It's often said that a good grinder is the most important piece of equipment for making espresso, and I tend to agree. I've seen far too many people with expensive espresso machines, but who try to skimp and get by with a crappy grinder. You can not make good espresso unless you have a good quality grinder that produces a consistently even grind. Uneven grind can cause all sorts of problems, including too fast an extraction and channelling in the puck. In my thinking it is better to get a great mill and then skimp if necessary on the espresso machine, not the other way round.
If you don't have a nice, even grind, you can't make good espresso. You can't make great espresso with a blade grinder: inevitably, a blade grinder produces some powder that ends up as grit in the cup and some particles so large they underextract and also cause channeling. In addition to a consistent grind, a good grinder should avoid clumping of the grinds, keep the beans cool while grinding, and help to evenly distribute coffee in the basket.
Using the Correct Grind
When making espresso - you need to use a very fine, consistent grind. How fine? One easy test I use is to pinch the grinds between my thumb and forefinger. The coffee ought to clump in the center of the pinch, where the pressure is hardest, but not too much. If it does not clump at all, it is too coarse and will make a weak shot. If it clumps excessively, it is too fine and will produce overextraction. With the photos on the left, the top photo is just right. The middle one is too fine (you can see a thumbprint in it) and the bottom image is too coarse. The best feedback on your grind is extraction time; if the water pushes through the puck too fast, use a slightly finer grind. If it goes too slow, use a slightly coarser grind.
Freshness of the roasted beans and their degree of roast impact how the water is absorbed and pushes through the puck, so these are factors that effect grind as well. This means you may have to adjust the grind from one coffee to the next, or as you use a pound of pre-roasted beans and the beans age. It can be helpful to have a grinder with stepless adjustment so you can make slight adjustments to the degree of grind. Remember that making good espresso requires balancing several factors - grind, tamp, coffee, pressure. It is less a matter of making it "right" than making everything work together in concert.
Keeping Things Clean!
Any burr grinder will retain several grams of coffee in the burrs, the chute, etc. Typically, the larger the grinder, the more grinds it will retain. While this isn't really a problem for a coffee shop that grinds new coffee every minute, it can be a big issue for a home user: the old grinds stale and negatively impact flavor. Equally problematic is that home users often change from one coffee to another, and the flavor from a previous coffee might contaminate a new one. In the end, not much can be done about grind retention: it's just the price we have to pay for using commercial equipment in a home setting. Keep your machine as clean as possible, and flush a few grams of beans through the machine if you haven't used it in a while, or if you're switching coffees. You should clean your burrs frequently by running instant rice or a dedicated grinder cleaner (we sell Grindz) through the machine.
All burr grinders will eventually need to have their burrs replaced. Burrs are rated for how many pounds they can grind before replacement — typically dozens of pounds for home grinders and up into the hundreds for commercial machines — check your grinder's manual for detail. Replacing burrs is usually a pretty easy process, and replacement burrs currently cost about $30-$60, depending on the grinder.
Grind Distribution Basics
So, you've gotten a good grinder, good coffee, and a good machine, but your shots still aren't great? Chances are good that your distribution might be off. "Distribution" means how evenly distributed the grinds are in the portafilter. Classic evidence for poor distribution is a shot that is looks good for a few seconds, and then suddenly gushes out a light-blonde mess. What's happening is "channeling" – the pressurized water for brewing has pushed a hole in your puck and is flowing mostly through that hole rather than being pushed through the grounds. This means the water is not extracting flavor evenly and so the resulting shot will taste wrong.
The first key to fixing distribution is to think about it. When you're leveling off your coffee before you tamp, try to think about spreading the grinds around evenly, making sure to fill any fissures and to push grinds all the way to the edge. When you're done pulling a shot, before you throw away the puck, take a look and see if you can spot any tiny pinholes (or larger holes!) on the top of the puck – these are probably signs of uncorrected clumping – or holes at the side of the puck, where water has pushed between the puck and the filter basket. With the fine grind needed for espresso, grounds can often form clumps, which can be broken up as you level the coffee before you tamp. For the truly obsessed, you might consider getting a bottomless portafilter, where the entire bottom (spouts and all) has been cut off to expose the bottom of the filter basket. The thought here is that you can see specific places where channeling is occuring. Chances are good that, unless you're very lucky or very experienced, you'll be amazed by how much chaneling is going on. Be warned, though: when channels break through, they often shoot water in strange directions, so when using a bottomless portafilter be prepared for a mess!
No grinder doses coffee perfectly evenly into the basket, so it is usually beneficial to rotate the basket while dosing in order to evenly distribute from the beginning. Remember: it's much easier to distribute evenly while dosing than to correct poor distribution afterwards! There are a wide variety of distribution techniques out there, each strongly supported by many great baristas. For those interested, seaching "espresso distribution" on video websites should yield countless results.
A Last Resort for Distribution Problems
If, after all of the above, you still get shots that suffer from chaneling, it might be time for the so-called "Weiss Distribution Technique," aka WDT. The basic idea here is to stir the grinds in the portafilter to break up clumps and distribute everything evenly. For much more information on the WDT, visit the home-barista page