Machining and surface finishing converge

New way to produce precision surface finishes improve quality, productivity

In ECM, a charge exchange takes place between the cathode and the anode (component) in an aqueous electrolyte solution, by means of which the workpiece is accurately machined in a targeted fashion. Photo courtesy of EMAG ECM GmbH.

Today the production of precision parts is characterized by highly demanding specifications in terms of tolerances and surface quality. Innovative, advanced processes for deburring, surface finishing, and shaping are making it possible to improve quality and productivity, as well as economic efficiency.

In addition to the actual manufacturing processes, more and more attention is being focused on intermediate and downstream processes such as deburring and surface finishing in the production of high-quality components.

On the one hand, this attention is targeted at burr-free components and workpieces with defined edges and fillets, or a surface finish that minimizes friction, wear, and noise, and increases performance and service life. On the other hand, manufacturing steps for precise shaping are required as well, and in this respect, machining and surface finishing are converging more than ever before.

To resolve these challenges, innovative, advanced process are available that can be matched to the task at hand with high levels of productivity and economic efficiency and that deliver reliable, reproducible results.

Electrochemical Machining

Electrochemical machining (ECM) anodically removes metal from the surface of the workpiece and is commonly used in the aerospace, automotive, toolmaking, medical, and energy industries.

This procedure makes deburring possible in difficult-to-access areas such as internal bore intersections and pockets, and it permits burr-free shaping processes. The machining tool, which in this case is a cathode, and the component, which acts as an anode, are connected to a generator that serves as a direct voltage source for the machining process.

The component is machined highly accurately, independent of the metal’s amorphous structure, by means of the charge exchange that takes place between the cathode and the anode in an aqueous electrolyte solution. This makes it possible to produce even very small, thin-walled contours, fillets, ducts, slots, and wash-outs in workpieces made of practically any conductive metal. Because processing is contactless, the tooling does not wear during the machining process. It also isn’t exposed to thermal or mechanical influences.

The characteristics and the shape of the toolholder determine the location and amount of material that is removed from the workpiece. Generator power is selected based on the size of the surface to be machined at any given point in time, and it determines the speed at which material is removed and the achievable degree of surface roughness. Newly developed generators can produce Ra values of 0.1 µm or better, depending on the initial state. New-generation generators also prevent “stray machining” that may lead to worse machining results at the anode’s peripheral areas.

3-D, Precision ECM

As far as the actual processes are concerned, ECM and precision electrochemical machining (PECM) are both based on the same principle. Differences include the distance from the cathode to the workpiece and the use of an oscillating cathode in the PECM process. Like electrical discharge machining (EDM), this makes it possible to produce extremely accurate 3-D shapes, contours, and structures with high levels of surface quality. Ra values as low as 0.03 µm can be achieved.

In addition to shaping, the PECM process is also used for microstructuring of surfaces, for example to optimize tribological properties.

Abrasive flow machining improves surface roughness by a factor of 5 to 8 as compared with initial surface condition. Surface tension is reduced at the same time. Photo courtesy of 4MI GmbH.

ECM for Additive Component Manufacturing

Components produced by additive manufacturing processes already have established themselves in various industry sectors such as aerospace and medical technology. However, poor surface finishes after 3-D printing, as well as blobs that remain on the part after removing the support structure, are still a challenge.

A new ECM process, Coolpulse™, was specially developed for, among other applications, surface finishing of 3-D printed, metallic components. It makes it possible to improve both micro- and macrostructures on internal and external surfaces in a single process, and specified surface characteristics can be reproducibly obtained with short cycle times. Furthermore, support structure remnants and surface defects that might be caused by 3-D printing processes also can be removed.

Abrasive Flow Machining

In addition to the ECM processes, abrasive flow machining (AFM) can be employed. AFM primarily is used for processing difficult-to-access workpiece areas and internal surfaces of components made of metal and ceramics that cannot be processed by conventional procedures.

Typical applications of AFM include rounding, polishing, and deburring, as well as geometry optimization and the minimization of surface tension.

In this process, the workpiece is clamped for processing in one or more fixtures inside the AFM machine. The processing medium -- abrasive particles -- are paired with a specific task with regard to type, size, and concentration, are embedded in a polymer mass of defined viscosity. These particles flow through or over the areas of the components to be processed in alternating directions at a defined pressure level by means of hydraulically powered pistons. The grinding medium functions like a liquid file.

Process parameters are continuously monitored to ensure reproducible results.

The AFM process makes it possible to improve surface roughness by a factor of 5 to 8 as compared with the part’s initial surface condition. It’s used, for example, in the automotive, plastics, and tool- and mouldmaking for the processing of impression dies, tablet moulds, and deep-drawing dies.

Barrel Finishing

Surf, stream, and pulse finishing processes involve barrel finishing for individual part processing, which can be easily integrated into automated production lines.

These new developments permit highly accurate, reliable deburring, edge rounding, smoothing, grinding, and polishing of high-quality, geometrically complex components such as cutting tools and implants, as well as motor, gearbox, and turbine components.

The effects of pulse finishing are based on ideally matched relative motion between the processing medium and the workpiece. For example, the workpiece typically is secured in a clamping collet and accelerated to a speed of up to 2,000 RPM, decelerated, and accelerated again in a rotating bowl within a very short time frame.

The AFM process pushes particles over or through areas of the components in alternating directions at a defined pressure level by means of hydraulically powered pistons. Photo courtesy of Fraunhofer IPK.

Interaction between the inertia of the processing medium – caused by the different speeds of the workpiece -- and the abrasive particles results in targeted grinding action with accurate deburring, even in areas which previously have been inaccessible for barrel finishing, such as cross-holes in hydraulic components.

Polishing With Plasma

Like electropolishing, plasma polishing is also an electrolytic process, but it works with high voltage and an electrolyte based on a salt solution, which is considered ecologically harmless.

This process works by the formation of plasma after the anodically polarized metallic workpiece has been immersed into the electrolytic bath. The plasma coats the workpiece, resulting in reduced roughness, as well as the removal of organic and inorganic contamination, with just a minimal loss of mass.

Depending on the material specification, material abrasion typically lies between 2 and 8 µm per minute, and achievable roughness values are less than 0.01 µm. The geometric shape of the component remains nearly unchanged.

Contributing writer Doris Schulz can be reached at doris.schulz@pressetextschulz.de.