More older 4 exams Flashcards
(22 cards)
CO2 laser sources are not employed for robotized Laser Metal Deposition process because
(indicate the CORRECT answer):
o They have larger spot diameter with respect to active fiber sources
o They cannot be employed with pulsed or continuous wave emission
o Fiber sources typically provide an easier beam delivery mechanism to the process
head
o Keyhole mode processing is unavoidable
o They have larger spot diameter with respect to active fiber sources
o They cannot be employed with pulsed or continuous wave emission (FALSE, CO2 lasers can do both)
+ Fiber sources typically provide an easier beam delivery mechanism to the process
head **(Fiber sources allow for flexible beam delivery, ideal for robotics)
o Keyhole mode processing is unavoidable (FALSE - deposition is usually conduction mode and CO2 can be controlled)
Which of the following is WRONG regarding keyhole welding:
o The use of a lens with a longer focal distance hinders keyhole formation
o Beam quality does not impact on keyhole formation
o The use of transport fibers with a smaller diameter makes keyhole formation more
challenging
o It is preferable to realize high penetration joints
o The use of a lens with a longer focal distance hinders keyhole formation (longer focal length reduce energy -> keyhole formation harder)
o Beam quality does not impact on keyhole formation **(strongly affects keyhole welding, as better beam quality leads to more efficient energy concentration)
+ The use of transport fibers with a smaller diameter makes keyhole formation more
challenging (small fiber diameter -> better beam focus -> higher energy density -> facilitates keyhole formation)
o It is preferable to realize high penetration joints (desired for deep, narrow welds)
The productivity of a laser micro-machining process is directly proportional to the pulse
energy. Which of the following is WRONG?
o Lower duty cycle will decrease the productivity
o Increasing the pulse duration will increase the productivity
o Higher pulse repetition rate will decrease the productivity
o Greater peak power will increase the productivity
o Lower duty cycle = fewer pulses → lower productivity
o Longer pulses = more energy per pulse → potentially better material removal
+ Higher pulse repetition rate will decrease the productivity (higher pulse repetition rate increases productivity by more pulses per second)
o Higher peak power = better energy delivery
Considering the need to cut high thickness mild steel, which of the following is WRONG
o Higher emission power allows to cut with higher cutting speed
o Pulsed wave emission is always detrimental on the cut quality
o Melt and blow cutting is the preferred cutting method
o An exothermic reaction of the material with the assist gas allows to achieve higher
cutting speeds
o Higher emission power allows to cut with higher cutting speed (More power -> deeper cut)
o Pulsed wave emission is always detrimental on the cut quality (PWE worsens cut quality)
+ Melt and blow cutting is the preferred cutting method (For thick steel, reactive cutting/oxidation with oxygen is more effective)
o An exothermic reaction of the material with the assist gas allows to achieve higher
cutting speeds (exothermic assist gas helps)
Which of the following sentences regarding the characteristic curve of a waterjet pump is
correct?
o It cannot represent the behaviour of an orifice in terms of pressure and flow rate
o It can be obtained through experiments where pressure is selected on the machine
and flow rate is measured
o It varies according to the orifice diameter
o It is not possible to work in the area under the curve
o It cannot represent the behaviour of an orifice in terms of pressure and flow rate (FALSE, it does that)
+ It can be obtained through experiments where pressure is selected on the machine
and flow rate is measured (the curve is generated by fixing pressure and measuring flow)
o It varies according to the orifice diameter (yes, but secondary factor)
o It is not possible to work in the area under the curve (it’s common to operate below the ideal line)
Which one of the following sentences is true when milling thin walls?
o The mill diameter must be always small
o Forces perpendicular to the wall should can be large as they do not play a role
o In roughing, the two sides of the wall should be machined alternatively dividing the
wall height in steps
o The last finishing pass should be made in down milling
o The mill diameter must be always small (mill size depends on wall geometry)
o Forces perpendicular to the wall should can be large as they do not play a role (NO, they can cause wall deformation)
+ In roughing, the two sides of the wall should be machined alternatively dividing the
wall height in steps (Alternating and stepwise cutting helps prevent deflection or bending)
o The last finishing pass should be made in down milling (not always best -> may pull the wall)
What factor should be considered when acquiring forces in milling?
o The bandwidth of the load cell plays a role
o The milling force signal is constant over time
o The milling force signal is intrinsically made of two harmonics
o The resultant of the cutting forces acting on a tooth has a constant direction respect
to the machine
+ The bandwidth of the load cell plays a role (milling forces are dynamic, so the load cell must be fast enough to capture rapid variations)
o The milling force signal is constant over time (FALSE)
o The milling force signal is intrinsically made of two harmonics (OVERSIMPLIFIED)
o The resultant of the cutting forces acting on a tooth has a constant direction respect to the machine (FALSE, it rotates with cutter position)
What is the difference between EDM and WEDM??
o The polarity
o The material removal physical principle
o One can work on conductive material while the other cannot
o One can work harder materials
+ The polarity (differ in polarity (tool vs workpiee) affecting the material removal efficiency)
o The material removal physical principle (Same material principle - Spark erosion)
o One can work on conductive material while the other cannot (Both CAN)
o One can work harder materials (Both CAN)
Which of the following sentences is true when machining a deep and thin pocket with EDM?
o Flushing is not critical
o There is a risk of creating a conical pocket
o The tool must stay absolutely inside the pocket during all the operation
o Dielectric fluid can only be fed through the workpiece
o Flushing is not critical (FALSE, Flushing is very important to remove debris)
+ There is a risk of creating a conical pocket (Due to side spark erosion, EDM pockets can become wider at the top)
o The tool must stay absolutely inside the pocket during all the operation (FALSE - often electrodes moves slightly to aid flushing and spark access)
o Dielectric fluid can only be fed through the workpiece (FALSE - can be through electrode, side flow or workpiece)
Which sentence is true for ultrasonic machining?
o It is mostly devoted to machine metallic alloys
o It is a roughing process
o It better machines hard materials
o It cannot drill holes
o It is mostly devoted to machine metallic alloys (USM is not ideal for metals)
o It is a roughing process (Not necessarily - USM is non-thermal and non-chemical, can produce fine details)
+ It better machines hard materials (TRUE - USM is suited for brittle, hard, non-conductive materials like ceramics or glass)
o It cannot drill holes (FALSE - It CAN drill holes, especially in glass and ceramics)
Which sentence is true for tube hydroforming?
o It is not useful when the final tube presents undercuts
o The guided zone of the mould has always to keep the tube fixed in position during the
process
o The pistons displacement is almost null when the tube is long and complex
o Pressure is high at the beginning to start the bulge formation
o It is not useful when the final tube presents undercuts (FALSE - hydroforming can manage undercuts with proper tooling)
o The guided zone of the mould has always to keep the tube fixed in position during the
process (Not always - some axial movement is often allowed)
+ The pistons displacement is almost null when the tube is long and complex (In complex geometries, axial feeding can be minimized to avoid buckling or wrinkling)
o Pressure is high at the beginning to start the bulge formation (FALSE - Usually axial feeding preedes high internal pressure)
What does a waterjet pump mandatorily provide?
o High flow rates (tens of liters per minute) at low pressure
o High pressures at low flow rates (usually less than 5 liters per minute)
o High power, usually in the range of 100 kW and more
o High pumping frequency
o High flow rates (tens of liters per minute) at low pressure
+ High pressures at low flow rates (usually less than 5 liters per minute) **(Ultra-high pressures WJ systems, and flow rates)
o High power, usually in the range of 100 kW and more (FALSE - Typically around 20-40 kW)
o High pumping frequency (Pressure and flow rate are more important)
Q1: Which of the following material/energy source combinations IS NOT a feasible solution?
a) Mould making with a tool steel/Plunge EDM
b) Cutting of alumina (Al2O3)/Transferred arc plasma cutting
c) Welding of pure Cu/Fiber laser
d) Cutting of cardboard paper/Pure WJ
e) Cutting of paper/CO2 laser
a) Mould making with a tool steel/Plunge EDM (Steels are conductive, and EDM is used for precise mould cavity creation)
+) Cutting of alumina (Al2O3)/Transferred arc plasma cutting (alumina is a NON-conductive ceramic, so plasma is ineffective because it relies on electrical conductivity)
c) Welding of pure Cu/Fiber laser (Feasible for high-power fiber lasers, because Cu reflects IR)
d) Cutting of cardboard paper/Pure WJ (Pure/abrasive-free WJ are great for soft materials)
e) Cutting of paper/CO2 laser (Commonly used)
Q2: Laser heat treatment shows surface hardness variability. Which is NOT a plausible explanation?
a) The material properties may vary slightly in different positions.
b) The beam intensity profile may be non-uniform.
c) The material thickness may be variable along the irradiated region.
d) The surface roughness may be homogenous around the irradiated region.
e) The surface may have partially oxidized regions prior to the heat treatment.
a) The material properties may vary slightly in different positions. (TRUE - local microstructire or composition changes affect absorption and hardness)
b) The beam intensity profile may be non-uniform. (TRUE - a non-flat-top beam causes uneven heating)
c) The material thickness may be variable along the irradiated region. (TRUE - thicker areas dissipate heat faster -> less hardness)
+) The surface roughness may be homogenous around the irradiated region. **(Doesn’t explain variability, uniform roughness wouldn’t cause local treatment differences)
e) The surface may have partially oxidized regions prior to the heat treatment. (TRUE - oxide layers can affect absorption and conductivity)
Q3: In order to carry out a remote laser welding operations which of the following consideration is
WRONG?
a) Remote welding in the keyhole mode can be more easily achieved with single mode fiber lasers.
b) The single mode fiber laser generates a small Rayleigh length due to the low M2 value.
c) Laser wobbling can be used to increase the seam width.
d) The use of a shielding gas is more difficult compared to proximity welding.
e) The use of a cross-jet to protect the scanner optics is a feasible option.
a) Remote welding in the keyhole mode can be more easily achieved with single mode fiber lasers. (TRUE - high intensity helps form keyholes)
+) The single mode fiber laser generates a small Rayleigh length due to the low M2 value. (SIngle-mode have longer Rayleigh length, thanks to better low quality (low M2))
c) Laser wobbling can be used to increase the seam width. (TRUE - beam oscillation helps in joint tolerance and seam width)
d) The use of a shielding gas is more difficult compared to proximity welding. (TRUE - nozzle is far from the surface, so local protection is trickier)
e) The use of a cross-jet to protect the scanner optics is a feasible option. (TRUE - protects scanning head from spatter)
Q4: Which of the following statements regarding laser micromachining is WRONG?
a) Ultrashort pulses provide highest quality.
b) Nanosecond pulses provide higher productivity with reduced quality.
c) Millisecond to microsecond pulses are usually not used for ablation as they can generate excessivemolten material.
d) Concerning femtosecond pulses the higher energy content provides a deeper ablated region.
e) Defocusing the laser beam with femtosecond pulses provides a smaller ablated diameter in all Cases
a) Ultrashort pulses provide highest quality. (TRUE - low HAZ and precise ablation)
b) Nanosecond pulses provide higher productivity with reduced quality. (TRUE - more molten material = more recast layer)
c) Millisecond to microsecond pulses are usually not used for ablation as they can generate excessive molten material. (TRUE - not suitable for precision micromachining)
d) Concerning femtosecond pulses the higher energy content provides a deeper ablated region. (TRUE - depth scales with pulse energy)
+) Defocusing the laser beam with femtosecond pulses provides a smaller ablated diameter in all Cases **(It can increase spot size, lowering fluence below ablation threshold)
Q5: Concerning the fiber delivery of a laser beam which of the following statements is WRONG?
a) In order to trap the light in the fiber core the cladding should have a lower refractive index.
b) A step refractive index profile improves the beam quality.
c) A single mode fiber can be achieved with a small core diameter.
d) A smaller refractive index of the core decreases the number of modes.
a) In order to trap the light in the fiber core the cladding should have a lower refractive index. (TRUE - total internal reflection requires n_cladding < n_core)
+) A step refractive index profile improves the beam quality. (FALSE - Step-index profiles are more prone to multimode propagation, reducing beam quality)
c) A single mode fiber can be achieved with a small core diameter. (TRUE - limits the # of propagation modes)
d) A smaller refractive index of the core decreases the number of modes. (TRUE - index contrast and core size define modal capacity)
Q6: In a melt and blow laser cutting operation of 2 mm-thick stainless steel with nitrogen you observe that the cut quality is degraded throughout the cutting operation with fixed process parameters. Which of the following IS NOT a plausible cause?
a) The material generates excessive oxidation enthalpy resulting in kerf enlargement.
b) The material thickness locally increased due to scarce tolerances.
c) The nozzle distance changed due to the complex surface geometry.
d) The gas pressure decreased due to a failure in the piping system.
+) The material generates excessive oxidation enthalpy resulting in kerf enlargement. **(Not plausible since oxidation enthalpy isn’t relevant with nitrogen (inert gas)
b) The material thickness locally increased due to scarce tolerances. (Plausible - leads to higher energy need, worse cut)
c) The nozzle distance changed due to the complex surface geometry. (Plausible - changes beam/gas focus = cutting instability)
d) The gas pressure decreased due to a failure in the piping system. (Plausible - less force to remove molten metal = bad kerf)
Q7: During an ultrasonic machining operation, which of the following options IS feasible to increase the material removal rate concerning deep pockets?
a) A stepped sonotrode should be used to increase the vibration amplitude.
b) The abrasive slurry flushing rate should be reduced.
c) Materials with low hardness such as stainless steel should be considered as the workpiece.
d) A magnetorestrictive transducer should be used without a control winding to increase frequency
and amplitude.
e) Abrasive particle size should be reduced
+) A stepped sonotrode should be used to increase the vibration amplitude. (stepped sonotrode aplify vibration amplitude via mechanical resonance)
b) The abrasive slurry flushing rate should be reduced. (WRONG - flushing is critical, especially in deep holes)
c) Materials with low hardness such as stainless steel should be considered as the workpiece. (WRONG - USM is ideal for hard, brittle materials)
d) A magneto restrictive transducer should be used without a control winding to increase frequency
and amplitude. (Not effective - can’t control amplitude/frequency well)
e) Abrasive particle size should be reduced (Less agressive)
Q8: During the roughing phase of a plunge EDM operation carried out on stainless steel you observe
excessive tool wear. Which of the following IS NOT a plausible cause?
a) The tool material has poor thermal conductivity.
b) Inverse polarity is used with high spark duration.
c) Continuous arc is formed between the tool and the workpiece.
d) The dielectric fluid flushing is not sufficient.
e) Low spark frequency is used.
a) The tool material has poor thermal conductivity. (plausible - retains heat = fast wear)
b) Inverse polarity is used with high spark duration. (Plausible - more energy sent to tool = more wear)
c) Continuous arc is formed between the tool and the workpiece. (Plausible - unwanted discharge = extreme wear)
d) The dielectric fluid flushing is not sufficient. (Plausible - leads to debris buildup and instability)
+) Low spark frequency is used. (low spark freq = longer time between sparks -> allowing tool cooling -> less wear)
Q9: Which of the following DOES NOT influence the cut surface quality during an AWJ operation?
a) Water pressure
b) Suspension mixing ratio
c) Abrasive flow rate
d) Nozzle diameter
e) Cutting speed
a) Water pressure (Higher pressure = more kinetic energy -> better cut)
+) Suspension mixing ratio **(Irrelevant)
c) Abrasive flow rate (influences kerf quality, surface finish)
d) Nozzle diameter (controls jet coherence)
e) Cutting speed (Too fast = striation, poor finish)
Q10: Which of the following statements concerning plasma arc cutting is CORRECT?
a) It provides higher productivity compared to laser cutting at low material thicknesses.
b) It is suited to producing holes on sheets with large thicknesses.
c) The plasma beam provides both the heat and the mechanical pressure required to eject the molten
material.
d) Cu- and Al-alloys cannot be cut due to high electrical and thermal conductivity.
a) It provides higher productivity compared to laser cutting at low material thicknesses. (FALSE - lasers are better for thin sheets)
b) It is suited to producing holes on sheets with large thicknesses. (FALSE - precision and hole quality are poor in thick material)
+) The plasma beam provides both the heat and the mechanical pressure required to eject the molten
material. **(Thermal and mechanical effects to eject molten metal from the kerf)
d) Cu- and Al-alloys cannot be cut due to high electrical and thermal conductivity. (FALSE - they CAN be cut with proper gas and current settings)
