"Machining" may be divided into five processes:--
_First._ When the stock leaves the beater it is run into a large "stuff" chest, and is continually being stirred so that it will not be lumpy. By this time the pulp is about as clean as possible and is ready for the paper machines. The first thing to be done on the machine is to dilute the stock with pure water to the consistency of buttermilk, according to the thickness of the paper required. Then this liquid stock runs through what are called "sand settlers," which are supposed to collect what dirt, iron, etc., remain.
_Second._ From the sand settlers the stock runs on to a screen, through which it is drawn by means of suction. This process prevents fibres which are lumpy and too long from getting on to the machine, and allows only those of a certain size and length to go forward to be made into paper. An endless and very fine wire cloth, which is continually moving at the same rate of speed as the rest of the paper machine, takes the stock after it has been screened. This is the first step toward making the material into actual paper. Thick rubber straps on each side of the wire determine the width of the paper. This wire shakes a little in order to weave the fibres together while in a state of suspension. At this period the stock looks like thick cream, but soon changes its appearance to the form of a sheet more or less solid on coming to the end of the wire, where there is what is called a "dandy,"--a roll covered with similar wire cloth pressing lightly on the paper as it runs along the wire. Designs in relief on the surface of this roll produce the well-known marks called "water marks." Just beyond the "dandy," underneath the wire, is a suction box which draws enough of the water out so that the paper can go through the "couch" roll at the end of the wire without being crumbled.
[Illustration: Cross-section of a Paper Machine.]
_Third._ The couch roll is a small hard roll covered with a thick felt called a "jacket," and is used on the paper machine to prevent the paper from being crushed, for it presses out much of the water and flattens the paper so that it will pass from the wire to the felts without breaking and through the press rolls without crushing. From this couch roll the paper leaves the wire and is carried along on an endless woollen felt to the press rolls, which are made of hard rubber, steel, or brass. These rolls press the fibres together well, squeezing out more of the water and flattening the sheet.