Preparing start holes for microEDMing

Preparing start holes is a critical micromachining operation. One common method is sinker EDMing start holes prior to running a wire EDM operation. Appropriate electrode preparation and workpiece fixturing are vital to the successful machining of a start hole. The following case study examines how microscale start holes were machined in a challenging application.

Magnified view of finished holes after wire EDM process. All photos courtesy Makino.

Application background

This application required accurately producing and placing 72 start holes, finished to a diameter of 0.1778mm, with minimum centerline spacing between holes of 0.254mm. The thickness of the material at the location of the holes was 2.5mm. Hole diameter tolerance was 0.001mm/-0.000mm, and the tolerance of the centerline pitch between holes was ±0.002mm.

Due to the exceptionally tight tolerances of the finished-hole diameter and location, the start holes were sinker EDMed and the final hole size was wire EDMed. To leave sufficient finishing stock for wire EDMing, the start-hole diameter had to be less than 0.127mm. This stock amount provides for adequate engagement of the wire during roughing and skimming to allow for proper creation of the finished form. To stay under the maximum target size of the start hole, a 0.080mm-dia. tungsten pipe electrode material was selected, which is manufactured by Japanese supplier Sumiden Fine Conductor.

A V-block aligns two guide bushings, creating a controlled centerline for simple insertion of a smaller pipe into a larger pipe.

Flushing through pipe electrode.

To effectively hold the 0.080mm-dia. pipe electrode and provide through flushing, it was joined with a larger pipe electrode. This made it easier for the machine’s fine-hole tooling chuck to clamp and seal on the larger, more rigid assembly. The following assembly steps were followed:

• A precision ground V-block served as the base fixture.
• The larger diameter copper pipe electrode (above, left), measuring 0.300mm OD and 0.100mm ID, was inserted into a 0.305mm precision guide bushing.
• The smaller tungsten pipe electrode has a 0.080mm OD and 0.040mm ID. (This electrode was inserted into a 0.085mm guide bushing.)
• With the centerline of each pipe electrode precisely aligned, the smaller diameter pipe was inserted into the larger pipe to a depth of at least 0.5" or more.
• One drop of Super Glue was applied to bond the two pipe electrodes. (Yes, Super Glue. In the world of micromachining, we often have to find creative solutions that get the job done.)

Inserting the assembly into the machine’s chucking device was a four-step process.

1. The 0.300mm pipe electrode was inserted into the subcollet, leaving ½" of the pipe extending from the back of the subcollet. The subcollet was then tightened firmly to ensure stability of the assembly.
2. A rubber seal was installed around the electrode by pushing the electrode through the rubber seal, creating a positive seal for fluid flow. This rubber seal ensured that the high pressure flow of dielectric oil would be contained internally and forced to flow through the pipe electrode.
3. The subcollet seal and electrode assembly was then inserted into an Erowa fine-hole holder and tightened in place using a locking nut.
4. Finally, the completed fine-hole tool assembly was installed in the machine chuck. A fluid pressure test was performed to check for leaks and verify consistent flow of dielectric oil through to the very tip of the 0.080mm diameter electrode.

Three key factors contributed to the success of this fine-hole EDMing operation: proper flushing, rotation and guiding of the electrode. Taking them in order, an active flow of fresh, cool dielectric oil was presented to the tip of the pipe electrode. Second, the 0.080mm-dia. electrode most effectively machines when rotated at 1,200 rpm or higher. Finally, the tip of the electrode was guided with a precision bushing to ensure precise hole positioning and shape accuracy.

Part holding fixture

To provide the highest degree of workholding precision, chucking repeatability and interchangeability between the sinker and wire EDMs and the measuring microscope, a workpiece holding nest, machined by the wire EDM, was used.

The locating window dimensions were machined 0.002mm larger than the outside dimensions of the workpiece blank, providing precise part alignment and location relative to the wire EDM, where the final microholes were machined in the part.

This precision two-part holding nest mounted to an Erowa integrated tooling nest was machined in place on a wire EDM.

(External part dimensions are 10.500mm × 12.80mm × 19.75mm.) This fixture nest was mounted to an Erowa precision part-holding fixture (manufactured for use in the Makino UPJ-2 ultraprecision wire EDM tooling), which provided a total chucking repeatability of 0.7µm. The tooling interface allowed for simple, precise chucking when moving the part between multiple machines.

By combining novel approaches to electrode assembly, electrode tooling, precision workholding and advanced machine tool technologies, start holes were successfully created for this micromachining application. µ

About the author: John W. Bradford is micromachining R&D manager at Makino Inc., Lacey, Wash. Telephone: (360) 252-2737. E-mail: john.bradford@makino.com. Web: www.makino.com.