Asking the right questions and understanding the cutting process
(The Fabricator - October 1998) By Richard Ward, Richel, Inc.
Are you thinking about using or purchasing a waterjet cutting system? Are you sure that you will make the right decision? Do you know which questions to ask? Do you understand the differences among manufacturers' products? Some common questions to ask when considering a waterjet system are:
* Would a personal computer (PC) or computer-numerical control (CNC) be best, and what are the differences between them?
* What tolerances should the gantry hold, and how should they be specified?
* What is the speed range needed to offer a full-contract cutting service that can process all of the required materials?
* How is waterjet work priced, and what will the market bear?
* When is it better to use plasma, oxyfuel, or laser?
* What is the competition?
* How difficult is it to secure work, and what size gantry is needed to handle 80 percent of the work?
* What size pump would be best, and why use multiple heads?
* What are the relationships among pump horsepower (HP), orifice and nozzle sizes, abrasive selection and flow, water pressure, cutting speed and quality of cut to edge finish, tolerance and taper?
* What about the waste product? Is it safe, and how is it disposed of?
* How important is software, and could an existing in-house, computer-assisted design/ computer-assisted manufacturing (CAD/CAM) program be used efficiently?
* Is it possible that the business could grow and require a second and third system?
* When should the system be installed to ensure that high-quantity work can be performed, as well as the infrequent jobs?
* Is it worth attending an independent course on waterjet cutting to learn the answers to all of these questions and the hundreds more that will arise?
The questions regarding waterjet technology are endless. Most will go unanswered without a dedicated effort to research the industry. To answer these questions, time is needed to travel, interview, operate systems in person, and evaluate the 10-inch stack of papers filled with information about different waterjet systems. Time is also needed to sort though all of the contradictions and evaluate why each manufacturer believes it has the best system and all others are inferior. But will the final decision be the right one?
The Consultant's Responsibilities
It is possible to make the correct decision when purchasing a waterjet. Before purchasing the system, it is important to know how much cutting work is available and to secure clients.
Through market research, independent consulting firms that specialize in waterjet cutting can assist a company to evaluate whether or not the amount of available cutting work can support its purchasing a waterjet system.
Consultants can also help in obtaining price quotes and follow up with potential clients. Compiling business plans for presentation to financial institutions can also be handled by a consultant. Additional consultant responsibilities include:
1. Assisting with the specification of a waterjet system, yet leaving the final choice to the client.
2. Verifying that installation and commissioning of the system are done properly.
3. Training operators in the field, including focusing on how to manipulate the controller to perform work efficiently.
4. Installing software for tracking business leads and standardizing quotation procedures.
5. Training on CAD/CAM software.
6. Following up with business growth when required.
7. Answering technical questions regarding specific cutting applications.
Evaluating Consultants
A quick reference check should be considered when evaluating a waterjet consulting firm.
The best references to check are, of course, former clients of the consulting firm. When checking references, a company can obtain first-hand information from the reference, such as if it felt pressured into making a specific decision. In addition, a company can verify that information provided by a consultant is factual, unbiased, and clear. Before buying a waterjet system, it is important to understand the differences among the machines. A full understanding of these characteristics beforehand leaves no surprises. If all the facts are known ahead of time, there should be no regrets regarding the purchase after it has been made. That a specific original equipment manufacturer (OEM) manufactures a system best suited for the job because it has sold many machines is often the only "guideline" clients use.
Evaluating Waterjet Systems
What is waterjet cutting, and why are there so many differences among manufacturers? What is forcing change in the industry, and how are changes being addressed?
Waterjet cutting is a unique form of processing and shaping materials, best described as a controlled, accelerated erosion process. With water pressures, abrasive flow, orifice life and nozzle wear variables, combined with the motion of a CNC gantry, waterjet cutting is often considered an art more than a science.
The drag of the exit of the stream can be as much as inches behind the entrance. Added to this equation is the constantly changing kerf resulting from the increasing inside diameter of the nozzle or focusing tube, which immediately begins growing as the waterjet is activated.
In the early days of waterjet cutting, tolerances were severely limited as the carbide nozzles made to focus the water and abrasive mixture lasted only a few hours, growing from 0.040 to 0.080 inch.
This rapid change in kerf, and the limitations of CNC, restricted the financially viable applications for waterjet cutting compared to traditional methods of cutting or machining, especially because part tolerances were very loose.
Because of the short nozzle life, little need existed to produce high-tolerance gantry systems to mount the waterjet cuttings heads. Software to generate CNC code was simple and effective.
The development of long-life nozzles with 0.020 inch of expected growth, which provided 60 to 100 hours of effective nozzle life depending on the application, was quickly followed by a call for higher-tolerance gantries.
Waterjet users were then able to produce parts within +/- 0.005 inch in thinner materials up to 3/4 inch thick.
However, the waterjet stream still includes some objectionable characteristics. The stream's drag on exiting the material will always trail the entrance from the top. The effects of drag can only be seen when a sharp change in direction is needed.
If the exit is 0.5 inch behind the entrance and a 90-degree corner is taken, obtaining a good, clean cut on the exit side is impossible. The ability to ramp down and up at corners, allowing the stream to "catch up" while simultaneously adjusting for kerf growth associated with the slower speeds, is important. The ways in which lead-ins and lead-outs are configured and where they are located in relation to each other are crucial to good part production.
Waterjet cutting software controls the stream, allowing the production of higher-tolerance parts. However, without a controller that is able to read and execute the necessary commands and a high-tolerance gantry capable of acting on these commands, tolerances will still be limited.
Every controller has its own characteristics, each with pros and cons. Some are very limited in the way they read and act on the code presented, while others are more advanced. Some are more complex to operate than others. Some have limitations regarding relationships between kerf size and tool compensation-limiting the smallest line or arc that can be cut to greater than that of the kerf diameter. With others, there is no limitation.
Often explaining the differences among just the controllers can require hours. Software customized to waterjet cutting is also important to cutting success. As jobs become larger, nesting parts to maximize the use of sheets or remnant sheets is vital.
Waterjet cutting can be used effectively in many applications, such as processing alloys and other specialized materials.
Additional opportunities exist for using waterjet cutting in some unobvious areas. For example, the absence of a heat-effected zone (HAZ) when using a waterjet eliminates the grinding and machining typically associated with the removal of material from the newly cut edge. The edge quality and tolerances achievable with waterjet can reduce production to a single process. Waterjet cutting can also be used for near net shape cutting, in which parts are produced with an additional 0.015 inch of material that will be machined in subsequent operations.
Summary
Waterjet cutting can be simple or complex depending on the application, the controller, the software, the tolerances of the gantry, the structure of the supporting table, the marketing, and the pricing strategy.
Research has not stopped. On the contrary, manufacturers and independent operators are working hard to break new ground in the quest to provide higher-tolerance parts in less time with a lower production cost. We are in the age of rapid development and changes. Waterjet cutting is no exception.
