The high velocity of the jet creates a Venturi effect, or vacuum, in the mixing chamber located immediately beneath the orifice. Abrasive, typically garnet, is metered from a mini-hopper through a plastic tube down to the cutting head and is sucked into the waterjet stream in the mixing chamber. Cutting speed will increase with more abrasive until a saturation point is reached where speed starts to decrease. If the abrasive amount is increased too high, ultimately the mixing tube will clog.
One of the exciting advancements in waterjet cutting in the past few years is the appearance on CNC- controlled abrasive metering systems. These systems precisely control the amount of abrasive that is allowed to flow to the cutting head. During the piercing process, cutting pressure and abrasive amount are reduced and the cutting head makes small, circular motions in the X and Y axes. This piercing procedure allows for difficult applications, such as piercing of glass and stone, to be processed with relative ease.