Water-Jet Deburring for Complex Metal Parts

Hydro-deburring or water-jet deburring is quickly gaining acceptance as an effective deburring technology for complex metal parts requiring a high level of precision and repeatability. By harnessing the kinetic energy of a high-pressure (5000-psi/340-bar) water jet and precisely aiming it at a hole or edge, chips can be dislodged or burrs can be broken off, leaving the part burr- and chip-free. The same process can be used to clean machining oils and debris.

Workpiece Cleaning and Deburring Challenge

A hydraulic valve block used in modern ABS (anti-lock braking system) or ESC (electronic stability control) hydraulic units clearly illustrates the potential of water-jet deburring. However, the component could be any number of valve blocks, hydraulic manifolds or components with numerous complex hole geometries and intersections requiring cleaning and deburring as part of the manufacturing process.

Modern automotive ABS/ESC hydraulic units consist of an aluminum valve body that holds the solenoid valves and pump, low pressure accumulators, an electric motor and an electronic control module.

The valve body is two to three times a cigarette pack in size. During the manufacturing process, the valve body may require as many as 200 milling, drilling and tapping operations to produce the stepped bores, blind threaded holes and intersecting holes required for operation. Production rates are generally in the order of 100 pieces per hour. As can be expected, these machining operations produce chips and burrs of all kinds.

To ensure proper operation of the ESC or ABS unit, prior to assembly, the valve body must be free of debris and burrs that could detach during operation and cause an obstruction or interfere with a close tolerance sliding motion. Sharp edges and burrs can also damage seals during assembly. The complexity of such components and importance relative to passenger safety and product reliability make it essential for manufacturers to invest in cleaning and deburring technologies that produce a high level of repeatability.

Cleaning and Deburring Processes

Thermal deburring or thermal energy method (TEM) is a very popular high production method of deburring valve bodies. With TEM, parts are loaded into a combustion chamber and charged with a mixture of oxygen and combustible gas, which is ignited to create a controlled explosion. The short blast of intense heat vaporizes the burrs, without affecting the remainder of the part. TEM is not suitable for thick burrs resulting from dull tooling because it tends to melt the burr rather than vaporize it. Prior to TEM, parts must be free of oil, chips, nested chips and long hanging burrs to produce a quality part. After TEM, traces of thermal ash are likely to remain, requiring repeat cleaning.

Another deburring method used with valve bodies is electrochemical deburring, or ECD. In this type of reverse electroplating, a tool (cathode) comes in close proximity to the burr area (anode), and an electrolyte is pumped between the two to close the circuit for a DC current, thereby dissolving the burr. As with TEM, the parts must be thoroughly cleaned beforehand—especially loose chips that may cause short circuits between the tool and workpiece, producing undesirable results. Burr orientation must be consistent from part to part to ensure uniform deburring, and quite often complex, feature-specific tooling is required. Cleaning is also required after ECD to remove the electrolyte and other byproducts of the process.

Robotic water-jet deburring is earning its place as a viable alternative to, or companion technology for, the deburring methods described above. A robot-controlled water jet is precisely aimed at a hole or edge to dislodge chips or tear-off burrs. Its greatest advantage over TEM and ECD is its ability to clean parts in the process by simply adding a specialized cleaning agent to the water. It also outperforms traditional methods of valve body cleaning because the water jet can dislodge stubborn chips from deep intersecting holes. However, water-jet deburring is not a form of machining; it is unable to remove thick burrs or add a radius to an edge. One must qualify burrs for water-jet deburring, not expecting the process to compensate for tool wear or unrefined machining processes.

Qualifying Burrs for Water-Jet Deburring

A typical valve body can have as many as 30 holes, many intersecting each other. During the machining process, chips can get lodged into holes, and burrs of all sizes and types can develop. Measuring burrs is a subjective process because much is based on visibility. Several standards exist to classify edge finish, but water-jet deburring can be effective for removing a variety of burrs and/or chips, including feather-edge burrs, hanging burrs, wire-edge burrs, grinding burrs and lodge or nested chips.

These burrs are generally visible at 4× magnifications and can be broken off using a de facto qualifying method known as the 0.5-mm mechanical pencil test. This test is a variation of what is known in burr inspection technology as the No. 2 pencil point test. Basically, the lead of a 0.5-mm mechanical pencil is pushed out by 7 mm and used to break off the burr or dislodge the chip. If either can be done without breaking the lead, it is said to be a good candidate for water-jet deburring. Otherwise, the burr is deemed to be parent material and requires other methods or hand deburring to remove. Parent material is typically caused during the machining process and will begin to be more predominant as tooling wear sets in. Parent material is understood to remain solidly attached to the part and unlikely to detach in service, thereby resulting in a possible product failure.

Today, almost every automotive engine, brake or fuel system manufacturer uses some form of water-jet deburring. The most demanding applications usually require that the water jet be precisely controlled by some form of automation—CNC equipment or articulating robot. Since it can both deburr and clean parts in a single eco-friendly process, water-jet deburring offers a proven alternative to traditional deburring methods. The high pressure water jet’s intense force, when precisely directed, effectively knocks-off loose burrs and dislodges nested chips in deep intersecting holes without damaging the part. With a cleaning additive, the same process is able to cleanse away lubricants, oils and shop dirt. Water-jet deburring has its limitations, as does every method of deburring. Nonetheless, when the limitations are understood, the potential is worth investigating.

References

• Gillespie, L.K., Hand Deburring: Increased Shop Productivity, Published by SME, 2003
• Gillespie, L.K., Deburring and Edge Finishing Handbook, Published by SME, 1999
• Carley, L., ABS Series: Teves Mark 20, Brake & Front End, December 2000