For best cut quality, an optimum distance between the nozzle and the work piece should be maintained. Typically, between 0.0625" and 0.125" (1.5 – 3.0 mm) is the optimum height for abrasive waterjet cutting. As the distance increases above 0.125", rounding on the top edge of the cut will result. This occurs because the waterjet stream looses coherence as it travels through open air. Increased nozzle height will also result in increased kerf angle. If the distance between the nozzle and the work piece is increased by ¼", cutting speeds must be reduced by approximately 20% to achieve similar results with respect to tolerance and edge quality. Automatic height control is the most reliable and accurate way to maintain proper standoff distance. Figure 7 shows that if the nozzle height is increased too far, the jet will not have enough power to fully penetrate material that it would easily cut at much lower heights.