Quality Scan: Getting EDM Up to Speed in Cut Quality
By Greg Langenhorst
Technical Marketing Manager
MC Machinery Systems, Inc.
Wood Dale, IL
Improving component quality is an ongoing process. As new technologies are ushered in, care must be taken to ensure component quality isn’t just maintained, but improved. When we talk about quality improvement in EDM technology, the rubber really meets the road in the tangible, physical characteristics of the finished component itself—the surface quality.
Precision work has always been available, but it traditionally has been time-consuming. Many older EDM machines, the Mitsubishi V Series for instance, while fast, couldn’t get down to that single-digit, 6–8 μin. Ra surface finish—such jobs almost needed a special machine. The next-generation Mitsubishi FA Series solved that surface finish requirement, but sacrificed some of the V Series machine’s speed.
Recently, we’ve been seeing an expanding demand for improved surface finish, especially in the aerospace and medical industries. For example, a jet engine hub that holds engine blades made from a high-nickel material such as Inconel. These components have a very specific recast layer requirement, so high-precision, fine surface finish is a necessity. Medical implement parts can always be improved with regards to surface finish—consider joints or other metal-on-metal applications.
We also see such requirements in the high-speed stamping industry, where punches and dies are made from carbide. A finer finish on these components means less galling on the edge. The finer the finish, the smoother the die runs and the longer punches last. We are seeing a real performance gain in production part cutting on the wire EDM where a lot of jobs that were produced required secondary finishing, like extrusion finishing or a vibratory finisher with polishing stones that round edges and corners. Our goal has been to get the surface quality right off the machine, thus maintaining part geometry and edge quality, in less time.
This has been a hallmark issue of the design changes in the MV Series EDM. A new power supply system is now monitoring each spark individually, and shaping it so that it provides the best cutting action per energy input. Previously, the power supply has been monitoring groups of sparks and averaging the results over time, rather than spark by spark.
The MV series implements a new Digital Matrix Sensor and a high-speed all fiber optic internal communication system that is able to read each spark and optimize performance. The MV Series constantly and seamlessly adjusts on time, off time, and amplitude (which basically controls power and frequency). Wire size, material type, thickness variations, flushing conditions, and whether you’re going straight-line or turning sharp corners is all taken into consideration in real time.
Because of this real-time feedback, an operator has finer control of what’s going on than ever before. Plus, less electricity and less wire is used to get a better product. That’s another key factor; the MV Series consumes as much as 60% less wire to get to the same or improved results.
This has helped Mitsubishi cut an equal quality part faster, or a better quality part in about the same time as previous technologies. To simplify wire EDM, there’s an initial rough cut, where a shape is programmed and cut with the wire electrode, like a bandsaw through a piece of wood. If better accuracy or surface finish is required, skim cuts follow, reducing energy output and going back across the first cut surface removing smaller amounts of material. By reducing the energy, an operator improves straightness, corner accuracy and surface finish. Depending on what you need for surface finish controls the number of skim cuts. The best possible surface finish of a 6–8 μin. Ra, (the rough cut is about an 80–90 μin. Ra), is completed with one rough cut and four skims.
The new technology is able to better and more accurately control energy output, so as the surface finish number decreases the accuracy improves. In addition, because the sparks are controlled so much better the desired surface finish is achieved in fewer passes. Generally speaking, you can reduce your entire cycle time by one pass and still hit the accuracy/finish requirements compared to older machines.
Tough-to-meet surface finish requirements for high-quality components once required special machinery, a secondary process, or had been too time consuming. These are now possible on a single machine, using less wire and power, and hitting the surface finish mark in fewer passes—an efficiency that is opening up new doors in several industries. ME
This article was first published in the July 2013 edition of Manufacturing Engineering magazine. Click here for PDF.