Abrasive Waterjet Technologies

The paradigm shift that started the era of precision cutting with abrasive waterjets was the change from cutting heads entraining abrasive carried in water to entraining abrasive carried in air (AWJs). In the 1970s the technologies for the development of abrasive waterjets existed along with a conducive environment created through the exchange of ideas at the International Water Jetting Conferences organized by the British Hydrodynamics Research Association (now BHR Group). At the end of the 70s cutting systems that entrained abrasive carried in water were being manufactured. These systems were inefficient, difficult to operate and maintenance intensive because of problems with generating and handling abrasive water mixtures. The logical and paradigm change from entraining abrasive carried in water to entraining abrasive carried in air was first commercially demonstrated by Flow Research in Seattle.

Since the introduction of AWJs there has been no paradigm shift in the way abrasive waterjets are generated but there have been very substantial improvements in AWJ cutting performance. Improved cutting performance is the result of incremental developments in ultra high pressure pumps, cutting heads, software and control systems. Improving cutting performance, combined with advances in machine tool design and innovative marketing and sales activities, has resulted in AWJs becoming one of the three major non contact cutting methods; the others being lasers and wire electric discharge machining (WEDM).

Probably the most important development leading to widespread commercialization of AWJ based machine tools was the adoption of reacted tungsten carbide for cutting head focus tubes - a paradigm shift in super hard materials technology by a major chemical company (Dow Chemical Company), exploited by a nozzle manufacturer (Boride Products Inc, now part of Kennametal Inc). A twenty times improvement in focus tube life to 50 to 100 hours transformed the prospects of abrasive waterjets from a niche market to a main stream machine tool.

The basic design of entrainment cutting heads developed in the early 1980s has remained unchanged. The design is largely dictated by the extremely erosive environment within a cutting head with little regard to fluid dynamic principles. One of the reasons for no fundemental changes in cutting head design is lack of research. Abrasive waterjets do not have the research kudos of lasers and EDM. They are considered to be "mechanical engineering" with little scope for fundemental developments. This is a misconception which needs to be addressed.

2. AWJ Paradigm

A paradigm is the way a group/community perceives and thinks about a topic based on a particular version of reality. This means a paradigm has a mental model associated with it. At present the abrasive waterjet community has a single paradigm for generating cutting jets for precision machining – AWJ paradigm.

Initially, industry does not need the mental model associated with a paradigm to represent reality. Rather it needs a model that is a convincing explanation of "how" a technology works so that funding can be obtained for research, product development and exploitation. A mental model that closely represents reality is advantageous for product development and commercial success - but it is not essential.

It has been assumed that an AWJ cutting head is a form of jet pump (Hashish 2002). Jet pumps are first and foremost energy dissipaters as their energy efficiency is typically less than twenty five percent. Clearly one does not want a cutting head to act primarily as an energy dissipater. An AWJ cutting head is not a jet pump it is a unique, two stage, fluid dynamic device (See Section 3.1 in Technology).

A new industry, such as the abrasive waterjet industry, can develop rapidly using a plausible explanation for the process it is exploiting. A research community is, however, in a very different position as it needs to rapidly discover the physical processes industry is exploiting, otherwise it generates more and more research papers that support a flawed mental model of physical processes. In other words, a research community can become trapped into a self-perpetuating process of research projects whose conclusions support a flawed mental model associated with a paradigm. In this situation a research community finds it hard to innovate and it loses the support of industry and research funding organizations.

The Research Community, and particularly Universities involved with abrasive waterjets play an important role in building awareness in industry of the capabilities of AWJs and in demonstrating new applications for the existing technology. However, there is a need for fundemental research into the generation of abrasive waterjets. Extending the operation of abrasive waterjets to micro machining opens up oppertunities for researchers; the scale of the equipment needed for research becomes more managable within a University enviornment.

Successful technologies evolve through incremental improvements and as a result of paradigm shifts. Incremental improvements are predictable but paradigm shifts may occur many years after the technology for a shift becomes available. Delays in adopting paradigm shifting technologies are often the result of proponents of existing and new technologies having different mental models of the physical processes involved.

An individual working alone is usually responsible for creating a new mental model that revitalizes a research community. Invariably the individual will not have been deeply involved with the existing paradigm (Gosling 1994) and have little credibility with proponents of the existing mental model or credibility with the companies who will be the ultimate beneficiaries of new technologies. It is, therefore, uncertain if, when and where new paradigms for abrasive waterjets will emerge.

3. Paradigm Shifts Needed in Abrasive Waterjet Technologies

Because AWJs are the only abrasive waterjets people have known the capabilities of AWJs are viewed as what is possible. We need to think “outside of the box”. The author has demonstrated that abrasive particles as small as 50 nanometers will cut stainless steel. As a fluid dynamists the author sees no physical reason why abrasive waterjets could not operate down to a few microns diameter. However, there are plenty of practical reasons why minimum cutting jet diameters are limited to 30 microns or so by current technologies.

Technologies that allow cutting jet diameters to be reduced below 200 microns are also likely to allow multiple cutting cycles per second. Abrasive being present in a cutting jet on starting cutting will overcome a major problem with AWJs of damaging brittle and composite materials by waterjet impact without abrasive at the start of a cut.

Although a paradigm shift may be possible the ultimate decisions leading to exploitation of the paradim shifting technology are business related. Some of the factors that will influence business decisions are considered in the following Sections.

4. Past Attempts at an Abrasive Waterjet Paradigm Shift

In the 80s and 90s attempts were made to develop an abrasive waterjet paradigm shifting technology for macro machining using suspension abrasive waterjet technology. It was demonstrated that a 400% improvement in cutting speed, relative to AWJs, was possible by passing an abrasive suspension through a cutting nozzle. However, these attempts failed because flow circuits and equipment were too complex, user unfriendly and unreliable, with no possibility of overcoming these problems.

The work on suspension jets is likely to be an important input to any abrasive waterjet paradigm shift.

5. Macro Machining - AWJs

The potential user base for AWJs for macro machining is probably ten times the present user base. With such market potential manufactures concentrate on selling systems based on existing technologies and on making incremental improvements to their existing products.

Leading abrasive waterjet manufactures have a sufficiently large user base for companies specializing in spares and consumables to enter the market and seriously erode the profitable after-market of leading players. Many “me too” competitors have entered and continue to enter the market, competing for abrasive waterjet consumables, equipment and complete machining systems.

Leading players can maintain market share through the quality of their products, their ability to meet demands for sophisticated machining systems and through their marketing and sales capabilities. Given these factors it is not surprising that there is little pressure from major abrasive waterjet manufacturing companies for a paradigm shift in the way abrasive waterjets are generated.

The cost of cutting parts has steadily reduced over the past 30 yerars through improved AWJ performance and by:

Other developments, such as dynamic control of cutting heads to eliminate edge taper, have allowed abrasive waterjets to produce parts to a higher quality. However, until a paradigm shift occurs in abrasive waterjet generation further reductions in cut part costs and improvements in cutting quality will be modest.

Further increases in pump pressures above the current maximums of around 6000 bar will be difficult to justify for the wider market because of reliability and costs issues. There is also the problem of focus tube life, with arguments being made to restrict water pressures to minimize overall operating costs. It is known that cutting performance can be improved by reducing the ratio of focus tube bore to waterjet orifice diameter but again focus tube life suffers.

Given that improved cutting performance is dependent on having a satisfactory focus tube life, and that no new nozzle materials are likely in the foreseeable future for macro diameter AWjs, the available options are:

Over the past ten years the wall plug efficiency of abrasive waterjets has improved by a percentage point to 3% or so energy transfer to abrasive particles. Over the same period laser wall plug efficiencies, have in some cases, risen from 3% to over 10% of energy transfer to cutting beams. In the same period the cutting capabilities of lasers have been extended to encompass materials and material thickness that were considered the preserve of abrasive waterjets. New types of lasers will continue to enter the market and further erode the competitiveness of abrasive waterjets for some applications.

Paradigm shifts for macro machining will involve new cutting heads and modified abrasive feed equipment, without affecting the major capital item in cutting systems - the pump. This means it should be cost effective to upgrade existing cutting tables with new cutting heads and to adapt existing abrasive feed systems.

6. Micro Machining

6.1 Entraining Abrasive Suspension

In their basic form AWJ cutting heads and their abrasive feed systems do not function well with cutting jet diameters less than 400 microns. AWJs are now operating down to 200 micron cutting jet diameter with modified and scaled down AWJ cutting heads combined with refined abrasive feed systems. The abrasive feed systems can reliably meter abrasive flows down to a kilogram per hour.

To reduce cutting jet diameters much below 200 microns it is necessary to change the abrasive carrier medium from air to water. Abrasive particles are statically suspended in water thereby overcoming particle to particle electrical forces that make dynamically suspended fine particles in airflow a problem.

Abrasive waterjets generated by entraining abrasive suspensed in water into a high velocity waterjet were used for on site cutting and under developed for general machining until they were replaced in the early 1980s by the then paradigm shifting technology of AWJs. AWJs were more effective, much simpler and easier to operate and more reliable than systems entraining abrasive suspension.

Misconceptions about entraining abrasive/water mixtures into a high velocity waterjet include:

In practice these disadvantages can be minimized and compensated for by other factors. In particular:

The water content of stabilized abrasive mixtures can be kept to 30% to 35%
Abrasive particles are accelerated more effectively in a water environment compared to a water/air environment
Higher particle acceleration rates allow the length to diameter ratios of nozzle bores to be reduced compared to AWJ nozzles with benefits in minimizing friction losses

Past problems with cutting systems based on entraining abrasive suspensions are avoided by limiting maximum cutting jet diameters to 200 microns or so . With cutting jet diameters less than 200 microns:

A cutting head that entrains an abrasive suspension can be powered by existing AWJ intensifier pumps so the perceived technical development are relatively modest. With modest percieved technical developments the major barrier to marketing of micro machining abrasive waterjets will be market awarness in industries that have so far not seen abrasive waterjets as candidates machining methods.

The general exploitation of FAWs requires ultra high pressure pumps that have water flows that are 5 to 20% of the flows from the smallest pumps used for AWJs. Relative to AWJ pumps, the development cost of a pump for FAWs will be considerably lower, thereby reducing the investment for a company seeking to start an abrasive waterjet manufacturing business.

It could be argued that a paradigm shift is needed in the abrasive supply chain for FAW technology to become a reality. A high level of quality control, from initial abrasive preparation through to abrasive mixture flowing into a cutting head is required, particularly for FAW cutting jet diameters less than 100 microns. Abrasive supply chain requirements for small diameter FAWs are similar to those for micro abrasive waterjets (MAWs) and are discussed in Section 6.
operate down to jet diameters of 300 microns by inducing additional airflow to a cutting head using auxiliary suction. Below 300 microns particle agglomeration and attachment to passage walls, caused by electrostatic forces and/or moisture, become particularly troublesome.

6.2 Suspension Jets for Micro Machining

It is not generally known that abrasive waterjets are capable of micro machining, so MAWs could be seen as a paradigm shifting technology looking for a market. However, MAWs will compete with and complement micro machining lasers in a similar way that AWJs complement and compete with general machining lasers. A market for MAWs is, therefore, already identified in micro machining laser job shops which can use MAWs to extend the services on offer.

In order to minimize technical risks and to achieve good reliability, water pressures for MAWs need to be limited to about 700 bar. At 700 bar the cutting jet energy density is equivalent to that of an AWJ operating at about 3000 bar.

Water flow rates for cutting jets less than 50 microns in diameter are under 2.5 litres per hour, making it practical to use pneumatically driven intensifier or direct driven plunger pumps to generate 700 bar water pressure. By utilizing materials and sealing technologies developed for AWJ pumps, MAW pumps will achieve considerably longer lives than AWJ pumps. Also capital costs for a MAW pump are a fraction of an AWJ pump.

Machining cycle times reduce as feature size decreases. If it takes 1 second to machine a unit feature with a 1 mm diameter tool, a 100 micron diameter tool would be expected to produce a unit feature in 0.1 seconds. Technologies to allow FAWs and MAWs to machine multiple features per second have been developed.

The business model for MAWs is likely to represent a paradigm shift with manufacturers of MAWs taking control over the abrasive supply, rather than the situation with AWJs where numerous suppliers compete to supply abrasive. It will be essential for MAW manufacturers to provide abrasive in order to ensure adequate quality control over abrasive and to obtain an appropriate financial return for the investment in bring the technology to market. The paradigm shift associated with abrasive supply is discussed in the next Section.

MAWs can be built as desktop systems with a particular attraction for small businesses and the hobby market. Ease of use, reliability and serviceability will be critical for these markets along with good presentational design.

7.Paradigm Shifts in Abrasive Waterjet Businesses

The abrasive waterjet industry is currently a single product industry in terms of abrasive waterjet generation. With the rapidly growing market for AWJ systems, being a single product industry is not a problem. However, the cyclic nature of the machine tool market has resulted in temporary over capacity with all the attendant financial problems for individual companies. This situation has been played out in many industries and is a fact of the machine tool business.

To mitigate the effects of market downturns, forward planning companies seek to diversify their product portfolio, with the expectation that market downturns are out of phase across different market sectors. An example of this is the laser machine tool industry where companies evolved from a single product – one cutting beam generation method – to companies providing a range of products – several cutting beam generation methods.

The laser research community played an important role in driving the development of new laser beam generating methods and in the creation of start-up companies to exploit new laser technologies. To diversify their product portfolios major laser companies were able to acquire new products by buying start up companies and/or using the research community for product development. The situation in the abrasive waterjet industry is very different from that of the laser industry, as no new methods of abrasive waterjet generation have come out of the research community. Also only now are innovative small companies developing that major abrasive waterjet companies could buy to acquire new products.

Paradigm

References

Research

History

Paradigm Shifts in Abrasive Waterjet Technologies

1. Background