HIGH-PRESSURE WATER DEBURRING

Increasingly, manufacturers are expected to deliver burr-free, totally clean parts to the point of use. To meet this challenge, parts manufacturers are turning to new technologies.

Traditional mechanical and abrasive deburring methods include hand and/or robotic mechanical deburring with deburring tools and rotary brushes or vibratory finishing. Abrasive Flow Machining (AFM), Thermal Energy Method (TEM) and, to a lesser extent, Electro-chemical Machining (ECM) deburring methods are well known to industry.

More recently, High-Pressure Water Deburring (HPD) has gained wider acceptance in the automotive industry and beyond as a particularly environmental- and part-friendly technology for removing part contaminates, burrs, chips and, at the same time, cleaning the part.

HPD has a number of advantages over other processes, first and foremost being that the part is totally clean and residue free after deburring. A brief comparison to other processes illustrates this point.

When hand deburring is employed, quality is not always consistent. The work is often labor intensive and internal features are very often difficult to reach. Even when deburred, the part still needs to be cleaned. With robotic brush deburring, internal features cannot always be reached. Very small or loosely attached chips cannot be removed with total certainty and, again, parts still need to be cleaned.

With AFM, the abrasive material is forced through the part and then must be flushed free from the part. ECM is employed primarily for edge and surface finishing. Parts are submerged in a salt solution and an electrical current is pulsed flowing from tool (cathode) to tool (anode) removing metal surface atoms without contact. The technology requires complex precision tooling with feature-specific geometry to remove material only where needed. Afterwards, parts must be washed to remove the salt and prevent corrosion.

TEM removes material by means of combustion. Burrs are burned off. Parts are put into a chamber; gas is injected and then ignited. Parts must be properly cleaned and dried before and cleaned afterwards to remove combustion residue. Tooling for TEM is simple. Cycle times are short because typically many parts are deburred in a single cycle.

With CNC HPD, a high pressure waterjet, typically between 5,000 and 10,000 psi, is directed along edges and specific part features to selectively deburr surfaces. Parts are feature-specific deburred and at the same time cleaned. Conditioned water (water with a rust inhibitor) is the deburring media.

The basic operating principle of HPD relies on the impact force of a high velocity waterjet exiting from a small diameter orifice to knock away chips, debris and burrs from the surface. The process does not cut or compromise the basic part features, nor is it intended to do so. It removes material that is an unintended consequence of the machining process.

HPD will remove material that is not solidly attached to the surface. The burr, in a sense, is qualified. Loosely attached burrs will come off, while firmly attached burrs that cannot be removed with 10,000 psi do not. Feather edge burrs, often only visible through a microscope, are removed. In general, HPD does not chamfer edges; in softer materials such as aluminum, edges are dulled. For harder materials, edges stay sharp.

The most suitable materials for HPD are soft metals such as aluminum, cast iron and materials of lower tensile strength. Harder materials require higher pressures; softer materials require lower pressures. The time it takes to deburr a part is a function of the type of machine, the power of the machine's pump, the sophistication of the nozzle tooling and, most importantly, the number of features that need to be deburred.

With generic tooling, cycle times are generally longer but fewer stations are needed. Pump sizing is a function of the size and number of orifices that are designed into a nozzle or the manifold. The greater the flow rate for a given pressure, the larger the pump power rating. Typically, it will take 5-10 seconds per part feature, and total cycle times between 30-60 seconds can be expected.

High-pressure water deburring is well suited for applications that require inaccessible features to be deburred, when parts must be very clean, when consistent quality is required or when parts cannot be subjected to heat or corrosive chemicals. The media, conditioned water, is very good in a number of respects. It's friendly to the environment and the process occurs at room temperature and does not use abrasives or corrosive chemicals.

Specific to the equipment itself, a CNC HPD deburring machine either moves the nozzle to the part feature or, better yet, the machine moves the part to the nozzle. Machines are either of an X, Y, Z configuration with one or more rotary axes (Cartesian orthogonal design), or sometimes a robot is used. In general, robots have less positioning accuracy, while X, Y, Z machine tool structures are used when greater accuracy is needed.

Part programming is also easier and simpler with an X, Y, Z-type machine. Parts, dimensioned in X, Y, Z coordinates, translate easily to CNC X, Y, Z coordinates for part program execution. Because the robot must be placed inside the work zone, machines that rely on a robot to move the part require more floor space. The robot is also exposed to continual high-pressure water spray within the deburr and wash chamber that, over time, will cut through pneumatic and hydraulic hoses and electrical cables and compromise exposed motors, encoders and sensors.

For X, Y, Z movement machines, there are a number of advantages when the part is moved to the nozzle instead of the nozzle to the part. Maintenance is considerably less, because with stationary deburr stations all high pressure lines are rigid piped and do not require high pressure flexing hoses that have a short life at high pressure.

Stationary workstations allow for more complex tooling, including simultaneous multiple feature deburring. The type of machine we manufacture is an X, Y, Z and C type of waterjet deburring system that moves the part (or multiple parts) to the nozzle. Only the overhead ram holding the part is in the wash chamber. Parts are linearly processed from station to station.

Horizontal and vertical part face operations can be performed in any of six workstations in this equipment. The part is moved and indexed to present the face to be deburred to the waterjet nozzle. This machine is a hybrid machine, in that both mechanical-power deburring and HPD are done in the same machine. Parts can first be carried deburred to a mechanical deburr station for a chamfering or brush operation, then moved to water deburring stations.

All axes are ballscrew-driven to give the machine the accuracy and rigidity required for mechanical deburring. Integration to a part in-feed and out-feed material delivery system is straightforward, as is robotic machine tending. The machine becomes the handling device, moving the part from conveyor (or part pickup point) station to station to part drop-off point. A quick change end-effecter allows the same machine to handle a wide variety of parts.

Other features can be incorporated, including a first operation pre-wash station, a post- deburr final part rinse station and an air blower drying station for complete part processing in one machine. When greater cycle time reduction is needed, multiple parts can be picked up and moved to the waterjet nozzle for simultaneous deburring.
Just as in machining, tool selection for HPD is very important for reducing part cycle time. Nozzle materials include HSS, carbide, ceramics, sapphire and more exotic materials. Harder materials result in longer nozzle life.

Direct nozzles create a solid stream or jet that is pointed at the feature to be deburred. Rotary lance nozzles are used for entering small diameter bores or cavities (down to 6 mm diameter). The waterjet exits at or near the end of the nozzle typically at 90 deg or 45 deg to axial direction of the nozzle. The nozzle is rotated as the part is fed, deburring the feature (feed/rev mode).

Rotary manifolds work like a milling cutter. Typically, three or more fan nozzles are rotated as the part is fed, deburring across an area as wide as the cutter (analogous to a shell mill). For high-volume applications, or when cycle time reduction is paramount, a custom manifold is designed that deburrs all features in one shot.

The heart of any high pressure water jet deburring system is the pump. Typically, electric motor-driven 3-cylinder positive displacement plunger pumps are employed because of their greater ability to create a constant (spike free) pressure. One or more high-pressure shifting valves direct water from the pump to the deburring station. Water returns from the wash chamber to the recovery water tank.

The recovery water is strained and filtered, then pumped back to the clean water tank, where it is again filtered and supplies water for the high-pressure pump. It's a closed-loop system. The pump power is dissipated as heat into the water and either a heat exchanger or water chiller is needed to keep the water temperature reasonably constant.

As the benefits of HPD and cleaning become more widely recognized, users from fields beyond automotive, such as the medical industry or the fluid power industry, should look to HPD as a way for delivering a clean and burr-free assembly-ready part.

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Richard Bertsche, President of Bertsche Engineering Corporation, 711 Dartmouth Lane, Buffalo Grove, IL 60089, 847-537-8757, Fax: 847-537-1113, www.bertsche.com.