What grinder should I use?
We highly recommend using a burr grinder dedicated to coffee only, since it grinds much more consistently than a blade grinder.
Burr grinders deliver the most consistent grind size because they cut the coffee beans in two stages, allowing them to get smaller and smaller as they travel through between the burrs.
Blade grinders, on the other hand, deliver a very inconsistent grind size - they produce coffee grounds that look like you've just smashed the beans up with a hammer!
Think about cutting up potatoes to be roasted: if you have some pieces that are much larger, they'll end up not cooking all the way through. If you have some pieces that are much smaller, they'll end up overcooking.
Coffee brewing follows the same logic! If your coffee grounds have very small particles mixed with very large ones, you will extract too much from some, and not enough from others - and the result will be not the best tasting cup.
For more info check out this guide.