AN INTRODUCTION TO PRECISION ELECTROLYTIC MACHINING

Manufacturers of metal components continue to search for better methods that are faster, more efficient, lower in cost, with high reliability. A new machining process, called PEM, is offering solutions to some of these needs. This machining process was developed in Germany and has recently been introduced in North America.

Electrolytic machining (which is also referred to as electrochemical machining, or ECM) has been used in manufacturing since the 1960s. Electrodes with a mirror image of the shape to be machined are separated by a gap which allows a salt-water solution to flow thru and remove the metal. The process has demonstrated advantages in machining difficult-to-machine metal alloys such as stainless steel and Inconel, and machines much faster than conventional single-point cutting since it is an area-machining process. However, since the ECM process removes material from the workpiece surface by ionic dissolution of the surface atoms via direct current flow, the process is inherently limited by current flow.

The precision electrolytic machining (PEM) process was developed to overcome the limitations of ECM that relate to precisely focusing a high metal removal current onto the workpiece with boiling the saltwater solution. The PEM process uses a coordinated combination of short DC pulses and a reciprocating electrode to allow a high current to flow for short periods while the electrode is approximately ten microns from the surface as graphically shown in Figure 1.

This innovative machining concept advances the electrode tool in a reciprocating motion to the workpiece (curve S) and a current pulse is produced while the electrode is in close proximity to the workpiece surface ("I" in Figure 1). A salt-water electrolyte is injected between the electrode and workpiece which provides for current conduction without the electrode contacting the workpiece. As electrons cross the gap during the pulse, material on the workpiece is electrolytically dissolved, forming a mirror image on the workpiece.

The PEM process provides the ability to perform high accuracy machining, deburring, and finishing in one operation. For example, high quality gears can be machined without the concern for burrs being introduced into the final product. Since the PEM process is a stress-free metal removal process that removes metal from the surface atom by atom, there is no possibility of forming burrs or mechanical deformation at edges. This is the case for all metals machined with PEM, including stainless steel and other alloys that are hard to machine.

Although surface finish is material dependent, many materials are machined to very smooth finish, with some as good as 1 µin to 4 µin Ra. Gears for precision meters, as shown in Figure 2, are machined burr free, six at a time, in a standard PemTec machine.

Machining usually starts with a solid blank or a per-form of the component geometry. Figure 3 shows a turbine wheel with 45 airfoils machined from a blank. The electrode is a thin stainless steel plate with the airfoil shapes wire EDM'd as a negative of the turbine wheel geometry less the machining gap for electrolyte. Eight turbine wheels are machined in one 45 minute machining cycle. The mold insert for date marking is shown in Figure 4 along with the electrode. Four parts can be machined simultaneously in nine minutes.

Larger components can also be precision machined, such as the filter plate of Figure 5 that is made of a proprietary alloy and is difficult to machine conventionally. The passages cannot have any burrs under high magnification and all edges must have radiuses. This component is machined on one side, then machined from the opposite side at 90 deg. Machining time is five minutes per side.

Punch dies are another example of the machining efficiency that PEM offers. Hardened punch blanks are machined to final form and surface finish for a variety of shapes as seen in Figure 6. Not only can PEM machine these in the hardened state, a uniform undercut of the full geometry can be machined as is the case with the dies of Figure 7. The undercut is accomplished by simply varying parameters in the part program. The typical machining time for these dies is 60 to 80 minutes. One of the advantages in the sun and star punch is that the multiple shapes can be machined on one punch and not four separate punches, which also results in savings of a smaller die set.

A PEM machining center (Figure 8) consists of four major components that are delivered as plug-together modules for ease of installation. The machine is made of granite to guarantee the thermal and mechanical properties necessary to ensure high accuracy repeatability. The reciprocating Z axis uses a patented high accuracy design that maintains precision throughout the ten to sixty hertz reciprocation frequency range. The PEM power module is available in 2K, 4K, 6K and 8K ratings to supply precisely controlled current pulses.

The PEM Aqua module provides process electrolyte at ten liters per minute. The Aqua system automatically controls temperature, pH, and conductivity to the exact values required for the machining process. An ultra fine filter system is incorporated to remove all material machined from the workpiece and uses an automatic back flush system to clean the filters. The system control is an intuitive touch screen interface with simple programming and graphical display of the machining process and all system functional components.

Application possibilities for PEM exist in aerospace, automotive, consumer product, medical, and fluidics markets for features such as heat transfer surfaces, nozzle geometry, fluid and gas path features, surgical and implant component surface geometry. The ability of the process to meet requirements for full-form high volume machining, with non-consumed tooling, is very attractive to manufacturing. Some of the advantages that will be realized from this process are reduced work-in-process, improved component strength, increased production and fewer rejects, fast machining time, and excellent, stress-free surfaces.

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Donald Risko is vice president of PEM Technologies, 532 Greenleaf Drive, Monroeville, PA 15146-1200, 201-724-0967, Fax: 574-830-2310www.pemtechnologies.com, [email protected].