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 important both on the removal rate and on the obtainable quality.

The erosion 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 in the material; even the shape of the tool will be changed, so it must be designed properly to obtain the wanted shape.

Variants

• Fluid conveyed by holes inside the tool, conveyed through the workpiece
• Vertical fed and rotary movement combined, grinding
• Plane milling, sheaving, continuous 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 arc that moves like a cylinder.

2) PLASMA FORMATION

When the breakdown voltage is reached, a plasma channel is formed.

Plasma is made by charges that moves (ions & electrons), electrons go up and ions go down.

At the very beginning, the most important contribution is given by the electrons (higher mobility and lower mass) which start to hit the anode, thus wearing it.

While tau increases, also the positive ions start flowing getting accelerated towards the cathode producing a greater erosion.The cathode can reach even the 99% of total erosion.

At different times, we have different material removal rates.

3) DISCHARGE

The negative and positive ions migrate respectively towards the anode and the cathode.The current rises and the tension is reduced.A vapor channel forms around the plasma beam and the melting process starts.

These are the phases with the largest material removal rate. The channel expands, the current and the tension stabilize. Temperature increases (4000 - 10000 °C) and also the vapor pressure (up to 20 MPa).

The electric spark and the heat reach their maximum intensity.The vapor bubble expands.Finally the electric circuit is opened: the current will stop.

TOOLS

Main properties that a tool must have:

• Low wear (not too much erosion wear)
• High thermal conductivity
• High melting point
• Good electric conductivity
• Good machinability
• Low cost

We want MRR_tool ≪ MRR_piece, so let’s apply Weller’s formula (actually it can be used because it applies only on the workpiece, cathode)

[T_m^1.25]_tool ≫ [T_m^1.25]_piece

The tool will dissipate heat before reaching T_m, no high power required to reach it

Correcting

[K_ρC_pT_m²]_tool ≫ [K_ρC_pT_m²]_piece

Themoconductivity: K, density: ρ, specific heat at constant pressure: C_p

ρC_pT_m = heat to melt a specific volume

K_Tm = proportionality to the power quantity dissipated by conduction

The materials having

[K_ρC_pT_m²]_tool > [K_ρC_pT_m²]_piece are:

• Steel (not suitable as a tool)
• Graphite (easy to be machined but dusty when unworked)
• Copper (good resistance to erosion, easy to be machined)
  • Very suitable to machine TiC, most used (cheapest)
• Copper-Tungsten/Silver Alloys (sintered, expensive)

TOOL WEAR

• End wear: not a problem, adjusted with feed rate/gap control
• Side wear: can be compensated only if uniform, you have to take it into account
• Corner wear: it is the most important since it determines the accuracy of the final cut
  • If an electrode can successfully resist erosion at its most vulnerable points, the overall wear will be minimized.

Electromagnetic fields tend to concentrate at the electrode corners thus inducing faster wear. The sharper the angle, the more sparks are generated in this area and the more heat build up. Blunt corners will wear less than sharp angle ones. Corner wear can be minimized by choosing a smooth radii size electrode material (fine-grains graphite) that has high strength and high density.

ECM

ECM is based on electrolysis, so the anodic dissolution of the workpiece (anode, +) while the tool is the cathode (-).

DC or pulsed voltage is applied, current flows between the tool and the workpiece inside an electrolyte.

The electrolyte is flushed through the machining area. Dissolved material generates sludge.

The obtained shape is almost the negative of the used tool.

The difference with EDM is that now the entire gap is continuously under erosion. The smaller the distance, the faster will be the corrosion rate.

Here, if we want vertical walls, we have problems of uniformity: we have to involve vertical walls of the tool but still there will be some corrosion.

The shape is unpredictable but of a very good quality in terms of roughness.

This process is extremely repeatable but it's difficult to predict the first shape.

The electrolytic liquid must be injected where needed. The problem is that the MRR depends on the local speed of the flow: if you have a gradient in speed, it will influence the MRR but this is difficult to be controlled.

The resulting profile is a wave. I can try to compensate it with another wave at the second step, but this is a too difficult problem and no one wants to face it.

DEBURRING

If I want to do deburring, I don't have that problem. I want to remove burrs. They are usually small.

PROCESS CHAIN

• CAD 3D
  • CAD SYSTEM
• STL FILE
  • ORIENTATION
    • SUPPORTS
    • THICKNESS
    • TRAJECTORY
  • AM MACHINE SOFTWARE
• LAYER BY LAYER MANUFACTURING
  • AM MACHINE
• REMOVAL AND POST-PROCESSING
  • MANUAL MANIPULATION
• PART

The STL can be given directly to the machine or to a slicer for cheaper machines.

The slicer takes many decisions:

• Orientation: Sometimes it's obvious, but you need to choose the best one.
• Supports: we have to understand if needed unsupported longings, you would have to start from zero. Sometimes it's possible, other you need pillars.
• Thickness of each layer: the larger it is, the faster is the process but the lower is the surface quality.
• Trajectory: outer contour, way of filling (zig zag, parallel, etc.)

WHY SO RELEVANT

The current and potential success of AM relies in 3 key factors:

• It is a completely digital technology, easy to share between manufacturers (unlike conventional forming and casting)
• It requires no dedicated tools (easy building up time but no specific tool needed)
• It allows to produce very difficult geometries (undercuts, complex shapes...)

It has a tremendous flexibility and can perform optimized topology; possibility to increase mechanical properties putting material only where needed, maximize the stiffness minimizing the weight → software of optimization.

AM represents the step from mass production (rigid automation) to mass customization (digital, flexible automation)

Mass customized, low volume, high mix

Tremendous flexibility + relatively high productivity

ex: dental plants

Sum of all Manufacturing types

Mass customization

Mon-produced high volume low mix

Mass production options

Mass production

Craft

1900 1910 1980 2000