Beneath the cylinder, and fully enclosing it, is a grid made of sheet metal provided with transverse slots. This is designed to remove impurities and maintain constant airflow conditions. However, since the cleaning effect is extremely small, some manufacturers, such as Rieter, have replaced the grid with a closed sheet metal casing. This enables the multitude of small air vortexes that tend to arise at the slots to be prevented. A closed sheet gives better fiber orientation on the cylinder surface and often reduces the number of neps at high cylinder speeds. Covering of the cylinder between the licker-in and the flats, and between these and the doffer, takes the form of protective casing. One of these protective sheets, near the flats at the front of the machine, is specially formed as a knife blade. The level and quality of the flat waste can be influenced by adjusting the distance between this blade and the cylinder. Narrow spacing produces little waste and wide spacing produces more strippings.This setting option is, however, not suitable for use as a means of adjusting the waste extraction effect of the flats. If, for example, an attempt is made to eliminate more short fibers by raising the flat waste level, it will not succeed. More long fibers rather than short fibers will be eliminated in the flat strippings. Fiber loss will be increased. Once an optimum has been established (mostly by the manufacturer), the setting should not be altered without excellent reasons.