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Introduction to Manufacturing Technology –Lecture 5

Introduction to Manufacturing Technology –Lecture 5. Instructors: (1)Shantanu Bhattacharya, ME, IITK, email: bhattacs@iitk.ac.in (2)Prof. Arvind Kumar, ME, IITK email: arvindkr@iitk.ac.in. Review of last lecture.

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Introduction to Manufacturing Technology –Lecture 5

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  1. Introduction to Manufacturing Technology –Lecture 5 Instructors: (1)Shantanu Bhattacharya, ME, IITK, email: bhattacs@iitk.ac.in (2)Prof. Arvind Kumar, ME, IITK email: arvindkr@iitk.ac.in

  2. Review of last lecture Shaw’s model of projected grains and parity of modeled MRR with experimental MRR. Theoretical and actual trends of MRR with Feed Force, Frequency, Amplitude, Work hardness to tool hardness ratio, Mean grain diameter, abrasive concentration and viscosity. Numerical estimation of MRR and time of machining of square hole.

  3. Numerical Problem Determine the percentage change in the machining time for an USM operation cutting WC plates when the tool material is changed from copper to stainless steel.

  4. Numerical Problem

  5. Numerical Problem

  6. Ultrasonic Machining Unit • The main units of an Ultrasonic Machining unit are shown in the figure below. It consists of the following machine components: • The acoustic head. • The feeding unit. • The tool. • The abrasive slurry and pump unit. • The body with work table.

  7. Acoustic Head The Acoustic head’s function is to produce a vibration in the tool. It consists of a generator for supplying a high frequency electric current, a transducer to convert this into a mechanical motion (in form of a high frequency vibration). A holder to hold the head. A concentrator to mechanically amplify the vibration while transmitting it to the tool. Most transducers work on the magneto-strictive principle because of high efficiency, high reliability in the 15-30 KHz range, low supply voltage, and simple cooling arrangement. Stampings are used to reduce loss as in transformers. The dimensions are so chosen that the natural frequency coincides with the electric supply frequency.

  8. Acoustic Head • The main purpose of the concentrator is to increase the amplitude to the level needed for cutting. • Various types of concentrator are used. • The figure below shows how the amplitude of longitudinal vibration of the transducer-concentrator assembly is amplified. It should be noted that the system has to be held to the main body at a nodal point.

  9. Feed Mechanism • The objective of the feed mechanism is to apply the working force during the machining operation. • An instrument showing the movement of the tool indicates the depth of machining. • The basic type of feed mechanisms are the following: • Counterweight type • Spring type • Pneumatic and hydraulic type • Motor type.

  10. Design consideration for theTool • The tool is made up of a strong but ductile metal. • Stainless steels and low carbon steels are used for making the tools. • Aluminum and brass tools wear ten and five times faster than steel tools. • The geometrical features are decided by the process. • The diameter of the circle circumscribed about the tool should not be more than 1.5-2.0 times the diameter of the end of the concentrator. • The tool should be as short and rigid as possible. • When the tool is made hollow the internal contour should be parallel to the external one to ensure uniform wear. • The thickness of any wall or projection should be atleast five times the grain size of the abrasive. • In the hollow tool the wallshould not be made thinner than 0.5-0.8 mm. • When designing the tool consideration should be given to the side clearance which is normally of the order of 0.06-0.36mm, depending on the grain size of the abrasive.

  11. Abrasive Slurry • The most common abrasives are Boron Carbide (B4C), Silicon Carbide (SiC), Corrundum (Al2O3), Diamond and Boron silicarbide. • B4C is the best and most efficient among the rest but it is expensive. • SiC is used on glass, germanium and most ceramics. • Cutting time with SiC is about 20-40% more than that with B4C. • Diamond dust is used only for cutting diamond and rubies. • Water is the most commonly used fluid although other liquids such as benzene, glycerol and oils are also used.

  12. Summary

  13. Introduction to Abrasive Jet Machining (AJM) • In AJM, the material removal takes place due to impingement of the fine abrasive particles. • The abrasive particles are typically of 0.025mm diameter and the air discharges at a pressure of several atmosphere.

  14. Mechanics of AJM • Abrasive particle impinges on the work surface at a high velocity and this impact causes a tiny brittle fracture and the following air or gas carries away the dislodged small work piece particle.

  15. Mechanics of AJM • The process is more suitable when the work material is brittle and fragile. • A model for the material removal rate (MRR) is available from Sarkar and Pandey, 1980. The MRR Q = χ Z d3 v3/2 (ρ/ 12Hw)3/4 Where Z = No. of abrasive particle impacting per unit time. d = Mean diameter of the abrasive grain v = Velocity of the abrasive grains ρ = Density of the abrasive material Hw = Hardness of the work material χ = Is a constant.

  16. Process parameters • The process characteristics can be evaluated by judging (1) the mrr (2) the geometry of the cut (3) the roughness of the surface produced (4) the rate of nozzle wear. • The major parameters which control these quantities are: • The abrasive (composition, strength, size and mass flow rate). • The gas (composition, pressure and velocity). 3. The nozzle (geometry, material, distance from and inclination to the work surface).

  17. The Abrasive • Mainly two types of abrasives are used (1) Aluminum oxide and (2) Silicon carbide. (Grains with a diameter 10-50 microns are readily avialable) • For good wear action on the surfaces the abrasive grains should have sharp edges. • A reuse of the abrasive powder is normally not recommended because of a decrease of cutting capacity and clogging of the nozzle orifices due to contamination. • The mass flow rate of the abrasive particles depends on the pressure and the flow rate of the gas. • There is an optimum mixing ratio (mass fraction of the abrasive) for which the metal removal rate is the highest. • When the mass flow rate of the abrasive increases the material removal rate also increases.

  18. The gas • The AJM unit normally operates at a pressure of 0.2-1.0 N/mm2 . • The composition of gas and a high velocity has a significant impact on the MRR even if the mixing ratio is not changed. The nozzle • The nozzle is one of the most vital elements controlling the process characteristics. • The nozzle material should be hard to avoid any significant wear due to the flowing abrasive. [Normally WC (avg. life: 12-30 hrs.) or Sapphire (Appr. = 300 hrs.) are used] • For a normal operation the cross-sectional area of the orifice can be either circular or rectangular and between 0.05- 0.2mm2 .

  19. Nozzle to tip distance (Stand off distance) • The nozzle tip distance (NTD) or the stand off distance is a critical parameter in AJM. • The NTD not only affects the MRR from the work surface but also the shape and size of the cavity produced. • As shown in the figure below, the velocity of the abrasive particles impinging on the work surface increases due to their acceleration after they leave the nozzle. This increases the MRR. • With a further increase in the NTD, the velocity reduces due to the drag of the atmosphere which initially checks the increase in MRR and then decreases it.

  20. Photo graphs of the actual Machined Cavity profile at different NTD Profile of the machined cavity at different stand off distances (a) 2mm (b) 6mm (c) 10mm (d) 14mm (e) 16mm (f) 20mm

  21. Mixing and Mass Ratio • Mixing ratio (M) also influences the MRR. M = Volume flow rate of abrasive particles Volume flow rate of carrier gas = Ůa/ Ůg • In place of M the mass ratio α may be easy to determine. α = Ma/ Ma+c = Abrasive mass flow rate Abrasive and carrier gas combined mass flow rate

  22. Numerical Problem • During AJM, the mixing ratio used is 0.2. Calculate mass ratio if the ratio of density of abrasive and density of carrier gas is equal to 20.

  23. Numerical Problem • Diameter of the nozzle is 1.0mm and the jet velocity is 200m/s. Find the volumetric flow rate (cm3/sec) of the carrier gas and the abrasive mixture

  24. Abrasive Jet Machines The abrasive jet machines are manufactured by a single manufacturer M/s Airbrasives. (SS White and Co., New York) • The gas propulsion system supplied clean and dry gas (air, nitrogen, or CO2) to propel the abrasive particles. • The gas may be supplied either by a cylinder or a compressor. • In case of a compressor a filter or a dryer may be used to avoid water or oil contamination to the abrasive powder. • The gas should be non toxic, cheap and easily available and should not excessively spread when discharged from nozzle into atmosphere.

  25. Electrochemical Machining (ECM) • Electrochemical machining is one of the most unconventional machining processes. • The process is actually the reverse of electroplating with some modifications. • It is based on the principle of electrolysis. • In a metal, electricity is conducted by free electrons but in a solution the conduction of electricity is achieved through the movement of ions. • Thus the flow of current through an electrolyte is always accompanied by the movement of matter. • In the ECM process the work-piece is connected to a positive electrode and the tool to the negative terminal for metal removal. • The figure below shows a suitable work-piece and a suitably shaped tool, the gap between the tool and the work being full of a suitable electrolyte.

  26. Electrochemical Machining • The dissolution rate is more where the gap is less and vice versa. • This is because the current density is inversely proportional to the gap.

  27. Electrochemical Machining • The dissolution rate is more where the gap is less and vice versa (as the current density is proportional to the gap. • Now, if the tool is given a downward motion, the work surface tends to take the same shape as that of the tool, and at a steady state the gap is uniform. • Thus the shape of the tool is represented in the job. • In an electrochemical machining process, the tool is provided with a constant feed motion. • The electrolyte is pumped at a high pressure through the tool and the small gap between the tool and the work-piece. • The electrolyte is so chosen that the anode is dissolved but there is no deposition on the cathode. • The order of the current and voltage are a few 1000 amps and 8-20 volts. The gap is of the order of 0.1-0.2mm . • The metal removal rate is typically 1600 mm3/sec for each 1000 Amp. • Approximately 3 KW-hr. are needed to remove 16000 mm3 of metal which is almost 30 times the energy required in a conventional process.

  28. Electrochemical Machining • With ECM the rate of metal removal is independent of the work-piece hardness. • ECM becomes advantageous when either the work material possesses a very low machinability or the shape to be machined is complex. • Unlike most other conventional and unconventional processes, here there is practically no tool wear. • Though it appears that, since machining is done electrochemically, the tool experiences no force, the fact is that the tool and work is subjected to large forces exerted by the high pressure fluid in the gap.

  29. Electrochemistry of ECM process • The electrolysis process is governed by the following two laws proposed by Faraday. • The amount of chemical change produced by an electric current, that is, the amount of any material dissolved or deposited, is proportional to the quantity of electricity passed. • The amounts of different substances dissolved or deposited by the same quantity of electricity are proportional to their chemical equivalent weights. • In the quantitative form, Faraday’s two laws state that • m α I t ε • Where, m = weight (in grams) of a material dissolved or deposited, • I = Current (in amperes) • t = time (in seconds) • ε = gram equivalent weight of the material.

  30. Ion-Ion and ion-solvent interaction • Although strong electrolytes are completely ionized, their ions are not entirely free to move independently of one another through the body of a solution, except when this is infinitely dilute. • The following things happen in such a situation: • Ions will move randomly wrt each other due to fairly violent thermal motion. • Coulombic forces between ions of same and opposite kinds will be present which leads to a time averaged ion atmosphere of one kind wrt. To a central ion of the opposite kind. • Movement of such ions under an external electric field will be very slow. So the atmosphere moves to the opposite direction as the primary central ion resulting in a continouous disruption and reformation of the atmosphere. (atmosphere assymetrically distributed around the central ion) and electrophoretic effect(viscous drag of the atmosphere)

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