Grinder burr types explained (flat, conical, DRM)

Grinders are one of the keys to exceptional espresso. Discuss them here.
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#1: Post by HB »

Jim's enlightening lesson on grinder burrs was buried in the Bench's Versalab M3 thread. I have copied it below for easier reference.
another_jim wrote:The M3 has arrived at its new temporary home, a person whose tasting and grinder tweaking acumen I trust far more than my own. With luck, we'll get a more authoritative tasting report than I can provide.

I completed my taste tests using SOs. Although my subjective impression was that the M3 stood out more in this area than with regular espresso blends, my scores were completely in-line with those posted on my taste report.

And that brings me to the subject of the post.

I took the grinder apart prior to shipping it. I wanted to check how much grinds it retained, and what their condition was. The inspection confirmed Versalab's claims about grounds retention -- I swept the lot into a container, and it was less than what stays behind in my doser after I sweep it out, maybe 3 to 4 beans worth of ground coffee. It was hard to tell from their condition how cool the grinder is running. However, the grounds seemed brown and dry, rather than the black, oily double shot's worth of compressed grinds I find when cleaning the Mini's chamber.

As I was doing this, I chanced to look at the way the conical burrs were spaced. First I looked at the upper burrs with the lower ones removed:


Hmm, I thought, the outer conical burr is really far back. So I mounted the lower (inner) conical burr, and could still see a lot of daylight.


If you look at the left side, you'll see the amount of clearance. In fact, you can toss through the coffee beans ...


... and they emerge completely unscathed. A little testing confirmed it, the conical burrs are too widely spaced to affect the beans if the flat burr is not in place.

On the other hand, trying to grind coffee without the inner conical mounted turns into a failure too, the beans simply won't enter the flat burr set, which is too narrowly spaced to admit a whole bean. I tried to examined the beans mid-grind (with the grinder properly assembled) trying to disassemble it so the beans being ground remained in position. I made a mess of it and wasn't able to get good photos. It does appear that the conical burrs augur the beans down against the flat burr entrance, where they get crushed against one another, and the metal walls, into pebble size.

Schomer has written a lot about conical grinding and its wonders, especially in connection with this burr-set. Most of this appears to be complete nonsense. Basically, grinders force the beans into a more and more constricted space, so they break into finer and finer particles. The burrs don't do much cutting, they are merely paddles that criss-cross like scissors, pushing the bean fragments into the tighter space.

Here's a schematic cross-section for flat burrs:


The typical burr grinder has three angles where the bean breaking, coarse and fine grinding respectively occur. As the burrs are put together, these angles create a progressively constricted space the beans are forced into. At the outer diameter, the grindstones are perfectly parallel for the final grind to a powder.

Now, armed with this mental schema, lets look at the DRM/M3 burrs and the Mini Mazzer burrs (standing in for all flat burr configurations):


Looking at the Mini burrs, you can see the three zones very clearly, in the breaking zone (marked by green shading) the burrs are widely spaced. as one transitions to the coarse grind zone (blue shading), the burrs become tightly spaced, creating a narrow channel for the grinds, the final fine grind stage (red shading) is a very narrow outer perimeter.

Now over to the DRM flat burr. The fine grind area is equally narrow. But there's no bean breaking area, and the coarse grind area is huge in comparison to the mini's. So where's the green bean breaking area? That's where the conical burrs come in. They force the beans into the flat burrs, breaking them as they enter.

In other words, the only thing the conical burrs do on this grinder is allow the flat burr to have a much enlarged coarse grinding area. Whether and how this contributes to an improved grind is anyone's guess.

A final thing caught my eye that may be relevant to the discussion of particle distribution in espresso grinds. On the DRM grinder, the burrs are not cut all the way to the perimeter; instead there are little channels that allow some coarse particles to escape. On the Mini, the burr channels are cut all the way through, also giving an escape path for some coarse particles (see enlarged sections). I'm wondering if grinder manufacturers can "tune" their grindstones by varying the size of these escape channels, in order to affect particle distribution?
lennoncs wrote:very interesting have my undivided attention.
another_jim wrote:In that case ... I wasn't planning on continuing the grinder 101, but here's a look at a pure conical burrs grinder. This is the Trespade burr set from the Lux, Pavoni, Innova home grinders and the lowest end commercial model from Iberital. The burrs are quite tiny, but the configuration of the grinder is commercial, albeit cheap. BTW, at around $150 to $200, these grinders are the true entry level for serious espresso grinding -- loud, messy, slow, but grind quality equal to the mini's.

Here's the grind chamber from the Pavoni PGC, my travel grinder, with the static outer burr removed.


As you see the layout is conventional, including the mass of grinds not spat out by the paddles.

Here is the outer burr assembly:


The mount is hard plastic instead of brass, and the mounting threads aren't tight, but the burr is well attached by two screws. The assembly is driven by a worm gear driving the teeth shown on the outside. Despite the poor threads, this gives very good grind adjustability.

Now to the business side. the burrs are slightly out of focus, but (starting from the bottom of the picture) the bean breaking, coarse grinding and fine grinding areas are apparent. Note that the outer burr is roughly tube shaped, whereas the inner burr is conical, so one has the same constricting space as with the flat burr.

The grinding areas on the inner burr are a lot clearer:


Again note the changing surface angles as one moves from the bean breaking to coarse to fine grinding zones. One of the reasons conical burr grinders may be better than flats is also apparent here -- this is a tiny burr set, yet the coarse grinding area is larger than the mini's (although not as large as the M3's. On a true commercial conical like the Conik or robur, these areas must be huge.

It also becomes obvious that the DRM/M3 conical is missing the coarse and fine grinding zones, and only has the augur-like bean breaking section.

When one puts the burrs together, one finds the same flush contact along the fine grinding zone as in the flat burr grinders:


OK, that's it for grinder 101. I'm not sure if this tells one which grinder is best; but it does let one at least see what's going on with the grind stones.
another_jim wrote:OK, one more 101.

How about less then steller grinders:

Scoot on over to long time altie Ken Wilson's page on burr sets, which includes all the cheapies.

The low end flat burr grinders seem to missing anything but the bean breaking part. The Solis conicals don't have a fine grind section. The Zassenhaus burrs on the other hand look perfectly legit -- I suppose motorizing one of these could save someone some money compared to going with that 1K plus commercial conical.
Dan Kehn


#2: Post by mathias »

The Santos ( grinders uses burrs similar to the M3/Cimbali, i.e there is no "bean breaking" area on the burrs. The M3 does not work without the conical burr according to Jim. Differencies easily spotted is the higher RPM and the hexagonal nut (instead of a bare axle when conical burr is removed on the M3) on the Santos.

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HB (original poster)

#3: Post by HB (original poster) »

Related thread Conical burr - worth the upgrade?, including John's further insights on Versalab M3 versus DRM burrs:
Jepy wrote:I always thought there was a certain amount of pre-breaking from the conical part in the DRM, so I took a closer look. The Versalab cone and cone ring are slightly different from the original DRM it was designed after. Maybe in the Versalab the cone is just an auger, but in the DRM, the cone and ring do a considerable amount of pre-breaking.

It's almost like a gauge, with the smaller beans slipping past without damage:
Dan Kehn

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HB (original poster)

#4: Post by HB (original poster) »

Also see Mazzer Mini Component Photos and extreme closeup Mazzer Mini burr photos by Al Ruscelli.
Dan Kehn