An essential part of the brewing process is crushing the malt so that the enzimes can attack the starches and sugars contained within the husks. The soluble substances are extracted more quickly from smaller particles than large lumps. However the husks must be left as intact as possible. This is one of the examples of a situation where a brewer must decide which way is best. Large breweries use large 6 roll mills with decreasing gap sizes and sieves between them. The first roller gap is approximately 1,8mm to crack the husks with little damage to them. The second roller gap is about half the first and separates the husks from the insides. The third set of rollers are set to between 0,2 and 0,4mm and crushes the malt starches to a fine powder referred to as flour.
The above description is a very brief over view of what is done by industrial brewers with big machines and plenty of money. Home brewers and even most micro breweries can not afford the luxury of a multi roll mill. However, centuries have passed us by an acceptable results have been obtained from 2 roll mills. If you have a mill that has adjustable rollers you could try crushing the malt with the rollers set at a large gap. You can then set the gap smaller and then pass the crushed malt over a sieve to separate the husks from the flour. The flour can then be put through the mill again with the rollers set at a fine gap. Before mashing in the husks and the flour must be re combined to facilitate lautering.
We do not want to crush the husks to fine as this will result in the mash bedding down to much and a stuck mash forming.
Another method to prevent the husks being broken during milling is to wet the malt before it is put through the mill. In a process called wet milling the malt is steeped in water below mash temperature for up to 30 minutes. During steeping the endosperm is also hydrated and becomes more difficult to mill. A development of the process hydrates the malt within a minute of the grain passing through the first rollers either using hot water (50° to 70° C) of steam in a steeping chamber immediately above the mill.
To see if the gap of the mill is correct put some malt in the mill and crush it. The husks should not be broken to more than half their length. In a good crush the husks are split longitudinally in half. A white powder with few lumps should be formed from the starches in the malt. If the husks are still whole and there is very little powder reduce the gap. If however there is plenty of white powder and plenty of straw coloured fibres open the gap. After a little playing around with a mill you will soon see what makes a good crush and what doesn't work. Remember that if you are getting poor yields from your mash the course could be from a too course a crush, on the other hand if you are getting stuck mashes you crushing too finely.