ELECRO CHEMICAL MACHINING

•      ECM, a dc voltage (10-25 v) is applied across the gap between a pre-shaped cathode tool and an anode workpiece.

•      The workpiece is dissolved by an electrochemical reaction to the shape of the tool.

The electrolyte flows at high speed (10-60 m/s) through the gap (0.1-0.6 mm) to dissipate heat and wash away the dissolved metal

•      The material removal rate by ECM is given by:

      MRR = C Ih

      where, MRR=mm³/min,

      I=current in amperes,

      h=current efficiency, which typically ranges from 90-100%,

      C is a material constant in mm³/A·min.

•      Feed rate (mm/min): f = MRR / A₀

      Assuming a cavity with uniform cross-sectional area A₀

Advantages of ECM

•      The components are not subject to either thermal or mechanical stress.

•      No tool wear during ECM process.

•      Fragile parts can be machined easily as there is no stress involved.

•      ECM deburring can debur difficult to access areas of parts.

•      High surface finish (up to 25 µm in) can be achieved by ECM process.

•      Complex geometrical shapes in high-strength materials particularly in the aerospace industry for the mass production of turbine blades, jet-engine parts and nozzles can be machined repeatedly and accurately.

•      Deep holes can be made by this process.

•     
Limitations of ECM

E      ECM is not suitable to produce sharp square corners or flat bottoms because of the tendency for the electrolyte to erode away sharp profiles.

•      ECM can be applied to most metals but, due to the high equipment costs, is usually used primarily for highly specialised applications.

•      Complex cavities in high-strength materials, esp. in the aerospace industry for mass production of turbine blades.

ULTRASONIC MACHINING - USM

1.Introduction

Ultrasonic machining was a non-traditional machining process. USM was grouped under the mechanical group NTM processes. briefly depicts the USM process.

            In ultrasonic machining, a tool of desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15 – 50 μm over the workpiece. Generally the tool was pressed downward with a feed force, F. Between the tool and workpiece, the machining zone was flooded with hard abrasive particles generally in the form of a water based slurry. 

As the tool vibrates over the workpiece, the abrasive particles act as the indenters and indent both the work material and the tool. The abrasive particles, as they indent, the work material, would remove the same, particularly if the work material was brittle, due to crack initiation, propagation and brittle fracture of the material. Hence, USM was mainly used for machining brittle materials {which are poor conductors of electricity and thus cannot be processed by Electrochemical and Electro-discharge machining (ECM and ED)}.

2. Mechanisms of Material Removal in USM and its modelling

As had been mentioned earlier, USM was generally used for machining brittle work material. Material removal primarily occurs due to the indentation of the hard abrasive grits on the brittle work material. As the tool vibrates, it leads to indentation of the abrasive grits. During indentation, due to Hertzian contact stresses, cracks would develop just below the contact site, then as indentation progresses the cracks would propagate due to increase in stress and ultimately lead to brittle fracture of the work material under each individual interaction site between the abrasive grits and the workpiece. The tool material should be such that indentation by the abrasive grits does not lead to brittle failure. Thus the tools are made of tough, strong and ductile materials like steel, stainless steel and other ductile metallic alloys.

Other than this brittle failure of the work material due to indentation some material removal may occur due to free flowing impact of the abrasives against the work material and related solid-solid impact erosion, but it was estimated to be rather insignificant. Thus, in the current model, material removal would be assumed to take place only due to impact of abrasives between tool and workpiece, followed by indentation and brittle fracture of the workpiece. The model does consider the deformation of the tool.

3. Machine

The basic mechanical structure of an USM was very similar to a drill press. However, it had additional features to carry out USM of brittle work material. The workpiece was mounted on a vice, which could be located at the desired position under the tool using a 2 axis table. The table could further be lowered or raised to accommodate work of different thickness. The typical elements of an USM are

•        Slurry delivery and return system

•        Feed mechanism to provide a downward feed force on the tool during machining

•        The transducer, which generates the ultrasonic vibration

•        The horn or concentrator, which mechanically amplifies the vibration to the required amplitude of 15 – 50 μm and accommodates the tool at its tip.

The ultrasonic vibrations are produced by the transducer. The transducer was driven by suitable signal generator followed by power amplifier. The transducer for USM works on the following principle

•        Piezoelectric effect

•        Magnetostrictive effect

•           Electrostrictive effect

Magnetostrictive  transducers  are  most  popular  and  robust  amongst  all.

Fig. 9.2.8 shows a typical magnetostrictive transducer along with horn. The horn or concentrator was a wave-guide, which amplifies and concentrates the vibration to the tool from the transducer.

4.Applications

•        Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics, carbides etc.

•        Used for machining round, square, irregular shaped holes and surface impressions.

•        Machining, wire drawing, punching or small blanking dies.

5.Limitations

•      Low MRR

•      Rather high tool wear

•      Low depth of hole

ELECTRON BEAM MACHINING --- EBM

Process

 

Equipment used in the Electron Beam Machining:

• Cathode, Bias grid, anode, electromagnetic lens, electromagnetic coils, deflector coils, telescope, vacuum gauge, throttle valve, diffusion pump.

• The electron beam machine consists of an electron beam gun used to produce free electrons at the cathode. The high velocity particles are moving through the small spot size. The cathode (tool) was made of tantalum or tungsten material. The cathode filaments are heated to a temperature of 2500to 3000and the heating leads to thermo – ionic emission of electrons. The magnitude varies from the 25 mA to 100 mA. The solidities lies between 5 Ac to 15 Ac .The emission current was influenced by the voltage that was nearly 150kV, and the current was applied between the anode and cathode to release the electrons in the direction of work piece.
• Bias grid: It was also known as grid cup. The grid cup was a negative that was subjected with respect to the filament. So, the electrons generated with the help of the cathode would directly flow towards the anode. During the flow of the electrodes no diversions are seen. The anode attracts the electrons and gets accelerated; the electrons would gain a high velocity.
• The cathode controls the flow of the electrons, and the grid cup used to operate the gun in pulsed mode only.
• After the anode the electron beam passing through the magnetic lens and the apertures are connected in series. The magnetic lens was used to shape the electron beam and reduce the diversion factor.
• The apertures allow the convergent electrons to permit and caught the low energy divergent electrons from the fringes.
• Finally the electron beam passes through the electromagnetic lens and deflection coil. Then the deflection coil sends the electron beam through the hole, to improve the shape to machine a hole.
• The vacuum was created between the work piece and the electron beam gun, and there was a series of rotating disc with slots.
• The disc allows the electron beam to pass over the material for machining, and it prevents from the fumes and vapors generated during the machining.
• Work piece was placed on the CNC bench. Then holes of any shape are made on the work piece material. In the gun beam flection and CNC control are used to shape.
• Vacuum was maintained in gun, and the vacuum ranges from Suitable vacuum was maintained because the electron as it does not lose their energy, and where the life of the cathode was obtained. By using the diffusion pump and rotary pump the vacuum was maintained.
• Diffusion pump should act as an oil heater. If the oil was heated then the oil vapor rushes upwards. The nozzle changes the direction of the oil vapor and starts moving in the downward direction at high velocity. The oil vapors are reduces in the diffusion pump; this was because of the presence of the cooling water cover.
  Capability of Electron Beam machining process
• Electrical beam machining makes a hole ranges from 100 to 2 mm.
• Compares to the steels aluminum and titanium was freely machined.
• Based upon the type of the material, power density, depth of cut holes diameter, which are the reasons for the number of holes drilled per second on the material.
• Holes could be elongated along with the barrel shape or depth.
• Reverse tapper could also be performed below the surface of the work piece material.
• In the electron discharge machining Cut formation was not observed
• With the help of the electron discharge machining we could machine the wide range
• The depth of cut must be 15 mm with a length to diameter ratio of nearly 10.
• of materials like stainless steel, aluminum, steel, plastics, ceramics etc.
• In EBM the heat affected zone was narrow; this was because of the short pulse occurrence. The heat affected zone was nearly 20 to 30
• The EBM does not apply any cutting forces on the material.
• During the process very simple investment was required for work
• EBM process allows machining of brittle and fragile materials.
• Holes are drilled at an angle of 20 to 30

Electron Beam Machining – Advantages and Limitations

                           

EBM provides very high drilling rates when small holes with large aspect ratio are to be drilled. Moreover it could machine almost any material irrespective of their mechanical properties. As it applies no mechanical cutting force, work holding and fixturing cost was very less. Further for the same reason fragile and brittle materials could also be processed.The heat affected rather less due to shorter pulses. EBMcan provide holes of any shape by combining beam deflection using elelctromagnetic coils and the CNC table with high accuracy. However, EBM had its own share of limitations. The primary limitations are the high capital cost of the equipment and necessary regular maintenance applicable for any equipment using vacuum system. Moreover in EBM there was significant amount of non-productive pump down period for attaining desired vacuum. However this could be reduced tosome extent using vacuum load locks. Though heat affected zone was rather less in EBM but recast layer formation cannot be avoided.