98mm Burrs for Brew - Page 15

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

#141: Post by Jake_G »

rmongiovi wrote:It's not like these are the holes that the ground coffee comes out of.
Watching slow motion videos of grinding with a clear grind chamber, I can say that this is indeed where most of the coffee comes out.
coffeeOnTheBrain wrote:And with a different angle to answer the original question once and for all.
They are cut with the sharp edge forward.

coffeeOnTheBrain

#142: Post by coffeeOnTheBrain »

Jake_G wrote: ...
They are cut with the sharp edge forward.
Let's call it even :)

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rmongiovi

#143: Post by rmongiovi »

Jake_G wrote:Watching slow motion videos of grinding with a clear grind chamber, I can say that this is indeed where most of the coffee comes out.
I would like to see those videos. I'd like an explanation for why nothing comes out of the 360 degree gap between the burrs and instead queues up waiting for those notches.

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Jake_G
Team HB

#144: Post by Jake_G »

coffeeOnTheBrain wrote:Let's call it even :)
Deal! :P
rmongiovi wrote:I would like to see those videos. I'd like an explanation for why nothing comes out of the 360 degree gap between the burrs and instead queues up waiting for those notches.
Unfortunately they are not public at this time, and I'm not the author, so I can't make them so. :(

However, the explanation is not hard to grasp. The burrs work a bit like a pump when grinding in that there is a large volume in the center where whole beans are introduced with progressively smaller working volume between the burrs as the beans are crushed and forced outwards. As the burr rotates, the beans are forced into regions of smaller and smaller working volume between the burrs, and the fragments tend to follow the path with the largest volume available at any given point along the way.

A critical point in the beans' path to the periphery of the burrs is where the final teeth are cut into the upper surface of the burrs. The final teeth are cut -in every flat burr that I've seen that has flat segments along the periphery, at least- in such a way that the flat surface comes to a point, shown below:


Those points are important, because they are the closest points of contact between the two burrs at this point of the grind path and -because they are points- represent basically zero percent of the volume between the burrs; whereas bean fragments will flow into the comparatively large open volume of the final stage teeth, they will run into the points and tend to be guided into the channels on either side, which lead to the outfall.

Sure, some fines will make it between the flats, but it is a simple geometry exercise to see that even if coffee flowed evenly out of all the available cross-sectional area for it to do so, there would be preferentially more exiting from the outfall because there is far more open area there than there is between the flats.

Furthermore, there is significant shearing action between the flats as the rotating burr passes the fixed burr that force trapped fragments into the open area of the teeth and towards the outfall. There is no "queuing up", stuff just takes the path of least resistance, especially when it is rigid stuff that is too big to fit into the places of more resistance. While some fragments can and certainly do pass through the flats, the vast majority are too large to fit and as they are crushed and sheared and rolled and polished on their way to the outfall, they tend not to hop back up into the slim gap between the flats.

At least this is what I have observed on large horizontal flats. I'm sure vertical and slanted burr sets behave somewhat differently, though I am unsure how much differently.

Cheers!

- Jake