Essentially, there are only two things that happen in a mill: grinding and separating....
The process used to produce white flour in a modern roller mill is referred to as a gradual reduction. The endosperm is gradually reduced in particle size by running it between a series of pairs of rotating hardened steel rollers, either corrugated or smooth, and is separated from the bran and germ by running it over sieves. All pairs of rollers in a mill have one slow mill and one fast one. The rolls turn in opposite directions, toward each other, pulling the stock (material being ground) between them. The cut, depth, and spiral of the corrugation, together with the rotation differential, determine the aggressiveness of the milling at any particular step.
There are five roll systems in a flour mill: break, sizing, midds (for middlings), low grade, and residue.
In the break system, the kernel is opened, the bran flattened and the endosperm broken into large chunks. Al-though some flour is produced here, the goal in the break system is not to produce a lot of flour but to maximize separation of bran from endosperm. Because the break rolls are at the beginning of the milling process, the quality of the work done here effects each subsequent step, thus determining both the yield of flour and the quality of that flour. If the rolls are too aggressive, portions of the bran may be torn or ground into dust that will be impossible to separate from the endosperm in later steps.

http://home.earthlink.net/~ggda/The_Milling_Process.htm
..................................................................................................
The whole milling process can be seen as a repetition of two processes – grinding and sifting. The wheat is first passed over a series of fluted break rolls. A pair of break rolls do not turn at the same speed, the higher roll usually turns about twice the speed of the lower roll.
Wheat travelling between the break rolls is ripped apart and the white endosperm material is released. After passing through each set of break rolls the particles are sorted on a sifting machine. The flour is removed and the coarse branny material is returned to the break rolls in order to separate out any flour still attached to the bran.
Semolina, which is chunks of endosperm, is also produced in the break system and this Semolina is passed onto a new series of rolls called reduction rolls. Eventually all the wheat going through the break system is removed as either flour, semolina or coarse bran.

http://www.bakeinfo.co.nz/Facts/Wheat-Milling/Wheat/Milling-...
................................................................................................

The wheat milling and rye milling processes are very similar because flour is the product of the break rolling system. In durum wheat flour milling, the intent is to produce as little flour as possible on the break rolls. As in wheat flour milling, the intent in rye milling is to make as much rye flour as possible on the break rolls. Consequently, there are more break rolls in proportion to reduction rolls in a rye mill than in a durum wheat flour mill.

http://www.epa.gov/ttnchie1/ap42/ch09/final/c9s0909-1.pdf

--------------------------------------------------
Note added at 1 day10 hrs (2012-01-16 22:18:43 GMT)
--------------------------------------------------

Breaking process

T h e most effective means yet found for meeting the above
requirements is a number of successive graduated treatments of the grain between pairs of spirally fluted chilled-iron rolls driven at different speeds. T h e rolls are progressively set closer together and more finely fluted throughout the four or five breaks usual in this country.
(an excerpt from a Cambridge Journal, provided by bonafide1313)

--------------------------------------------------
Note added at 1 day10 hrs (2012-01-16 22:20:55 GMT)
--------------------------------------------------

Flour

The process of manufacturing flour is carried out differently by different manufacturers, so far as its details are concerned, but the main features are the same. The wheat as it comes to the mill is first of all cleaned, by screening to get rid of any large foreign substances that may be present in it, and by "scouring" to get rid of the fine dirt that may adhere. The next process is that known as breaking. The wheat is cut by corrugated iron rollers provided for the purpose.



http://chestofbooks.com/health/nutrition/Food-Dietetics/Cere...