EDM, ECM, Additive manufacturing

EDM

Electric Discharge Machine

Thermal removal process by melting and vaporizing the material by an electric discharge.

It is mainly a thermal process (an electrically conductive material is eroded by a sequence of electrical discharges).

Some features, such as the structure or the hardness or tensile strength of the material processed are irrelevant both on the removal rate and on the obtainable quality.

The removal of the material does not require direct contact between tool and workpiece.

Fundamental Properties

1. Possibility of working metals or hard alloys difficult to machine with conventional methods (tool steels, very hard or tungsten carbides, expensive to be machined with traditional machining).
2. Ability to copy any geometric shape in two or three dimensions.

Used by builders of dies, tools, moulds, punches, stamping.

Confined to small tools (the table of the machine is not very large).

Die Sinking (or Plunge) EDM

Shaping process in which a series of electrical discharges are triggered between the tool electrode and the workpiece (conductive material) causing erosion.

The negative form of the tool is copied onto the workpiece.

The special die penetrates slowly into the workpiece: even the shape of the tool will be changed, so it must be designed properly to obtain the wanted shape.

EDM

Electric Discharge Machine

Thermal removal process by melting and vaporizing the material by an electric discharge.

It is mainly a thermal process (an electrically conductive material is eroded by a sequence of electrical discharges).

Some features, such as the structure or the hardness or tensile strength of the material processed are irrelevant both on the removal rate and on the obtainable quality.

The removal of the material does not require direct contact between tool and workpiece.

Fundamental Properties

1. Possibility of working metals or hard alloys difficult to machine with conventional methods (tool steel, very hard or tungsten carbides, expensive to be machined with traditional machining).

2. Ability to copy any geometric shape in two or three dimensions.

Used by builders of dies, tools, moulds, punches, stamping.

Confined to small tools (the table of the machine is not very large).

Die Sinking (or Plunge) EDM

Shaping process in which a series of electrical discharges are triggered between the tool electrode and the workpiece (conductive material) causing erosion.

The negative form of the tool is copied onto the workpiece.

The special die penetrates slowly into the material: even the shape of the tool will be changed, so it must be design properly to obtain the wanted shape.

The tool slowly sinks in the material, thanks to the vertical movement.

It also oscillates, to help the recirculation of fluid and debury.

It works creating a spark of short duration (μs) at a very large frequency.

One spark happens in a specific location, where there is the minimum electric resistance. The spark is triggered at the minimum distance tool-workpiece, while there is the maximum local conductivity of the fluid.

Only one spark at time.

At the beginning it happens at the corners (CORNER EFFECT), when they are rounded the location at minimum distance will be specific.

If the fluid is full of deburies, a spark will happen with them.

In order to increase the efficiency of discharges, these are released within a dielectric fluid, not in the air. The dielectric promotes the electric arc creation and decreases its section.

VARIANTS

Some variants of this process are:

• Fluid conveyed by holes inside the tool, conveyed through the workpiece
• Vertical feed and rotary movement combined, grinding
• Plane milling, sheaving, contoured milling, circular and band sawing
• Wire EDM (filament of copper as tool, cheap and convenient)

The tool must be first machined (usually graphite is used) or you can use a generic one that moves i_nside (like a cylinder).

System

The machine structure is very rigid (stiffness) to avoid vibrations and deformations so allowing to obtain tight tolerances.

MRR (100A): 0 ÷ 4,5 cm³/min

Dimensional tolerances: ± 0,02 ÷ 0,12 mm

Surface roughness: 0,2 ÷ 12 μm

• Fine machining
• Rough machining

Flexible tolerances changing the process parameters.

Lazarenko's Relaxation Circuit (1943)

It is the finest relaxation circuit invented by Lazarenko ➔ RC circuit

ΔV of the capacitor is the same ΔV at the gap.

When the break down voltage is reached in the gap a discharge takes place (not in the capacitor, it's desired not to make that happen).

When the dielectric is broken, the capacitor is discharged at the gap in micros