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Micro-EDM: Dielectric oil matters December 29, 2012

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The four basic functions of dielectric oil are:

1. Insulation. The dielectric must insulate the workpiece from the electrode. The disruptive electrical discharge must take place across a gap that is as narrow as possible. As gap width decreases, achievable process accuracy increases.

2. Ionization. Optimal conditions for the production of an electrical field must be created as quickly as possible, then a spark path must be provided. After the impulse, the spark path must be rapidly deionized or extinguished so the next discharge can be made. The dielectric must constrict the spark path to achieve high energy density, which also increases discharge efficiency.

3. Cooling. The EDM process involves elevated temperatures. Because the discharge spark has a temperature range of 8,000°C to 12,000°C when it punctures the workpiece, dielectric oil must cool both the workpiece and the electrode. In high-precision microEDMing, centralized dielectric chillers must be able to effectively remove absorbed heat from the dielectric oil to maintain overall operating temperatures within ±2°C — a critical factor in achieving part and feature accuracies below 2μm.

4. Removal of waste particles. Eroded material particles must be removed from the discharge area to avoid EDM process disruptions. Excess debris will “short circuit” the gap, decreasing process efficiency. A high-efficiency fluid filtration system must be part of any high-precision microEDM. Average particulate filtration media, used in-line with a standard dielectric reservoir, should be rated at 5μm to 8μm. If the EDM has a fine-hole machining option, a 1μm prefilter should be installed in the high-pressure pump reservoir.

Ref: Bradford, J.W. (2008). Low-viscosity dielectric oils improve microEDM operations. Micromanufacturing, Vol. 1(2), pp.8-11.

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EDM discharge dressing December 28, 2012

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Using the EDM process to prepare the electrode geometry is becoming more common as the part and feature size requirements get smaller. Conventional machining processes can no longer provide the required feature size and definition of many microparts.

When EDMing parts smaller than 1mm × 1mm and part features smaller than 0.1mm × 0.1mm, the challenge of electrode-shape preparation becomes significant.

By holding an electrode for shaping, the vibration and heat generation can be difficult. An innovative remedy to these challenges is EDM discharge dressing. This approach involves applying an associated dressing block, or alternate device, mounted to the machine table to create an inverse geometry on the electrode or part mounted to the
machine spindle.

The dressing device can consist of one of many materials that exhibit lower wear characteristics than the material mounted to the spindle. Ideal wear ratios are 50 percent or less, relative to the amount of material that is removed from the spindle-mounted tool.

Commonly used dressing block materials are traveling brass wire, silver tungsten, copper tungsten and various types of tool steel capable of dressing the graphite tools mounted to the spindle. A relatively short aspect ratio (30:1 or less) of electrode could be effectively prepared.

Advantages of discharge dressing
– It is a highly repeatable process.
– Final tool runout will be no greater than that of the spindle-bearing runout of the machine used.
– Noncontact preparation of the spindle-mounted electrode will permit dressing to sizes less than 0.010mm.
– The entire dressing and machining process can be unmanned, thereby reducing overall part manufacturing costs.
– By implementing a precision guide, the tip of the electrode (or workpiece, if mounted in the spindle) can be accurately held in place during the machining process. This is especially crucial when producing long, small-diameter parts or pins.
– Parts with a high Rockwell hardness can be easily and accurately machined.
– The electrode tip is guided during dressing, eliminating vibration.
Ultrasmall electrode diameters (as small as 0.005mm) can
be EDM-dressed to sizes not commercially available.
– Unlimited shapes and geometries can be produced with contour machining programs.
– The workpiece can be mounted to the spindle and EDMed to size by a traveling wire mounted to the worktable.

Ref: Bradford, J.W. (2008). Discharge dressing prepares electrodes, workpieces for microEDMing. Micromanufacturing, Vol. 1(1), pp.10-11.

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Micromachining Technologies (2): MicroEDM January 24, 2011

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วิบุญ ตั้งวโรดมนุกูล (2010) Micromachining Technologies ตอนที่ 2: MicroEDM, วารสารส่งเสริมเทคโนโลยี, August-September 2010, Vol.37 No.212, 83-86.

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