A micro abrasive waterjet is generated by passing a suspension of abrasive in water through a cutting nozzle. A 50 micron (0.002 inch) diameter MAW will consume about half a kilogram (1 pound) of abrasive per hour and 2.5 litres (0.7 US gallons) of water. Water pressure is typically 700 bar and waterpower at a cutting nozzle is less than 100 Watts. Although small MAWs cut the same materials as AWJs, albeit through thinner sections.
Passing abrasive particles in pressurised water through a nozzle is a very efficient process compared to entraining abrasive into a high velocity waterjet. The benefit of operating a MAW at 700 bar, as against 3000 to 4000 bar of AWJs and FAWs, has to be set against having abrasive particles in pressurised water, rather than abrasive suspended in air or water at near atmospheric pressure.
A cutting jet can be created inside a MAW flow circuit if a significant pressure differential occurs. For instance, if a conventional shut off valve was closed or opened on an abrasive suspension at 700 bar the valve's seats could be seriously damaged. This means flow circuits, components and control strategies for MAWs must be designed to meet the following requirements:
The effect of meeting theses requirements can be seen on a desk top MAW cutting system of Figure 7.1, and the schematic flow circuit for a MAW shown in Figure 7.2. The abrasive storage vessel is located immediately above the cutting head so that abrasive only passes through a short vertical flow passage.
The system shown on Figure 7.1 has a catcher tank that can be removed by hand to allow recovery of miniature parts by washing parts out of the catcher onto a collection screen. For easy of construction the system has "plug and play" X-Y linear axis mounted underneath the catcher tank. For a small table it is easy to protect the linear motion unit and also to put the catcher tank in a "bag" that seals to the cutting head so that abrasive and water are contained.
Figure 7.1 Experimental desk top MAW cutting system
Pressurised water from a pump is fed to a flow controller, Figure 7.2. The flow controller has a number of functions:
Pre-mixed abrasive in a cartridge, Figure 7.3 is installed into an abrasive storage vessel that has a barrel with an interrupted thread for quick opening and closing.
The pumping duty for MAWs can be met by:
Figure 7.4 shows a pair of pneumatically driven intensifier plunger pumps for 700 bar pumping duty.
Figure 7.4 Pumping Unit
MAWs need to be capable of multiple abrasive on/off cycles per second. Raising water to 700 bar compresses water by about three percent. Water compressibility results in several seconds of water flow through a cutting nozzle being stored in an abrasive storage vessel. It is, therefore, not possible to stop cutting by depressurizing a MAW flow circuit, as is done on a FAW. Instead an abrasive on/off valve is located just prior to a cutting nozzle, as shown on Figures 7.2 and 7.3.
A patented abrasive on/off valve has been developed that uses diamond seats along with control strategies that minimize abrasive being present when a valve opens or closes.
Cutting nozzles with bore diameters between 40 and 60 microns have been produced in a number of super hard materials including diamond . Cutting nozzles have a conical or radiused inlet, and bore length to diameter ratios of about 20.