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30 MMS August 2017 mmsonline.com RAPID TRAVERSE Machining Technology in Brief Methods Machine Tools, the U.S. importer of FANUC RoboCut wire EDMs, says that a new slug-retention technology introduced on the α-CiB series of machines makes it possible to hold the core in place with an automatic routine in the CNC program. This routine, which can be entered by the operator at the machine's CNC unit, adds a "stitch" at a few strategically placed locations around the periphery of the core. This stitch is created by intentionally causing the wire to go into a "reverse electrical discharge machining" mode in which brass from the wire is deposited in the kerf between the core and the parent material. This brass deposit forms a tab that is a few mil- limeters deep and wide at the top of the channel formed by the wire-cutting action. This tab is sufficient to hold the core in place until the job is completed. After the finished plate, which is still holding all the stitched cores in place, is removed from the machine, a tap of a small hammer is all that is needed to break the tab and enable the cores to drop away. The remaining portions of the tab can be removed with a few strokes of a file or abrasive deburring stick. In the past, procedures to retain the slug involved stopping the machine so that a dab of glue or a small brass strip could be inserted to hold the core in place. After each stop, the operator had to initiate a wire rethreading routine to commence cutting again. Usually, these "glue stops" were written into the CNC program during the program- ming step. The core-stitch function on the FANUC machines does not have to be pre-programmed. Stitch points are set directly at the machine, spar- ing the programmer of this task. According to Methods, incorporating these stitching functions lengthens the actual machin- ing time somewhat, however, eliminating other steps to secure the slugs reduces overall process- ing time to complete the job. With the core-stitch function, the more openings that are being made at one time, the greater the overall time savings. In addition, this stitching function is automatic, enabling multi-workpiece cutting processes to run in an extended unmanned mode. How the EDM wire-cutting process is able to create the tab that stitches the core in place is worth a note because it reveals an understanding of the "spark erosion" concept. An energized electrode wire is able to remove material efficiently because the machine's power supply rapidly turns on and off the flow of electrical current, with each pulse creating a small discharge or "micro explo- sion" between the sur face of the wire and the surface of the workpiece. These pulses of energy occur many thousands of times a second. Although this discharge is highly localized (on a microscopic level, in fact), the discharge of energy creates a temperature high enough to melt the workpiece sur face and create an expanding gas bubble beneath the spot. When this bubble bursts, it disperses molten material as tiny particles that quickly solidify to be swept away by the flushing action of the flowing dielectric fluid. Normally, controlling numerous variables related to the pulse of electrical energy enables the elec- trical wire to minimize loss of its mass during the cutting process. However, these variables can be manipulated to cause a form of electroplating. In this mode, particles of brass from the wire are released and allowed to migrate to the side walls of the kerf being cut in the workpiece. To create the tab, the table travel slows down so that the amount of electroplating of the brass wire to the top of the ker f will be enough to fill in the ker f completely behind the wire as it continues to move forward in the direction of the cut. When a tab of sufficient length is complete, the power supply returns to the settings for standard cutting. At the same time, table travel goes back to its normal speed, and the process no longer leaves any tab. The RoboCut α-CiB series includes the C400iB, C600iB and C800iB models. They are designed to deliver high accuracy via thermal displacement compensation to accommodate room tempera- ture fluctuation. The company reports that the machines provide 0.0002-inch positioning accu- racy and 0.0001-inch repeatability. A moving-tank design also contributes to accuracy. Unlike a moving-column design, which can make the lower guide position susceptible to inaccuracy due to pitch and yaw of the column, the column and lower arm of the moving-tank design are f i xe d so that the ta ble a nd wor k pie c e move instead of the column. Methods Machine Tools, call 978-443-5388 or visit methodsmachine.com.