EDM Process Design Guidelines

Understanding Electrical Discharge Machining: A Foundation for Modern Manufacturing

Electrical Discharge Machining (EDM) has revolutionised precision manufacturing across industries, from aerospace component fabrication to medical device production. For manufacturers throughout Atlantic Canada and beyond, understanding the fundamental principles and design guidelines for EDM processes is essential for achieving optimal results while minimising costs and production time.

EDM operates on a straightforward yet sophisticated principle: controlled electrical sparks erode material from a workpiece submerged in dielectric fluid. This non-contact machining method allows engineers to cut intricate shapes in hardened materials that would be impossible or impractical to machine using conventional methods. For Nova Scotia's growing advanced manufacturing sector, particularly in industries serving offshore energy, defence, and aerospace applications, EDM capabilities represent a critical competitive advantage.

This comprehensive guide explores the essential design considerations, process parameters, and best practices that engineers and technical managers must understand when specifying EDM operations or designing components intended for EDM fabrication.

Types of EDM Processes and Their Applications

Before establishing design guidelines, it is crucial to understand the three primary EDM variants and their distinct capabilities:

Wire EDM (WEDM)

Wire EDM utilises a thin brass or stratified wire, typically ranging from 0.1 mm to 0.3 mm in diameter, as the cutting electrode. The wire travels continuously through the workpiece, allowing for precise two-dimensional cuts that can be angled up to 45 degrees from vertical. This process excels in producing:

• Precision dies and punches with tolerances of ±0.002 mm

• Complex gear profiles and spline forms

• Extrusion dies for aluminum processing

• Medical implant components requiring burr-free edges

• Aerospace structural components from hardened alloys

Wire EDM machines can achieve surface finishes as fine as Ra 0.1 μm with multiple skim passes, making them ideal for components requiring minimal post-processing.

Sinker EDM (Ram EDM)

Sinker EDM employs a custom-shaped electrode, typically machined from graphite or copper, which "sinks" into the workpiece to create complex three-dimensional cavities. This process is particularly valuable for:

• Injection mould cavities with intricate detail

• Forging dies for automotive components

• Turbine blade cooling passages

• Thread forms in hardened materials

• Blind cavities with sharp internal corners

Sinker EDM can achieve internal corner radii as small as 0.05 mm, depending on electrode size and flushing conditions.

Hole Drilling EDM (Fast Hole EDM)

This specialised variant uses tubular electrodes to drill small, deep holes with aspect ratios exceeding 100:1. Common applications include cooling holes in turbine blades, starter holes for wire EDM operations, and fuel injection nozzle orifices with diameters as small as 0.1 mm.

Critical Design Parameters for EDM Success

Effective EDM process design requires careful consideration of numerous interrelated parameters. Engineers specifying EDM operations must balance material removal rates, surface finish requirements, dimensional accuracy, and economic factors.

Discharge Energy and Pulse Parameters

The fundamental EDM parameters that control material removal and surface quality include:

• Peak Current (Ip): Typically ranges from 0.5 A for fine finishing to 500 A for roughing operations. Higher currents increase material removal rates but produce rougher surfaces and larger heat-affected zones.

• Pulse Duration (Ti): On-time periods range from 0.5 μs to 2,000 μs. Longer pulses remove more material per discharge but increase crater size and surface roughness.

• Pulse Interval (To): Off-time allows for dielectric deionisation and debris flushing. Typical ratios of Ti:To range from 1:1 for finishing to 1:3 for stable roughing.

• Gap Voltage: Open circuit voltages typically range from 40 V to 400 V, with spark gaps maintained between 10 μm and 100 μm depending on conditions.

Material Considerations

EDM can machine any electrically conductive material regardless of hardness, but material properties significantly influence process parameters and outcomes:

Tool Steels: Commonly machined materials like D2, H13, and S7 tool steels respond well to EDM, with typical material removal rates of 200-400 mm³/min during roughing. The heat-affected zone (HAZ) depth typically ranges from 5 μm to 50 μm depending on discharge energy.

Carbides and Ceramics: Tungsten carbide machines approximately 40% slower than steel, while conductive ceramics require specialised parameter sets and often benefit from negative electrode polarity.

Superalloys: Nickel-based superalloys such as Inconel 718, commonly used in Maritime aerospace and energy sector applications, require reduced discharge energies and careful attention to recast layer formation, which can affect fatigue life.

Design for EDM: Geometric Considerations

Components intended for EDM fabrication benefit significantly from design optimisation. Understanding process limitations helps engineers create manufacturable designs that meet functional requirements while minimising production costs.

Internal Corner Radii

EDM cannot produce perfectly sharp internal corners. The minimum achievable radius depends on the electrode or wire size plus the overcut (spark gap). For wire EDM, specify internal corner radii no smaller than half the wire diameter plus 0.015 mm. Using 0.25 mm wire, the minimum practical internal radius is approximately 0.14 mm.

For sinker EDM, internal corner radii are limited by the minimum practical electrode feature size and spark gap, typically yielding minimum radii of 0.1-0.2 mm for graphite electrodes.

Aspect Ratios and Depth Limitations

Flushing efficiency decreases significantly with increasing depth-to-width ratios. Design guidelines include:

• Wire EDM: Maintain workpiece thickness-to-wire diameter ratios below 200:1 for reliable cutting. A 0.25 mm wire can reliably cut through material up to 50 mm thick.

• Sinker EDM: Cavity depth-to-width ratios should remain below 4:1 for conventional flushing. Deeper cavities require through-electrode flushing or orbital motion techniques.

• Hole drilling EDM: Achievable aspect ratios of 100:1 or greater, though accuracy decreases with depth.

Wall Thickness and Fragile Features

Minimum wall thicknesses depend on material and overall component geometry. General guidelines suggest:

• Hardened tool steels: Minimum 0.5 mm wall thickness for robust features

• Carbides: Minimum 1.0 mm due to brittleness concerns

• Unsupported thin sections should maintain thickness-to-height ratios above 1:10

Surface Integrity and Post-Processing Requirements

EDM processes create unique surface characteristics that engineers must understand and account for in their designs, particularly for components subject to fatigue loading or corrosive environments—common considerations for Maritime industrial applications.

Recast Layer Formation

The EDM recast layer, also called the white layer, forms when molten material resolidifies on the workpiece surface. This layer exhibits different properties than the base material:

• Thickness: Ranges from 2 μm for fine finishing to 50 μm for aggressive roughing

• Hardness: Typically 20-50% harder than base material due to rapid quenching

• Composition: May contain elements from the electrode material and cracked dielectric

• Microstructure: Characterised by tensile residual stresses and potential microcracking

For fatigue-critical applications common in aerospace and offshore energy components, recast layer removal through grinding, polishing, or chemical methods is often specified. This requirement should be anticipated in design tolerances and cost estimates.

Heat-Affected Zone Management

Beneath the recast layer, a heat-affected zone exhibits tempered or rehardened characteristics depending on the base material. For hardened tool steels, this zone may show reduced hardness, potentially affecting wear resistance in die applications. Specifying appropriate finishing parameters or post-EDM heat treatment can mitigate these effects.

Surface Finish Specifications

EDM surface finishes are typically specified using Ra (arithmetic average roughness) values:

• Roughing: Ra 6.3-12.5 μm

• Semi-finishing: Ra 1.6-3.2 μm

• Finishing: Ra 0.4-0.8 μm

• Mirror finishing: Ra 0.1-0.2 μm (requires multiple skim passes)

Achieving finer finishes requires progressively reduced discharge energies, resulting in longer machining times. A mirror finish may require 10-15 times longer than a standard finishing cut.

Process Planning and Economic Considerations

Effective EDM process planning balances technical requirements with economic realities. For Nova Scotia manufacturers competing in global markets, optimising EDM operations can significantly impact profitability.

Electrode Design and Manufacturing

For sinker EDM operations, electrode costs often represent 30-50% of total job costs. Effective electrode design strategies include:

• Multiple electrodes: Using roughing, semi-finishing, and finishing electrodes with progressively tighter tolerances optimises material removal while achieving fine finishes

• Electrode material selection: Graphite electrodes cost less and machine faster but wear more quickly than copper electrodes. For production applications exceeding 10 impressions, copper-tungsten electrodes may prove economical despite higher initial costs.

• Undersizing: Electrodes must be undersized by the spark gap plus any desired clearance. Typical undersize values range from 0.1 mm per side for roughing to 0.02 mm for finishing.

Machining Time Estimation

Accurate time estimation enables competitive quoting and production scheduling. General material removal rate guidelines for steel include:

• Wire EDM roughing: 150-250 mm²/min cutting rate

• Wire EDM finishing: 30-80 mm²/min cutting rate

• Sinker EDM roughing: 200-500 mm³/min removal rate

• Sinker EDM finishing: 10-50 mm³/min removal rate

These rates vary significantly based on material, flushing conditions, and accuracy requirements.

Quality Control and Inspection Protocols

Maintaining dimensional accuracy and surface quality in EDM operations requires robust quality control protocols aligned with industry standards commonly referenced in Atlantic Canadian manufacturing contexts.

In-Process Monitoring

Modern EDM systems incorporate adaptive control systems that monitor spark gap voltage, discharge frequency, and short-circuit occurrence. These systems automatically adjust machining parameters to maintain stable conditions and consistent quality.

Post-Process Inspection

Critical EDM components typically require:

• Dimensional inspection: Coordinate measuring machines (CMM) verify critical dimensions against tolerances, typically ±0.005 mm for precision work

• Surface roughness measurement: Stylus profilometers or optical systems verify Ra, Rz, and other specified parameters

• Metallographic examination: Cross-sectional analysis verifies recast layer thickness and HAZ characteristics for critical applications

• Crack detection: Fluorescent penetrant inspection identifies surface cracks in the recast layer

Partner with Experienced Engineering Professionals

Successful EDM process design requires balancing numerous technical parameters while meeting quality requirements and economic constraints. Whether you are developing new products for the aerospace industry, designing tooling for manufacturing operations, or solving challenging machining problems for the offshore energy sector, working with experienced engineering professionals ensures optimal outcomes.

Sangster Engineering Ltd. provides comprehensive engineering services to manufacturers throughout Nova Scotia, Atlantic Canada, and beyond. Our team combines deep technical expertise with practical manufacturing knowledge to help clients optimise their designs for EDM and other advanced manufacturing processes. From initial concept development through production support, we deliver engineering solutions that meet your technical requirements while respecting budget and schedule constraints.

Contact Sangster Engineering Ltd. today to discuss your EDM design requirements and discover how our engineering expertise can support your manufacturing success.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.