Problems of the Processing Accuracy for Electro-erosion erosion and Electrochemical Machining Processes

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12th ESAFORM Conference on material forming Twente,, Nederland, 27 29 April 2009 MS13: Non-conventional processes Problems of the Processing

1 12th ESAFORM Conference on material forming Twente,, Nederland, April 2009 MS13: Non-conventional processes Problems of the Processing Accuracy for Electro-erosion erosion and Electrochemical Processes by Institute for Fundamental Electrical Engineering and Electromagnetic Compatibility

2 Outlines 1. Motivation 2. Kind of short circuits 3. processes 4. 1/39

3 Fear of the short circuit through the machining-person Accuracy Gap width influence Accuracy of the clamping system Accuracy of the guidance system Accuracy of the machining process Defined local effects Wear effects Spreading effects Contamination effects Short circuit effects 2/39

4 Fear of the short circuit through the machining-person & Small gaps are one requirement for high processing accuracy & Critical particle size (removal rate) is boundary of process stability & Short circuit is the extremely case of process instability & Short circuit is a necessary phase of the remove process & How can you analysis the short circuit condition? & How can you analysis the condition before the short circuit? & Micromachining had a desire for small work gaps. 0.1 mm from 1,000 mm 0.01% Macro machining 100 times better as 1 µm from 100 µm 1.00% Micro machining 3/39

5 EDCM ECoDCM / EKB EDM ECM/PECM µpecm / ECF Short circuit probability Uncritical range Critical range Supercritical range Inter-electrode gap width 1 mm 1 µm 1 nm Development area for Micro- and Nano- 4/39

6 Contamination as the origin of short circuits (EDM, single pulse) Pulse current: 5 A Pulse duration: 5 µs Gap width: 15 µm Tool diameter: 0,1 mm Pulse current: 25 A Pulse duration: 10 µs Gap width: 15 µm Tool diameter: 0,1 mm Pulse current: 45 A Pulse duration: 25 µs Gap width: 15 µm Tool diameter: 0,1 mm Burning phase Pulse end phase Work fluid: n-dodecane Lit.: [1],[2] [1] Schulze, H.-P., Wollenberg, G., Mecke, K., Läuter, M.: The development of different gas bubble morphology in small working gaps at spark erosion. CAPE 2005 Melbourne, Australia, (CD) [2] Schulze, H.-P., Wollenberg, G., Matzen, S., Mecke, K.: Origins of gas Bubbles in a small work gap during the Micro-EDM. 15th ISEM 2007 Pittsburgh, Pennsylvania, USA, pp /39

7 Typical kinds of the Type G (gas) Bubbles Bridging Type S (solid) Particles Tool Flow Propagation Workpiece Gap < 1 µm Soft Short Circuit Hard Short Circuit 6/39

8 Type S (solid) Particles Flow Direct contact (more points of the surface) Feed rate A Gap < 1 µm Workpiece Hard Short Circuit A A* A and A* electrical conductivity Joule heating with short circuit current (additional gas bubbles); Destruction of short circuit elements and/or damages of electrodes surface; Thermal breakdown (spark or arc erosion). A and A* insulating material Local deformation of electrical field strength; Fixing of the constant gap width; Mechanical destruction in area of short circuit elements; Discharge channel splitting. A (conductive) and A*(dielectric) Local gap reduction (structure damages). 7/39

9 Type S (solid) Particles Flow L Particle layer Soft Short Circuit B Workpiece Gap < 1 µm Particle bridging No or minimal destruction of electrode surfaces! B short time reaction (ns - µs) L reaction for longer time (µs ms) B and L electrical conductivity Local gap width reduction (instable);-l;b Multiple discharge channel splitting;-b Plasma channel base movement;-b;l Tendency of the arc or spark discharge;-b B and L insulating particles Changing of gap capacitance;-l Changing of partial gap width;-b;l Multiple discharge channel splitting;-b;l Process interrupt;-l (Instability) Plasma channel base movement;-l;b 8/39

10 Type G (gas) Bubbles Critical Phases Soft Short Circuit Partial in solution Flow Propagation Layer Discharge splitting Collapse ECM interrupt Gas discharge Passivation?? Gas formation on the electrodes surface Layer Gas formation into the working medium - Bubbles 9/39

11 Type G (gas) Bubbles Critical Phases Soft Short Circuit Gas-solid-interface Gas-fluid-interface Flow Tool Partial in solution Workpiece bubble bases & Gas-fluid-interface as area with higher contamination; & Bubble expansion (oscillation) as reaction of the inner energy; & Difficult task to analysis the critical gas bubble; & Gas discharge within the bubble (contact with the electrodes); & Changed gap conditions within bubble volume; & Small bubbles (after collapse) coalesce with solid particle. 10/39

12 Type G (gas) Bubbles Soft Short Circuit Partial in solution Flow Tool Workpiece & Layer formation as dielectric layer (volitional effect for process combinations); & Changed gap conditions through different gas-fluid-solid interfaces; & Origin of partial discharges (PD); & Problem of the complete gas layer Demands in topology. 11/39

13 Pulse ElectroChemical - PECM Defined pulse energy Minimal critical pulse energy Processing with smaller average gap width Higher removal rate per pulse is possible Hard short circuits are critical process conditions - Process analysis during pulse duration and fast process interrupt. - Minimal pulse energy rerouting (bypass). - Pulse pause regulation with measured of the gap conditions. Soft short circuits are uncritical process conditions - Process control after pulse duration. - Self-regulating through flushing conditions and adjustment of the duty cycle. 12/39

14 Pulse ElectroChemical - PECM Hard Short Circuit - PECM Normal gap conditions 10 V Parasitic components Process analysis Trigger point of over-current - du/dt 10 A 250 ns Current probe - 16 ns delay 13/39

15 Pulse ElectroChemical - PECM Starting conditions B ec, R* Cathode - Tool B ec, R B con, R B pas, RGC Temporal conditions for PECM Anode Column (k) 1 2 k n Soft R* R R 0 1 High density of contaminations (local) Row (l) m Temporal conditions for Passivation 14/39

16 Pulse ElectroChemical - PECM Soft & Primary type G through the Hydrogen reduction of the cathodic surface; & Sludge development and no solution through electrolyte (type S); & Passivation through solid and/or gas layers; & For Micromachining (small working surface) is the gas bubble propagation an additional part of the gap flushing. & For acid electrolytes are the sludge development no problem. & Higher removal rate by using of no-acid electrolytes (NaCl, KCl, NaNO 3, ) 15/39

17 µ-pulse ElectroChemical - µ-pecm High processing accuracy Gap width smaller than 2 µm Defined pulse energy < Minimal critical pulse energy Short reaction time between Soft - and Hard - short circuit Hard short circuits are critical process conditions - Process analysis during pulse duration and fast process interrupt. - Origins are big particle (S) or contacts on micro surface topology (S). - Pulse pause regulation with measured of the gap conditions. Soft short circuits are uncritical process conditions - Main problem is the sure method of the differentiation of soft - and hard - short circuits. - Self-regulating through flushing conditions and adjustment of the duty cycle. 16/39

18 µ-pulse ElectroChemical - µ-pecm type 2 - Process energy source - type 1 Gap carrier current Gap voltage Short circuit defined by limited gap current!? 17/39

19 µ-pulse ElectroChemical - µ-pecm 3,0 2,0 µpecm - Short circuit Current in A 1,0 0,0-1,0-2,0-3,0 Cathodic pulse Gap voltage ~3 V Gap current ~ 500 ma Voltage in V 0,5 0,0-0,5-1,0-1,5-2,0µ -1,0µ 0,0 1,0µ 2,0µ Time in s 18/39

20 Micro-Electrical Discharge - µedm Different Conditions to ECM: Local remove of the workpiece surface dependent of the gap width; Uniform removal rate before the next feed rate step; Necessary inhomogeneities of the surface as ignition conditions; Small pulse energy is a premise for a high accuracy; Contamination is a problem when the high concentration in special areas; Critical conditions through Hard - and Soft - short circuits are possible. More measurement systems! 19/39

21 Micro-Electrical Discharge - µedm Type S (solid) Particles Critical Phases How do you can measure the short circuit effect before it effects! Flow Bridging Workpiece Gap < 1 µm The regeneration of the gap conditions reduced the active machining time extremely. Accuracy High Productivity Layer Slime Solution is low mass material-dependent Influence of the Double Layer Effects on EDM 20/39

22 Micro-Electrical Discharge - µedm Type L (liquid) Additives Flow DL-Double Layer Gap < 1 µm Type L is the result in series of additives and pyrolysis effects EDM Working fluid (hydrocarbon or deionised water)- Joule heating Effects of the Double layer! Pyrolysis Undefined Additives Additive most shorter and faster or/and Charge-defined. 21/39

23 Micro-Electrical Discharge - µedm Tool - Cathodic Ignition kernel Double Layer area Streamer Propagation Gap ~ 5 µm Phase 1: Pre-ignition to ~20 ns Workpiece - Anodic Phase 2: Pre-ignition to ~80 ns Phase 3: Pre-ignition to ~120 ns Base of the Plasma channel Pure liquid discharge! 22/39

24 Micro-Electrical Discharge - µedm Tool - Cathodic Gap ~ 1,5 µm Gap ~ 5 µm Gap variation for Micromachining Phase 1: Pre-ignition to ~20 ns Phase 2: Pre-ignition to ~80 ns Phase 3: Pre-ignition to ~120 ns Workpiece - Anodic 23/39

Micro-Electrical Discharge - µedm Current in A Voltage in V 20 15 10 5 0-20µ -10µ 0 10µ 20µ 30µ 40µ 50µ 60µ 70µ 80µ 90µ 100µ 110µ 120µ 700 600 500 400 300 200 100 0-20µ -10µ 0

25 Micro-Electrical Discharge - µedm Current in A Voltage in V µ -10µ 0 10µ 20µ 30µ 40µ 50µ 60µ 70µ 80µ 90µ 100µ 110µ 120µ µ -10µ 0 10µ 20µ 30µ 40µ 50µ 60µ 70µ 80µ 90µ 100µ 110µ 120µ Time in s Double Pulse Analysis Soft 10 A erosion current 25 µs pulse duration 10 µs pause duration 0 ms 0,9 ms 24/39 11,7 ms

26 Micro-Electrical Discharge - µedm Double Pulse Analysis 5 A erosion current 25 µs pulse duration 10 µs pause duration Current in A Voltage in V -20µ -10µ 0 10µ 20µ 30µ 40µ 50µ 60µ 70µ 80µ 90µ 100µ 110µ 120µ Short circuit 0-20µ -10µ 0 10µ 20µ 30µ 40µ 50µ 60µ 70µ 80µ 90µ 100µ 110µ 120µ Time in s Arc 25/39

27 Micro-Electrical Discharge - µedm ms ms ms ms ms ms 5 A erosion current 5 µs pulse duration 5 µs pause duration 37 µs image duration N-dodecane 10 µm gap width ms ms ms 26/39

28 Micro-Electrical Discharge - µedm 27/39

29 Micro-Electrical Discharge - µedm uncritical Needle Pulse Generator For Micro EDM and Finishing Voltage in V -2.0µ -1.0µ µ 2.0µ pulse duration: pulse current: pulse sequence: 100 ns 30 A 2 MHz Current in A µ -1.0µ µ 2.0µ Time in s 28/39

30 Micro-Electrical Discharge - µedm Process Energy Source Feeder System Working gap Influence of the gap variation Positioning and Feed rate Process Control System The Bode-Plots show better gap conditions for n-dodecane De-ionised water leads to electrochemical effects De-ionised water show big changes in the capacitive feature The pure capacitive gap characteristic of the n-dodecane give the possibility to calculate with a simple equivalent circuit. Changed between capacitive and ohms-capacitive conditions 29/39

31 ELESIN ED-EC-Combination 30/39

32 ELESIN ED-EC-Combination Small frontal and lateral gap width high accuracy in the lateral gap; Fast critical stages through gas bubble; First passivating case is the gas bubble layer (dielectric) Main case Second passivating case is a dielectric solid layer Special case Third case short circuit through bridging Critical case Process control only for the critical cases. 31/39

33 ELESIN ED-EC-Combination No breakdown conditions in the lateral gap maximal removal rate up to Passivation pure ECM. Minimalisation of the frontal gap through Passivation, Feed rate and contamination low gap voltage for the Breakdown. - Small pulse energy better machining accuracy. - High pulse energy better removal rate. - Special case glance surface 32/39

34 ELESIN ED-EC-Combination Ideal process control, when the ED-remove is equal the feed rate. Critical stage: No high gap width after ED-remove as phase for gap regeneration. Galvanic processes on the electrodes. 33/39

35 Micromachining for EDM, ECM and hybrid machining Current i Adjustable current magnitude OP EDM Source characteristic i total Breakdown i EC OP ECM 0 u lim u bv Adjustable Gap characteristic voltage Voltage u Micro hole sinking i ecm in lateral gap 34/39

36 Micromachining for EDM, ECM and hybrid machining Source-Gap-Characteristic Transition time Fall time Rise time Pulse pause duration 35/39

37 ECoDCM multiple combination 36/39

-80 2 1 0-1 Short circuit Splitting of cutting

38 ECoDCM multiple combination Fused cutting surface Voltage [U] Current [ka] Short circuit Splitting of cutting surface Time [s] 37/39

39 Short-circuits avoid are the best principle! Short-circuits recognize are the largest problem! Each procedure requires its own short-circuit analysis. A condition is complete understanding of the process. Most non-conventional manufacturing processes show large gaps in the process analysis (short circuit detection). The problems with the extension of the operational area are the unsettled process changes. The variety of the kinds of short-circuits makes their classification more difficult. 38/39

40 Exact defined micromachining conditions are possible! Selection of applications Micromachining conditions are result of technological parameters, processing size and removal rate process Micromachining make demands on equipment new solutions and a greater parameter ranges Quality of the Micromachining is a result of the quality of the Process Energy Source and a minimization of the critical short circuits. Otto-von-Guericke-University Magdeburg Institute for Fundamental Electrical Engineering and EMC Werner-von-Siemens-Building Tel.: , Fax: hans-peter.schulze@ovgu.de 39/39

41 12th ESAFORM Conference on material forming Twente,, Nederland, April 2009 MS13: Non-conventional processes Problems of the Processing Accuracy for Electro-erosion erosion and Electrochemical Processes by Institute for Fundamental Electrical Engineering and Electromagnetic Compatibility

Ultra-short pulse ECM using electrostatic induction feeding method

Available online at www.sciencedirect.com Procedia CIRP 6 (213 ) 39 394 The Seventeenth CIRP Conference on Electro Physical and Chemical Machining (ISEM) Ultra-short pulse ECM using electrostatic induction