analysis and performance of slotted tools in electrical discharge drilling

Analysis and Performance of Slotted Tools in Electrical Discharge Drilling

Ramy Nastasi, Philip KoshyMcMaster University

Canada

NANTES

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64th CIRP General AssemblyNantes, August 25, 2014

Analysis and Performance of Slotted Tools in Electrical Discharge DrillingR. Nastasi, P. Koshy

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omThe objective of this research was to enhance productivity in electrical discharge drilling (EDD)

The research involved aspects of gap flushing, and computational fluid dynamic analyses of dielectric flow

horizontal EDM3/20



Gap flushing is of critical importance in EDM processes in terms of removal rate, geometric accuracy and surface integrity

through-tool flow

jet flushing jump EDM

orbital EDMReuleaux kinematics

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Computational fluid dynamics software are being widely used for modelling fluid flow in EDM

Wang & Han (2014)Simulation of debris transport

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Cetin et al. (2004)Flow fields in jump EDM

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Okada et al. (2009)Flow analysis in wire EDM

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In EDD, at low rotational speeds, fluid in the frontal gap flows along concentric flow patterns, which redistributes the debris without ejecting it

frontal gap

With increasing speed, the emergence of centrifugal forces drives a radial fluid outflow, but the close proximity of the lateral workpiece wall tends to recirculate the fluid

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This work examined the efficacy of introducing geometric features into cylindrical rotating tools to enhance flushing in the drilling of blind macro holes

In particular, the emphasis was on the design, analysis and testing of slotted tools

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Computational fluid dynamics was used as a tool to generate corresponding flow fields

This enabled insights into experimental observations, and the assessment as well as optimization of the geometric features

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Meshing of the fluid domain proved to be challenging due to the gap being small relative to tool diameter and machining depth

Ironically this is much the same issue that renders flushing difficult

Frontal and lateral gaps of 200 µm were assumed

Typical computational cycle times were < 20 minutes on a desktop computer

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φ15.8 mm

Over the entire speed range tested, the slotted tool outperformed the cylindrical and helical tools

At small depths flushing is not quite an issue

single start | 22° helix angle

machining depth < 1mm 4 slots | 2 x 1.5 mm2

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60 rpm tool speed

For the cylindrical tool, at an aspect ratio of ~3, the removal rate is just 30% of that at the surface

The removal rate of the slotted tool is 200% of that of the cylindrical tool

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Computed flow fields highlight the distinct difference between the flow patterns of the helical and slotted tools

ingress and egress through distinct channels within slot

slotted

ingress through lateral gap;egress through helical channel

helical

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Particle tracking was activated in the numerical model to characterize flushing effectiveness

Flushing was quantified in terms of the steady state flux of particles ejected through the annular gap at the workpiece surface

Particles were dispersed in the frontal gap, and their flow through and out of the lateral gap was tracked

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Good linear correlation validates the numerical model

Open data points that deviate the most from the linear trend correspond to combinations of extreme machining depths and tool speeds

computed tool performance relative to arbitrary datum

removal rates were experimentally measured over machining depths of 1-50 mm and tool speeds of 60-240 rpm (cylindrical electrodes)

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Slot height wields more influence than the slot width

This is important in order to not compromise machining power

Bands of fluid ingress & egress are aligned along the slot height

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Tool with a single slot is comparable to the helical tool

More slots enhance tool performance

For identical slot area & width, a tool with a single deep slot performs better than the one with several detached slots

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The removal rate of the tool with a single extended slot was verified experimentally to be 300% of that of a cylindrical one

300%

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Computed pressure distribution indicates the single extended slot to correspond to a higher pressure gradient

This combined with the larger radial sweep all but eliminates recirculation near the tool center

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Conclusions

Helical and radial slot features in cylindrical tools enhance flushing and removal rates in electrical discharge drilling

Numerical flow modeling was developed to gain qualitative insights into flushing performance, and as a quantitative tool for the design, assessment and optimization of tool geometric features

A tool with a single slot approaching the tool center related to the most effective flushing, and yielded a 300% increase in removal rate relative to that of a conventional rotating cylindrical tool, at a hole aspect ratio of just 3

An enhancement of such magnitude was attained at just a 6% decrease in tool section area, which does not compromise the machining power that scales with the tool frontal area

Thank you for your kind attention!

Natural Sciences & Engineering Research Council of Canada

Canadian Network forResearch & Innovation in

Machining Technology

analysis and performance of slotted tools in electrical discharge drilling

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