A more systematic approach to ram EDM

Modern Machine Shop, Feb, 1992 by Tom Beard

For most of 45 years as a pattern maker, the tools Dale Ziegler used to finish molds were files, scrappers and emery cloth. His tools are very different these days. Now the Harrison, Ohio shop, Northbend Pattern, uses ram EDM to put the final touches on the core molds they make for sand casters all over the Midwest. But just as important as the EDMs themselves is the equipment employed to make electrodes. And what Northbend uses is state of the art.

The process of ram EDM is, after all, more than power settings, orbiting patterns and oil. It begins with capturing complex three-dimensional geometry and somehow transferring that model to electrode tooling. This is the portion of the process that is so painstakingly slow for many shops, but where Northbend has become so accomplished.

There are four major components to Northbend's process; a digitizing machine to capture 3D geometry from hand-crafted patterns; a special graphite milling machine to create electrodes; conventional tracer mills to rough workpieces; and two CNC ram EDMs to finish. All but the tracer mills were manufactured by LeBlond Makino Machine Tool Company (Mason, Ohio). What the new technology has done is integrate all stages of mold making--from the first pattern to the final burn--into a single, systematic process. Here's how it all fits together.

The Electrode Model

Ever since Mr. Ziegler first realized that "you can't machine (internal) square corners with a round tool," he has been looking for a better way to finish molds. Hand finishing was laborious, but worse, it was inconsistent, which has become an increasing liability over time. Sand casters these days must deliver tolerances once only expected of machining, and are willing to share very little of their dimensional leeway with a mold supplier. This, and the ever shrinking supply of skilled pattern makers, made the move to EDM almost inevitable. But how Northbend does EDM goes several steps beyond the norm.

Frank Schilling leads the EDM operation. He says that the process usually begins with a hand-carved wood or plastic pattern since customers often request a physical model and that is still the most efficient way to generate the complex forms typical of Northbend's work. From this original pattern, they create a poured plastic "electrode model," which in many ways is the centerpiece of the entire process. Besides being a precise 3D representation of the complete mold cavity, the electrode model will be a digitizing master, tracing pattern and process plan all rolled into one.

The first step is to decide which features of the model will be reproduced with the EDM and which will be machined. The philosophy at Northbend is to machine wherever possible since it is much faster. EDM is applied where a cutting tool won't provide the necessary detail, such as in internal corners or very intricate features, and on some hard-to-machine materials. The EDMed areas are kept relatively small, which means that multiple electrodes will often be used on a single workpiece. Once the surface areas, or patches, to be EDMed are determined, they are marked right on the model.

Now the model is ready to be probed, and is mounted on the table of the Makino Accutrace digitizing machine. The first probe used is of a diameter that fits snugly into locating pin bushings previously inserted in the model. By probing two or more of these points, the model is located within the digitizer's 3D coordinate system. Then an origin point for the model is indicated, which is extremely imporant since it will be the location reference for the rest of the process.

They don't have to digitize the entire model, just the features that will be turned into electrodes. The rough markings that define those features are now replaced by precise scribe lines on the model done by a quill mounted in the spindle of the digitizing machine. This process describes the X-Y boundaries of the patch, which will correspond exactly to that of an electrode. An origin point will be designated for the patch, and its location relative to the workpiece origin will be recorded, as this information will be critical when it comes time to EDM the mold.

Though the digitizer does have the ability to scale a model, they will usually work full size. However, they do want the electrode to be slightly undersized to compensate for the volume of the orbiting pattern and spark gap. This will be done simply by selecting a probe slightly smaller in diameter than the corresponding tool that will ultimately be used to machine the electrode. No data manipulation is required since the digitizing process is essentially emulating a conventional tracer mill. But rather than directly controlling the tool path, as a tracer does, the digitizer is capturing X-Y-Z coordinate points as it moves back and forth across the model surface. These points will later be turned into a part program.

There are several standard digitizing paths to choose from. Resolution is also selectable. It can be set simply as a standard increment between points. It can also be set as a dimensional tolerance, so that the digital model deviates no more than the specified value from the actual surface. Mr. Schilling usually opts for the latter option, as it is more efficient in terms of the amount of data required to adequately define the form. The idea is to gather enough points to provide the necessary accuracy, but not so many to make the digitizing routine or data file excessively long.