Low-viscosity dielectric oils improve microEDM operations

Advances in dielectric fluid have played a key role in improving the efficiency and acceptance of microEDMing. Choosing the right dielectric fluid is critical for successful operations. This article focuses on the role of dielectric oil, most commonly applied to sinker EDMs, high-precision hole sinker EDMs and ultraprecision wire EDMs. Using specially formulated, low-viscosity dielectric oils can lead to signifi cant operating improvements.

Basic functions

Dielectric media, circulated between the electrode and workpiece, must be carefully selected and applied to maintain peak performance and control of the electrical spark. Another key factor is the dielectric media filtration system, which helps maintain consistent gap performance and dielectric cleanliness. Due to space limitations, just fi ltration will be addressed in this article. Th e four basic functions of dielectric oil (specific to sinker EDMs and specially designed wire EDMs) 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 fi eld 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. Th e EDM process involves elevated temperatures. Because the discharge spark has a temperature range of 8,000° C to 12,000° C (14,430° F to 21,630° F) 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.

EDM pulse cycle

The following sequence illustrates the basic chronology of a complete ionization and discharge cycle:

1. Ionization. High-voltage pulse establishes discharge channel in dielectric oil.

2. Discharge. The voltage decreases and current flows through the discharge channel and melts the workpiece at contact point, generating a hydrogen gas bubble.

3. Resting cycle. Low-voltage current is shut off, the gas bubble implodes, the molten material globule is cooled by dielectric, and instantaneous cooling fractures a chip that the dielectric flushes away.

Proper application of dielectric oil is critical to effi cient EDMing of microparts and microfeatures (part sizes smaller than 1mm×1mm and feature sizes smaller than 0.1mm×0.1mm). One key is the kinematic viscosity of the dielectric. In the Makino microfabrication center, we have found that ideal viscosity levels should be at or below 1.8mm²/sec., at an operating temperature of 20° C (68° F). By conducting two tests, we quantifi ed the specifi c advantages of using a specially formulated oil with a relatively low kinematic viscosity, 1.8mm²/sec., compared to a standard dielectric oil with a kinematic viscosity of 3.2mm²/sec.

Case study

Makino prepared the following two tests to quantify dielectric flow characteristics and their impact on machining cycle time. Using a high-pressure flushing device, fluid was forced through a pipe electrode to determine how long it takes to fill a test tube to a certain volume. The high-viscosity oil took longer than the low-viscosity oil to fill the test tube to the same volume, thus confi rming total fl ow rate diff erences. Both tests used identical pipe electrodes and identical registered back pressures on the flushing pump.

Dielectric A droplet formation under high-pressure flushing operation. Photo taken while flushing high-viscosity oil dielectric through a 0.250mm-dia. pipe electrode (0.100mm ID). Even when using a high-pressure flushing pump at 10 Mpa, it is extremely difficult to force high-viscosity dielectric oil through a small-diameter object. High-viscosity oil can be forced through with enough pressure, but at a relatively low volume.

Dielectric B continuous flow under high-pressure flushing operation. Photo taken while flushing a low-viscosity oil dielectric through a 0.250mm-dia. pipe electrode (0.100mm ID). Compared to Dielectric A image, the lower-viscosity oil is forced through the same pipe electrode with the same pressure. Note the dramatic difference in dielectric oil flow rate. Lower-viscosity oil can flow in higher volume through smaller diameters at the same pressure.

Test 1: Dielectric flow.

Objectives:

1. Fill a test tube to a total volume of 10ml using a back pressure of 10 Megapascal (1,450 psi). The restriction device (pipe electrode) was a 0.25mm-dia. copper pipe with an ID of 0.100mm. Th e electrode length was 200mm.

2. Record and compare total fill time for respective oils tested. Dielectric A had a viscosity of 3.2mm²/sec. and a total fill time of 105 seconds. Dielectric B had a viscosity of 1.8mm²/sec. and a total fill time of 55 seconds, so use of lower viscosity dielectric B reduced machining time by 46 percent.

Test 2: Cycle time impact.

Objectives:

1. Burn 20 2.5mm-deep holes into 420 stainless steel using 0.250mm-dia. copper pipe electrode (described previously) and applying 10 Mpa (1,450 psi) flushing pressure. The electrode length was 200mm.

2. Record and compare machining cycle times for respective oils tested. Dielectric A had a viscosity of 3.2mm²/sec. and an average cycle time per hole of 68 seconds. Dielectric B had a viscosity of 1.8mm²/sec. and an average cycle time per hole of 45 seconds, so use of lower-viscosity dielectric B reduced machining time by 34 percent.

As these tests indicate, choosing the right dielectric media can greatly improve microEDM efficiency. This smalldiameter electrode requires a dielectric medium that provides effi cient and safe dielectric properties and low kinematic viscosity. The proper dielectric provides adequate flushing to remove debris and cool the discharge contact point on the electrode.

In other microEDM applications, lower-viscosity dielectric oils have also provided superior results—especially when electrode features are smaller, more fragile and thus more susceptible to hydraulic deflection. The standing graphite pin electrodes are 0.050mm in diameter × 2.0mm in length. As each pin is used in a sinker EDM, it will experience certain fluid dynamic eff ects as it moves either laterally, during orbital movement, or vertically, during pulse-type machining. This electrode must displace a certain volume of oil as it approaches the workpiece and must withstand certain vacuum forces as it moves away from the part. These hydraulic forces are reduced as the oil’s kinematic viscosity decreases, thus applying less outside force and infl uence on the electrode features.

As microEDM technologies and applications increase and as spark gap clearance, electrode feature sizes and flushing hole sizes all decrease, the application of specifically formulated, lower-viscosity dielectric oils becomes more critical.

Author’s Note: Oelheld GmbH, Hirschmann Engineering USA Inc., Single Source Technologies Inc. and E. Bud Guitrau contributed to this article.

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.