Today, the production of precision parts is characterised by highly demanding specifications with regard to tolerances and surface quality. In addition to the actual manufacturing processes, more attention is being focused on intermediate and downstream processes such as deburring and surface finishing for the production of high quality components.
On one hand, this is targeted at burr-free components and workpieces with defined edges and fillets or a surface finish which minimises friction, wear and noise, as well as increasing 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 to an ever greater extent. In order to master these challenges, innovative and advanced processes have been developed.
In the case of electrochemical machining (ECM), which is used in various fields from aerospace to toolmaking, metal is anodically removed from the surface of the workpiece. This procedure makes deburring possible in difficult-to-access areas such as internal bore intersections and pockets, and also permits burr-free shaping processes.
The machining tool, namely a cathode, and the component (as an anode) are connected to a generator which serves as a direct voltage source for the machining process. The component is machined accurately, independent of the metal’s amorphous structure, by means of the charge exchange which 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 washouts in workpieces made of practically any conductive metal. Since processing is contactless, the tooling is neither subject to wear due to the machining process, nor is it exposed to thermal or mechanical influences.
The characteristics and the shape of the tool holder determine where and how much material will be removed from the workpiece. Generator power is selected depending on the size of the surface to be machined at any given point in time, and also determines the speed at which material is removed and the achievable degree of surface roughness.
Newly developed generators can reach Ra values of 0.1 μm and better, depending on the initial state. Beyond this, they also prevent so-called stray machining which may lead to worse machining results at the anode’s peripheral areas.
3-Dimensional Accuracy With PECM
As far as the actual processes are concerned, ECM and precision electrochemical machining (PECM) are both based on exactly the same principle. Essential differences include the distance from the cathode to the workpiece on the one hand, and the use of an oscillating cathode in the PECM process on the other hand.
Similar to electrical discharge machining (EDM), this makes it possible to produce extremely accurate three-dimensional shapes, contours and structures with very high levels of surface quality. Ra values of down to 0.03 μm can be achieved. As compared with the EDM process, machining is more accurate with regard to component dimensions and tolerances, and it does not result in any thermal influences.
Significantly reduced machining time is a further advantage of the PECM process as opposed to conventional manufacturing. Comparisons, of a component that was produced by means of a conventional process involving spark erosion, milling, drilling, grinding, deburring and lapping, to a PECM process with subsequent grinding, reveal a 90 percent reduction in pure manufacturing time. In addition to shaping, the PECM process is also used for micro-structuring of surfaces, in order to optimise tribological properties.
ECM Process For Additive Manufacturing
Components produced by means of additive manufacturing processes have already established themselves in various industry sectors such as aviation and medical technology. However, poor surface finishes after 3D printing, as well as blobs which remain on the part after removing the support structure, are still a great challenge.
An ECM process called CoolPulse has been specially developed for, amongst other applications, surface finishing of 3D 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 can also be removed, which may result from 3D printing processes.
Abrasive Flow Machining
Schematic diagram of an AFM machine
Abrasive flow machining (AFM) is used primarily for processing workpiece areas which are difficult to access and internal surfaces of high-quality components made of metal and ceramics, which cannot be processed by means of conventional procedures.
Typical applications include rounding, polishing and deburring, as well as geometry optimisation and the minimisation of surface tension. The workpiece is clamped for processing in one or more fixtures at the AFM machine.
The processing medium is abrasive particles which are matched to the respective task with regard to type, size and concentration. They are embedded in a polymer mass of defined viscosity. This then flows through or over the area of the component 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 in order to assure reproducible results.
The AFM process makes it possible to improve surface roughness by a factor of five to eight as compared with initial surface condition. It is used, for example, in the automotive, plastics and aluminium industries, as well as in tool and mould making for the processing of, amongst other workpieces, impression dies, tablet moulds and deep drawing dies.
The process has proven its worth in other sectors as well including medical technology, aviation and aerospace, as well as in textile machinery manufacturing. Additive manufacturing of metallic components in modern industrial production is opening up an additional range of applications for AFM.
New Perspectives In Barrel Finishing
This stream finishing system with pulse drive, which is
integrated into mass production in the automotive industry,
is used for fully automated deburring, rounding and
smoothing of cam shafts. Reduction of peak-to-valley height,
for example from 0.2 to 0.1 μm, is accomplished in less
than one minute.
Surf, stream and pulse finishing processes involve barrel finishing solutions for individual part processing which can be easily integrated into automated production lines. These new developments permit accurate, reliable deburring, edge rounding, smoothing, grinding and polishing of geometrically complex components such as machine cutting tools and implants, as well as motor, gearbox and turbine components. These are tasks which have usually had to be completed manually in the past by means of time-consuming, costly processes because no automated solutions were available.
The effects of pulse finishing are based on ideally matched relative motion between the processing medium and the workpiece. For example, the workpiece 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 period of time. Interaction with the inertia of the processing medium—due to the different speeds of the workpiece and the abrasive particles—results in targeted grinding action with accurate deburring, even in areas which have previously been inaccessible for barrel finishing, for example crossholes 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 results in the formation of plasma after the anodically polarised metallic workpiece has been immersed into the electrolytic bath. The plasma coats the workpiece, thus 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 two and eight μm per minute and achievable roughness values are less than 0.01 μm. The geometric shape of the component remains nearly unchanged.
APMEN Cutting Tools, May/June 2017