When brewing coffee or extracting espresso, the grind's degree-of-fineness greatly affects how much extraction occurs, and the amount of extraction determines how many of the brewed coffee's characteristics ar tastable. The finer the grind, the more surface area is exposed to the brewing water, allowing more organic components to be drawn out of the ground into the water.
Matching the correct grind with a brewing method is important, and a grinder that allows for controlling the consistency (degree-of-fineness) for example as a stepped grinder is crucial, especially if different brewing methods are used at different times. If you only grind one kind of espresso or coffee beans, a stepless grinder might grant you the chance to adjust seamless the fineness of your grind.
The two main types of electric grinders are blade and burr. Blade grinders rather chop the beans at 20,000 to 30,000 RPM, the result is inconsistent in regards of fineness and the most of coffee flavor gets lost through the high temperature those grinders produce. With other words, blade grinders do not make any sense, rather grind hazelnuts with them.
Burr grinders use a basic mechanism invented by French company Peugeot in the 1840s, and one that is still employed in most pepper mills. The beans are ground between two grooved plates, or heads, usually cast metal or ceramic, one of which is in a fixed position, the other of which is turned by a motor housed in the base of the machine. Generally, the lower of the horizontally mounted plates is turned by the motor while the upper plate is stationary. The relative positions of the heads to each other determine the fineness of the grind. Usually, it’s the upper, stationary plate that is connected to the appliance's grind fineness/coarseness adjustment(s).
There are conical and flat burrs grinders on the market and the grinders spin at speeds between 500RPM and 1680RPM. Typically, the flat-plate machines spin at a faster rate, which generates heat (as in a blade grinder) which is potentially harmful to the beans. The higher heat causes quicker evaporation of the essential oils in the beans, and could impart a burned taste to the resulting brew. Low speed machines do have two other advantages over the faster spinning grinders: they create less static electricity in the grinds, which makes for easier handling and clean-up, and they’re usually noticeably quieter, disadvantage is they are more expensive. In general, performance and durability are determined by how well the components are machined and the quality of the construction, not by the configuration of the burr plates.
Grinders which operate at slower RPMs actually need more power to maintain constant speeds than faster motors. In Watt the motors have between 150 Watt and 350 Watt for spins around 1600 RPM and up to 900 Watt for spins in the 500 RPM area. In general, the slower the grinding plate spins, the less heat, noise, and static electricity that are generated. RPM range for burr grinders is between 400-1650.
Grind adjustment: The grind consistency (fineness) of most burr grinders is changed by adjusting the position of the top plate in relation to the bottom plate. By turning the threaded collar/ring, into which the upper grinding plate is set, the level of grind is raised or lowered. Now to adjust those settings there are stepped and stepless grinders. Stepped grinders you have clickable or preset grind levels. If you can change the grind settings seamless, we call it a stepless grinder. As stepped grinders are extremely convenient if you use different grind settings alternatively (e.g. espresso, drip, french press), the stepless grinder allows you to more detailed change grind settings, answering different humidity levels in the beans you want to grind (each batch might be slightly different and you want to the grind settings only a hint). The number of stepped settings range from 17-200+, with 40-50 clickable positions as a good value for burr grinders. More sophisticated grinders offer both clickable grind settings and fine control for gradations between each position. Adjustments are made by turning either the ring at the bottom of the hopper (or a lever attached to the ring, sometimes released by a separate clutch control on the side of the machine), or by turning a dial or slide control on the machine housing. Kings of the scene offer both: A worm drive (a two-gear differential configuration) which offers the finest degree of control possible. Please take into account how many people with what level of skills use the grinder for extraction shots. Unexpected changes of grinder settings can ruin easily the output for a whole shift in a restaurant.
Depending on your environment for your grinder, a doser grinder or a doserless grinder can be the better solution: If you use less than 40 shots per day, you are better off with a doserless grinder. If you use more shots per day or freshness of the espresso ground is not your priority no.1, you better should buy a doser grinder. The reason for is, that with a doser grinder the fresh grinds fall into a round tray divided into 5-6 wedge-shaped sections, each of which holds from 5.5-10 grams of grounds and you adjust the output per shot by turning a knob inside of the doser. The ground coffee is distributed into the portafilter underneath, a dose at a time, with the pull of a mechanical lever. For a double shot you pull twice and you are done. The doser refill with fresh ground coffee works either manual, timer based, or ground level based called automatic, mostly a question of price against convenience. Doserless grinder directly send the fresh ground from the burrs into the portafilter when the portafilter's rim hits the push button. Nevertheless there is a little left over from the last grind (and in so far may be old ground) in the connection tunnel between burrs and spout. Worst case such residual old coffee ground in the tunnel might bake into a kind of a brick over night and the doserless grinder on the next day does not work: It depends on the design of the doserless grinder, the kind of espresso beans you use and on the humidity in the environment of the grinder. Most doserless grinders use a timer based method to grind the right amount of grams into the portafilter, either executed by a simple timer with a knob to adjust time after the button is pushed by the portafilter. More expensive doserless grinders use amicroprocessor based time model with different settings for one-shot, two-shot or endless grinding.
Coffee grinds are messy by nature: The friction of the beans inside the grinder and the plastic surfaces of hopper and doser etc. with which the beans and later the ground come into contact generates static electricity, which exacerbates the messiness. Value grinders come with an anti-static agent to reduce the mess.