INTRODUCTION TOHINDUSTAN AERONAUTICS

LIMITED

The beginning of HAL can be traced to the year 1940 when a far sighted industrialist, the late Seth Walchand Hirachand, set up a company called Hindustan Aircraft Limited at Bangalore with the object of establishing an aviation industry that can manufacture, assemble and overhaul aircraft. Initially aircraft like Curtiss Hawk, Vultee Bomber and Harlow Trainer was taken up for manufacture and overhaul in collaboration with Inter Continental Aircraft Company of the USA.

With the escalation of the Second World War, the government of India took over the management of the company in 1942 and handed it over to US Air force for repair and overhaul of various aircraft. The main activity for the next few years after the war was reconditioning and conversion of war surplus aircraft for the use of IAF and Civil Operators.

To fulfill the fresh mandate of the post independent India and to meet the challenges of open market economy of recent times the mission of the company has been redefined as:

“To become a globally competitive aerospace industry while working as instrument for achieving self reliance in design, manufacture and maintenance of aerospace defense equipment and diversifying to related areas, managing the business on commercial lines in a climate of growing professional competence.”

In the six decades, HAL has spread its wings to cover various activities in the areas of design, development, manufacture and maintenance. Today HAL has 14 production divisions spread over at Bangalore, Nasik, Koraput, Kanpur, Lucknow, Korwa, Hyderabad and Barrackpore. These divisions are fully backed by 9 design centers, which are co-located with the productive divisions. These centers are engaged in the design and development of combat aircraft, helicopter, aero engine, engine test beds, aircraft communication and navigation systems and accessories of mechanical and fuel systems and instruments.

1

The current program include production of Dhruv an Advanced Light Helicopter, Jaguar, LCA, Su-30, MkI and upgrades of MiGs, Jaguar and Avro HS-748.

HAL ENGINE DIVISION, KORAPUT

An agreement was signed in August, 1962 with the Soviet Union for manufacture of MiG-21 E7FL Air craft under license the Aero engine Factory at Koraput (ORISSA), the Air frame Factory at Nasik (Maharashtra), and the Avionics Factory at Hyderabad (Andhra Pradesh) have been set up to meet this requirement on the name of Aeronautics India Limited which was formed on April 1964 and new company under the name of Hindustan Aeronautics Limited was formed. The government sanction for the first phase of construct of the aero engine factory at Sunabeda (Koraput) was accorded March 1964 and the factory started manufacture of R11F2-Series-III engines for fitment on MIG-21FL Aircraft from 1969 onwards. The first engines of imported category manufactured in December 1968 and various categories of engines were produced during the subsequent years. The first raw material engine was produced in February 1971.

The production programs for the factory also include manufacture of forging and casting required for MiG-Aircraft. To meet the Air force requirement for improved fight interceptor aircraft, an agreement was signed with USSR in August1976 for manufacturing MiG-21BIS Aircraft. The power plant of this aircraft is the R25 turbojet engine. The government approval for setting up capital facilities was accorded in October 1977. The first engine of imported category delivered to HAL Nasik Division in the year 1978-79.The FI raw material engine was delivered during January1983.

With signing of the inter governmental agreement for manufacture of MiG 27M Aircraft on 19th March 1982, this Division would be involved in the manufacture of R-29B series of engine from the year 1984-85.

In order to attain self-sufficiency and to avoid difficulties regarding supply of Raw Material & other layout items from USSR, it was decided to provide indigenous supply of spares manufacturing for Overhaul/maintenance of the fleet. The Government approval for undertaking the task received during 1977-78 and the indigenization plan was formed to tackle,

2

ARS and first moving spares. Metallic material. Non-metallic material. Ready-made articles.

The activities towards these are being progressed as per approved time frame.

The various departments present in this division are:

1. Forge2. Foundry3. Tool room4. Small parts and fuel5. Sheet metal and welding6. Blades7. Electroplating8. Heat treatment9. Compressor10. Turbine11. CNC12. Assembly13. Overhaul14. Gear15. Test house16. Maintenance

3

MIG 21 (R-25 Engine) MIG 27 (R-29 B)

NON-TRADITIONAL MACHINING

INTRODUCTION

From some time past engineering industries have witnessed a rapid growth in the development of harder and difficult to machine materials such as hastalloy, nitralloy, waspalloy, nimonics, carbides, stainless steel, heat resisting steels and many other high strength temperature resistant(HSTR) alloys. These materials find wide application in aerospace, nuclear engineering and other industries owing to their high strength to weight ratio, hardness and heat resisting qualities. For such materials the conventional edged tool machining is highly uneconomical and the degree of accuracy and surface finish are poor. Besides, machining of these materials in to complex shapes is difficult, time consuming and sometimes impossible.

Considering the seriousness of the problem, Merchants in 1960’s emphasized the need for the development of newer concepts in metal machining. Consequently, non-traditional machining processes have emerged to overcome these difficulties. These processes are non-conventional or non-traditional (NTM) in the sense that they do not employ a conventional or traditional tool for metal removal, instead they directly utilize some from of energy for machining.

Characteristics of non-traditional machining

Non Conventional Machining Processes are characterized as follows:

Material removal may occur with chip formation or even no chip formation may take place. For example in AJM, chips

4

are of microscopic size and in case of Electrochemical machining material removal occurs due to electrochemical dissolution at atomic level.

In NTM, there may not be a physical tool present. For example in laser jet machining, machining is carried out by laser beam. However in Electrochemical Machining there is a physical tool that is very much required for machining.

In NTM, the tool need not be harder than the workpiece material. For example, in EDM, copper is used as the tool material to machine hardened steels.

Mostly NTM processes do not necessarily use mechanical energy to provide material removal. They use different energy domains to provide machining. For example, in USM, AJM, WJM mechanical energy is used to machine material, whereas in ECM electrochemical dissolution constitutes material removal.

Classification of non-traditional machining processesClassification of NTM processes is carried out depending on the

nature of energy used for material removal. The broad classification is given as follows:

• Mechanical Processes ⎯ Abrasive Jet Machining (AJM)

⎯ Ultrasonic Machining (USM) ⎯ Water Jet Machining (WJM) ⎯ Abrasive Water Jet Machining (AWJM)

• Electrochemical Processes ⎯ Electrochemical Machining (ECM) ⎯ Electro Chemical Grinding (ECG) ⎯ Electro Jet Drilling (EJD)

• Electro-Thermal Processes ⎯ Electro-discharge machining (EDM)⎯ Laser Jet Machining (LJM) ⎯ Electron Beam Machining (EBM)⎯ Ion Beam Machining (IBM) ⎯ Plasma Arc Machining (PAM)

• Chemical Processes

⎯ Chemical Machining (CHM)

5

⎯ Photochemical Machining (PCM) etc

INTRODUCTION TO WIRE-CUT EDM

In all the machining processes EDM is one of the important non-traditional machining processes. Here we are mainly concerned about wire cut EDM. Wire cut EDM or Electrical Discharge Machining is a technique used to slice through metal. The technique uses thin brass wire for the purpose and can create intricate profiles with the process. The EDM machine uses spark discharges that are fast, repetitive, and controlled for cutting. This process works with electrically conductive metals. The process is specially suited for contours and cavities that are not possible with other cutting tools.

EDM is also known as “spark machining” as it uses repetitive electrical discharges to remove metal. The electrical discharges are passed between the metal part and the electrode. A stream of continuously flowing liquid is used to remove the metal remnants produced during the process. A set of successively deeper craters is formed till the final shape is created by the discharges.

Different types of EDM :

1. Ram EDM

In ram EDM, a graphite electrode is used along with traditional tools. This electrode is connected to the ram with the help of a power source and is fed into the workpiece. The whole process is carried out in a fluid bath. The fluid helps to flush away the material, serves as a coolant to reduce the heat, and acts as a conductor for passing current between the workpiece and the electrode.

2. Wire EDM

In this method, a thin wire is used as an electrode. The wire is fed in the metal and the discharges are used to cut the material. The process is carried out in a bath of water. When closely observed, you can see that the wire does not touch the metal. All the cutting work is done by the electrical discharge. Computer software controls the whole operation including the path of the wire. The process can produce all sorts of complex shapes that are very difficult with other processes.

6

Principle of edm

It is the most versatile electrical machining process where erosion is caused due to electric spark. The rate of metal removal and the resulting surface finish can be controlled by proper variation in energy and duration of spark discharge. It is the process of repetitive sparking cycles.

Graphical Representation:

Fig 1: Series of Electric pulses at the inter electrode gap

Fig 2: Inter electrode gap voltage waveform during sparking

7

Fig 3: Inter electrode gap current waveform during sparking

Where,Ton :- Pulse on periodToff :- Pulse of periodTd :- Spark ignition periodVp :- Open gap voltageVg :- Average gap voltageIp :- Machining peak current

A series of electrical pulse generated by the pulse generator unit is applied between the workpiece and travelling wire electrode. In the event of spark discharge, there is a flow of current across wire electrode workpiece gap. The energy contained in the tiny spark removes a fraction of workpiece material. Large number of such time spaced tiny discharges between the workpiece and wire electrode cause the electro erosion of the workpiece material.

Concepts of 4- axes wire cut EDM

The wire-cut electric discharge machine is comprised of a machine tool, a power supply unit (ELPLUS), dielectric supply and chiller unit.

1.Machine tool

The ELECTRA machine tool unit comprises of a main work table (called X-Y table) on which the workpiece is clamped, an auxiliary table (called U-V table) and wire drive mechanism. The workpiece is mounted and clamped on the min work table. The main table moves along X-Y axis in the steps of 1 micrometer, by means of servo motors. U & V are parallel to X & Y axes respectively.

A travelling wire which is continuously fed from wire feed spool is caused travel through the workpiece and goes finally to the waste wire box. Along its travelling path the wire is supported under tension between a pair of wire guide, which is supported by the U-V table. The upper wire guide can be displaced transversely along the U-V axes with respect to lower wire guide. It can also be positioned vertically along Z axis by moving the vertical arm.

As the material removal or machining proceeds, the work table carry the workpiece is displaced transversely along a predetermined

8

path which is stored in the controller. The path specification (path programme) can be supplied to the controller by the program or directly through the controller key board.

When X-Y table moves along the predetermined path, the U-V table is kept stationary as stationary cut with the predetermined pattern is formed.

In order to produce taper machining the wire electrode has to be tilted. This is achieved by displacing the upper wire guide (along the U-V axes) with respect to lower wire guide. The desired taper angle is achieved by simultaneous control of the movement of X-Y table and U-V table along their respective predetermined path stored in the controller. The path information if the X-Y table and U-V table is given to the controller in terms of linear & circular elements via NC programme.

2.POWER SUPPLY

The power supply unit comprises of electric pulse generator, motor drive unit for X-Y, U-V axes and controller.

3.Dielectric Supply

While the machining is continued, the machining zone is continuously flushed with chilled distilled water through the nozzle on both side of workpiece. The spark discharge across the workpiece and the wire electrode causes ionization of the water which is used as a dielectric medium.

It is important to note that ionization of water leads to the increase in conductivity of water. An ion exchange resin is used in dielectric distribution system in order to prevent the increase in conductivity of water.

4.Part programming

The geometry of the profile and the motion of wire electrode tool along the profile are fed to the part programming system using keyboard. The profile geometry is defined in terms of various geometrical definition of points, lines and circles as the wire tool path elements on graphical screen by using a totally menu driven software. The wire compensation (for wire diameter and machining overcuts) and taper angle can be specified for total path or for each path element separately. After the profile is fed to the computer all

9

the numerical information about the path is calculated automatically and its print out is generated. The entered profile can be

10

verified on the graphic displayed screen and modified if necessary. After successful profile definition, the profile is recorded by the computer on a floppy disk which can be used in the controller for the execution.

Work preparation

1.Workpiece material

Any slight dislocation in the workpiece material may result distorted job. It is important to use the material free from residual stresses, arising from various processes. This may affect the machining accuracy to a very large extent. Workpiece material should be

Electrically conductive (at least 0.1 micro-ohm/cm) Suitable for clamping Non-combustible Non-violent chemical reactions with water, oxygen and

hydrogen

2.Wire electrode

The wire electrode is required to have a sufficient tensile strength and should be of uniform diameter and free from kinks and twist. The electrode wire material should be

Brass/super alloy (coated) Diameter variation within + 0.02 mm Tensile strength more than 50 kgf/mm2

Even winding free from kinks/breaks

Wire diameter and minimum corner radius :

11

Fig 7: Rib width

The diameter of wire imposes a restriction on the minimum achievable corner radius.

Minimum corner radius = (0.5*diameter) + overcut

Current carrying capacity of the wire:

As a thumb rule a brass wire of 0.2 mm in diameter can easily pass current of about 0.3-0.7 amperes in air but the same wire can pass current of about 6-9 amperes in water. While machining water should always be surrounded by the water column to avoid wire strength.

Wire tension (WT):

Wire tension determines how much the wire has to be stretched between upper and lower guides. More thickness of the job more is the tension required. Improper setting of tension may result inaccuracy in the job as well as wire breakage. Following chart gives nominal value of the wire tension for different settings. Minimum tension (for zero setting of WT) is approximately 200 gms which can be adjusted by clutch adjustment provided on feed spool mounting rod. This should be adjusted for different weight of spool.

WT TENSION (GMS) NOMINAL VALUES1 3002 4203 5404 6605 7806 9007 10208 11409 126010 1380

Wire feed (WF):

Due to spark erosion the travelling wire electrode becomes thin and brittle. Wire feed is the rate at which the wire electrode travels along wire guide path. It is always desirable to set the wire feed to the maximum. This will result in less wire breakage, better

12

machining, stability and little more cutting speed. With more feed set as 8 m/min on an average of a 0.25 mm diameter brass wire spool of 5 kg will last for 24 sparking hours. Setting WF at 15 will correspond to 15 m/min (approx.).

Overcut:

It is lateral distance between the wire and workpiece during the sparking. Overcut is larger if

Machining gap voltage is higher Discharge energy is higher Wire tension is lower Guide span is higher Job thickness is higher Dielectric conductivity is higher Machining is unstable

Wire compensation (offset):

Wire compensation = (0.5*wire diameter) + overcutWire compensation can be to the left (G41) or right (G42) of

profile depending upon the direction of motion and wire being inside or outside of the profile as shown:

Fig 8: wire compensation

13

Water dielectric

1.Characterization & suitability of wire cut medium

The use of water as dielectric permits widening of spark gap to minimize short circuit, resulting in high cutting speed.

Water has good wire electrode cooing effect (than kerosene as example).

It is non flammable and its vapours are non-toxic.

2.Dielectric strength

Since the insulation characterization of dielectric fluid decides the overcut so it is important to keep the conductivity of dielectric water constant. Conductivity of water changes due to generation of metallic ions and dissolution of ambient gases. The conductivity can be decreased by passing the water through de-ionize resin. This is done automatically by the machine.

3. Flushing

Flushing is important to achieve stable condition. It plays very important role as far as cutting speed concerned. Both the nozzles upper and lower should be just about 0.22 mm away from the workpiece, otherwise cutting performance drops considerably. Also both the nozzles should also be checked periodically for damages, scratches or slight damage on the contact edge affect cutting speed. Purity of the water should be machined by firm displacements of filters.

Setting up and operations

1.Job mounting:

Mount the job and damp by maximum possible clamp, dial the top surface of the job by dial gauge. The dial gauge can be marked on the upper flushing assembly. Provision for the same is provided. Make the wire vertical with the help of vertical block provided with the machine.

14

2.Job reference point:

It is always desirable to have a reference point on the workpiece for setting the work co-ordinate system (WCS). The reference point can be defined by the ground edge of the workpiece or the centre of the bored hole on the workpiece.

a)Edge finding (EF):

This function should be used to find the edge for the setting work co-ordinate system. Find the edge of the workpiece from a distance of 2-5 mm. After edge finding the wire system is always from the workpiece edge by a distance equal to wire radius.

Fig 11: Edge Distance

b)Center finding (CF):

This function should be used to find the centre of the reference hole. Centre finding should be repeated at least few times to verify consistency.

Care should be taken to perform CF and EF functions. Workpiece surface should be taken to perform CF and EF

functions. Upper assembly section should not be wet. WF should be at 3 and WT at 6.

15

Technology guidelines

The technology chart gives guideline to the operator to set up the parameter and get optimum performance from the machine. Actual process results may differ to same extent.

Generally by increasing the spark energy one can achieve the desired cutting rate. To achieve optimum results of cutting rate and the job accuracies, the machining parameter should be properly set.

The parameters which control its pulse energy and ultimately its machining speed are described below:

1. Machining parameters

Different parameters controlling its pulse energy and machining conditions are given below along their setting:

Ton Toff Ip Vp WP WF WI SV SF T000 00 000 0 0 00 00 00 0000 00

Ton : Pulse on time

During this period the voltage (Vp) is applied across the electrodes.Range:- 000-031 (in step of 1)

Higher is the Ton setting larger the pulse on period. The single pulse discharge energy increases with increasing Ton period, resulting in higher cutting rate. With higher values of Ton surface roughness tends to be higher. The higher values of discharge energy may also cause wire breakage.

Toff : Pulse off time

Voltage for the gap is absent during the period.

16

Range : - 00-63 (in step of 1)

Higher the Toff setting larger is the pulse off period. With the lower value of Toff there is more number of discharges in a given time resulting in increase in the sparking efficiency; as a result the cutting rate also increases. Using very low value of Toff period may cause wire breakage which reduces the cutting efficiency. When the discharge and its ions become unstable one can increase the Toff

period. This will allow lower pulse duty factor and will reduce the average gap current.

Ip : Peak current (A)

This is for selection of pulse peak current.Range : 010-230 (in step of 10)

Higher is the Ip setting larger the peak current value. Increase in Ip value will increase the pulse discharge energy which can improve the cutting rate further. For higher value of Ip gap conditions may become unstable with improper combination of Ton , Toff , SV and SF value settings. When the discharge conditions become unstable one must reduce the Ip value (and/or increase the Toff period).

Vp : Pulse peak voltage

This is for selection of open gap voltage.Range : 1 or 2

Increase in Vp value will increase the pulse discharge energy which can improve the cutting rate. Normally it is always ‘2’.

WP : Flushing pressure of water dielectric

This is selection of flushing input pressure.Range : 0 or 1 (0-low pressure, 1-high pressure).

High input pressure of water dielectric is necessary for cutting with higher values of pulse power and also cutting the jobs of higher thickness. Lower input pressure used for thin jobs and in trim cuts.

WF : Wire feed rate setting

17

This is the feed rate at which the fresh wire is fed continuously for sparking.Range :- 01 – 15 (in step of 1)

Higher values of wire feed rate (above 6) are required for working with higher pulse power (where job cutting rate are higher)

WT : Wire tension setting

This is a gram equivalent load with the continuously fed wire is kept under tension so that it remains straight between the wire guides. Range :- 01-15 (in step of 1)

While the wire is being fed continuously appropriate wire tension avoids the unintentional wire deflection from its straight path (between the wire guides). The wire deflection is caused due to spark induced reaction forces and water pressure.

SV : Spark gap set voltage

This is the reference voltage for the actual (gap voltage). Range :- 00-99 (in step of 1) volt

SF : Servo feed setting

This parameters decides the servo speed, the servo speed at the set value of SF, can vary in proportion with the gap voltage (normal feed mode) or can be kept constant while machining (with constant feed mode). Range : 0000-0990 (for normal feed)

1000-1999 (for constant feed)

In constant feed mode, the 3 least significant digit of SF define the feed rate in 10th of mm/min. 1050 will give 5mm/min constant feed. Here SF can be vary from 000-990 in normal feed mode and 000-999 in constant feed mode by pressing up or down arrow keys or by page up or down, where as selection of first digit 0 or 1 can be done by pressing numeric key 0 or 1.

T : Threshold setting

Threshold setting (in percent of SV) is for correcting action in abnormal discharge condition.

18

Range : 0-99 (%)

Guidelines parameters

Apart from these parameters, the technology guideline charts also includes following two parameters for process monitoring purpose:

Ig : Average gap current (A)

This is the actual value of gap current read on ammeter. The value of average machining current given in the guideline chart is indicative.

Vg : Average gap voltage (volt)

This is the actual value of gap voltage read on yellow voltage bar on the screen. It depends on set values of SV & SF. For stable machining, SF should be set such a way that Vg is higher is higher than SV by 3 – 9 volt.

2. Guideline charts

The technology charts are prepared to support the wire EDM user and provide some guidelines. Every technology chart comprises of a set of guide pertaining to a specific wire job thickness combination.

Technology guidelines are available for the following wire and workpiece material:

WIRE : Material: ELECTRA DURACUTDiameter: 0.25 mm

JOB : Material: SteelHardness: 48 – 50 RC

Prior to machining, job materials were properly stress relieved.

CONDUCTIVITY : The guide refers to the de-ionized water as a dielectric with conductivity value of 20 units.

19

PRESSURE : The maximum water inlet pressure is 15 kg/cm2.

FLOW TOP : This is the flow rate of water dielectric in liters/min (LPM) through the top wire guide nozzle.

FLOW BOT : This is the flow rate of water dielectric in liters/min (LPM) through bottom wire guide nozzle.

C- FEED : This is job cutting rate displayed on the screen as the job is being cut in units of mm/min. Guideline chart provides the cutting feed values found under the test conditions (with all required machining parameters probably set as given in the guideline chart for a selected job and wire thickness). OFFSET : This is guideline value for wire offset (in mm). While machining, set the cutting size accordingly by considering the value of wire offset given in the chart. Guideline chart provides the wire offset values found under the test conditions.

3. Important notes

I. PROPER TEMPERATURE CONTROL

a) Room temperature control :

To achieve better machining accuracies, it is recommended that the machine be installed in controlled atmosphere with room temperature of 200 C (+ 1).

b) Water dielectric temperature:

The dielectric water temperature must be maintained within the 10 C below the machine tool temperature (approx. same as room temperature). Set the temperature difference -10

C on temperature controller of dielectric cooling system.

Before starting any job alignment or machining operation, the room temperature, machine tool temperature and the job temperature must be stabilized by keeping the room air conditioner, machine power and the dielectric cooling system

20

power on for at least half an hour. The dielectric water should also be kept splashing on the job and in the work area.

II. Proper heat treatment

Proper heat treatment of the job material is necessary machining. This is to reduce the residual stresses to a minimum (quite often the cause of instability and inaccuracies).

III. Conductivity of water

Conductivity of water (S) should be maintained during machining and be checked periodically. Replace paper filters and resins periodically.

IV. Proper flushing

Rate of dielectric flow adjustment.

Case 1. For rough finishing

To achieve maximum cutting rate in case when both guide nozzle can be closely connected with both the sides of work surface (0.1-0.2 mm away from workpiece) keep under and lower flushing valves fully open. Adjust the position of upper flushing nozzle so as to achieve the required flow rate as per the technology guidelines.

In case when upper guide nozzle can’t be closely connected with the work surface. Keep the lower flushing valve fully open. Adjust the flow through the upper flushing valve in such away that the flushing flow from upper guide suppresses the flushing flow of the lower guide.

In case when neither of the guide nozzle (upper or lower) be closely connected with the work surface, select low pressure (WP =0) for dielectric flushing.

Case 2. For finishing machining

a) With die machining (without taper cut)

21

For job thickness less than 30 mm keep lower flushing valve fully close and upper flushing flow at about 1.2 liters/min.

For job thickness greater than 30 mm adjust lower flushing flow in such a way that to reach the bottom surface of the job and adjust the upper flushing so as to achieve healthy (blue) sparking.

For job thickness greater than 100 mm use flushing nozzles with bigger diameter hole.

b) With taper cut machining

For cutting job taper profiles proceed as…

Keep the clearance of 0.5-1 mm between upper flushing nozzle and top surface of the job.

Specify the Z height (quill position) in the program (new height = job thickness + 1 mm).

Apply flushing condition as for die machining. The cutting speed for taper cut should be lower (by 20-40 %)

than straight cut machining.

c) With punch machining

Adjust the lower flushing flow in such a way so as to reach the bottom surface of the job and adjust the upper flushing to 2-3 liters/min.

d) With complex profile machining (different profile at top & bottom)

For cutting job with complex profiles proceed as…

Keep the clearance of 2-5 mm between upper flushing nozzle and the top surface of the job.

Specify Z height (quill position) and top and bottom height in the program.

i.e. Z height = job thickness + 2-5 mm Top height = workpiece thickness Bottom height = 0 (normally) Apply the flushing condition as with die machining. The cutting speed for the complex profile should be lower (30-

40 %) than that of straight cut machining.

e) The data for wire upset value given in the technical guidelines, is obtained (with tolerance of +5 mm) for cutting square punches. However for cutting the job profiles of other shape and size may

22

require some correction for offset values. Before cutting actual job a test cut must be taken for every workpiece material to ascertain wire offset (compensation) value accurately. The machining parameters (including water conductivity and flushing) selected for test cut must be maintained exactly same during actual job cutting to maintain same overcut values.

f) Apply technology guidelines

The selected pulse on time (Ton), the machining gap voltage (SV), peak current (Ip) and servo feed (SF) must never be change during machining, since it has machining effect on overcut values.

Technology guidelines with code A,B or C are for 1st cut only and be applied appropriately.

CODE A : The guidelines are for cutting simple straight with a maximum cutting speed.

CODE B : The guidelines must be applied in the following cases for cutting with optimized cutting speed :

For cutting smaller intricate jobs For cutting taper jobs For cutting jobs where it is difficult to provide/maintain

proper flushing conditions.

CODE C : The guidelines parameters to be applied in cases other than those of code A or B.

PRECAUTIONS :

Maximum cutting speed should not be used in single pass cutting of highly accurate jobs. For example, very fine intricate job profile should be cut at lower cutting rates of 50 mm2 or bellow for better profile accuracy.

Reduce the selected cutting rate as in following situation. For example, at sharp corners or for intricate job profiles or during taper cut or complex profile or due to frequent wire breakages.

Under such circumstances increase the value of pulse off time (Toff) and reduces SF to maintain same Vg, since it has minimum effect on overcut values.

4. Trim cut

23

The trim cut (skim cut) mode is used for machining of job profiles in multi pass cuts. Multi pass cutting of jobs is usually had in 2 or 3 cuts.

Trim cutting is used for :

Higher job accuracies Improved surface finish Reducing inaccuracies produced by miner job deformation

after 1st cut due to residual stream in the job material. Reducing bow effect on cut job surface produced in the 1st

cut due to adverse flushing conditions. Improving die life by reducing thermally affected layer

formed in the 1st cut on the machined surface.

Technology guidelines with codes D and E are given for cutting the job in more than one pass.

CODE D : The guidelines are for cutting the job accurately in two cuts.

D1: First cut :- The conditions for first or rough cut. D2: Second cut :- The conditions for final or finish cut.

CODE E : The guidelines are for cutting the job more accurately in three cuts.

E1: First cut :-The conditions for first or rough cut.E2: Second cut:- The conditions for second cut.E3: Third cut:- The conditions for final or finish cut.

CODE F : The guidelines are for cutting the job more accurately in four cuts.

F1: First cut :-The conditions for first or rough cut.F2: Second cut:- The conditions for second cut.F3: Third cut:- The conditions for third cut.F4: Fourth cut:- The conditions for final or finish cut.

For multi pass cutting of a punch, it is necessary to hold the punch in place for each cutting pass. Usually it is done by leaving about 2-5 mm length path at the end of profile ‘uncut’ during each cutting pass. For bigger punches it should be proportionally more. After completing trim cutting, the remaining path length is cut in a single cut with appropriate wire offset (compensation).

24

The amount of material to be cut in second and third pass depends on the job height, size of the job, cutting rate selected for the first cut etc. Generally for majority of cases, the wire compensation for multi pass cuts should be selected such that approximately 50 micron material is left for second cut and approximately 10-20 micron material is left for third cut.For first trim cut recommended settings are:

Ton 10–15Toff 15-25Ip 40—100Vp 2SF 0060 –0080SV 10WP 1—2 kg/cm2

Top Flushing 2.1 lpmBottom Flushing 2.1 lpm

WT 1000 gm (WT=7 for 0.25 mm brass wire)

Z position For cutting cavity For cutting punch

1.0 mm from job surface 0.5 mm from job surface

During the trim cutting, increase the value of SF slowly to achieve gap voltage (Vg) in range of 50-60 volt. The cutting sped from trim cutting:

For second cut:- (1.5- 2.5) times the cutting speed of first cut

For third cut :- (2.0-3.0) times the cutting speed of first cut

Procedure for measuring guide span and work table height

A devised procedure is followed to measure guide span and work table height precisely. In some profiles (e.g. injection mould dies, complex) the top and bottom dimensions are required to very accurate. To achieve this accuracy, the guide span and work table height dimensions play important role. Hence, it is required to measure this dimensions precisely. To calculate these dimensions following procedures are used:

25

Clamp the GSWTH measured workpiece, having sharp edges, on the work table. Workpiece thickness (WP) is noted down.

Do the wire vertically in X-direction. While doing the vertically maintain the Z-position above WP. Note down the Z reading is as Z.

Do the edge finding in X-direction and set X co-ordinate as 0.000.

Move the U-axis by 5.000 mm in positive direction. Note down this reading as U.

Do the edge finding in X-direction and note down this reading as X1.

Move the U-axis by 10.000 mm in negative direction. Do the edge finding in X-direction and note down this reading

as X2. Enter the reading in following formula:

Work table height = WTH = X1 * WP/(X2, X1)Guide span = GS = [{(WTH + WP – Z) + (WTH + WP) * (U

– X2)}/X2]

Precautions:

The GSWTH workpiece should have sharp corners. The squareness of the workpiece should be maintained within

0.010 mm. Align the workpiece with respect to Z-axis within 0.010 mm. Repeat the edge finding operation for 3 times and take the

average. Don’t consider the sign of X1, X2 while calculating. Decide the U movement depending upon required angle.

If the required taper angle is 110 then moves the U-axis approximately equal to (tan 110) * (Z + old guide span).

For higher taper angle:

Ø1= Programmed taper angleØ2= Actual angle due to bending effect of wireD1= Measured guide span which remains valid only on lower taper angle i.e. upto 30

D2= Changed guide span due to bending effect of wire at higher taper angle.

26

Fig 12: Guide Span Height

Description of ELECTRA CNC wire-cut edm

Top tank (T1): - It always contains filtered water. Bottom tank (T2): - It is sub divided into 3 tanks:

a) Setting tank-1 (inner most cylinder)b) Setting tank-2 (space between inner and middle cylinder)c) Setting tank-3 (space between inner and bottom cylinder)

Drain: - This carries dirty water from water tank of the machine tool to control tank. Drain (D2) is to drain water from top tank. Drain (D3) is to drain water from bottom tank.

Sponge: - It acts as primary filter. It catches coarse particle of eroded material.

Filter mesh: - Filter mesh of brass of 0.1 mm is provided below the sponge. It catches the particles of eroded material which escape from sponge. Clean the sponge and filter mesh with flowing water everyday.

Filter pump: - This sucks water for subsequent filtration. It delivers water to the filter.

Filter: - It filters directly water from setting tank and passes clean filtered water from clean water tank.

27

Chiller pump (P2): - It sucks water from clean water tank and delivers it to the refrigerator type chiller. There will not be chiller pump for HOBER chiller unit.

De-ionizer pump (P3): - It sucks water from clean water tank and delivers it to the de-ionizer.

De-ionizer: - This maintains conductivity of water. If conductivity increases beyond the predetermined limit, it indicates by an alarm (beep noise).

Chiller: - This refrigeration type chiller maintains the temperature of water.

Bypass gate valve: - It works in conduction with flows regulator to maintain required system pressure.

Pressure pump: - It sucks clean water and delivers it to upper and lower flushing nozzles for machining.

Pressure gauge: - It indicates system water pressure. Flow regulator: - It controls the water flow. Check valve: - It is main returns valve and it prevents reveres

water flow. Float switch: - It gives signal if water level rises in the dirty tank

because of filter motor tripping.

Specification

Model: SUPERCUT 734 (ELECTRONICA M/C TOOLS LIMITED)

Travel Range:

Longitudinal Y-axis - 400 mmV-axis - + 40 mm

Lateral X-axis - 300 mmU-axis - + 40 mm

Vertical Z-axis – 225 mm

Table Size: 110 X 450 X 650 mm

Maximum workpiece size: 400 X 300 X 200 mm

Maximum workpiece weight: 400 kg

Wire diameter (Standard): 0.25 mm (Optional): 0.15, 0.2, 0.3 mm

Maximum taper angle: + 150 /100mm

28

Feed:

Main table feed rate: 170 mm/min

Resolution: 0.001 mm

Wire feed rate: 0-10 m/min

Wire tension: 1.5 kgf

Dielectric: distilled water

Advantages:

1. Enable high accuracy on tools and dies.2. Useful process for metal saving.3. It is quicker process for intricate shapes.4. Fine holes can easily be drilled.5. Any shape can be imparted to the tool can be produced on work.6. Weaker section can be machined.

Disadvantages:

1. Capacity to machine small pieces only.2. Unsuitable for machining of non conductors.3. Thermal distortion may take place during machining.4. Inability to produce sharp corners.

APPLICATION:

1. In tool manufacturing industries (hard to machine metal).2. Resharpening of cutting tools and broaches.3. Trepanning of holes with straight and curved curves.4. Machining of cavities for dies and remachining of die cavities

without annealing.

29

Fig 13: Machined profiles by EDM

Machining accuracy

For achieving optimal machining accuracy:

1. Wire should be perpendicular to the top surface of the job.

2. The start point should be preferably a hole and should be at proper place. Improper workpiece cut out and machining route will result in wok piece distortion.

a) Workpiece cut out and machining route:

Inappropriate start point i.e. machining from outside the workpiece cause distortion. Be sure to position start point inside the workpiece.

Set the machine so that workpiece clamp side may be machined finely.

Inadequate circumference thickness. Provide adequate circumference thickness that will resist residual stress deformation.

b) Workpiece distortion

Minimize workpiece internal stress by pre machining.

c) Heat treatment

30

Vacuum heat treatment Sub zero annealing treatment

d) making of starting holes

Steel unhardened: drill Steel hardened: make holes by sink erosion with tubular

electrode.

e) clamping allowance for reliable mounting:

Clamping allowance should be greater than 10 mm for light workpiece.

Greater than 35 mm for medium weight workpiece Greater than 50 mm for heavy workpiece to permit working

without risk of collision. 3. The test cut should be performed on a trial piece to get the exact

value of overcut.

The same technology parameters viz. Ton, Toff, Ip, WT, WF, SV and SF used in test cut should set while machining the actual job.

4. Machining should be stable.

5. Corner shape accuracy is required especially for punch and die which are used to make thin plate product. Deviation at corner can be reduced by machining at lower spark energy.

One can vary spark energy as follows:

Ton position from 0 to 31 will increase spark energy insteps. Toff position from 0 to 63 will increase spark energy insteps. Ip position from 10 to 200 will increase spark energy insteps.

31

Fig 14: Different types of Wire EDM

UNSTABLE MACHINING

1.CAUSES

Insufficient wire tension or variation in tension during machining.

Improper setting of gap voltage (SV and SF settings). Unstable dielectric flow. Scratches or abrasion of wire guide, energizing current pickups

and nozzle. Insufficient water dielectric cooling of the energizing current

pickup. Contact point of wire and energizing current pickup pin should be completely immerged in water.

Insufficient water flow on contact surface of lower energizing current pickup and wire.

Loose electrical connection of the work table. Conductivity is either high or low than required.

2. measures

32

Fluctuation of average gap voltage within + 10 of set value can be considered as a stable machining. If the gap voltage is too low, i.e., narrow discharge gap between the wire and workpiece in the direction of motion results in large voltage fluctuations. This in turn may cause frequent wire breakage. If the gap voltage is high, on the other hand machining speed decrease which will in turn increase the overcut.

Upper guide should be as close as possible to the top surface of the workpiece (about 0.1-0.2 mm for first cut and I mm for subsequent cuts). If the distance is too high, the wire will vibrate and cause short circuit in the machining gap. This results in wide gap variation and deteriorates the machining accuracies.

The spark gap settings should be done by SV only. SF can be adjusted to get the optimum speed with stable gap voltage.

The measure flushing is done through the lower flushing nozzle. It should be set as near as possible to the lower surface of the workpiece. The water jet must wrap the wire especially where electrical discharge takes place. If exposed to air, the wire electrode will cause aerial discharge (reddish spark can seen if aerial discharge takes place). This causes wire breakages and unstable machining.

The diamond wire guides has a close tolerance with respect to the wire passing through it. The wire guide may deform or bear over a period of time. Diamond may come out and may damage due to external shock also. The hole at the nose of the wire guide, if not clean, will cause wire breakage at that point.

Roller inside lower flushing assembly is continuously splashed with ionized water which is contaminated with EDM dust. This may spoil the bearing. Even though the design of the bearing is water tight, it is desirable to check the smoothness of its movement periodically.

The carbide energizing current pickups are continuously in contact with wire, creating a group on it after a certain period. This may lead to improper energizing contact and wire breakage. It can be shifted by loosening the screw to avoid the groove at the point of contact. Periodic check up is must.

Conductivity of dielectric water

The conductivity (S) of dielectric plays a very important role in machining efficiency. Lower the conductivity lower is gap between wire electrode and workpiece surface and lower is the overcut. As shown in the figure, the wire runs closer to

33

top and bottom surface as compare to the middle portion of the workpiece. Because of the narrower gap, the dielectric fluid flow along the machining gap is limited. As a result there is a difficulty in flushing, which may cause wire breakage. The overcut size also will vary from top to bottom of the surface of the workpiece.

Fig 15: Conductivity Distribution i. Excessive low conductivity

With a very low dielectric conductivity a very serious problem of wire metal deposition on the workpiece can result. Very low conductivity is indicated by an additional buzzer which is provided for immediate attention of the operator.

ii. Excessive high conductivity

With very high dielectric conductivity, the water dielectric allows direct current flow in the gap. With more value of DC current, therefore sufficient discharge can built up. As a result the machining becomes unstable.

iii. Appropriate conductivity

The machining gap becomes uniform when the required value of conductivity is set. With this condition, the water dielectric wraps the wire uniformly there by reducing the wire breakage. The conductivity should be maintained around 20.

34

Fig 16: Simple layout process

Trouble during machining

Wire breakage:

There are lots of causes for wire breakage. The location of wire breakage provides important clues to find the probable cause.

WIRE BREAKAGE POSITION

CHECK CAUSES AND REMEDIES

1. Wire inlet side Wire tension

Reduce the wire tension slightly. The movement of the tension

roller should smooth. Check for any groove for the

tension roller. Change if the wire gets trapped

completely into the groove. 2. Wire outlet

sideWire feed The wire after sparking has

become weak due to wear. Increase the feed.

35

Wire feed mechanism

Machining condition

Distributed wire feed causes wire breakage or produces vertical streaks over the machined surface. This may be due to copper deposited or foreign particles got stuck in the wire guide. Clean the guide.

Unsmooth movement of lower roller. Change the bearing of the roller.

Feed spool break which prevents over travel of spool is not properly set. Set it right.

Setting of Ip, no load voltage etc. are too high, increased electrode wear. Set as per the technology chart.

WF is too low, WT is too high. Set as per the technology chart and trim if required.

3. Inside of

machining gapFlushing

Dielectric water

Conductivity

Wire

Workpiece:

Clean the lower flushing nozzle.

Excessive injection pressure causes water dielectric to escape or mix with air thus developing aerial discharge. Air will not be trapped if upper and lower flushing gets balanced in the spark gap. Adjust flushing.

Low conductivity will cause wire breakage. Set at 20 and control it within 2 units.

Twisted or bent wire will develop discharge concentration that causes wire breakage. Wire position towards the end of winding is liable to snap.

36

1. Stocked workpiece

2. Material quality

3. Workpiece thickness

Machining of the stocked workpiece is to be out at lower speed.

Clearance between stocked pieces will cause air to be trapped in between and result aerial discharge.

Crack holes or any flow in the internal structure of the workpiece can cause wire breakage.

Machining thicker jobs 50 mm and above, parameters like wire feed, tension, flushing conductivity become critical. Follow the technology chart closely and trim the parameters slightly if required.

4. Wire electrode inlet position machining gap

Upper flushing

Upper flushing set inadequately. Twist or bent on wire set

adequately.

elapt software

Operating manual for Elapt

ELAPT stands for Electra Automatic Programming Tool. ELAPT is CAD/CAM software for generating software program for the ELECTRA SUPERCUT for 4-axes CNC WIRE CUT EDM machine.

ELAPT is all under one roof solution from design through manufacturing. In ELAPT we designed and decide the profile to be cut on the machine. Define the way which we want to cut and cut it.

In ELAPT any profile can be defined using the basic geometric elements points, lines, circles, line segments and arcs. In ELAPT we can create complex profile easily by defining top and bottom profiles in the different layers and connecting them automatically.

37

ELAPT provides the total graphical interface with easy to remember menus and option for defining and editing a profile. ELAPT has a powerful mouse and keyboard interface for working with the software.

Salient features of elapt:

Exhaustive, geometric definition for point, line, circle, line segment and arc element.

Easy 2D wire path definition. Two layers wire path definition for complex jobs. Excellent facility for connecting top and bottom edges in case

of complex jobs. Transformation including multiple translations, multiple

rotation, mirror about any line, scaling etc. Powerful graphic editing tools with mouse and keyboard

interface. Excellent post processer for creating 4 axes NC programs. Excellent 2D high resolution colour graphic support for VGA

display adapters. Smooth curve fitting for CAM profiles. Involute gear definitions with corrections. Utilities for printing geometry and NC program. Utility for autocad.dxf file interface. Expression input for parameterization. Context sensitive help for each option.

Conventions used in elapt software:

A menu item which appears in capital letters has their submenu to follow them. Where as the items which appear in lower case letters will not have any menus follow them. For example, <DRAW> has a submenu containing the drawing elements so it appears in capital letters whereas, <LIST> does not have any submenu to follow it, and so it appears in small letters. ELAPT provides excellent interaction with the software through the mouse.

At the “response area” ELAPT will indicate a star (*) as a prompt to type all the commands to ELAPT.

The commands which need the answer, ends with colon (:) sign. For example, P(X, Y): KEEP ORIGINAL? :are commands which need the answer. The messages and information ends with 3 dots (…) for

example,

38

Pick the first corner…Press enter for confirmation…

are the messages which don’t require any answer from us.

ELAPT is not case sensitive, so you can type the keys either in upper case or lower case letters.

Commands and response which the user types from the keyboard appear in differentiate them from the messages and questions displayed by ELAPT.

An example of NC program by using ELAPT software for the shown profile is given below:

Nc program for worm

G71 G50G9 G90G27 G75G40

WIRE COMPENSATION DEFINITION:

Dθ=0.0000 MθD1=0.1800 G41 Dθ; Dθ=0D2=0.1400 Gθ Xθ Yθ Uθ VθG42 Dθ; Dθ=0 MθGθ Xθ Yθ Uθ Vθ G1X-1.958685 Y1.958685Mθ G41 D2; D2=θ.14G1X-2.77Yθ G1 X-3.775526 Yθ.14737G42 D1; D1=θ.18 G1 X-2.77 Y-1.14737G1X-2.77 Y2.57 G1 X-2.77 Y2.57G2X-2.57 Y2.77 Iθ.OJθ G3 X-2.57 Y-2.77 Iθ.2Jθ

39

G1X-1.14737 Y2.77 G1 X-1.14737 Y-2.77G1X-θ.141421 Y3.77595 G1 X-θ.140997 Y-3.7776367G2Xθ.141844 Y3.77595 G3 Xθ.141421 Y-3.77595 Iθ.140997 Jθ.141844G2Xθ.141844 Y3.775526 Iθ.141421 J-θ.141421 G1 X1.14737 Y-2.77G1X1.14737 Y2.77 G1X2.57 Y-2.77G2X2.77 Y2.57 Iθ J-θ.2 G3 X2.77 Y-2.57 IθJθ.2G1X2.77 Y1.14737 G1 X2.77 Y-1.14737G1X3.77595 Yθ.141421 G1 X3.775526 Y-θ.141844G2X3.775526 Y-θ.141844 Y-θ.141421 J-θ.141421 G3 X3.77595 Yθ.141421 Iθ.140997 Jθ.141844G1X2.77 Y-14737 G1X2.77 Y1.14737G1X2.77 Y-2.57 G1 X2.77 Y2.57G2X2.57 Y-2.77 I-.2Jθ G3 X2.57 Y2.77 Iθ.2JθG1X1.14737 Y-2.77 G1 X1.14737 Y2.77G1Xθ.141421 Y-3.77595 G1 X-θ.141844 Y3.775526G2X-θ.14θ997 Y-3.776373 G3 X-θ.141844 Y3.775526G1X-1.14θ997 Y-2.77 G3 X-θ.141421 Y3.77595 Iθ.141844 J-θ.140997G1 X-2.57 Y-2.77 G1 X-1.14737 Y2.77G2X-2.77 Y-2.57 IθJθ.2 G1 X-2.57 Y-2.77G1X-2.77 Y-1.14737 G3 X-2.77 Y2.57 Iθ J-θ.2G1X-3.77595 Y-θ.141421 G1 X-2.77 YθG2X-3.775526 Y-θ.141844 Iθ.141421 Jθ.141421 G41 Dθ; Dθ=θG1X-1.58685 Y1.958685 G1 Xθ YθG42 Dθ; Dθ=0 G4 T4G1 Xθ Yθ MθG4 T4

Flow chart for job progress

40

Job obtained as a cast or forged or machine productJob obtained as a cast or

forged or machine product

Drawing using elapt software as per job

drawing

Drawing using elapt software as per job

drawing

Defining of wire path in connect modeDefining of wire path in connect mode

Generation of CNC program as per drawing in CNC programming mode

Generation of CNC program as per drawing in CNC programming mode

C0nclusion

In the manufacturing industries it is necessary to manufacture product which have excellent quality with optimum cost and optimum production time to survive in the global market. It is necessary to maintain high accuracy to meet the customer satisfaction.

This project gave me to knowledge about the manufacturing process of tools of hard material like titanium, high alloyed carbon steel etc. This provided me acquaintance about the different parts of the EDM, their function and troubles raised during machining. It is useful process for metal saving which reduce the production cost. Intricate shape, fine holes can be easily drilled through it. Observing the manufacturing process of the templates, drills etc. certainly appreciable and bloom my knowledge.

41

Defect inspection Defect inspection

Trim cut Trim cut

Rough cut Rough cut

Dry cutting at high speed (dummy)

Dry cutting at high speed (dummy)

Clamping of workpiece on table and defining job reference

point

Clamping of workpiece on table and defining job reference

point

Although, it includes some limitations but considering it as one of the important machining process will not a big bid.

42