Home Tempering, Grinding, and Bolting Wheat to get High Extraction Flour
Success begins with perfectly tempered wheat. Tempering consists of adding water to dry grain and allowing the grain to rest for a period of time before it is milled. The purpose of tempering is to toughen the bran and thus make it resist being broken into small particles during milling and to soften or "mellow" the endosperm and make it easier to grind. It also helps obtain bran with lowest possible starch content and flour that has ideal quality and higher extraction.
Temper at 77°F or longer if colder. Cooler temps increase flour output and ash. Hotter temps shorten tempering time and can improve gluten properties. 72 hours is not too long to temper.
For milling it is essential that the tempering water is fully absorbed and evenly distributed in the endosperm. Too short a tempering time results in more granular flour, and more power required to mill. Don't mill cold wheat. Milling cold wheat will tend to cause the bran to shatter and not flake off making it hard to sift off.
Ideal Moisture and Tempering Time
For roller mills
- Spring Wheat: 17.5% / 48 - 72 hrs.
- Hard Wheat: 16.5% / 36 - 48 hrs.
- Soft Wheat: 15.5% / 12 - 24 hrs.
- Durum Wheat: 17.0% / 4 - 12 hrs.
Use lower percentages for other milling methods.
These moisture levels may seem high. These are not necessarily the moisture levels used at commercial mills. They can't sell flour with a moisture level over 14% as it is much more likely to mold, draw insects, and have bacterial problems. This is not and issue for home milling if you are going to use in right away. Try to use your flour within 24 hours of milling.
The bite method
What moisture level is the grain you are starting with? No grain moisture meter? No problem. Take a couple grains of your wheat and bite it. The harder the grain is more water it will need to be tempered with. Long ago millers didn't have moisture meters. They used the bite method. Practice and learn the feel of grain. To achieve ideal tempering is to learn to feel and to observe.
How to Temper
Add the desired amount of water to wheat cover container and shake until water is dispersed and for 10 seconds every minute for 5 minutes. Don't add more that 5% per day. Temper at 77°F for 2 to 3 days. If your temperature is much lower than 77°F add a day.
Warning! Protect your mill. Don't grind any grain in your mill you are not comfortable with. You know your mill better then I do. Grind at your own risk.
I used Wheat Montana Hard Red Spring Wheat for the milling tests.
Every one says don't temper wheat for impact mills, but I have used my impact mill with estimated moisture of 14% and it worked great. About 9% of the bran was large and sifted out easily. I also tried 20% moisture in the impact mill and I think it was too high but it didn't ruin the mill and was not hard to clean up. It also had large bran but not an improvement over the lower moisture batch.
The Kitchen Aid Grain Mill (KAGM) worked well but not as well as the impact mill. It has very limited ability to grind fine.
The Wonder Junior Mill is hand cranked and it is easier to mill if the wheat is tempered. It works well at 13% to 15% moisture. I ran 20% through it and it glazed the stones. They were easy to clean with water and then let dry.
Bolted Flour aka High Extraction Flour
Bolted flour contains almost all of the germ, and the softer parts of the bran. Bolted flour was historically sifted through a piece of cloth. It is now typically sifted through a metal or plastic screen. The higher the percentage the closer to whole grain flour it is. The total flour out of the sifter divided by the total grain in to the mill would give you the extraction percentage. 100% extraction is whole wheat.
Home grinding and bolting wheat will get flavor that can't be beat.
History of flour bolting http://www.angelfire.com/journal/millbuilder/boulting.html 
After grinding sift your flour through a sieve. I have a 55 mesh I got here. http://www.fantes.com/sifters-shakers.html  also a 30 mesh I have had for years. Using a magnifying glass helps to inspect your work.
I have been tempering 10% moisture HRS wheat to 15% moisture by adding 50g of water to 1000g of wheat and putting it in my 80 °F proof box for 3 days. Grind the wheat in my impact mill on fine. Sift through a 30 mesh sieve and remove about 9% pure bran. Sift through a 55 mesh sieve and save the 74% very white flour. Take the middlings that were caught by the 55mesh sieve and look at them under a magnifying glass. You will see small flat brown bran and small roundish sand looking endosperm. Run this through a stone mill and sieve through the 55 mesh. Stone mill and sieve one more time. Discard the bran. You should have 84% to 88% extraction flour. I have been using about 65% hydration with this flour. It tastes wonderful.