Roller Mill Grinder

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Sardonic_Sardine
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Postby Sardonic_Sardine » Dec 16, 2013, 8:39 am

Do roller mills produce a more uniform grind compared to flat burr grinders?

I haven't found all that much information on how the two compare, but I'm wondering if a roller mill type grinder would have a narrower particle distribution curve, and hence produce a better cup.

I know that roller mills for grinding coffee have primarily been large, expensive, industrial machines. Though I'm wondering if a compact version can be made for home use.

This could possibly be built from mills such as these (I think they're made for milling grains for home-brewed beer):

http://www.mashmaster.com/p/4571858/mas...-mill.html

OR

http://www.monsterbrewinghardware.com/p..._main.html

Would this be a worthwhile exercise though? Thoughts?
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bostonbuzz
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Postby bostonbuzz » Dec 16, 2013, 1:18 pm

Burr grinders don't crush as much as "slice" beans apart. Take a look at a pair of burrs and you will see what I mean. It's hard to imagine slicing a roasted bean, but that's how the burrs are designed.

Theses rollers will be crushers. Some bulk grinders have crushing burrs resembling flat burrs with many little pegs. Theses are generally slightly inferior to standard flat burrs with machined sharp ridges.

My guess is that these rollers will work (if the beans get fed through at all) but will not be superior to existing technology. What will actually happen is another matter. If you follow through let us know and post some videos or photos please!
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RapidCoffee
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Postby RapidCoffee » Dec 16, 2013, 1:30 pm

There is no such thing as a grinder that slices coffee beans, including whirly blade grinders and burr grinders. Roasted coffee beans are brittle pieces of cellulose, and get smashed or crushed in the grinding process. This is borne out by direct SEM observations.

I have no experience with roller grinders, but per Wikipedia:
In a roller grinder, the beans are ground between pairs of corrugated rollers. A roller grinder produces a more even grind size distribution and heats the ground coffee less than other grinding methods. However, due to their size and cost, roller grinders are used exclusively by commercial and industrial scale coffee producers.
Water-cooled roller grinders are used for high production rates as well as for fine grinds such as Turkish and espresso.
John

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Sardonic_Sardine
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Postby Sardonic_Sardine » Dec 16, 2013, 10:27 pm

To be honest, I'm not sure what happens to coffee beans in flat burr grinders (whether they get 'sliced', 'crushed', or 'smashed'). I'd certainly appreciate it if someone could explain the mechanics of a grinder to me! My sense (which could be inaccurate, or merely a case of semantics) is that roasted coffee beans are quite brittle and when enough pressure is applied to them, they will 'shatter' into smaller fragments.

I'm certainly interested in how a roller mill would perform compared to flat burr grinders. It's something which I feel hasn't received that much attention in the context of home brewing, given how no non-industrial, coffee-specific grinder mill seems to exist (at least to my knowledge).

In the meantime, here's a link I found (courtesy of searching the home-barista forums) from a producer of industrial roller mills:

http://www.mpechicago.com/coffee/news/resources/
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pacificmanitou
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Postby pacificmanitou » Dec 17, 2013, 5:10 am

A similar description is given in Illy's book. I always assumed they were used on commercial coffee.
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TomC
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Postby TomC » Dec 17, 2013, 5:24 am

They are. The setup cost is greatly prohibitive to a home user, not to mention their size. The serial reduction in size of the particles moving thru various gapped rollers prior to hitting the last roller gap is what helps with the tight particle size distribution. There has to be a tinkering home engineer out there playing with a belt driven/pulley system involving roller mills for the home user.

There's a popular beer/home-brewing supply company that had their grain mill ( a simple roller mill) reach stratospheric levels of demand once the supply dried up. They put the necessary engineering and effort into building a high quality mill. I'm sure someone could do it for coffee as well, but keep it somewhat counter friendly. I imagine it would end up looking like a modern timepiece built by a master, with various cogs and wheels and gears, etc.

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TomC
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Postby TomC » Dec 17, 2013, 5:29 am

And it's interesting to note, large scale breweries that focus on waste reduction like the wonderful Sierra Nevada Brewery in my hometown, use their own version of the Ross Droplet Technique (RDT) in milling their barley. Grinding grains like barley produce a lot of dust. The dust builds up, creating excess heat, and can eventually gum up the roller mills output area if not attended to. Everyone has to battle fines :twisted: But their main purpose in doing so is that if they slightly pre-moisten the surface of the grain, no dust is created, and their efficiency is improved since they'll end up having more starch to begin the brew with.

Alan Frew
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Postby Alan Frew » Dec 17, 2013, 6:05 am

Roller mills are the ultimate in achieving precise particle size distribution. I have a dozen or so years experience with them in the printing ink industry, but they are also used for grinding cacao into chocolate, grinding various flours and many other industries where you need to turn big particles into small ones.

Buhler http://www.buhlergroup.com/global/en/pr...rAchyzxuUk are the mills I mostly worked with. The important thing with roller mills is that if you want to get precise particle sizes, the gap between the rollers has to be exact. Grinding relies on friction, friction produces heat, so all the rollers have internal cooling systems (usually refrigerated water) built in. When you're dealing with micron sized particles you DON'T want the metal rollers to expand or contract with temperature changes.

There are "laboratory" versions of roller mills produced by the same companies that make the production mills. They generally come with reverse cycle heat exchanger systems that need to be plumbed and wired in, and $10k+++ price tags. This is definitely not an industry suitable for amateur involvement, just producing the rollers requires a megabuck investment in technology.

Alan

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farmroast
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Postby farmroast » Dec 17, 2013, 11:18 am

Here is a recent pic of a grindmaster brew grind of a DP ethiopian. I haven't done any espresso pics yet. I talked with a friend who worked with roller mills (not coffee) a few years ago. She thought I'd still end up with fines at the espresso level due to the particle structure of beans.

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RapidCoffee
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Postby RapidCoffee » Dec 17, 2013, 12:53 pm

No question about it: every coffee grinding method produces fines. As the beans are crushed, small fragments (fines) break off. More crushing/breaking takes place with finer grinds, and hence more fines will be generated.

Nice pic, BTW. Any idea of the diameter of the large particles?
John

 
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