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Burr Grinding 101 and 102
BoldJava
More than you ever wanted to know about burrs - from HB/Jim Schulman. Hadn't bumped up against the post previously:

http://www.home-b...t1672.html

B|Java
Edited by BoldJava on 02/11/2009 5:07 AM
http://sidewalkmy...
Dave Borton
Milwaukee, WI
 
Dan
BJ, Thanks for posting that here. Jim Schulman does a good analysis of grinder mechanics. And using the M3 is fun and smart, since many consider it the best (and most expensive) grinder around.

I did some research into grinder mechanics for a design project a few years ago. In the coffee industry grinders are called mills.

Where a lot of people get confused is that they think a coffee grinder actually grinds coffee. It does not. Grinding is presenting a material against an abrasive, often grindstones. Jim falls into this pit for a moment. Notice that in one picture he mislabels burrs as grindstones. the problem is that people use the 'grinding' perspective to try and explain what is going on in a mill, instead of using 'milling' terms and perspective.

What really happens is called particle reduction. Any given set of mills or burrs reduces the size of the particle (beginning with a whole bean) into smaller and smaller particles. They do this by shear forces (not abrasive tearing). Just like a nut cracker that uses teeth to concentrate force in a small area, causing the particle to shear. It is a cracking action. Now that you know this, the shape of the burrs should make sense.

But why the complex shapes and angles on the burrs? That's easy too, once you know the physics. In particle reduction, the most you can reduce a particle size at any given step is about 1/5th the original size. The answer to getting something finer than 1/5th is to use a series of progessively finer burrs. Each step "cracks" the particle into smaller pieces. Jim shows this clearly in the photo he marks up showing three steps on the burrs. He gets the first step right: bean breaking, but the next two wrong: coarse grind, fine grind. Actually, they are: particle reduction step one, two, and three. Three steps seem to do the trick for coffee. Some materials only need one or two steps, some need five.

So, to wrap this up. the first step takes a coffee bean, let's say it is 0.5" long, and reduces it about 1/5th to 0.1", step two reduces it again by 1/5th to 0.02", and the final step reduces it by 1/5th again to about 0.004". Opening the burr set then allows larger particle sizes to pass through.

Dan
Edited by Dan on 02/12/2009 7:58 AM
 
seedlings
Thanks for the 101, 201, and now 301, Dan!

CHAD
Roaster: CoffeeAir II 2# DIY air roaster
Grinder: Vintage Grindmaster 500
Brewers: Vintage Cory DCU DCL, Aeropress, Press, Osaka Titanium pourover
 
BoldJava
Super post. Edit: That is Jim Schulman's post off of Dan's HB site. It is short compared to Jim's book on water.

B|Java
http://sidewalkmy...
Dave Borton
Milwaukee, WI
 
Dan
BJ, Thanks, I've changed my post to reflect that. :)
 
cfsheridan
Thanks for the information, gents.
 
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