There are two basic methods of generating abrasive waterjets, Figure 2.1:
Figure 2.2 shows the schematic arrangement of the abrasive feed systems for the two methods.
Important characteristics of the entrainment method include:
Dynamic Suspension – abrasive particles dynamically suspended in a flowing gas (air or steam). Dynamically suspending abrasive results in a delay after starting a waterjet before abrasive reaches a cutting head. There is also a time delay in clearing abrasive from a cutting head before a waterjet can be turned off. Time delays in turning a cutting jet on and off makes drilling blind holes, etching, marking and many milling operations difficult or impractical.
Static Suspension – abrasive particles suspended in water allowing abrasive to remain in a cutting head and feed system when a waterjet is turned off. The continuing presence of abrasive in a cutting head allows cutting to be started and stopped multiple times per second.
The relationship between cutting jet diameter and acceptable cutting jet on/off times is discussed in Section 2.3
Important characteristics of the suspension method include:
To continuously discharge abrasive through a suspension cutting nozzle requires an abrasive feed system that has at least two pressure vessels, eight valves and a sophisticated control system. Four valves operate in an abrasive environment and abrasive can reach other valves during upset conditions. Such abrasive feed systems are too complex to be made “industry worthy” and this is indicated on Figure 2.1.
For cutting jet diameters less than 50microns (MAWs), abrasive consumption per hour is under 0.5 kg. Batch operation of MAWs is practical using a small abrasive storage vessel that holds sufficient abrasive in a cartridge for an hour or so of cutting.
Cutting jet on/off cycle times of AWJs are typically several seconds. A cutting cycle time of seconds is very slow in comparison to competitive machining technologies. However, for most AWJ applications an on/off cutting cycle time of seconds is not a problem. Important exceptions are damage to brittle, composite and thin workpieces at the start of a cutting cycle. Auxiliary devices in AWJ abrasive feed circuits are used to minimize the time between starting a waterjet and abrasive being entrained.
Economic and practical considerations require cutting jet on/off times to be shortened as cutting jet diameters are reduced. The reason for this can be understood by considering the time to machine a unit feature - that is a feature one cutting jet in diameter and depth.
Referring to Figure 2.3, keeping the cutting energy density constant, the time to machine a unit feature decreases linearly with decrease in cutting tool/jet diameter. Another way of looking at the effect of reducing cutting jet diameter is to consider the time to profile a workpiece that is scaled down in the same proportion as a cutting jet. Cutting speeds around a profile remain the same for the two workpieces, so the time to profile the scaled down workpiece is reduced by the scale ratio. This means abrasive on/off cycle times and cutting head repositioning times must also decrease linearly to maintain the ratio of workpiece machining time to the total machining cycle time.
Abrasive waterjets are turned on and off mechanically so can never match the electronically switched characteristics of lasers. With current technologies abrasive waterjets are capable of:
FAW and MAW systems designed specifically for dynamic operation could achieve a higher number of cutting cycles per second.
Figures 2.4 and 2.5 shows the capabilities of FAWs and MAWs in carrying out multiple machining operations per second. The elongated shape of the features in Figure 6 reflects the poor vibration characteristics of the cutting head motion system on which the FAW cutting head was mounted. The adverse effects of the motion system on the feature shapes highlights the importance of designing FAW and MAW cutting tables for dynamic machining.
