HYDRODYNAMICS OF THE ELECTRICAL DISCHARGE MACHINING

HYDRODYNAMICS OF THE ELECTRICAL DISCHARGE MACHINING PROCESS (EDM) Patrick Haas, Prof. HES hepia-cmefe

2 http://www.cmefe.ch

HEPIA-CMEFE GROUP


Dominique Aegerter (Moto2)

Avintia (MotoGP)

Drone de mesure de la pollution

HEPIA-CMEFE GROUP


AUDIT OF F1 TEAMS !  2010-13 hepia-cmefe is an official partner of the Formula

One Teams Association (FOTA) !  Roberto Putzu and Patrick Haas act as auditors regarding

the implementation of the « Aerodynamic testing and CFD simulation Regulation » Formula One Teams

Association

Force India

HEPIA-CMEFE GROUP


F1 WIND TUNNEL CHARACTERISTICS

!  Regulation adopted for cost restrictions ! !  60% scale models max. !  50 m/s maximal air speed !  Rolling road (belt) !  Boundary layer succion !  Test for several angles of pitch, yaw and roll !  More than 500 complete runs (offsets and all angles) per

month, in addition to CFD work

Aerodynamic Testing Restrictions (FOTA)

HEPIA-CMEFE GROUP


CFD CAPABILITIES OF F1 TEAMS

!  The calculation power of CFD jobs regarding the car external aerodynamics is limited

!  Up to 1’500 CFD jobs per month (50 per day mean!)

!  Up to 18 h for a full car job on the cluster

Aerodynamic CFD Restrictions (FOTA)

Sauber F1

HEPIA-CMEFE GROUP


CFD CAPABILITIES OF F1 TEAMS Computer and tools

!  Commercial and home made codes (Fluent, Star CCM+, ecc.)

!  CFD finite volume approach !  Highly parallelized cluster (HPC) of 64 bits

processors !  Up to 4’000 cores !  Up to 15’000 Gb RAM !  Infiniband DDR 48 Gbit/s interconnects or faster !  Approx. 100 kW electrical power supply !  Approx. 150 kW heat at the condenser tower

Sauber F1

HEPIA-CMEFE GROUP


HYDRODYNAMICS OF THE ELECTRICAL DISCHARGE MACHINING PROCESS (EDM)

« Use of electrical discharges to remove material »

The processes available today are the following :

!  EDM die-sinking (ex. credit card injection moulds)

!  Wire-EDM (ex. extrusion moulds)

!  EDM hole drilling (ex. engine injectors diam. < 0.100 mm)

!  EDM milling (ex. complex parts, small, hard material). New method developed by Agie Charmiles with hepia (Prof. J. Richard)


SOME BACKGROUND : THE EDM PROCESS

1.  Application of a potential between the electrode and the material

2.  A ionized channel is created

3.  A plasma is formed. The spark generator decrease the potential to stabilize the current.

4.  After an imposed time, the current is stopped. A plasma implosion is created (low pressure in the plasma zone). Dielectric cooling effect.

5.  Liquefied material is removed from the surface.

3 4 5

When the plasma disappears, liquefied material is ejected in the dielectric. Solid particles are formed.


THE EDM PROCESS

Views of craters


THE PARTICLE SHAPES AND DIMENSIONS

IA = 32 A, TA = TI = 25 s, low injection 2[ ]

µ

µ

Particle diameter between 1 to 20 micrometers [2]


WHY HYDRODYNAMICS IS A KEY FACTOR ?

!  The spark starts at the location where the dielectric resistance is the smallest (gap, fluid conductivity, ecc.)

!  The EDM process can work only if the next spark moves

to the smallest gap location (another location !) !  There is a necessity to find a way to regenerate the

dielectric and to clean the gap, removing the particles. Although, the spark stays all the time at the same location and the process can not work.

The production speed is partially governed by hydrodynamics. Project motivation (CFD, test rig, on site production tests)


THE DIE SINKING SCENARIO

The production scenario is : 1.  Spark cycles are generated for a while 2.  The spark generator detect the presence of

particles (analysis of the current and potential curves) and control the CNC

3.  When the quantity of particles is too high, the cleaning process starts : « the electrode is moved up and down to generate a flow through the lateral gap and in the cavity »



The challenge : « What is the most efficient movement ?»

!  Amplitude ? !  Speed ? !  Sequence of

small and big jumps ?


NON-DIMENTIONAL NUMBERS FOR SIMILTUDES

–  Reynolds number (inertia vs viscosity effects): –  Weber number (inertia vs surface tension effects): –  Bond number (gravity vs surface tension effects): –  Froude number (inertia vs gravity effects): –  Morton number (inertia vs interfacial energy):

Physics: - Viscous flow (injection or parts in motion) - Transport of particles - Gas bubble production

- Bubble movement with possible aggregation


GAS BUBBLES EFFECT

Forces acting on a bubble:

•  Gas weigth

•  Surface tension

•  Drag force

•  Lift force

•  Inertia (or added mass)

•  Archimede

•  Momentum rate of change

Bubble flow modes

Perreira Dias (4)



1) Experimental investigations

!  On the EDM machine : scale 1:1

!  On a test rig : scale 20:1and 50:1 in similitude

50:1 scale test rig



2) CFD investigations (Fluent) !  Transient model !  2D and 3D studies !  RANS k-w turbulence model

Some CFD models


1. At the beginning of the movement, two symetrical vortices are generated

THE DIE SINKING FLOW ANALYSIS



2. The electrode going up, the aspect ratio of the two vortices increases



3. The vortices break into several ones aligned vertically (more stable configuration). The solid particles are mixed.



4. When the electrode is near 5 to 10 times the width of the cavity, the intensities of the vortices decrease and the particles start to fall. It is not interesting to go higher with the electrode.



5. When the electrode goes down, some particles stay under the horizontal part, the others are ejected through the vertical gap.






THE WIRE EDM PROCESS !  A dielectric injection is applied continously by 1 or 2 nozzles !  Idea : We can improve the production by working on the nozzle design !  We used 3 approaches: theoretical, experimental and CFD

Material

Upper nozzle and wire guide

Dielectric injection Wire in movement (mecanichal tension and electrical potential)

Lower nozzle and wire guide

Dielectric


THE DESIGN OF WIRE EDM NOZZLE 1) Experimental investigations

Pressure measurements in the gap


-800

-700

-600

-500

-400

-300

-200

-100

0

100

0 5 10 15 20 25 30 35 40 45

P-Pa

tm (m

bar)

Depth (mm)

Measured pressure in fontcion of depth (z = 0.25 mm) 50 mm high part

a

b

c

d

e

f

g

THE DESIGN OF WIRE EDM NOZZLE


THE DESIGN OF WIRE EDM NOZZLE

Annular vortices are generated


THE DESIGN OF WIRE EDM NOZZLE 2) CFD investigations and design (Fluent)

Map mesh (hexahedra) Tgrid patch conform mesh (tetrahedra)


THE WIRE EDM SCENARIO

!  Free jet in the dielectric (without part)

Jet speed in m/s


•  Jet during production (with part) The maximal speed can reach 30 m/s The flow is unstable at the slot entry

Jet speed in m/s

CFD INVESTIGATIONS

Flow understanding leads to nozzle design optimisation


!  Some nozzle prototypes have been produced using rapid prototyping techniques

3D systems machine

PRODUCTION OF PROTOTYPES

Protoype nozzle in resin


!  Very high speed jets give flow induced vibrations acting on the wire. The best nozzle found is one still having a part of pressure energy at the end. Difficulties to design a standard nozzle !

!  The distance between the nozzle and the part remains a production parameter !

!  The production speed obtained practically on a WEDM machine is then dependent of the part geometry (not only the part thickness)

FINAL RESULTS

!  Production speed measurements have shown an increase until 20% if an efficient nozzle design is used

!  New ways for the gap cleaning are under

investigations. They include a nozzle design / wire movement optimisation.


Thanks, questions ?

Patrick Haas, Prof. HES hepia – cmefe Groupe de compétences en mécanique des fluides et procédés énergétiques

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HYDRODYNAMICS OF THE ELECTRICAL DISCHARGE MACHINING