Manufacturing Technology I. Exercise 14. Material removing machining techniques. (Selecting and designing techniques)

Transcription

1 Lehrstuhl für Technologie der Fertigungsverfahren Laboratorium für Werkzeugmaschinen und Betriebslehre Manufacturing Technology I Exercise 14 Material removing machining techniques (Selecting and designing techniques) Werkzeugmaschinenlabor Lehrstuhl für Technologie der Fertigungsverfahren Prof. Dr. - Ing. F. Klocke RWTH - Aachen Steinbachstraße Aachen

2 Inhaltsverzeichnis Table of Contents 1 Selecting the technique Variants of the technique Areas of application for material removing production techniques Selecting techniques in tool and mould manufacture Designing electrical discharge machining operations (EDM) Symbols and abbreviations Collection of equations Tasks Designing Electro-Chemical Machining (ECM) Symbols and abbreviations Formulae Task Literature Fertigungstechnik I - Übung 14 2

3 Selecting techniques in tool and mould manufacture 1 Selecting the technique 1.1 Variants of the technique The material removing manufacturing techniques include the following machining operations with their associated variants: Electro Discharge Machining, EDM), - Electrical discharge machining, - Electrical discharge cutting, using wire electrode which is running off; Electro-chemical material removal (Electro Chemical Machining, ECM), - Electro-chemical machining, - Electro-chemical deburring; Chemical removal, - Etching. Electrical discharge material removal and electro-chemical removal, both of which are widely used in industrial practise, are presented in this exercise. 1.2 Areas of application for the material removing production techniques The technological and economic characteristics of the material removing production techniques make them particularly suitable for use in tool and mould manufacture. The general advantages of the techniques include the non-contact machining mode, which produces only low levels of process force and permits materials of any level of hardness to be machined. It also provides the option of machining complex and filigree contours. The drawback of electrical discharge machining is the thermal damage to the external zone, which results from the thermal material removal principle. In electro-chemical machining, account must be taken of the gap expansion in deep cavities. Some examples of possible areas of application are shown in Table 1.1. Fertigungstechnik I - Übung 14 3

4 Selecting techniques in tool and mould manufacture Electrical Discharge Machining, EDM Electrical discharge machining - Tool & mould manufacture: Manufacture of injection and compression moulds, Forging dies, start holes for wire erosion - Turbine manufacture, cooling ducts in turbine blades Electrical discharge cutting with a trailing wire electrode - Tool and mould production, manufacture of cutting tools punches and dies - Plastics processing, manufacture of extrusion dies - Manufacture of stator blades Electro Chemical Machining, ECM) Electro-chemical machining - Turbine production, manufacture of turbine laufern and cooling ducts Turbine blades - Tool making, manufacture of deburring dies Electro-chemical deburring - Deburring and contour machining operations on ratchet wheels synchronizing disks and axis parts Table 1.1 Areas of application for the material removing manufacturing techniques Tool and mould manufacturing is dominated by single part and small batch production, i.e. in the majority of cases, only a few specimens of any particular tool are manufactured. The wide range of different tools confronts the tool and mould manufacturer with a demand for a high degree of flexibility. Tool and mould manufacture can be subdivided into various types. A basic distinction is drawn between samples and prototypes, auxiliary tools and serial tools. Samples, prototypes and auxiliary tools are used in the early stages of the product or tool development process. These will not be examined at this point. In contrast, serial tools are generally used in pre-series and pilot lots, in product implementation phases and after the launch of a product on the market. Serial tools, in turn, can be divided into various categories, whereby characteristic features are allocated to different categories. Distinctions can be made between injection and compression moulds, forging dies, drawing and pressing tools. The materials used, the form of the raw material and, above all, the geometry or the contour, provide characteristics for each individual category, Table 1.2. The features are associated with characteristic demands, which must be met by the techniques used to manufacture the tools and moulds. Fertigungstechnik I - Übung 14 4

5 Selecting techniques in tool and mould manufacture Tool / Form Materials Raw material Contour / Geometry Injection & 40 CrMnMo 7 Raw bloom Hollow mould Press mould X 38 CrMoV 5 1 (large material complex contour R m = N/mm² removal) highly filigree large engraving depth Forging dies 56 NiCrMoV 7 v Raw bloom Hollow mould X 38 CrMoV 5 3 (very large material less complex contour R m = N/mm² removal) slightly curved surfaces large rounding Drasing& GG 25 CrMo Cast blank Hollow mould Pressing tools GGG 70 (constant less complex contour Zamak allowance) slighty curved surfaces HB 30 (Grey cast iron) large rounding Cutting tools PM S 653 Raw bloom Brakedown PM X 210 CrW 12 some complex, filigree PM X 155 CrVMo 12 1 cutting line geometry HRC high levels of precision in R m 2000 M/mm² some cases Table 1.2 Dividing serial tools into categories A typical forging die for connecting rod is shown in Fig.1.3. This connecting rod die is usually manufactured in an electrical discharge machining operation. The tool electrodes which are required, are generally shaped as forming electrodes, i.e. the contour to be produced, is present in the tool. The die is produced by die-sinking the electrodes into the workpiece. A graphite tool electrode is shown in Fig These electrodes are used to manufacture the base for the roof railing of the Mercedes C-class. The electrodes are generally manufactured in a cutting, i.e. milling, turning, grinding operation. Fig. 1.3 Forging die manufactured in a spark erosion operation Fertigungstechnik I - Übung 14 5

6 Selecting techniques in tool and mould manufacture Graphite electrode 215 x 65 x 45 mm Material R8510 Up to surface quality VDI 30 Graphite electrode with 34 ribs 215 x 65 x 45 mm Material R8710 Up to surface quality VDI 24 Source: SGL Carbon, Mercedes-Benz Finished part TPE Fig. 1.4 Graphite electrode and plastic injection moulded part Turbine manufacture is an important area of application for electro-chemical rib machining. Whole turbine wheels are produced in this manner, using electrochemical machining, Fig However, on the other hand, even oval cooling boreholes can be drilled into the curved turbine blades. This cannot be achieved when other manufacturing techniques are used. Fig. 1.5 Electro-chemically manufactured turbine wheels Fertigungstechnik I - Übung 14 6

7 Selecting techniques in tool and mould manufacture 1.3 Selecting techniques in tool and mould manufacture The production techniques to be used in tool an mould manufacture are selected on the basis of various criteria which can be summarised under the categories technology, geometry and economic efficiency, Fig Metal cutting techniques such as milling and grinding as well as the metal removing techniques, are suitable for rough and finish machining operations. The characteristics and properties of the metal removing techniques EDM and ECM are shown in Fig Economic efficiency - time spenting - necessary machines and appliances - automatable - personell-intensity Technology - Material - Surface - Frige area influence - Form- and positional tolerance - material removal efficiency / wearout Geometry - Tool geometry - Material geometry Milling EDM ECM Grinding - all electrically conductive materials can be machined - geometrically complex grooves possible - high-strength materials can be machined, because there are no developments of forces - all metalic materials can be machined - geometrically complex grooves possible - high-strength materials can be machined, because there are no developments of forces Fig. 1.6 Criteria for the selection of manufacturing techniques in tool manufacturing The dressing operation can be followed by a finish-machining operation, depending on the level of surface quality required. Although manual re-working is common in industrial practice, it has the disadvantages of a lack of reproducibility and a high requirement for personnel. Alternatively, EDM or electro-chemical machining methods can be applied, Fig Each of the alternative finishmachining techniques has its own specific advantages and disadvantages. EDM polishing is not fundamentally different from other forms of EDM machining. Only the discharge energy is reduced considerably, permitting high levels of surface quality to be achieved. The disadvantages are the long machining times and the process related thermal workpiece damage which can be minimised, but never completely avoided. Fertigungstechnik I - Übung 14 7

8 + + Selecting techniques in tool and mould manufacture Electro-chemical finish-machining provides reproducible removal depths, good levels of surface quality and high dimensional, form and positional accuracy. This technique also lends itself well to automation. However, the work required to build the devices, for example, is so high that it is usually economically efficient to machine only serially produced parts in this way. However tool and mould manufacture is dominated by single part and small batch production. Shot / Sand blasting - Removal not reproducible - Dimensional integrity Grinding/Honing/Lapping - Kinematic & technological limitations by complex 3D forming Abrasive flow machining - Low form & dimensional accuracy - technological limits for complex shapes EDM polishing + - Limited area - time consuming - therm. workpiece damage EDM polishing with silicon powder + - Suitable from R max < 10 µm - form contortion in depth engraving - therm. workpiece damage Manual polish - labour intensive - not reproducible Electrochemical finish-machining + - reproducible mat. removal - easy to automate - high surface quality - Form- and position exact Fig. 1.7 Finish-machining techniques in tool and mould manufacture Fertigungstechnik I - Übung 14 8

9 Auslegung funkenerosives Senken (EDM) 2 Designing EDM cutting operations 2.1 Symbols and abbreviations A mm 2 Cross-sectional area EDM - Electro Discharge Machining F D N Wire pretensioning force I A Average working current, strength of current P W Power, abrasive flow Q W mm 3 /min Material removal rate R a µm Arithmetic mean deviation of the R m N/mm 2 tensile strength, T h Machining time U V Average working voltage V mm 3 Volume of material removed V E mm 3 /min Electrode wear rate V W mm 3 /min Material removal rate V W mm 2 /min Cutting rate W e mj Discharge energy b µm Bulging b R µm Width of the external zone b U µm Width of the phase transformation zone d mm Diameter f e Hz Effective pulse frequency f p Hz Pulse frequency h mm Workpiece height, workpiece thickness Fertigungstechnik I - Übung 14 9

10 Auslegung funkenerosives Senken (EDM) i e A Discharge current i(t) A Flow of the current over time î e A Maximum discharge current i e (t)a Progression of the discharge current over time m g Mass of material removed q mm 3 /s Flow rate r mm Radius r mm Planetary radius s µm Machining gap s F µm Frontal machining gap s L µm Lateral machining gap s m mm Middle cutting track s o mm Upper cutting track s u mm Lower cutting track t 0 µs Pulse interval time t d µs Ignition delay t e µs Discharge duration t i µs Pulse duration t p µs Pulse cycle time u, v mm Excursion of the upper wire feed u(t) V Voltage curve over time u e V Discharge voltage u e (t) V Discharge voltage over time û i V no-load voltage v m/s Speed Fertigungstechnik I - Übung 14 10

11 Auslegung funkenerosives Senken (EDM) v A mm/min Removal rate v D m/s Wire run-off speed δ R µm Crack depth λ % Frequency ratio ϑ % Relative wear ρ g/cm 3 Density ρ Leg g/cm 3 Density of the alloy τ - Duty factor 2.2 Formulae 1. Discharge duration t e is the time of the current conduction during discharge. 2. The ignition delay time t d, is the time which elapses between switching on the voltage pulse and arcing through the discharge path, i.e. until the current increases. This time is required in order to build up the discharge channel and therefore depends on the condition of the machining gap. 3. The pulse duration t i, is the time of the switched on voltage pulse (adjustable at the generator). It equals the sum of the discharge duration and the ignition delay time: t i = t e + t d. In the case of iso-frequent generators, the discharge times may vary as a result of different ignition conditions in the machining gap when the pulse duration is fixed. 4. The pulse interval time t 0 is the interval between two voltage pulses (adjustable at the generator). The discharge path of the previous discharge is de- ionised in this time, permitting the subsequent discharge to ignite at a different point. 5. The pulse cycle time t p is the time between switching on one voltage pulse and switching on the following voltage pulse. It is equal to the sum of the pulse Fertigungstechnik I - Übung 14 11

12 Auslegung funkenerosives Senken (EDM) duration t i and the pulse interval time, t 0 : t p = t i + t The duty factor τ is the ratio of the pulse duration t i to the pulse cycle time t p : τ = t i / t p. 7. The pulse frequency f p, is the number of voltage pulses switched on per unit of f p = 1 / t p. time: 8. The no-load voltage û i occurs as a maximum value on the discharge path when no current is flowing. It can usually be adjusted in several stages at the generator and determines among other things the width of the machining gap in which a discharge can ignite. 9. During discharge, the discharge current i e flows through the discharge path. As a rule, the average discharge current i e is usually specified. It is limited by the efficiency of the power module of the generator and can be adjusted in stages at the generator. Since the conditions in the machining gap change constantly, the erosion process is generally a stochastic sequence of voltage and current progressions. The following parameters are defined accordingly. 1. The effective pulse frequency f e is the number of actually ignited spark discharges per unit of time in the discharge path. 2. The frequency ratio λ is the ratio of the effective pulse frequency f e to the pulse frequency f p : λ = f e / f p. This quantity is an informative value which can be used as a basis on which to evaluate the quality of the erosion process. 3. The discharge voltage u e, occurs on the discharge path when the discharge has ignited and the current is flowing. Since this quantity is time-dependent, the mean discharge voltage u e is usually specified. The level of the mean Fertigungstechnik I - Übung 14 12

13 Auslegung funkenerosives Senken (EDM) discharge voltage u e, is dependent on the material combinations involved and lies between 15 and 30 V in the majority of cases. 4. The average working voltage U, is the arithmetic mean value of the voltage on the discharge path during the machining operation. 5. The average working current I, is the arithmetic mean value of the current which flows through the discharge path during the machining operation. The average working voltage U, and the average working current I, are two measured quantities which can be used to adjust and monitor the erosion process. 6. Pulse energy W e, is the energy consumed on the discharge path during one discharge. The following applies: W = u (t) i (t) dt u i t t e e e e e e. e The volume of the individual discharges and, to a considerable degree the structure of the surface after erosion, is determined by the pulse energy. 7. The material removal per discharge V We is the volume of the workpiece removed by one discharge. 8. The wear per discharge V Ee is the volume of the tool electrode removed per unit of time. 9. The material removal rate V W is the volume of workpiece material removed per unit of time. 10. The electrode wear rate V E, is the volume of tool material removed per unit of time. 11. The volumetric relative wear ϑ, is the ratio of the electrode wear rate V E to the material removal rate V W : ϑ = V E / V W. 12. The arithmetic mean surface roughness value R a, and the average peak-tovalley height R z, are used to evaluate surface quality. Fertigungstechnik I - Übung 14 13

14 Auslegung funkenerosives Senken (EDM) 2.3 Exercises Exercise 1: Plans have been drawn up to finish-machine a forging die for a crankshaft in an EDM operation. The die has been pre-milled with an allowance of 3 mm, leaving a material volume of 60 cm³ to be removed in the EDM operation. a) Outline three reasons in not form, for finish-machining the die in an EDM operation. b) A pulse duration t i = 350 µs and a pulse interval time t 0 = 50 µs, have been set for the static pulse generator of the electrical discharge machining facility. What frequency response ratio is required in order to ensure that a machining time T, of 200 min can be achieved in the erosion operation when the material removal is V we = 2.5 * 10-3 mm³ per discharge? c) After the erosion operation, a reduction in the tool electrode mass of g was measured (density ρ = 1.8 g/cm³). Please calculate the volumetric relative wear υ. d) In order to achieve worthwhile material removal rates in smoothing operations, the tool electrode has the form of a four-channel electrode and the die is eroded using the multi-channel technique. What voltage serves as the regulating variable for the feed motion of the tool electrode? Minimum working voltage Average working voltage of all channels Maximum working voltage No-load voltage Discharge voltage Please give reasons for your answer Fertigungstechnik I - Übung 14 14

15 Auslegung funkenerosives Senken (EDM) A R 1 R 3 R 2 A section A-A 40 Fig Precision punch a) The intention is to produce a punch in an EDM operation using a trailing wire electrode in multi-cut technology. How many re-cuts are required when a surface quality of R a = 0.4 µm is specified? Please describe the approach pursued by this technology. Use Table for your solution Surface quality Cutting rate V W [mm²/min] R a [µm] Main cut Re-cut 1 Re-cut 2 Re-cut Table EDM cutting in multi-cut technology b) Why is the cutting rate higher in all re-cuts than in the main cut, although the pulse energy or the discharge current is reduced considerably? Fertigungstechnik I - Übung 14 15

16 Auslegung funkenerosives Senken (EDM) c) How high is the manufacturing cost in an EDM process when the length of the cutting path S, including the approach path is 360 mm, the die is 30 mm high, assuming a machine hourly rate of 75 /h. Please use Table to answer this question. d) The punch used, has a step for attachment in the precision cutting tool (c.f. Fig ). Why can the punch not be manufactured in an EDM cutting operation? e) Constant flushing of the machining gap is planned in order to minimise the machining time required. Sketch the arrangement of the machining electrode and tool in the diagram below. Label the dielectric flow. Assume that the tool electrode was produced in an EDM cutting operation. Machining direction punch Finished contour f) The punch is to be machined in a rough followed by a finish machining operation. Use Diagram to determine the minimum machining time t, required, when the workpiece weighs 2.8 kg prior to machining, 1.2 kg after rough machining and 0.96 kg after finish-machining. The density of the HSS material used, is 8 kg/10-3 m³. Fertigungstechnik I - Übung 14 16

17 Auslegung funkenerosives Senken (EDM) τ = 0,9 u i = 240 V Material removal rate w 500 i e = 90 A λ = 0,8 mm³ min 50 i e = 10 A % i e = 90 A relativ wastage 10 5 i e = 10 A µs Pulse duration t 5000 Diagram Material removal rate and relative wear g) The lateral gap width in the rough machining operation is 250 µm and 30 µm in the finish-machining operation. What measure can be taken in order to ensure that it is not necessary to produce a new finish-machining electrode? At what contour radii (R 1, R 2, R 3 ) in Fig , do process-related limitations take effect? h) How long is the feed path covered by the machining electrode in the roughmachining operation when wear occurs only in the front area of the electrode, the setting conditions are selected as listed for f) and the machining gap measures 250 µm? Fig and Diagram are required in order to answer this part of the question. Fertigungstechnik I - Übung 14 17

18 Designing Electro-Chemical Machining (ECM) 3 Designing Electro-Chemical Machining (ECM) 3.1 Symbols and abbreviations A mm 2 Electrode area C kg/100l H 2 O Electrolyte concentration ECM - Electro-Chemical-Machining F A s/mol Faraday constant I A Average working current, Strength of current J A/mm 2 Current density J min A/mm 2 Minimum current density M g/mol Molar mass M i g/mol Molar mass of alloy element i Me n+ - Metal lion with the ionic charge n+ P W Power Q A s Electrical charge Q W mm 3 /min Material removal rate R Ω Resistance R a µm Arithmetic mean peak-to-valley height R m N/mm 2 Tensile strength U V Average working voltage U V Polarisation voltage U el V Voltage drop in the electrolyte solution V mm 3 Volume of material removed V sp mm 3 /(A min) Specific volume of material removed h mm Height, width of workpiece Fertigungstechnik I - Übung 14 18

19 Designing Electro-Chemical Machining (ECM) m g Mass removed q mm 3 /s Flow rate r mm Radius s mm Gap width s mm Gap expansion s 90 mm Front gap s α mm Normal gap s max mm Maximum gap width s min mm Minimum gap width t s Machining time v a mm/min Material removal time v f mm/min Feed speed z - Change in electro-chemical valency z i - Change in electro-chemical valency of the alloying element i α Angle of contour inclination, conicity κ S/m Specific electrical conductivity θ a C Electrolyte temperature on electrolyte flow exit θ e C Electrolyte temperature on electrolyte flow entry ρ g/cm 3 Density ρ Leg g/cm 3 Alloy density Fertigungstechnik I - Übung 14 19

20 Designing Electro-Chemical Machining (ECM) 3.2 Formulae Faraday s Law m = M z F I t Material removal rates given - Non-passivating electrolyte solutions v = V J - Passivating electrolyte solutions va = Vsp ( J - J min) A sp Current density J I = A Gap width s = Uel κ. J Voltage drop in the electrolyte solution U el = U - U Fertigungstechnik I - Übung 14 20

21 Designing Electro-Chemical Machining (ECM) 3.3 Task Fig Motor cycle brake disk (according to Köppern GmbH, Hattingen) 12 bore-holes (diameter d = 14 mm) are required in order to accommodate the connecting pins for the production of the motor cycle brake disk illustrated (material X20Cr13, density ρ = 7.8 g/cm³, thickness 6 mm). Since the brake disk has been hardened and the brake disk holder has not been tempered, this cannot be achieved in a conventional drilling operation (the drill would slip into the untempered material). Electro-chemical machining is suitable since no influence is exerted by process forces when this technique is applied. Three brake disks are laid on top of one another (with the holder inserted) and are machined in an electro-chemical operation using a 20 % in weight sodium nitrateelectrolyte solution (conductivity κ = 15 S/m). With an average working voltage U, of 12 V, a material removal rate v a, of 4 mm/min is achieved. The polarisation voltage is U = 4 V and the mean electro-chemical valency is z = 3 under the prevailing machining conditions. The intention is to set the Faradayconstant F, to A*s/mol and the average molar mass M, to 55 g/mol. Fertigungstechnik I - Übung 14 21

22 Designing Electro-Chemical Machining (ECM) a) What strength of current is required for this electro-chemical machining operation? b) How long does the machining operation take? c) What size is the gap width s, between the tool and the workpiece? Fertigungstechnik I - Übung 14 22

23 Designing Electro-Chemical Machining (ECM) 4 Literature König, W. Fertigungsverfahren, Vol. 3, Abtragen EDM: P ECM: P VDI-Verlag, Düsseldorf, Spur, G., Stöferle, Th. Handbuch der Fertigungstechnik, Vol. 4/1, Abtragen, Beschichten EDM: P ECM: P Hanser-Verlag, München, Fertigungstechnik I - Übung 14 23