Milling increases the surface area of the grain, making the starch more accessible and separates the seed from the husk.
Crushing your grain is an important part of preparing your malt for the mashing process regardless of which of Grainfather brewing unit you have. To ensure that the starch reserves are sufficiently milled, care must be taken to avoid damaging the husk, and providing coarse enough grits so that a good filter bed can be formed during sparging.
If your grain is not crushed enough, the individual kernels will not be able to absorb enough water and the conversion of starches to fermentable sugars will be very slow. If the grain is crushed too fine, water will not be able to correctly flow through the grain so you may risk extracting harsh tannins from the grain husks or cause a 'stuck' mash.
Most homebrewers use a standard 2-roller grain mill. Roller mills also tend to keep the husk more intact while still crushing the grain effectively. We recommend a medium-sized crush for the Grainfather Grain Mill. The useful range for 2-roller mills is 0.024" to 0.039", on the Grain Mill this will be settings 4-6. A great rule of thumb to use when setting the space between your rollers is to measure the gap using a credit card, or similarly sized card.
For a breakdown on which gap size settings suit which grains, see here.
For what to do if the crush is too coarse, see here.
For what to do if the crush is too fine, see here.