Large conical vs flat burrs: Consistency and pour characteristics?

Grinders are one of the keys to exceptional espresso. Discuss them here.

#1: Post by GlennV »

It is often claimed that the large conicals are more consistent; I was wondering what the latest thinking on this is. I have a largish (75mm) flat, and it's certainly consistent - in that returning to the same setting with the same dose of the same coffee usually gives the same pour. However, different coffees can require very different settings, and can have very different flow characteristics. In particular, whilst N.Italian style roasts pour very evenly - with the flow just switching on and staying constant for the duration - the lighter roasts I prefer require a much finer grind and the pours tend to start slowly and accelerate. The worst offenders in this regard become almost impossible to dial in, with the dwell time extremely sensitive to grind setting.

I am tempted to try out a large conical (in the guise of a Pharos) because of claims like:
another_jim wrote:There is one large difference between conicals and flat burr grinders that is relevant to some home users. Large conicals are easier to adjust than large flats.

With a conical, the same grind setting will produce the same flow rate for given dose, say 15 grams, of just about any coffee (other than Sumatra or MM) at just about any usable age.
Yet in the recent roasting thread "Body vs. Clarity", endlesscycles (also with a K10) says:
endlesscycles wrote: ... As espresso, the brewing roasts require a very fine grind ...

which suggests I might still have the same issues with a large conical.

Taste is fine. This is not a question about taste, it's about being able to work with a wider range of coffees.


#2: Post by malling »

Jim is pretty much spot on.

when you change coffee on a BIG flat like the Major, you definitely have to adjust the grind.

If I use the same dose for example 19g On my Major, I need to make a fine adjustment. But since I get coffee from All over Europe I tend to use different doses.

Therefore I Can Certainly live with the minor annoyance, until I Can afford (allowed) a k10

I also prefer not to grind to fine, therefore I would rather updose whenever I use lighter roast.

What I know from experience is that conic grinders needs allot less adjustment and are easier to dial in compared to big flat.

On the negative side Is that they have a much higher grind retention.

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#3: Post by orphanespresso »

Although I do not possess the expertise of another_jim and others on this subject, I do spend a fair amount of time thinking about grinding burrs and the grinding of coffee.

I find that it is a helpful thought experiment to separate the idea of hardware from the concept of reducing a coffee bean to particles by whatever method and visualizing just what happens during this process. Start with a roasted bean and a rock and move through technology to a fully formed grinder and try to visualize the results...

If you crush a coffee bean with tamper, for example, you can observe various particles, and assuming identical water content of the bean, you will see a variety of particle sizes, including the micro fine particles which form the smallest component of the desired bimodal grind distribution of the ideal espresso grind. Sort out the larger pieces and crush them again and you observe again, the micro fines and various other particles. AS the light roast beans are harder (not necessarily drier), to crush this bean you will have to exert more force and this extra force can result in the production of more of the micro fines in the crushing event (you are not only crushing the intact whole bean but the extra force also crushes some of the larger pieces).

To continue the thought experiment, imagine your grinding burr floating in space with no axles or motor or chutes, but only the burr. The conical burr, developed centuries ago for coffee grinding, naturally produces the bimodal particle distribution for espresso. This is in a way an accident since the burr design came about to use gravity as the force to enable the beans to enter the burr. The top large scoops in the conical burr do the first break of the bean and the smaller cutters process the particles to the desired final size. The larger scoop size will allow sometimes an entire bean to be crushed in the initial contact in an explosive moment to produce an event similar to the tamper/bean crush. Smaller scoop size will nibble the bean and require more events to process the bean into particles small enough to enter the secondary cutters for final grooming.

It is my theory that the large conical burr "sets up" the micro fine particle matrix of the espresso grind on the first crush of the bean in the large scoop cutters, as these large cutters can process beans almost in their entirety and not with multiple nibbling and crushing action. As the larger particles are reduced in size, this reduction requires fewer crushing impacts thus producing fewer micro fines, or at least a more defined amount of micro fines. It also helps that the conical burrs use a shearing action to process the particles and that the big conical burr can process more efficiently (same amount of coffee at a slower rpm), again maintaining the proper bimodal distribution for espresso.

What happens upon changing the adjustment on a big conical is that the amount of micro fines remains fairly constant due to the physical action of the burr while one is basically manipulating only the target particle size. To me this helps visualize the "forgiving" nature of the big conical.

Forgive the long thought experiment, but these same ideas can be applied to small conicals and flat burrs of all sizes to help one think of the process bean grinding. I also have an 83mm flat burr grinder and it seems to behave very similar to the large conical in controlling the espresso extraction on grind vs dose, although I do observe a similar behaviour on the pour as described by the OP.

Thanks for the time.

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#4: Post by EricBNC »

As my K30 is processing 4g per second I have to wonder if gravity alone is at work - seems too quick - something else must be going on - is the burr snatching the bean and forcing it into a journey of reduction?

As for the final size, I suspect the gap between the burrs is tiny at the espresso end of the dial and nothing will get through but the right size piece of bean - a tiny piece. What am I missing?
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Author of "The Bell Curve: Instructions for Proper Herd Mentality"

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#5: Post by orphanespresso »

I think an analysis of this whole thing is book length no forum post length....sorry to leave so many gaps :oops:
Oh yes, other forces move the bean bits through the cutters...look at the scoops on your Krups as they form wedge shape rams that force the smaller bits into the secondary cutters. The movement of the burr drives the movement of the particles (quite elegant system actually). This is why some coffee is left in the small cutter burr faces, there is nothing to force the last bits through...but if the burr is spinning fast enough these bits will be cast out by centrifugal force (another component of the particle movement, but not on a hand grinder).

Likely there are very thorough engineering analysis of these processes, I am just trying to get to my own understanding of big conical vs small, and conical vs flat, and how it all works.

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#6: Post by EricBNC »

I feel like the same forces that move the beans (wedge shaped cutters) down to the business end of the burrs on the Krups Die Cast Conical Burr grinder have more in common with the 68mm conical burrs found on the Compak K10 and the OE Pharos than they do with the 65mm flat burrs on the Mahlkönig K30.

I can visualize this more easily than the process that sends the same beans for processing into the larger flat burrs. In the end only particles as small as or smaller than the gap in the burrs will make it through to the portafilter. I can't see a lot of fines in the coffee after using the Mahlkönig 65mm burrs or the 58mm flat Mazzer Mini burrs on another grinder I use.

To be honest, both flat burr grinders make what looks like fluffy powder when grinding for espresso, as does the Preciso and the Krups.
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Author of "The Bell Curve: Instructions for Proper Herd Mentality"

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#7: Post by orphanespresso »

I agree that the processing of beans in a conical burr is more evident than flat burrs since flat burrs both spin VERY fast and therefore centrifugal force is a big driving event in these burrs to move the particles through the cutters. Still, I think the production of micro fines is accomplished in the same way, the breaking of beans and larger particles produce the fines naturally (this is part of the reason stale beans won't produce a good espresso grind) and the fines are essential for a good espresso grind. And the larger the flat burr, the longer the fine cutting grind path is and the larger the primary cutters are as well making a larger flat burr perform more and more like a conical, except that centrifugal force and coffee back pressure generally blow the micro fines through the system.

Extremely fast spinning flat burrs (or conicals for that matter) have to be supported by good bearings and high tolerance machined or cast carrier systems to maintain the proper burr gap. Also, burr speed leads to the cutters making "mistakes" as they do not care what they cut (two particles side by side get recognized as one and instead of having 2 of the right size you get 4 that are smaller than your target set size). As consumers of coffee hardware we are subject to the wishes of the mass consumer public and the emphasis is on speed at all costs and even though we all claim that we simply want a great cup of espresso, when it comes to grinders we also want it FAST. I have never in following discussions on forums for many years seen anyone complain that their grinder was too fast, but speed can be a real enemy for good consistent grinding, especially on low end machines.

Another thought experiment is to consider a stone type grist mill and the various reasons that such a grinder does not perform for espresso even though it is great for grinding flour.

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#8: Post by EricBNC »

If speed is the problem then the 58mm Mazzer Mini Burrs on my KitchenAid Proline should defeat this foe with the relatively slow 450 RPM's.

I can say the grind from the modded KitchenAid Proline is nice but I cannot say it is nicer than the grind produced by the 65mm burrs on my Mahlkönig K30. That grinder isn't loafing with speeds that must be very fast based on production. The Quality is not compromised as much as you would think if just looking at the numbers.

In spite of the speed the grinder is capable of assisting the espresso machine in producing a nice cup - more layered than the conical burr Krups, Baratza Preciso, or La Pavoni PBC - these small conical grinders can be one-dimensional with an emphasis on the brighter note - my flat burr grinders pick up this same flavor note but it is not the only note - if the coffee is right a deeper sweetness shows up too - sometimes (with Stumptown Hairbender for example) you get some hard to place flavors too.
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Author of "The Bell Curve: Instructions for Proper Herd Mentality"

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#9: Post by orphanespresso »

Eric...I have the same experience with the small conicals that you observe and like you, I am approaching this idea of coffee grinding as a physical process which can be understood on strictly physical terms. I don't think there is any magic in there with the big titan burr grinders...just a good burr and some superior build quality to run it. It is just the same with espresso machines and magic in there and it seems that if one can understand and control all of the parameters then one should get a predictable result.

With grinders, there are no real good tools to see what is happening on the grind and everything is interpreted after the fact through pulling a shot and getting in to some subjective evaluations. You like the K30 for the shots you can make when using it. I would probably like it as well. I wish I had one. I always enjoyed our B. Vario for various reasons but now prefer either the big conical burr or big flat burr in other formats...I am just a lot more able to tweak the shots with these larger burr grinders and that is interesting to me...not better, just a phase I am going through I suppose. A lot of it is personal taste.

Again, as I said, I am not an engineer or coffee scientist, simply spend a good bit of time thinking about these things, like many other people do. :)


#10: Post by decaf_Ed »

orphanespresso wrote: ... I find that it is a helpful thought experiment...
Thanks for the insightful (and civil) post.