Waterjet: Cut Quality and Cutting Speed Explained

April 1, 2010

Canadian Industrial Machinery asked James Daugherty, product manager, Flow International Corp., and Steve Szczesniak, national waterjet product manager, Mitsubishi, to discuss the relationship between cut quality and speed in waterjet cutting applications. Here is what they had to say.

Dynamic Waterjet cutting with active tolerance control incorporates the motion system, articulated cutting head, and mathematical cutting models to keep quality as high as possible, even at high cutting speeds.

CIM: How important is cut quality in waterjet metal cutting applications?

Daugherty: Cut quality, as it pertains to edge finish, is extremely important in many waterjet applications. Machine shops often choose waterjet because of high cut quality and no need for secondary finishing. Of course, there are always those that require a lower surface quality, which also can be easily achieved by increasing or decreasing cut speed.

Cut quality, as it relates to accuracy, is of primary importance. With options such as Flow’s Dynamic Waterjet, which automatically takes the guesswork out of producing a part, this has become much easier to accomplish.

CIM: How does cut quality differ for thin, thick, or even stacked parts?

Daugherty: Cut quality is excellent on thin, thick, and stacked parts. Many waterjet shops cut stacked thin parts to increase their overall productivity, and thick metals, such as 5-in.-thick titanium, can effectively be cut only with waterjet technology.

CIM: How does cut speed affect cut quality and surface finish?

Daugherty: Cut quality and surface finish are indeed related to cut speed. Two characteristics of waterjet technology are stream lag and taper. Stream lag increases with higher cutting speeds. Cutting too fast will result in stream lag that reduces cut quality.

Taper affects surface finish by creating a cut that is wider at the top than the bottom, or vice versa; this can result from cut speed that is too slow.

CIM: How do software features help ensure high quality?

Daugherty: Software that models the stream behavior across varying thicknesses is very important to achieve good results across multiple thickness ranges.

Flow features SmartStream technology in our software packages, which allow an operator to simply choose a material type from a library within the Flow software, enter thickness and desired surface finish, and the software takes care of the rest.

The best cut quality is achieved by accurately predicting the behavior of the jet. As mentioned, the software is critical in this endeavor, and features such as SmartStream are imperative to achieve the very best cut quality and accuracy without compromising cut speeds.

CIM: Can five-axis cutting affect cut quality?

Daugherty: Moving to multiaxis cutting increases the complexity in predicting taper and stream lag. As with 2-D cutting, reducing cut speed significantly will improve cut quality to some degree. A better way to achieve this is by using a product similar to our newly released Dynamic Waterjet XD. It combines 60 degrees of motion with the optimal cut speed gains and precision traditionally associated with our Dynamic Waterjet product.

CIM: How does the material being cut affect cut quality?

Daugherty: The material being cut has an effect on cut quality. FlowMaster and FlowXpert software automatically takes into account material hardness and cut speed to minimize taper while maximizing overall surface finish.

CIM: Does abrasive usage affect cut quality?

Daugherty: Traditionally, the more abrasive you use, the higher speeds you can achieve; you do hit a point of diminishing returns, however.

By using a higher-pressure pump, such as our 94,000 Hyperpressure, we can use 40 percent less abrasive while increasing cut speed and maintaining cut quality.

CIM: How can a user find that “sweet spot” between cutting speed and cut quality?

Daugherty: Cutting speed and quality are directly related. If you want higher quality, you must sacrifice speed. Without the use of a taper-compensation head of some sort, some metals approximately 0.750 in. thick can be cut at a reasonably high quality and productive speed. However, with Flow’s Dynamic Waterjet taper, control cut speeds can be increased up to 400 percent while virtually eliminating taper and improving cut part tolerances on almost all material thicknesses.

CIM: How is cut speed delivered by a waterjet?

At higher PSI a longer, straighter stream can be maintained for longer periods of time, and pierce times can be reduced.

Szczesniak: Several variables determine the cutting speed of a waterjet.

First is the heart of the system, which is the pump. The more horsepower, and therefore PSI of pressure, your pump delivers, the faster the cutting speeds that can be reached. The two primary pressures that we offer in our pumps are 60,000 PSI and 90,000 PSI. That change of 30,000 PSI does a couple of things. First, water velocity increases from Mach 2 to Mach 3, so you are carrying your cutting tool, the abrasive, at a faster velocity. A 90,000-PSI pump typically will give you a straighter stream for a longer period of time, so it’s ideal for harder materials and taller workpieces. With higher PSI you also get the benefit of faster pierce times.

Second is abrasive amounts and abrasive type. Remember that the abrasive is the true cutting tool and the water is just the delivery system.

There are different types of abrasives. Alluvial has more rounded edges, and mined abrasives are crushed and therefore have more facets and sharper edges. Even though the mesh size may be the same on these abrasives, the sharper abrasive will cut faster.

Third, mesh size will also affect cut speed. The coarser the abrasive, the faster you can cut.

CIM: How important is cutting speed in waterjet applications?

Szczesniak: Speed is generally of the utmost importance in waterjet cutting.

Waterjet is an alternative to conventional machining of parts whether it is chipping, laser, or EDM. It really depends on the application, but usually people are waterjetting parts because of speed.

Some materials can’t be cut on a laser cutter, and that’s where waterjet fits it. We don’t compete with our laser division, for example, we complement our laser division.

CIM: How important is understanding the material that is to be cut?

Szczesniak: We have a database of materials from 0.062 in. all the way up to 6 in., and each material has five different cut quality levels. The quality of the cut determines the speed as you typically don’t get the fastest cut speed with the highest accuracy level. The slower you cut, the better the surface finish you will achieve and the better the accuracy will be.

CIM: Does cutting speed affect lag?

Szczesniak: Yes. The faster you cut, the more lag you have at the bottom of the jet.

CIM: What is the relationship between cut speed and taper?

Szczesniak: Taper is a natural phenomenon in waterjet cutting. That is why we offer the option of tilting the head to counter the taper effect. With our machine, the control and database will counter taper by tilting the jet and put that taper into the scrap rather than the part itself.

CIM: How does a part’s shape affect cutting speed?

Szczesniak: When entering a corner the machine will decelerate, make the corner, and then accelerate away. Variables to this speed are the quality of the part, the radius of the corner, and the thickness that you are cutting.

For more information, visit www.flowcorp.com and www.mitsubishi-world.com.