Die-Sinking (EDM) Working Principle, Applications, Process Parameters, & Equipment

Die Sinking Machine in Operation | Image Source: S&T Engineers

In this article, you will read about Die Sinking EDM, how works, its applications in manufacturing dies, molds, and parts, the advantages over machining, limitations, electrode materials, and design,  machines, optimizing process parameters, accuracy, surface finish, electrode wear, safety, and more. The article covers all aspects of the die-sinking EDM process including applications, tooling, parameters, troubleshooting, and careers.

Die-sinking, also known as die-sinking EDM or sinker EDM, is a manufacturing process that uses an electrically charged electrode to precisely cut or shape metal workpieces. This article provides a comprehensive overview of die-sinking EDM, explaining what it is, how it works, its applications, advantages and disadvantages, and other key information.

What is Die-Sinking EDM?

Die-sinking EDM, short for die-sinking electrical discharge machining, is a machining process where an electrode tool sinks into the workpiece to erode away metal and form the desired shape. It is an electro-thermal production process where electrical discharges (sparks) are used to selectively remove metal and cut cavities in conductive materials.

The electrode and workpiece are submerged in an insulating dielectric fluid like deionized water or hydrocarbon oil. When voltage is applied, an arc is created between the electrode and workpiece at very small gaps, generating a plasma channel. The high heat from the plasma channel vaporizes and melts the metal, which is then flushed away by the dielectric fluid, causing erosion. By precisely controlling the sparks, material can be removed from the workpiece to mirror the electrode shape.

How Does Die-Sinking EDM Work?

The die-sinking EDM process works by creating controlled electrical discharges between an electrode and the workpiece. Here are the key steps:

• The conductive workpiece is submerged in a tank of dielectric fluid along with the shaped electrode or 'tool' connected to a power supply.
• When the power supply is switched on, it creates a large difference in potential voltage between the tool and workpiece.
• As the electrode approaches the workpiece, dielectric breakdown occurs in the fluid, allowing small sparks to jump across the gap.
• Each electrical discharge vaporizes and melts a minuscule amount of material on the workpiece surface.
• The melted metal instantly solidifies and is washed away by the dielectric fluid flow.
• By repeating these controlled sparks, the electrode progressively sinks into the workpiece, eroding away material to achieve the desired cavity, hole or profile.
• Servo motors precisely control the electrode position and movement to achieve the programmed shape with micron-level accuracy.
• Varying the voltage, pulse duration and interval controls the amount of metal removed.

Die-Sinking EDM Applications

Die-sinking EDM is used to produce dies, molds and components with complex cavities, holes, slots and 3D shapes that would be impossible or very costly to machine by conventional methods.

Typical applications include:

• Making injection molds for plastics and die casting molds for metal casting
• Manufacturing dies for extrusion, forging and stamping processes
• Producing parts for aerospace and aviation components
• Cutting intricate holes and cavities in automotive, surgical and fuel injection parts
• Making prototypes, prototypes, electrodes and components from difficult-to-cut alloys and hardened metals
• Machining components made of conductive ceramics like silicon carbide

Die-Sinking vs Milling/Machining

Unlike conventional machining processes that use cutting tools to mechanically remove material, die-sinking EDM uses electrical sparks so no cutting forces are involved. This makes EDM suitable for machining:

• Very hard materials over 68 HRC like hardened tool steel, carbides, titanium alloys
• Materials that are difficult to conventionally machine like ceramics, composites
• Complex 3D shapes, cavities and holes with sharp corners and edges
• Delicate tiny sections and weak slender shapes prone to tool chatter/breakage
• No tooling contact means no mechanical stresses, vibrations or chatter

Advantages of Die-Sinking EDM

Die-sinking EDM provides several benefits over traditional machining:

• Cuts all electrically conductive metals regardless of hardness - no materials limitation
• Does not affect material hardness, so workpiece retains original strength
• No direct contact so no mechanical forces, allowing delicate shapes
• High dimensional accuracy and surface finishes to mirror electrode
• No burrs, chatter marks or mechanical damage
• Ability to produce complex 3D shapes not possible by other methods
• Faster machining of small cavities with lower tooling costs
• Lower energy use and waste compared to conventional processes

Limitations of Die-Sinking EDM

Die-sinking EDM does have some disadvantages:

• Relatively slower for removing large volumes of material
• Restricted to electrically conductive materials
• Electrode must be erosion-resistant with a conductive core
• Special power supply and dielectric system is required
• Additional time required for electrode design and fabrication
• Dielectric fluids require filtering, maintenance and disposal
• Not ideal for visual quality surfaces

Die-Sinking EDM Electrode Materials

The electrode shape in die-sinking EDM is eroded during the process so it must made from materials capable of withstanding the spark erosion. Common materials include:

• Copper: Most commonly used electrode material due to high erosion resistance and ease of machining
• Graphite: Relatively low cost but higher wear rate than copper
• Copper-tungsten: Contains around 10-25% tungsten to improve wear resistance
• Brass: Used for fine detailing and surface finishing of eroded electrodes
• Zinc alloys: Cost effective alternative with medium erosion resistance
• Silver tungsten alloy: Better wear resistance than copper but more difficult to machine

Die-Sinking EDM Dielectric Fluids

Die-sinking EDM uses dielectric fluids to conduct current between the electrode and workpiece. These specialized EDM fluids have higher insulating strength to better control the spark discharge. Common types include:

• Hydrocarbon oil: Petroleum-based oil that readily flushes away debris, most commonly used
• Deionized water: Has excellent insulating properties and is a lower cost option
• Synthetic oil: Made from silicone or ester-based oils for specific applications
• Kerosene: Low viscosity option good for flushing fine particles

Die-Sinking EDM Process Parameters

By precisely controlling various process parameters, different EDM outcomes can be achieved:

• Voltage (V): Higher voltage increases material removal rate but decreases surface finish
• Current (A): Current level affects spark intensity and material removal
• Pulse on-time (μs): Duration of each electrical spark discharge
• Pulse off-time (μs): Interval between sparks allows flushing of debris
• Gap voltage: Voltage between electrode and workpiece to maintain arc
• Flushing pressure: Force of dielectric flow to remove melted material
• Electrode feed rate: Speed electrode advances into workpiece
• Polarity: Straight polarity (electrode negative) improves surface finish

How to Achieve High Accuracy in Die-Sinking EDM

Micron-level accuracy is possible with die-sinking EDM but requires:

• Precisely machined electrodes that match CAD models
• Rigid and thermally stable EDM machine construction
• Low wear electrode materials like copper-tungsten alloy
• Close control of spark parameters (voltage, current, timing)
• Accurate servo/CNC positioning of electrode
• Vibration damping on EDM machine structure
• Sufficient flushing pressure to remove debris from gap
• Holding temperature constant by cooling dielectric fluid
• Allowing sufficient sparking time for finer surface finishes
• Final polishing operation to clean surface asperities

Die-Sinking EDM Electrode Design

As the electrode shape is mirrored into the workpiece, careful design is required considering:

• Uniform wall thickness and minimal cross-sections to avoid early breakage
• Rounded corners and tapered walls to prevent electrode jamming
• Accessibility of fluid flow to avoid uneven erosion
• Electrode extend beyond final shape to allow grinding/polishing
• Providing sufficient support ribs and bridges to strengthen electrode
• Slightly oversizing electrode dimensions (~0.1-0.2mm) to offset erosion
• Dividing complex shapes into multiple electrodes for roughing/finishing

Die-Sinking EDM Electrode Fabrication

EDM electrodes can be machined by several methods:

• CNC Milling: Most common approach for machining electrodes from blocks of graphite or metal. Allows 3D shaping.
• Wire EDM: Used for directly cutting electrodes from metal plates to 2D profiles. No tooling required.
• Ram/Sinker EDM: EDM can also erode electrodes by spark machining other electrodes.
• Additive Manufacturing: 3D printing electrode shapes using binder jetting or fused deposition modeling.
• Plated Copper: Thin copper layer electrodeposited onto a master pattern then separated.

Die-Sinking EDM Machines

There are several designs of EDM sinking machines:

• CNC Die-sinking EDM: Modern CNC-controlled servo motors position the electrode for complex 3D shapes. Most versatile and accurate.
• RAM EDM: Electrode is manually positioned with slide mechanisms. Limited to 2D profiles and sinking capacity. Oldest EDM technology.
• Small Hole EDM: Specialized machines to drill small, precision holes. Used in aerospace industries.
• Micro EDM: For microscopic parts below 5mm. Requires vibration isolation, microfluidics.

Factors That Influence EDM Machining Speed

The material removal rate and cutting speed in die-sinking EDM depends on:

• Voltage and amperage settings - higher values increase removal rate
• Electrode and workpiece materials - affects melting temperature
• Pulse timing parameters - longer on-times increase sparks
• Gap size - smaller gaps = higher spark intensity
• Flushing pressure/method - clears debris for continuous erosion
• Electrode feed rate - faster sinking increases MRR
• Open circuits vs. RC-type power - open circuit has higher MRR
• Dielectric fluid - affects insulation strength and flushing

Die-Sinking EDM Process Optimization

Optimizing the EDM process involves finding the best settings for:

• Fastest material removal rate without excess electrode wear
• Best surface finish based on final application
• Maximum dimensional accuracy and fine profile details
• Minimizing electrode shaping/dressing during operation
• Achieving required workpiece surface integrity
• Preventing unwanted side-effects like recast layers and micro-cracks

This requires adjusting parameters like voltage, current, pulse timing, flushing, polarity and electrode type. The optimal settings can vary widely for different electrode/workpiece materials.

Die-Sinking EDM Surface Finish

Several factors affect the surface finish, including:

• Voltage level - higher voltage reduces surface finish quality
• Current setting - higher amperage worsens finish
• Gap size - smaller gaps improve finish
• Pulse duration - shorter pulses enhance finish
• Flushing method - pressure flushing gives cleaner surface
• Electrode material and wear - high wear causes rough finish
• Straight polarity vs reverse polarity - straight polarity improves finish

Typical surface finishes range from 1-10 μm Ra, with finer finishes achievable by post EDM processes like grinding, honing and polishing.

Die-Sinking EDM Electrode Wear

During the EDM process, the electrode erodes due to the electrical discharges. Copper and graphite electrodes experience wear rates around 10-30% of the depth eroded into the workpiece.

Key factors causing electrode wear include:

• Thermal shock from rapid heating/cooling during sparking
• Vaporization and melt ejection of electrode material from high heat
• Flow of high amperage currents through the electrode leading to internal heating
• Chemical attack of the dielectric fluid on electrode
• Accumulation of debris increasing local erosion

Wear reduces electrode dimensions sothey require periodic restoration by grinding or EDM dressing.

Recast Layers in Die-Sinking EDM

Recast layers are common issues in die-sinking EDM, where the resolidified melted material forms unwanted deposits on the surface. Recasts:

• Give poor surface finish and reduced accuracy
• Can contain micro-cracks reducing strength
• Increase risk of uneven or excessive electrode wear
• Reduce dimensional precision

Recast formation can be minimized by:

• Using lower discharge energies and shorter pulses
• High pressure flushing to remove debris
• Polarity switching between cycles
• Electrode/workpiece materials with high melt/boil points

Die-Sinking EDM Process Monitoring

Monitoring the EDM process allows for control over:

• Electrode wear rate: To schedule timely electrode restoration/dressing
• Material removal rate: For optimizing production efficiency
• Surface roughness/recast layer: To determine if in-process improvements are needed
• Sparking gap: To adjust settings if unstable or large gap occurs
• Discharge ratio: Percentage of effective sparks for performance
• Short-circuiting: Detects any unsafe direct contact between electrode and work

Die-Sinking EDM Tooling Costs

Tooling costs for die-sinking EDM include:

• Electrode blank material: Graphite or copper rod/block stock
• Electrode machining: Milling/turning for shaping electrode or wire-cut EDM
• EDM machine electrode guides: Fixtures to hold electrode
• Electrode dressing tools: For restoring worn electrodes
• Recast removal tools: Grinding wheels, files, etc. for finishing
• Workholding fixtures: Vices, clamps, bases to secure workpiece

Overall tooling cost for a die-sinking EDM project can range from 20-60% of total operational costs depending on part complexity.

Die-Sinking EDM Safety Hazards

Safety considerations for die-sinking EDM:

• Electric shock risk from open high voltage circuits - insulated machine covers are mandatory
• Toxic smoke/fumes from vaporized dielectric fluid and molten metal particles - exhaust ventilation required
• Fire hazard due to use of flammable hydrocarbon oils - proper fluid maintenance and containment needed
• Noise hazards from high frequency sparking - sound isolation or suppressors should be used
• Slips/falls due to spilled dielectric fluid - maintain clean, dry workspace
• Electromagnetic interference can affect pacemakers - warning signs needed

Career Opportunities in Die-Sinking EDM

Die-sinking EDM operators need training and experience in:

• Setup, programming and process optimization
• Electrode design principles
• CNC programming/machining for electrode fabrication
• EDM power supply, generator and servo technologies
• Metallurgy and melt characteristics of exotic alloys
• Measuring and inspection

With advanced EDM skills, technicians can progress into roles like lead operator, EDM programmer, applications specialist and supervisor.

Common EDM Terms

• Anode - The positively charged electrode in the EDM circuit that is eroded. Typically the electrode tool.
• Cathode - The negatively charged electrode that is machined by EDM. Normally the workpiece.
• Discharge channel - The plasma channel created by spark between electrode and workpiece.
• Dielectric - The fluid used to insulate electrode from workpiece and flush away debris.
• Erosion - The melting and displacement of material by EDM sparking.
• Gap voltage - The voltage potential between the electrode and workpiece.
• Kerf - The actual cut width made during the EDM process.
• Overburn - Excessive erosion caused by uncontrolled discharge energy.
• Recast layer - Redeposited melted material layer on EDM machined surfaces.
• Residual stresses - Stresses remaining after EDM cutting from rapid melting/solidification.
• Spark gap - The distance between the electrode and workpiece maintained for sparking.
• Wear ratio - The ratio of electrode wear to workpiece erosion. Normally ranges from 1:8 up to 1:30.

Die-Sinking EDM FAQs

Here are answers to some frequently asked questions about die-sinking EDM:

What materials can be cut with die-sinking EDM?

All electrically conductive metals and alloys can be machined, including steel, titanium, nickel, aluminum, and metal carbides. Non-conductive materials cannot be directly EDM machined.

How accurate is die-sinking EDM?

Modern CNC EDM machines can hold tolerances of at least ±0.005 mm and achieve surface finishes of 1-2 microns Ra. Accuracy depends on electrode precision, machine rigidity and process parameters.

Is surface finish good on die-sinking EDM parts?

EDM produces matte gray surfaces with some roughness from spark craters. Finish can be 1-10+ μm Ra based on parameters. Further polishing is often required for cosmetic surfaces.

What thickness can be cut in one sink with EDM?

Standard sinkers can erode 100-300 mm deep depending on workpiece hardness. Very thick sections may need multiple electrodes and restsinking steps.

Does EDM affect material hardness?

No, there is negligible change in the bulk hardness after EDM. Only a very thin recast layer on the surface is affected.

Can EDM cut small holes?

Yes, wire EDM can cut holes down to 0.1 mm diameter. Small hole EDM machines specialize in drilling holes from 0.05 mm diameter and above.

What does EDM stand for?

EDM means Electrical Discharge Machining. It refers to the sparks used to erode material.

Is EDM more expensive than milling or turning?

Due to the specialized equipment and electrode fabrication needed, EDM is generally more expensive than conventional machining for producing parts. But it is cost-competitive and sometimes cheaper when doing complex 3D shapes in hard materials.