Unconventional Machining Processes MCQ Quiz in मल्याळम - Objective Question with Answer for Unconventional Machining Processes - സൗജന്യ PDF ഡൗൺലോഡ് ചെയ്യുക

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

Explanation:

Abrasive jet machining (AJM)

• In abrasive jet, machining material removal takes place due to the impingement of the fine abrasive particles. These particles move with a high-speed air (or gas) stream. The abrasive particles are typically are of 0.025 mm diameter.
• When an abrasive particle impinges on the work surface at a high velocity, the impact causes a tiny brittle fracture and the flowing air carries away the dislodgedged small workpiece particle.
• This process is more suitable when the work material is brittle and fragile.
• One of the most important factors in the AJM is the distance between the work surface and the tip of the nozzle normally called The nozzle tip distance (NTD). NTD affects not only the material removal rate (MRR) from the work surface but also the shape and size of the cavity produced. When the NTD increases, the velocity of the abrasive particle impinging on the work surface increases due to their acceleration after they leave the nozzle. This in turn increase MRR. With the further increase in NTD, the velocity reduces due to the drag of the atmosphere which initial checks the increase in MRR and finally decreases it.
• The variation of MRR with NTD is given below.

​

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:

In ECM, there is no mechanical contact between the workpiece and the cutting tool and hence there is no possibility of tool wear. The material from the workpiece is removed by the chemical action of the electrolyte using Faraday’s law of electrolysis. Workpiece forms electrode (anode) and the cutting tool negative electrode (cathode).

The tool material used in ECM should have following property

• It should have high electrical conductivity
• It should be easily machinable and it should have high stiffness
• Its corrosion resistance should be high.

The advantages of ECM include

• Complex shapes can be made accurately
• The surface finish is good due to atomic level dissolution
• Tool wear practically absent
• Its material removal rate is highest.

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.