THE DRILLING PROCESS
In the small hole drilling process, rotating tube electrodes are advanced into the workpiece using a significantly modified version of the same EDM process employed in the traveling wire EDM application, which produces planar surfaces. Whereas in the wire application one is limited to relatively low currents to avoid wire electrode failure since a tensile stress is imposed on the wire, in the fast hole drilling application significantly higher currents are possible since tube electrodes are not subjected to any tensile stress. Therefore, the construction of tube electrodes has historically been limited to the homogeneous materials copper and copper-zinc brass alloys which were known to perform well in the wire application. The selection of the tube material is usually dictated by either the power supply of the machine tool or the workpiece material. Copper tubes are preferred for burning tungsten carbide and some complex aerospace alloys. However, most small hole drill machine tools are designed for and recommend brass tube electrodes. The tubular configuration can be single channel or multi-channel with a wide range of configurations to provide adequate high pressure dielectric flushing. Until recently, only standard brass or copper tubes of various geometries have been available for this rapidly growing application which has primarily been driven by the demands of cooling hole technology for turbines.
GIP COATED TUBE ELECTRODES
In fast hole drilling, the active surface resulting in metal removal by erosion is the exposed tube end which is perpendicular to the available tube surface for coating if a single channel tube is considered to be a wire with a hole along the wire axis. Whereas, the active surface represents 100% of a coated wire electrode, it only represents 10% - 15% of the active surface in a coated tube electrode. In addition, flushing in the wire application can be enhanced by manipulating the properties of a coated electrode whereas in the tube application flushing is accomplished by over powering the process with high electrical currents and high pressure dielectric fluid flow since tube fracture is no longer a consideration as it was in the wire application. Therefore, it is not surprising that historically coated tube electrodes for fast hole drilling have never been given serious consideration. However recently the technical team at GIP re-examined this EDM application and has developed an innovative, patent pending technology and process to produce coated tubular electrodes which offer significant improvements in machining efficiencies.
GIP COATED TUBE ELECTRODE PERFORMANCE
An evaluation test was designed to compare the fast hole drilling performance of GIP’s gamma phase brass coated single channel tube electrodes to uncoated brass single channel electrodes. Both electrodes were 300mm in length at a diameter of 1.0 mm. The tests were conducted on a major manufacturers CNC 32amp machine tool Center. The tests were performed using the manufacturer’s SKD (steel) 1.00 mm Technology on a 50.0mm thick block of D2 tool steel hardened to Rc 52–54 along with Carbide and Inconel. The coated electrodes can handle more aggressive settings than the plain brass electrodes but have also shown better results with the same settings.
The coated electrodes not only make a noticeable reduction in cycle time and wear in most cases when drilling, but they also show much improvement during the “break thru” of a thru hole. When the electrode loses the flushing at “break thru”, the coated electrode shows an improved performance for finishing a uniform hole diameter.