Unconventional Machining Processes MCQ Quiz in தமிழ் - Objective Question with Answer for Unconventional Machining Processes - இலவச PDF ஐப் பதிவிறக்கவும்

Concept:

Ultrasonic machining is an operation that involves a vibrating tool fluctuating at the ultrasonic frequencies to remove the material from the workpiece.

The process involves an abrasive slurry that runs between the tool and the workpiece.

It is typically used on brittle materials as well as materials with a high hardness due to the microcracking mechanics.

Explanation:

Ultra Sonic Machining (USM):

• Ultrasonic machining is a non-conventional machining process. USM has grouped under the mechanical group non-conventional machining processes.
• In ultrasonic machining, a tool of the desired shape vibrates at an ultrasonic frequency (19 ~ 25 kHz) with an amplitude of around 15 – 50 μm over the workpiece.
• Generally, the tool is pressed downward with a feed force, F between the tool and workpiece.
• The machining zone is 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 is brittle, due to crack initiation, propagation, and brittle fracture of the material.
• Hence, USM is mainly used for machining brittle materials which are poor conductors of electricity and thus cannot be processed by Electrochemical and Electro-discharge machining. 

Concept:

Ultrasonic machining (USM):

• Ultrasonic machining is an operation that involves a vibrating tool fluctuating at the ultrasonic frequencies to remove the material from the workpiece.
• The process involves an abrasive slurry that runs between the tool and the workpiece.
• It is typically used on brittle materials as well as materials with a high hardness due to microcracking mechanics.

• In ultrasonic machining, a tool of the desired shape vibrates at an ultrasonic frequency (19 ∼ 25 kHz) with an amplitude of around 15 – 50 μm over the workpiece.
• Generally, the tool is pressed downward with a feed force, F.
• Between the tool and workpiece, the machining zone is 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 work material, particularly if the work material is brittle (due to crack initiation, propagation and brittle fracture of the material).

USM MRR vs Feed Force

With an increase in the frequency of the tool head, the MRR should increase proportionally. However, there is a slight variation in the MRR with frequency. 

MRR increases with increasing feed force but after a certain critical feed force, it decreases because the abrasive grains get crushed under heavy load.

With increases of feed force, the material removal rate MRR is first increases and then decreases.

Confusion PointsThe relation of MRR with the parameter feed force has not been mentioned in the question and has been asked in the official exam in this manner only.

Explanation:

Electron Beam Machining (EBM): In this machining workpiece placed in the vacuum chamber and High‐voltage electron beam directed toward the workpiece. The energy of electron beam melts/ vaporizes selected region of the workpiece. The electron beam is moved by deflection coils.

The operation is performed in the vacuum to prevent the reduction of electron velocity.

Important Points

Concept:

\(Duty\ factor = \frac{Pulse \ on \ time}{Total\ time}\)

Calculation:

Given:

Frequency (f) = 10 kHz, Pulse off-time = 40 μs

\(Total \ time =\frac{1}{10\ \times10^3 }\) s

Total time = 100 μs

Pulse on time = Total Time - Pulse off-time

Pulse on time = 100 - 40 = 60 μs

\(Duty\ factor = \frac{Pulse \ on \ time}{Total\ time}\)

\(Duty\ factor = \frac{60}{100}\)

Duty factor = 0.6

Explanation:

The unconventional machining process and their characteristics and the application areas are discussed in the table below:

Explanation:

Ultrasonic machining (USM):

• Ultrasonic machining is an operation that involves a vibrating tool fluctuating at the ultrasonic frequencies to remove the material from the workpiece.
• The process involves an abrasive slurry that runs between the tool and the workpiece.
• It is typically used on brittle materials as well as materials with a high hardness due to microcracking mechanics.

• In ultrasonic machining, a tool of the desired shape vibrates at an ultrasonic frequency (19 ∼ 25 kHz) with an amplitude of around 15 – 50 μm over the workpiece.
• Generally, the tool is pressed downward with a feed force, F.
• Between the tool and workpiece, the machining zone is 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 work material, particularly if the work material is brittle (due to crack initiation, propagation and brittle fracture of the material).

​Additional Information

USM MRR (Material Removal Rate) vs Feed Force

With an increase in the frequency of the tool head, the MRR should increase proportionally. However, there is a slight variation in the MRR with frequency. 

MRR increases with increasing feed force but after a certain critical feed force, it decreases because the abrasive grains get crushed under heavy load.

With increases in feed force, the material removal rate MRR is first increases and then decreases.

Explanation:

Plasma Arc Cutting (PAC)

• Plasma cutting is a process that cuts through electrically conductive materials by means of an accelerated jet of hot plasma.
• It is a thermal removal process as it employs thermal energy to accomplish its work.
• In plasma arc machining, gas heated to very high temperatures by a high-voltage electric arc partially ionizes and consequently becomes electrically conductive, sustaining the arc.
• When gas is heated to the degree that electrons become ionized (electrically charged), the gas is called a plasma.
• Primary gases used for plasma arc machining may be nitrogen, argon-hydrogen, or air.
• The gas is forced at a high rate of speed through a nozzle and through the arc.
• As the gas travels, it becomes superheated and ionized.
• The superheated gas reaches temperatures of 25,000°C  to 30,000°C.
• A hot tungsten cathode and a water-cooled copper anode provide the electric arc, and the gas is introduced around the cathode. It then flows out through the anode.
• The size of the orifice at the cathode determines the temperature, with small orifices providing higher temperatures.
• The ionized particle stream is consequently a high-velocity, well-columnated, extremely hot plasma jet.
• Supporting a highly focused, high-voltage, "lightning-like" electric arc between the electrode and the workpiece.
• With such high temperatures, when the plasma touches the workpiece, the metal is rapidly melted and vaporized.
• The high-velocity gas stream then expels molten material from the cut. 
• Typical materials cut with a plasma torch include steel, stainless steel, aluminum, brass, and copper, also other conductive metals with high heat capacity and good oxidation resistance may be cut as well.
• Plasma cutting is often used in fabrication shops, automotive repair and restoration, industrial construction, and salvage and scrapping operations. 
• Due to the high speed and precision cuts combined with low cost, plasma cutting sees widespread use from large-scale industrial CNC applications down to small workshops.

Important Points

In Oxy-fuel cutting, the ignition temperature of the material must be lower than its melting point otherwise the material would melt and flow away before cutting could take place, therefore PAC cannot be used to cut the materials which cannot be cut by ordinary methods like oxy-fuel cutting.

Concept:

\(MRR = \frac{{AI}}{{\;ρ ZF}}\)

A = atomic weight, I = current, F = 96500 coulombs, Z = Valency

Calculation:

Given:

Current density = 80 A/cm², Cross-sectional area = 4 cm², Total area = 8 cm² (Two electrode cathode and anode)

ρ = 2600 kg/m3 = 2600 × 10^-3 gm/cm³

Now,

I_act. = 0.75 × 80 × 8

∴ I_act. = 480 A

Now,

\(MRR = \frac{{480\; × \;30\; × \;1000}}{{2600\; × \;3\; × \;96500}}\)

MRR = 0.01913 cc/s

MRR = 19.13 mm³/s × 60

∴ MRR = 1147.86 mm³/min.

Explanation:

EDM process is summarized as :

• With the application of voltage, an electric field build-up between the two electrodes at the position of least resistance. The ionization leads to the breakdown of the dielectric which results in the drop of voltage and the beginning of the flow of current.
• Electrons and ions migrate to anode and cathode respectively at very high current density. A column of vapour begins to form and the localized melting of work commences. The discharge channel continues to expand along with the substantial increase of temperature and pressure.
• When the power is switched off, the current drops; no further heat is generated, and the discharge column collapses. A portion of molten metal evaporates explosively and/or is ejected away from the electrode surface. With the sudden drop in temperature, the remaining molten and vaporized metal solidifies. A tiny crater is thus generated at the surface.
• The residual debris is flushed away along with products of decomposition of dielectric fluid. The application of voltage initiates the next pulse and the cycle of events.

Hence for the above procedure, it is necessary that wire and sample are electrically conducting.