Manufacturing Technology I Exercise 14 Material removing ... · PDF fileExercise 14 Material removing machining techniques ... 1.3 Selecting techniques in tool and mould manufacture

Lehrstuhl für Technologieder Fertigungsverfahren

Laboratoriumfür Werkzeugmaschinenund Betriebslehre

Manufacturing Technology I

Exercise 14

Material removing machining techniques

(Selecting and designing techniques)

WerkzeugmaschinenlaborLehrstuhl für

Technologie der FertigungsverfahrenProf. Dr. - Ing. F. Klocke

RWTH - AachenSteinbachstraße 53

52065 Aachen

Inhaltsverzeichnis

Fertigungstechnik I - Übung 14 2

Table of Contents1 Selecting the technique ..................................................................... 3

1.1 Variants of the technique................................................................... 31.2 Areas of application for material removing production techniques .... 31.3 Selecting techniques in tool and mould manufacture ........................ 7

2 Designing electrical discharge machining operations (EDM)............. 92.1 Symbols and abbreviations ............................................................... 92.2 Collection of equations .................................................................... 112.3 Tasks............................................................................................... 14

3 Designing Electro-Chemical Machining (ECM)................................ 183.1 Symbols and abbreviations ............................................................. 183.2 Formulae ......................................................................................... 203.3 Task................................................................................................. 21

4 Literature ......................................................................................... 23

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.



Electrical Discharge Machining, EDMElectrical 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 diesElectro-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.



Tool / Form Materials Raw material Contour / Geometry

Injection & 40 CrMnMo 7 Raw bloom Hollow mouldPress mould X 38 CrMoV 5 1 (large material complex contour Rm = 100 - 1500 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 Rm = 1300 - 2000 N/mm² removal) slightly curved surfaces large rounding

Drasing& GG 25 CrMo Cast blank Hollow mouldPressing tools GGG 70 (constant less complex contour Zamak allowance) slighty curved surfaces 220 - 270 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 57 - 64 HRC high levels of precision in Rm ≤ 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. 1.4.

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



Source: SGL Carbon, Mercedes-Benz

Graphite electrode215 x 65 x 45 mmMaterial R8510Up to surface quality VDI 30

Graphite electrode with 34 ribs215 x 65 x 45 mmMaterial R8710Up to surface quality VDI 24

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 electro-

chemical machining, Fig. 1.5. However, on the other hand, even oval cooling bore-

holes 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



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. 1.6. 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. 1.6.

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

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

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. 1.7. Each of the alternative finish-

machining 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.



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.

- labour intensive- not reproducible

- Kinematic & technological limitations by complex 3D forming

- Removal not reproducible- Dimensional integrity

+

+

+

- Suitable from Rmax < 10 µm- form contortion in depth engraving- therm. workpiece damage

+

EDM polishing withsilicon powder

Electrochemicalfinish-machining

EDM polishing

Abrasive flow machining

Grinding/Honing/Lapping

Shot / Sand blasting

Manual polish

+

- Limited area- time consuming- therm. workpiece damage

- Low form & dimensional accuracy- technological limits for complex shapes

- reproducible mat. removal- easy to automate- high surface quality- Form- and position exact

Fig. 1.7 Finish-machining techniques in tool and mould manufacture

Auslegung funkenerosives Senken (EDM)


2 Designing EDM cutting operations

2.1 Symbols and abbreviations

A mm2 Cross-sectional area

EDM - Electro Discharge Machining

FD N Wire pretensioning force

I A Average working current, strength of current

P W Power, abrasive flow

QW mm3/min Material removal rate

Ra µm Arithmetic mean deviation of the

Rm N/mm2 tensile strength,

T h Machining time

U V Average working voltage

V mm3 Volume of material removed

VE mm3/min Electrode wear rate

VW mm3/min Material removal rate

VW mm2/min Cutting rate

We mJ Discharge energy

b µm Bulging

bR µm Width of the external zone

bU µm Width of the phase transformation zone

d mm Diameter

fe Hz Effective pulse frequency

fp Hz Pulse frequency

h mm Workpiece height, workpiece thickness



ie A Discharge current

i(t) A Flow of the current over time

îe A Maximum discharge current

ie(t)A Progression of the discharge current over time

m g Mass of material removed

q mm3/s Flow rate

r mm Radius

r mm Planetary radius

s µm Machining gap

sF µm Frontal machining gap

sL µm Lateral machining gap

sm mm Middle cutting track

so mm Upper cutting track

su mm Lower cutting track

t0 µs Pulse interval time

td µs Ignition delay

te µs Discharge duration

ti µs Pulse duration

tp µs Pulse cycle time

u, v mm Excursion of the upper wire feed

u(t) V Voltage curve over time

ue V Discharge voltage

ue(t) V Discharge voltage over time

ûi V no-load voltage

v m/s Speed



vA mm/min Removal rate

vD m/s Wire run-off speed

δR µm Crack depth

λ % Frequency ratio

ϑ % Relative wear

ρ g/cm3 Density

ρLeg g/cm3 Density of the alloy

τ - Duty factor

2.2 Formulae

1. Discharge duration te is the time of the current conduction during discharge.

2. The ignition delay time td, 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 ti, 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:

ti = te + td.

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 t0 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 tp 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



duration ti and the pulse interval time, t0:

tp = ti + t0.

6. The duty factor τ is the ratio of the pulse duration ti to the pulse cycle time tp:

τ = ti / tp.

7. The pulse frequency fp, is the number of voltage pulses switched on per unit

of time:

fp = 1 / tp.

8. The no-load voltage ûi 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 ie flows through the discharge path.

As a rule, the average discharge current ie 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 fe 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 fe to the

pulse frequency fp:

λ = fe / fp.

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 ue, 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 ue is usually specified. The level of the mean



discharge voltage ue , 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 We, is the energy consumed on the discharge path during one

discharge. The following applies:

W u (t) i (t) dt u i tt

e e e e e ee

= ⋅ ≈ ⋅ ⋅ .

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 VWe is the volume of the workpiece

removed by one discharge.

8. The wear per discharge VEe is the volume of the tool electrode removed per

unit of time.

9. The material removal rate VW is the volume of workpiece material removed

per unit of time.

10. The electrode wear rate VE, 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 VE to the

material removal rate VW:

ϑ = VE / VW.

12. The arithmetic mean surface roughness value Ra, and the average peak-to-

valley height Rz, are used to evaluate surface quality.



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 ti = 350 µs and a pulse interval time t0 = 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 Vwe = 2.5 * 10-3 mm³ per discharge?

c) After the erosion operation, a reduction in the tool electrode mass of 10.80 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



R1

R3

R2

A

A

section A-A40

Fig. 1.3.1 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 Ra = 0.4 µm is specified? Please describe the approach

pursued by this technology. Use Table 1.3.1 for your solution

Surface

quality

Cutting rate VW [mm²/min]

Ra [µm] Main cut Re-cut 1 Re-cut 2 Re-cut 3

1.8 60

1.1 60 120

0.9 60 120 90

0.4 60 120 90 80

Table 1.3.1 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?



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 1.3.1 to answer

this question.

d) The punch used, has a step for attachment in the precision cutting tool (c.f.

Fig. 1.3.1). 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 1.3.1 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.



500

mm³min

50

10

5000µs5001005010Pulse duration t

100

%

10

5

1

Mat

eria

l rem

oval

rate

wre

lativ

was

tage

ie = 90 A

ie = 10 A

ie = 90 A

ie = 10 A

τ = 0,9

ui = 240 V

λ = 0,8

Diagram 1.3.1 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 (R1, R2, R3) in Fig. 1.3.1, do process-related limitations take

effect?

h) How long is the feed path covered by the machining electrode in the rough-

machining 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. 1.3.1 and Diagram 1.3.1 are required in order to

answer this part of the question.

Designing Electro-Chemical Machining (ECM)


3 Designing Electro-Chemical Machining (ECM)

3.1 Symbols and abbreviations

A mm2 Electrode area

C kg/100l H2O Electrolyte concentration

ECM - Electro-Chemical-Machining

F A s/mol Faraday constant

I A Average working current, Strength of current

J A/mm2 Current density

Jmin A/mm2 Minimum current density

M g/mol Molar mass

Mi g/mol Molar mass of alloy element i

Men+ - Metal lion with the ionic charge n+

P W Power

Q A s Electrical charge

QW mm3/min Material removal rate

R Ω Resistance

Ra µm Arithmetic mean peak-to-valley height

Rm N/mm2 Tensile strength

U V Average working voltage

∆U V Polarisation voltage

Uel V Voltage drop in the electrolyte solution

V mm3 Volume of material removed

Vspmm3/(A min) Specific volume of material removed

h mm Height, width of workpiece



m g Mass removed

q mm3/s Flow rate

r mm Radius

s mm Gap width

∆s mm Gap expansion

s90 mm Front gap

sα mm Normal gap

smax mm Maximum gap width

smin mm Minimum gap width

t s Machining time

va mm/min Material removal time

vf mm/min Feed speed

z - Change in electro-chemical valency

zi - Change in electro-chemical valency of the alloying

element i

α ° Angle of contour inclination, conicity

κ S/m Specific electrical conductivity

θa °C Electrolyte temperature on electrolyte flow exit

θe °C Electrolyte temperature on electrolyte flow entry

ρ g/cm3 Density

ρLeg g/cm3 Alloy density



3.2 Formulae

Faraday’s Law m Mz F

I t=⋅

⋅ ⋅

Material removal rates given

- Non-passivating

electrolyte solutions v V JA sp= ⋅

- Passivating

electrolyte solutions v V J - JA sp min= ⋅ ( )

Current density J IA

=

Gap width sU

Jel=

⋅ κ.

Voltage drop in the

electrolyte solution Uel = U - ∆U



3.3 Task

Fig. 2.3.1 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 nitrate-

electrolyte solution (conductivity κ = 15 S/m). With an average working voltage U,

of 12 V, a material removal rate va, 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 Faraday-

constant F, to 96.487 A*s/mol and the average molar mass M, to 55 g/mol.



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?



4 Literature

König, W. Fertigungsverfahren, Vol. 3, Abtragen

EDM: P. 3 – 63

ECM: P. 69 – 118

VDI-Verlag, Düsseldorf, 1990.

Spur, G.,

Stöferle, Th.

Handbuch der Fertigungstechnik, Vol. 4/1, Abtragen,

Beschichten

EDM: P. 60 – 134

ECM: P. 266 – 340

Hanser-Verlag, München, 1987.

Documents

Manufacturing Technology I Exercise 14 Material removing ... · PDF fileExercise 14 Material removing machining techniques ... 1.3 Selecting techniques in tool and mould manufacture