Prosthetic Arm Relies on Waterjet Cutting

Micro Waterjet LLC came from a partnership between Waterjet AG
and Max Daetwyler Corporation to
offer Abrasive Waterjet Micro Machining (AWJMM) technology in a micro-waterjet cutting machine. This partnership, and the years of research, results in an abrasive waterjet machine that bridges the gap often seen between existing cutting technologies.Artificial limbs that offer an individual enhanced capabilities for grasping, holding, and overall maneuverability has been the ultimate goal for many years. Recently, a contract given to The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD, will enable APL to manage the development and testing of the Modular Prosthetic Limb (MPL) system on human subjects, using a brain-controlled interface. The final design offers 22 degrees of motion, including independent movement of each finger, in a package that weighs about nine pounds – the weight of a natural limb. Providing nearly as much dexterity as a natural limb, the MPL is capable of unprecedented mechanical agility with a design that responds to a user's thoughts.

Designed and produced by HDT Engineering Systems, with the help of the micro-waterjet manufacturing system, are a number of components for the MPL.

According to Ezra Johnson, manager of systems engineering, HDT Engineering Services Inc., "For metallic parts, the cost of producing parts using the Micro Waterjet cutting system is less than wire EDM, and we were able to hold to tight tolerances. In the past, we were using punching for thinner parts, which required us to make a tool for each part. With Micro Waterjet's cutting process, we can have modified parts manufactured simply by sending them a new design file. Changes are now fast and cost-effective and the Micro Waterjet cut parts are more precise than punching."

Ezra continues that, "We have also used lasers in the past for smaller, critical parts, but some materials have compatibility issues since the laser tends to burn thinner materials. However, with the Micro Waterjet process we have no issues with burning or heat effected zones."

The biggest advantage HDT engineers have found with micro-waterjet cutting is material compatibility as compared to laser or wire EDM. The micro-waterjet system can cut a wider range of materials with a very high degree of precision. In addition, the cost of producing prototype parts is less with the micro-waterjet process when compared to punching or wire EDM.

When looking at the functionality of working with Micro Waterjet, Ezra notes that, "HDT has listed Micro Waterjet into our preferred database and they have a four-star rating in every category. Micro Waterjet's consistency and accuracy are dependable. So far, their parts have never been out of spec and we are happy to report that we have never had to send a part back for any reason."

Entering the Market
Many new materials experience a negative reaction to the heat generated during treatment with conventional machining processes. When processing parts with a waterjet, the material structure is not affected.Serving a wide range of markets – from medical and automotive to aerospace, electrical, and even watch components – Micro Waterjet LLC came from a partnership between Waterjet AG and Max Daetwyler Corp. to offer Abrasive Waterjet Micro Machining (AWJMM) technology in a micro-waterjet cutting machine. This partnership, and the years of research, results in an abrasive waterjet machine that bridges the gap often seen between existing cutting technologies.

"A process around since the 1970s, traditional waterjet cutting began as a process to cut through hard and/or thick plates to create 2D parts quickly and accurately," according to Steve Parette, product manager, Micro Waterjet LLC, Huntersville, NC. "This process also removes a small amount of material, creating less waste while offering better nesting opportunities."

Parette goes on to explain that, "The development of micro-waterjet cutting was mainly for cutting thinner materials with high accuracy and smooth finishes. Today, our machines can use a cutting stream down to 0.3mm (0.012")."

Precision Advantages
Waterjet cutting, in the literal form, is using high pressure water – typically around 60,000psi – through a very small hole. This is what concentrates the energy and makes it strong enough to cut through soft material.

"Abrasive waterjet cutting is where abrasive particles are introduced into the high velocity stream for cutting harder materials," Parette says. "Most materials that are cut using waterjet technology are actually cut by abrasive waterjet, because it is the abrasive that is performing the cutting action and it is the water that is the highway for the abrasive." Parette feels that one challenge facing AWJMM is the need to, "Change the old ways of thinking, because many engineers are convinced that waterjets cannot hold tight tolerances."

In truth, AWJMM features cutting accuracy of 0.0004", and surface-finish capability to 1.6μm Ra, depending on material and thickness. In addition, this technology is capable of micro-machining materials such as aluminum oxide, carbon fiber, and phenolic resins – all materials not for use with EDMs and lasers. Beyond the material aspect is the lack of a heat-affected zone, another deciding factor that essentially takes EDMs and lasers out of the picture.

"We can hold tolerances down to ±0.10mm (0.0004")," Parette states. "Now, as the material thickness, and in some cases the hardness, increases, the cut edge will have some taper or draft to it that has to be taken into account."

Garnet is the commercially-available abrasive used in the systems, however Parette notes that the garnet used in the machines from Micro Waterjet is typically finer than that used in traditional cutting systems.

Regarding water consumption, and looking at an environmental impact, Parette explains that, "Our machines use around 0.5 liters per minute, about 0.123 gallons per minute, when the cutting valve is open and running at a pressure of 60,000psi. Some traditional machines will utilize up to 1.2 gallons per minute."

Additional to Micro Waterjet's accuracy is the workholding and positioning systems. The work holder involves clamping the parts down rather than setting them on a base – or bricks – that is typical of other waterjet systems. Obviously, this is not a stable method if the material being cut is susceptible to turbulence from the water, meaning that if the material is moved on the micron level, it will alter the precision of the cut.

Regarding positioning of the nozzle for precision, the system in place ensures the nozzle accurately directs the abrasive and water at the part at 60,000psi. It is this design that provides the NC motion to cut the shapes, where the X-, Y-, and Z-axis make up the total positioning system. Together, this is what holds a- positioning accuracy of 3μm.

Benefits, Drawbacks
"If there are any major drawbacks of micro-waterjet cutting machines, perhaps it is the floor space required, when compared to smaller cutting equipment," Parette admits. "Although the machine itself is fairly compact, there must still be space for a high pressure pump and a filtration system to handle the water discharge."

That, apparently is the only drawback that Parette can find, unless one considers a slower cutting speed than other waterjet systems. This is because the design of the micro-waterjet cutting systems is to cut clear through a substrate, creating fine features with higher tolerances than conventional, so a somewhat slower speed is required.

However, even with a slightly slower cutting speed, it does not necessarily result in a more expensive process. When compared to conventional waterjet cutting or laser cutting, the slower process does result in longer cutting times, but the cost of an overall project may be marginally or considerably less when using Micro Waterjet machines if there is the elimination of additional operations. Micro-waterjet cutting delivers precision technology that offers low occurrences of process forces and thermal stress – with water cooling the processing spot and transporting chips away. It is because of this that cutting of even the most delicate contours occurs without any concern.

Micro Waterjet LLC
Huntersville, NC
microwaterjet.com