More heresy from the house of Londinium!
We see so much angst expressed on the internet whereby people are dismissive of a grinder on the basis that they have seen ground coffee come out in clumps from it. Clumps I say! Clumps do you hear? Clumps! Clumps! Clumps! The end of the world is nigh! Channelling supernova imminent! Stand back everyone!
In our experience clumping doesn't produce channelling. You can test this for yourself if you get a clump of ground coffee fresh from the grinder that you think is responsible for your channelling woes, and allow a single droplet of hot water to fall on it. You will be able to observe that the coffee grounds are hydrophilic, and therefore absorb the water droplet. As a result the coffee grinds swell, causing the clump to self destruct.
If you think about the manner in which water enters the portafilter from the shower screen it becomes obvious too. When water first enters the portafilter basket from the shower screen it does not immediately pressurise in the way it would if the ground coffee was an impermeable substance like wood. Instead the dry coffee grounds are highly absorbent until they become saturated, and so they soak up water almost as fast as it leaves the shower screen. As the grounds soak up the water any clumps you might have in your coffee puck are literally 'blown apart'. After a few seconds of water entering the portafilter the ground coffee in the top half of the puck at least becomes saturated and so the inflow of water exceeds the rate at which the ground coffee is able to soak it up, the headspace between the top of the ground coffee and the shower screen fills and then, because liquids are not able to be compressed, you finally get water pressure increasing against the coffee grounds and the espresso extraction process commences in anger.
If you are having issues with channelling try grinding your coffee more finely and reducing the dose. We think you'll find life a lot easier.
A Mazzer robur will happily produce clumped coffee grounds if you have very fresh, moist coffee grounds and are grinding fine. When we hear and read of people expressing dismay that a Robur is producing ground coffee with clumps and offering this up as a short coming or criticism of the grinder we have a chuckle. The Robur is one of the finest espresso coffee grinders made, and you can rest assured that we won't be disposing of ours in the foreseeable future, even though we can make it deliver grinds in clumps. Why? Clumping is not responsible for channeling in our view.
Run a few tests for yourself and see if you come to the same conclusion as we do.