L'ébavurage ne constitue certainement pas un processus de base de l'usinage. Jusqu'à présent il est plutôt considéré comme un mal nécessaire. Ceci conduit souvent au fait que, bien que les procédés d'usinage soient synchronisés de façon optimale, l'efficacité du système dans son ensemble diminue lors de l'ébavurage, parce que ...
EMAG ECM : Précis contre le dérèglement des arêtes
05/07/2010 - Oliver Hagenlocher
[Translate to French:] Workpieces with complex contours often feature sectors that are not easy to machine because they are difficult to access. Usually, undercuts, pockets and internal, overlapping bores present no major challenges to mechanical machining operations; but this often changes when such sectors have to be deburred – and even those parts of a workpiece that are difficult to access call for burrs to be removed cleanly and without negative impact on the material. With the mechanical, thermal and water jet-based technologies used up to now, intended output rates, economic viability and repeatability could frequently not be guaranteed. Medium size and large batch production in particular attach great importance to the best possible component quality, and internal burrs and lugs can badly affect component function. Practical applications show up another problem: the so called secondary burr, i.e. when burrs are removed using standard machining processes a secondary or „turned down“ burr can form and leave further, undefined finish-machining work to be done. These are the reasons why EMAG ECM GmbH, formerly Dorner GmbH, rely instead on the electro-chemical deburring process.
ECM – Electro-Chemical Machining
ECM stands for Electro-Chemical Machining and is – unlike spark erosion – a gentle, electro-chemical metal removal process that does not involve spark formations. An electrode is connected to a D.C. or pulse source to act as a cathode (tool), whilst the workpiece represents the other electrode and is poled as an anode. The charge in the electrode gap between cathode and anode flows in a watery electrolyte solution – usually sodium nitrate or sodium chlorite – and dissolves metal ions on the workpiece. The material thus removed can later be filtered out from the electrolyte solution as metal hydroxide. The contour of the cathode (tool) is made to fit the machining requirement. This ensures that deburring – without causing mechanical or thermal stresses – takes place only at the point of the workpiece where it is necessary to remove material. This is where the main advantage of the process lies. This pinpoint machining process allows for the most delicate components to be deburred with great accuracy and repeatability.
An example for such demanding deburring operations is the manufacture of a pump body for common-rail injection systems. The mechanical machining of these pump bodies and the deburring operations are supposedly state-of-the art procedures, with one prerequisite being: total control over the whole process. As it happens, machining, automation and deburring are all part of the EMAG Group’s range of applied technologies. It means that in future there will be only one interfacing contact. This is of enormous importance where deburring is concerned, as this initially minor problem can quickly develop into a major headache. And to prevent this from happening, it is important to define the machining direction right at the start when deciding on the machining layout and the sequence of operations. Only if this is done, can the burr later be removed economically and with precision. In reality, the big challenges of the past lay in the fact that deburring was put at the end of the machining process. This required cost-intensive efforts in mechanical deburring or alternative processes. In cases where ECM was used, the costs for fixtures and cathodes rose with the extent of the changes that had to be made. In the case of the pump body it will in future be possible to optimally design the whole process, including the interlinking of machines, right from the word go.
This also applies to the cylinder for the injection system. This component works at over 2,000 bar and features a number of different bores that have to be deburred or burnished to a particular surface finish. EMAG ECM GmbH with their highly developed power electronics can deal with these important demands in an economical way. Pulse form and current density ensure optimal surface finishes.
Contactless machining and economic viability add a new dimension
Quite apart from the precision deburring or burnishing of workpieces, on which it no doubt delivers, ECM can also be described as economically highly viable. As cycle times can be reduced and output rates increased in line with the degree of parallelisation, i.e. with the number of components that can be accommodated in a single fixture, cycle times per component can be set at below 10 seconds. The cylinder for the pump body, mentioned earlier, has – thanks to the EMAG technology - been grouped in fours and sixes, in the past; and, depending on the component, up to 20 workpieces have been machined in parallel before now.
The question of quality control EMAG ECM answers with its scalable power electronics, which allow for every individual cathode in a group of workpieces to be monitored separately. In fact, this system ensures that the size of the charge in the solution and the volume of metal removed by each cathode can be monitored.
Wear-resistant, precise, contactless and economically viable – these are all advantages connected with the use of ECM; and they are advantages that optimally complement the overall concept of the EMAG Group.
PECM – Precise Electro-Chemical Machining
An enhancement of ECM is the PECM process which EMAG ECM also uses and which the company has developed further. The “P” stands for “precise” and thus for the kind of precision that can be achieved with a pulsed current, an oscillating cathode and pulse-packets. The achievable quality largely depends on an efficient pulsed current source and a rigid machine.
A little incidental detail …
Besides the automotive industry the ECM and PECM processes are now also making inroads into the aerospace and the medical equipment industry.