Author
Paradigm
References
Research
  |
 |
 |
 |
History of Abrasive Waterjet Cutting
From onsite cutting to micromachining
The Birth
Using pressurised water to form a jet and carry out an industrial process, such as cleaning and washing out of minerals during mining, is centuries old. Entraining sand particles into a waterjet to enhance cleaning capabilities was a natural step to be followed by entraining abrasive particles for cutting. By the early 1970s entrainment abrasive waterjets were being exploited as an onsite Contractors cutting tool. Water at a few hundred bar pressure was used to generate a waterjet into which abrasive particles suspended in water were entrained. The cutting process was inefficient, labour intensive and the abrasive feed system difficult to operate. However, challenging onsite cutting tasks were being carried out profitably by Contractors on process, oil and gas and other installations.
By 1970 international interest in the use of high-pressure water jets had grown to the extent that the British Hydromechanics Research Association (BHRA now BHR Group) decided to run an international conference on the subject. The first conference was in 1972 and continues on a biannual basis. The BHR Group conferences lead to the setting up of the American Water Jet Technology Association (WJTA) conferences and the Pacific Rim conferences on waterjet technology. Early on in the BHR Group conferences the potential for precision cutting with abrasive waterjets was understood.
At the same time that BHR Group was enabling an international exchange of information on water jetting technology it was asked by a number of Contractors to improve the efficiency and ease of use of abrasive entrainment cutting systems. Towards the end of the 1970s research had led to a significant improvement in cutting head performance and in abrasive suspension feed systems. However, it was clear that effective cutting with entrainment cutting heads required higher water pressures than could be generated by Contractors pumps and that abrasive feeding arrangements had to be simplified.
Feeding Abrasive
The solution to the problems of feeding abrasive to cutting heads already existed in the form of semi-automated abrasive metering and feeding systems for air driven abrasive cleaning heads. Using compressed air to generate a supersonic air jet and entrain dry abrasive particles in air had been an established cleaning method for many years. The density of air is 4000 times less than that of abrasive so little momentum is available to be transferred from an air jet to abrasive particles. Because only modest abrasive particle velocities are achieved, cutting with compressed air systems is limited essentially to etching, although given time very precise cutting can be carried out on glass and other substrates for high technology industries, such as electronics.
Entraining abrasive carried in air into a waterjet requires airflow rates ten times or so the combined volume flow rates of water and abrasive. Generating such airflow requires ultrahigh high water pressures (>1000 bar). Critically, entraining abrasive in air de-couples the abrasive concentration in a cutting jet from the amount of abrasive carrier fluid entrained; more air can be entrained than is necessary to carry the abrasive to a cutting head without significantly reducing cutting performance. Using the same principle as an hour glass, dry abrasive is metered through a restriction in the base of a hopper to fall into airflow generated by a cutting head. By this means the abrasive flow rate can be set relative to the ultrahigh pressure water flow rate without regard to the airflow rate. With abrasive concentration in a cutting jet independent of airflow, dry abrasive feed systems were relatively easy to automate and have proven very reliable.
Ultrahigh Water Pressures
During the 1970s the cutting of soft materials with fine waterjet jets, generated by ultrahigh water pressures above 2000 bar, became an established production technique. Cutting of materials like cloth, paper, rubber and plastics in a 24 hour production environment was made possible by the development of reliable ultrahigh pressure pumps. An entrepreneurial and innovative Company, Flow Research Inc. in Seattle USA, were a leader in developing ultrahigh pressure pumps and systems for water cutting. With access to substantial US Government Small Business Research Funding, Flow Research Inc. developed cutting heads that entrained abrasive carried in air and integrate the components of abrasive waterjet systems to produce abrasive waterjet cutting machines.
Focus Tubes
At the start of the 1980s the development of reliable ultrahigh pressure pumps had allowed a paradigm shift that gave birth to the abrasive waterjet machine tool industry. Another paradigm shift, unconnected to abrasive waterjets, was to transform the market potential for precision abrasive waterjet cutting machines. This paradigm shift was in the manufacture of superhard tungsten carbide.
Increasing the cutting power of abrasive waterjets also increased erosive power within cutting heads to the extent that cutting head focus tubes, made of the best available tungsten carbides in the early 1980s, only lasted an hour or so. Such a short focus tube lives would have restricted abrasive waterjets to a very small market. Fortunately, a number of large companies invested heavily in R&D through the 1970s to develop new businesses based on superhard materials. One of these Companies was the Dow Chemical Company which had developed and patented a method of manufacturing an essentially binder less tungsten carbide. Sintering of tungsten carbide from fine grain particles was carried out very quickly at high temperature and pressure to limit crystal growth and thereby produce a superhard and homogeneous material. The material was given the trade name ROCTEC.
Focus tubes made from ROCTEC had a life 20 or more times those of conventional tungsten carbide and its homogeneous nature avoided problems with random rapid failures experienced with conventional tungsten carbides. With reasonable and predictable focus tube lives abrasive waterjet cutting had become an industrially viable machining method with widespread application.
Rapid Growth
Ultrahigh pressure pumps and entrainment abrasive waterjet cutting heads evolved through trial and error and were not “invented”. This meant there were no intellectual property barriers to Companies manufacturing abrasive waterjet machining systems and their manufacture spread rapidly around the world.
Abrasive Suspension Cutting
Through the 1970’s BHR Group had led the research on abrasive entrainment cutting but switch attention to abrasive suspension cutting as being the best means of cutting with Contractors pumping equipment. Passing abrasive suspended in pressurised water through a nozzle improved cutting effectiveness by a factor of four or so and thereby brought effective cutting within the capabilities of Contractors pumps. BHR Group called their direct injection of abrasive systems DIAJET. With abrasive suspension cutting there is only one hose connection to a cutting head and this makes it easier to deploy cutting heads tens to hundreds of meters away from the main equipment, such as when cutting sub-sea or de-commissioning munitions.
Abrasive suspension systems for major onsite cutting tasks are complex but they have been used very successfully by experienced teams to carry out a wide range of high value, difficult onsite cutting applications. Simplified versions of DIAJET have found applications as portable systems for small onsite cutting applications and for clearing unexploded bombs and other munitions.
Micromachining
Early on in exploitation of entrainment abrasive waterjets for precision cutting it was demonstrated that cutting jets down to 250 microns diameter could be generated. However, for 25 years problems with feeding abrasive particles for cutting jets less than 400 microns resulted in no abrasive waterjet manufacture guaranteeing operation of their systems below about 400 microns cutting jet diameter. Recently fine powder feeders, used in the pharmaceutical and other industries, have been adapted for metering of abrasive particles and the first abrasive waterjet cutting systems for operation with cutting jet diameters below 300 microns became commercially available in 2009.
Machining System Design
Since the mid-1980s manufactures of abrasive waterjet machining system have both exploited and contributed to advances in machine tool design, machine control systems, human machine interfaces (HMIs) and computer aided manufacturing (CAM) software. Of particular importance has been the development of CAM software by the leading cutting system manufactures that has resulted in abrasive waterjet machining systems becoming the machine tool that comes closes to “having the skill built into the machine”. The high level of abrasive waterjet CAM software support, relative to other machining methods, is made practical by there being no hard tooling parameters, such as required for turning or milling, and no adjustable cutting beam focusing parameters, such as with lasers.
A variety of cutting head motion systems have been adopted for abrasive waterjet cutting systems, ranging from 3 axis cantilever and gantry configurations to 6 axis robotic configurations. Predominantly workpieces are located on a grid fixed to a water filled jet catcher tank with the cutting head motion system isolated from the catcher tank. As cutting jet reaction forces on a motion system are small the structural design of the motion system is dictated by factors such as cost, positional repeatability and accuracy, speed, dynamic response and protection against abrasive.
With considerable care a system with precision ball screws and ridged machine structure can achieve accuracies of 100 microns on small components. This level of accuracy is adequate for precision machining and for near net shaping prior to a high precision finishing operations by another machining method, such as wire electric discharge machining (WEDM).
Micromachining Centres
With the advent of abrasive waterjets with cutting jet diameters below 300 microns there is the potential to achieve accuracies of 10 microns on some materials in thicknesses up to 10mm. This level of accuracy needs a repeatability of ±3 microns over the working area that can only be guaranteed from a high precision machine tool with:
- A rigid machine structure
- Workpieces fixtured to the machine structure
- A high precision motion system which preferable has linear motors on the X and Y axes. Compared to ball screws, linear drives have reduced wear and maintenance, do not suffer from backlash and can run at higher speeds and accelerations.
- Linear scales with nanometre interpolation
Micro abrasive waterjet machining centre (Finecut AB)