I used to sell this type of equipment.  You need what is refereed to as a screen or grate.  With the size opening of what you want your material to come out.  The smaller the opening the longer it will take to grind it down.  All of the machines that I worked with had either fixed hammers and bits, or swinging hammer mills.  There is a company in North Wilkesboro that manufactures some small ones for flour mills and can be used for wood on a small scale, I can get there name if any one wants it.  I hope this helps.