Fine abrasive waterjets have cutting diameters under 300 microns. FAWs are generated by entraining abrasive suspended in water into a high velocity water jet. Abrasive for FAWs has particle sizes that are too small for abrasive to be free flowing in air - particles agglomerate if moisture is present while electrostatic forces cause particles to attach to surfaces and to themselves if moisture is absent,.
The density difference between water and abrasive is four to one compared to four thousand to one between abrasive and air. This means abrasive particles in water do not need to be suspended by flow forces as is necessary with air. Instead particles are suspended in water by mixing before flowing to a cutting head at low velocity.
Depending on particle size, particle concentration and the settling distance, abrasive takes from seconds to days to settle in water. Abrasive settling occurs quickest in passages and tubing because settling distances are small. This makes the design and layout of FAW flow passages and tubing runs important in minimizing abrasive settling problems.
Preferably, abrasive mixtures are prepared with the minimum water content consistent with adequate flowability. Minimum water content enhances hindered settling of abrasive particles and minimizes the amount of water that has to be accelerated in a cutting nozzle bore.
Additives can be used to stabilize abrasive/water mixtures after they have been mixed. Whether or not additives are needed depends on cutting system use. The case for additives is strongest when a cutting system is used on an occasional basis. In an intensive cutting environment operational practices can reduce or avoid the need for additives.
What may seem surprising is that MAW cutting heads depend for their functioning on water vapour occupying a significant percentage of a cutting nozzle bore - vapour being the equivalent of air in an AWJ cutting nozzle. A state of super cavitation exists in a nozzle bore due to intense turbulence. The presence of vapour prevents waterjet kinetic energy being converted into pressure energy,a process that would be detrimental to momentum transferred from water to abrasive.
A waterjet nozzle is located close to a cutting nozzle inlet to prevent a waterjet interacting strongly with dense abrasive mixture outside a cutting nozzle bore. This makes for a compact cutting head, as shown by FAW cutting head profiling 50 micron thick stainless in Figure 6.1. It can be seen in Figure 6.2 that the view of the cutting process is very restricted compared to AWJ cutting head. An AWJ nozzle extends for tens of millimetres from a cutting head body, whereas on a FAW it can be less than a millimetre. Work is ongoing on cutting head design.
It is particularly important that a FAW cutting head has a waterjet nozzle that is able to pass abrasive without undue wear. Abrasive is not cleared from an FAW cutting head before stopping water flow, so abrasive is likely to enter a waterjet nozzle whenever water flow is stopped. Controlled penetration of abrasive into a waterjet nozzle can be advantageous because on starting water flow cutting begins instantaneously when a jet impacts on a workpiece.
For reasons given in Section 6.4 when a cutting head is used in dynamic machining mode the pump is used to start and stop water flow because water shut off valves cannot meet operate for sufficient cycles without seat failures.
A schematic arrangement of an abrasive mixture feed is shown on Figure 6.2. Figure 6.3 shows an abrasive mixture transportation bag temporally suspended near a FAW cutting nozzle as a feed system.
The content of the bag in Figure 6.3 is sufficient for several hours of operation of a FAW with a 100 micron diameter cutting jet. This illustrates the relatively small volumes of abrasive involved.
FAWs operate at the same water pressures as AWJs. Since AWJ intensifier pumps can operate down to zero flow, a FAW cutting head can be powered by an AWJ pump. However, a small diameter FAW needs a waterpower input that is less than ten percent of that of the smallest AWJ pump. There is therefore, a need for a new generation of ultra high pressure pumps specifically designed for FAWs.
FAWs can be said to have dynamic machining capabilities, albeit only five to ten cutting cycles per second against a laser’s thousand or so cutting cycles per second. At five cutting cycled per second it only takes a few hours to reach 100,000 cutting cycles. Valves for turning ultra high pressure water on and off can only achieve a 100,000 cycles or so before their seats need replacing. When a FAW machines multiple features per second the pump takes on the duty of starting and stopping water flow.
A high response, ultra high pressure, plunger pump has been developed for FAWs. The pump is driven by a servomotor and has a control circuit that provides both high response and avoids design water pressures being exceeded. A patent application has been made for the pump.