flavours, sour).”¹

That is the same argument this wiki’s Coffee grinding note makes about evenness beating the dial number, and the same argument Matt Perger made in 2014 about the Mahlkönig EK43 — that grind uniformity is what lets you pull more flavour out before the bitterness arrives. Alignment is the mechanical side of that argument: a grinder can have excellent burrs and still grind badly if they are sitting crooked.

Flat and conical burrs

Two shapes dominate. Flat burrs are a pair of discs with the cutting pattern on their facing surfaces, like two dinner plates pressed together. Conical burrs are a cone sitting inside a matching ring, common in hand grinders. Both can be misaligned, but the fixes differ — flat burrs are the ones people shim, because a flat disc can tilt.¹

The marker test

The standard home diagnostic costs nothing and takes about fifteen minutes.² It relies on the chirp point — the setting where the two burrs first touch each other and make a sharp chirping sound. On most grinders that point is also where the “zero” on the dial is set, so it doubles as a reference.³

The procedure everyone describes is the same:³²

1. Coat the flat edge of one burr — usually the stationary one — with non-permanent marker ink.
2. Reassemble the grinder and bring the burrs together to the chirp point, so the faces just barely rub.
3. Take it apart again and read the ink.

What you are reading is where the burrs actually touched. EspressoAF’s guide gives the interpretation: “If the marker is wiped off all around the circumference, which we call a ‘full wipe’, it indicates the burr is perpendicular to the rotation axis.”³ Ink gone in only one sector means that sector is the high point — the burr is tilted, and that side is riding closer than the rest.

Shimming

The correction is to lift the low side until the burr sits flat. The shim is a scrap of thin metal — kitchen aluminium foil is the usual choice, at roughly 0.016 mm (16 microns) thick.²

The important detail, and the one that is easy to get backwards: the foil goes under the low side — the side where the marker was not wiped off — not under the high point.² EspressoAF frames the same instruction from the other direction: “To shim the lower side of the burr, place shims under the screw opposite the high point.”³ It also warns against putting shims between screws, because “the screws on both sides will apply downward force on the burr, and the shims in the middle will form a fulcrum” — which bends the burr instead of levelling it.³

Then you do it again. Two independent guides put the realistic effort at 2–4 rounds of shim, reassemble, re-mark, re-check, with typical corrections needing 1–3 layers of foil (16–48 microns).² EspressoAF’s version is “repeat this process until a full wipe is achieved, then align the other burr using the same method”, and it does not pretend this is quick — it describes “countless disassemblies and reassemblies.”³

One free step comes before any foil. Most burrs mount on three screws, and the screw holes let you rotate the burr into several different positions. Coffee Navigated recommends trying them all first: “rotate the burrs individually to the different screwhole/burr combinations to check if some combinations are better aligned than others.”¹ The reason is manufacturing tolerance — “the burrs themselves should in theory have no differences in height around the burr, but in reality very small differences can occur”¹ — so one combination may cancel out more error than another before you have added anything.

Lapping, or “hyper-alignment”

Shimming levels the burr. It does not fix a burr chamber or a burr carrier that was machined slightly out of true in the first place. For that, Scott Rao published a more invasive method in January 2019: sand the mating surfaces flat.⁴

The outline, in his words and order:⁴

• Work through 220, 320, 400 and 800 grit sandpaper discs, optionally finer for polish, applying cutting oil to the paper.
• For the chamber phase, “tape your inner burr to the outer burr that’s still screwed into the burr carrier”, install the assembly, and rotate it under gentle pressure against the oiled disc.
• Most of the metal actually comes off early: “you should use 3 discs for at least 220 and 320, and can probably go down from there.”
• Then reverse it to flatten the carrier — “tape the outer burr, without carrier, to the inner burr” — so both mating surfaces end up in the same plane.

Rao also recommends a torque screwdriver when reinstalling the inner burr, “to keep things even.”⁴ That is not fussiness for its own sake: uneven screw tension is itself a source of tilt.

This is irreversible. You are removing metal from a machine, usually one still under warranty.

Doing it without wrecking anything

Three cautions repeat across the guides:

• Do not run the motor with the burrs touching. JayArr states it flatly: “never run the motor with the burrs in contact — it damages the cutting edges.”² The chirp test is done by hand, or with the motor off.
• Do not over-tighten. Steel screws in aluminium threads strip easily; snug is enough.⁴ JayArr notes that damaged mounting threads or burr surfaces are the kind of thing a warranty “would not” cover.²
• Do not go finer than the chirp. EspressoAF: “be careful not to adjust any finer to avoid damaging the motor.”³

Single-dosing

Single-dosing means weighing out one brew’s worth of beans, grinding exactly that, and leaving the hopper empty between uses — instead of keeping the grinder loaded with a day’s beans. The stated motivations are minimising retention (coffee stranded inside the grinder from the last dose, which then contaminates the next) and keeping unground beans out of the light and air, which connects to what Coffee freshness and degassing covers about staling.

Single-dosing and alignment travel together in enthusiast circles — the grinders marketed for one are usually the grinders marketed for the other — but the evidence linking them is not in the sources behind this note. See the callout below.

To verify

Everything specific about single-dosing. The research report behind this note defines single-dosing and asserts its benefits — lower retention, better brew-to-brew consistency — without citing a single source for any of it, and the report says so itself: none of its alignment guides “directly tie single-dosing to measured cupping scores, competition results, or auction prices.” The definition above is the common practitioner usage, not a documented finding. Retention figures, the claim that an empty hopper improves consistency, and any comparison between single-dosing and hopper-fed grinding are all unverified here.

The claim that top lots need aligned grinders. The same report notes it is “widely discussed in practitioner literature” that highly aligned grinders are preferred for high-scoring coffees from Cup of Excellence or Best of Panama, and then correctly refuses to state it as fact — there is no primary-source data connecting a grinder model or an alignment procedure to a documented cupping score or auction price. This wiki does not state it either.

To verify

The micron tolerances. Claims that these methods bring burr parallelism into the single-digit-micron range are qualitative. No source read here published measured before-and-after numbers — no particle-size standard deviations, no dial-indicator readings. The one hard figure in the research report, a lapping plate “accurate to within about 3 microns along its length”, comes from a YouTube demonstration⁵ and describes the flatness of the tool, not the alignment achieved with it. Treat “single-digit microns” as a goal people describe, not a result anyone has shown.

One source could not be read. The research report’s definitional source for burr alignment, an Espresso Outlet article, returned HTTP 429 when fetched for this note.⁶ Its core claims — parallel-and-concentric as the definition, fines on the tight side and boulders on the loose side — are corroborated by two other sources read in full,³¹ so they are stated as fact above; anything unique to that article is not.

Related

• Coffee grinding — grind size and why evenness beats the dial number; this note is the machine underneath it
• Extraction yield — what an even grind is ultimately protecting
• Total dissolved solids — the measurement that catches an inconsistent grinder
• Brew ratio — the recipe number alignment problems get blamed on
• Espresso machine — the method least forgiving of a misaligned grinder
• Pour-over brewing — where fines and boulders show up as a stalled or fast-draining bed
• Coffee freshness and degassing — the staling argument behind single-dosing
• Matt Perger — made grind uniformity a mainstream argument via the EK43
• Scott Rao — published the lapping method described here
• Specialty Coffee Association — publisher of SCA-320, the home grinder standard
• Cupping — the protocol that fixes grind by rule, so grinder differences show up plainly

Official links

• Official site — Specialty Coffee Association, Coffee Standards

Footnotes

1. Coffee Navigated — Grinder alignment ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶

2. JayArr — How to Align Coffee Grinder Burrs: DIY Marker Test Guide ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷

3. EspressoAF — Burr Alignment ↩ ↩² ↩³ ↩⁴ ↩⁵ ↩⁶ ↩⁷ ↩⁸

4. Scott Rao — A more effective way to align your grinder ↩ ↩² ↩³ ↩⁴

5. Burr Hyper-Alignment: Insane Precision (YouTube) — cited in the research report, not independently fetched ↩

6. Espresso Outlet — Burr Alignment: Its Impact on Flavor, Fines, and Grinding Performance — returned HTTP 429, not readable from here ↩