Engineering 5 Axis Abrasive Waterjet Cutting

By the WARDJet Team

Cutting with water and abrasive in 5 axis of motion poses some interesting challenges. Imagine taking an ultra high precision, multiple axis mechanism with everything from electronics, circuit boards, multiple motors and sensors, and giving one person a garden hose to spray all over the mechanism and simultaneously, giving another person buckets of sand to continuously throw over the mechanism while under power. Not quite everyone’s idea of a friendly working environment.

There is no ignoring waterjet cutting is forging its place into every area of manufacturing and engineering. New materials often demand waterjet cutting. With single and multiple head 5 axis waterjet cutting available on a single machine, the technology is becoming more and more accessible, and the lack of that capability is becoming a competitive disadvantage.

Waterjet is opening doors so fast it is difficult to keep up with the unlimited opportunities. Forgings are being replaced with parts nested into sheet material effortlessly cut into the most complex of shapes. Castings are being scanned and flashing trimmed, eliminating multiple tedious steps and labor. Near net shape cutting of everything from complex blades to helical rack and pinion to accurate cutting of composites, laminates of high nickel alloys, glass and phenolics, plastics, ceramics, armor plating of most every make and composition are easy. And this is no longer limited to flat, plane cutting. If you can imagine it, waterjet can probably be used to cut it. The principles of waterjet cutting are fairly simple. In a nutshell, water at pressure of 60,000 psi or more is channeled to a cutting head through sheathed, ¼” diameter autofrataged stainless steel tubing that in short lengths is not flexible. Abrasive is fed to the cutting head where the water pressure, as it transitions from potential energy to kinetic energy, accelerates the abrasive grains to over 2200 miles per hour in a nozzle that is 3” long. Sounds easy enough.

The endless applications for 5 axis waterjet cutting are phenomenal. The physics behind the process are impressive. But what are the challenges driving the design engineers who eat, sleep and drink 5 axis waterjet cutting? Let’s have a look at how WARDJet, a reputable waterjet manufacturer in Tallmadge Ohio, has gained respect for their Psy-Winder 5-axis waterjet cutting system.

Design

WARDJet’s Psy-Winder is a nutating/precession mechanism where the angle to the material is defined by one axis and the clock angle to the profile is another. The programming for this design is very simple and the G-code to describe a beveled circle can be done in 2 lines. Post processors are available for both 2D and 3D CAM software. Due to the nature of the Psy-Winder mechanism it is considered a fixed mechanical tool tip cutter so that the motion of the additional rotary axes does not affect the spatial position the cutting point. This is superior to a kinematically modeled mechanism which must offset its X, Y, and Z position based on the angles of the 4th and 5th axes and can accumulate additional error if the positions of the rotary axes are not perfect (i.e. deflected from the forces of a high pressure coil).

The goals WARDJet Inc. set for their Psy-Winder 5-axis cutting head were:

• Ensure the Psy-Winder is retrofitable to all waterjet systems ever produced by the company. This is a philosophy WARDJet strives to maintain and so far, any existing customer, no matter when they purchased a machine, can upgrade their present WARDJet waterjet to have single or multiple 5 axis cutting head capacity.
• Increased accuracy achieved by using harmonic drive rotary actuators with a hollow bore down the center and a positioning accuracy of 40 arc-seconds (.01°). Further accuracy can be achieved though laser tracking measurement and compensation.
• Controller servo cycle / update time of 0.00045 seconds or put another way, the location of the cutting head is checked over 2200 times per second
• Use of both hard and software limit and home switches
• Capacity of 12” to 24” vertical travel (Z axis) and upon request, to have flexibility to provide any Z travel specified
• Cutting angle from 0 degrees (vertical) to 90 degrees (horizontal)
• Combination height and crash sensor enabling consistent and accurate nozzle/plate spacing regardless of the behavior of the material being cut, all the while ready to stop on a dime if the nozzle unexpectantly encounters an obstruction.
• Fixed mechanical tool tip
• Ability to flood the tip of the nozzle with water even if 10” above the water level in tank, reducing sound and typical variations in cutting quality once air enters the stream.

Accuracy

In years past, when multi-dimensional waterjetting was new, the understanding was that there would be a sacrifice in tolerances. This is no longer the case. With advances in engineering of the nozzles and orifices used, the width of the waterjet stream is now predictable to 0.0005”, allowing manufacturers to support demands for precision 5 axis parts.

Key to creating an accurate 5-axis mechanism is the ability to measure the results. WARDJet employs a 3D spatial laser tracker to measure the actual mechanisms and correct for any “real world” inaccuracies that crop up in the manufacturing processes or from material properties. Standard laser interferometers are incapable of measuring all of the parts once they are assembled and operating in space. Once a final mechanism is measured, the data can be used to electronically compensate, on the fly, for any repeatable errors that occur, pushing the envelope on accuracy to new bounds.

High Pressure Water

Imagine your finger is the nozzle in a cutting head. Now wave it around and notice how flexible your wrist, elbow and shoulder are, not to mention the muscles as they twist and flex. So how do you get high pressure water bolted to a fixed position on the face of the Z carriage to the fixed position of the cutting head mounted on the other end of an ‘arm’ that twists, spirals and rotates? Any additional forces generated in flexing the high pressure tubing will typically cause variations in positional and repeatable tolerances. Coiling the high pressure tubing to reduce these forces normally shows unacceptable results in laser tracking.

WARDJet were able to virtually eliminate varying torsion, sheer and moment loads on the cutting head by directing the high pressure tubing through the center of rotary actuators. The combination of swivels and routing the ¼” high pressure tubing means no coiling is necessary. There is no wind up of the high pressure tubing often associated with 5 axis waterjet cutting and lengths of the high pressure tubing are short. All high pressure tubing eventually fatigues and fails, releasing high pressure water from hairline fractures. It is important to design a mechanism that fails in a safe manner and is easily repaired. WARDJet engineers added stainless steel sheathing around the high pressure water lines.

Getting Abrasive to the Cutting Head

The abrasive, which is typically garnet ranging from 150 to 50 mesh, is fed into the waterjet stream 3 to 4 inches above the tip of the nozzle. WARDJet has designed a revolutionary way to direct the abrasive through the middle of the actuator without any wind up of the 3/8” abrasive resistant hose. This also reduces the length of the path the abrasive travels from the mini hopper to the cutting head. An ingenious swivel system allows the abrasive to be passed through the center of the motor, collected and directed back into a flexible hose. The result is seamless delivery of garnet to the cutting head no matter what the cutting head position or angle of attack. Bulky tubing and awkward mechanisms subject to pinching, clogs and blockages are reduced into a simple straight path from abrasive hopper to cutting head.

Height and Crash Sensor to 50 Degrees

Any 5 axis programming revolves around a common mandate that the part does not change position while cutting. Fixturing for a 5 axis mill is often substantial to ensure the part does not move. In waterjet cutting one has to presume materials will move through the redistribution of stresses as material is cut.

A spring assisted, self locating crash sensor has a low voltage signal passed through the foot which when interrupted detects a crash. The height sensor is pneumatically controlled and can take periodic samples of the location of the material or continuously ride on the material. The analog feedback allows adjustments to be registered and responded to.

What is most impressive is the height and crash sensor can be deployed to work while the head is cutting at angles approaching 50 degrees off the vertical. This unique capability, as well as being able to integrate this information and respond in a 5 axis program, all on the fly, is all part of the advanced technology WARDJet has developed.

Combination of Processes

What if you need additional processes such as drilling, marking, tapping, reaming, liquid dispensing, bar code reading of parts, laser alignment, automatic loading and unloading of parts, 100% remote control and monitoring this all from a PDA in your pocket while on a business trip in Europe? Well, it’s all being done already.

Waterjet cutting is still in its infancy. Not because people are cutting simple shapes out of flat material, but because doors are opening with phenomenal technology allowing anyone who thinks outside the box, to catapult themselves into the future.

WARDJet Inc. is based in Tallmadge Ohio. Phone 330-677-9100  www.wardjet.com   sales@wardjet.com