20-06-2014, 12:53 PM
ULTRASONIC MACHINING
[attachment=65103]
• Ultrasonic machining (USM) is the removal of hard and brittle materials using an axially oscillating tool at ultrasonic frequencies [18–20 kHz].
• During that oscillation, the abrasive slurry is continuously fed into the machining zone between a soft tool (brass or steel) and the work piece.
• The abrasive particles are, therefore, hammered into the work piece surface and cause chipping of fine particles from it.
• The oscillating tool, at amplitudes ranging from 10 to 40 μm, imposes a static pressure on the abrasive grains and feeds down as the material is removed to form the required tool shape.
Machining System (Elements of USM)
• The machining system of USM is composed mainly from the
Magnetostrictor,
Concentrator or Horn,
Tool and slurry feeding arrangement.
Magnetostrictor
• It has a high-frequency winding wound on a magnetostrictor core and a special polarizing winding around an armature.
• Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization.
• Magnetostrictive materials can convert magnetic energy into kinetic energy, or the reverse, and are used to build actuators and sensors.
• The property (magnetorestriction) can be quantified by a factor called “magnetostrictive coefficient” or “coefficient of magnetostriction elongation”.
• It is the fractional change in length as the magnetization of the material increases from zero to the saturation value.
• The effect is responsible for the familiar "electric hum“ which can be heard near transformers.
• This effect is used to oscillate the USM tool at ultrasonic frequencies (18 to 20 kHz).
• The USM tool is mounted at the end of a magnetostrictor.
• Materials having high coefficient of magnetostrictive elongation are recommended to be used for a magnetostrictor.
\Factors Affecting MRR
1. Tool Oscillation or Vibration – Amplitude & Frequency.
• Amplitude of the tool oscillation has the greatest effect of all the process variables, MRR increases with a rise in the tool vibration amplitude.
• Vibration amplitude determines the velocity of the abrasive particles at the interface between the tool and work piece.
• Under such circumstances the kinetic energy rises, at larger amplitudes, which enhances the mechanical chipping action and consequently increases the MRR.
2. Abrasive Grains
• Both the grain size and the vibration amplitude have a similar effect on the removal rate.
• According to McGeough (1988), MRR rises at greater grain sizes until the size reaches the vibration amplitude, at which stage, the MRR decreases.
• When the grain size is large compared to the vibration amplitude, there is a difficulty of abrasive renewal.
• Because of its higher hardness, B4C achieves higher removal rates than silicon carbide (SiC) when machining glass.
• The MRR obtained with silicon carbide is about 15 % lower when machining glass, 33 % lower for tool steel, and about 35 % lower for sintered carbide.
• Both the grain size and the vibration amplitude have a similar effect on the removal rate.
• According to McGeough (1988), MRR rises at greater grain sizes until the size reaches the vibration amplitude, at which stage, the MRR decreases.
• When the grain size is large compared to the vibration amplitude, there is a difficulty of abrasive renewal.
• Because of its higher hardness, B4C achieves higher removal rates than silicon carbide (SiC) when machining glass.
• The MRR obtained with silicon carbide is about 15 % lower when machining glass, 33 % lower for tool steel, and about 35 % lower for sintered carbide.
• Water is commonly used as the abrasive carrying liquid for the abrasive slurry while benzene, glycerol, and oils are alternatives.
• The increase of slurry viscosity reduces the removal rate.
• The improved flow of slurry results in an enhanced machining rate.
• In practice a volumetric concentration of about 30 to 35 percent of abrasives is recommended.
• A change of concentration occurs during machining as a result of the abrasive dust settling on the machine table.
• The actual concentration should, therefore, be checked at certain time intervals.
• The increase of abrasive concentration up to 40 % enhances MRR.
• More cutting edges become available in the machining zone, which raises the chipping rate and consequently the overall removal rate.
3. Workpiece Impact Hardness
• MRR is affected by the ratio of tool hardness to workpiece hardness.
• In this regard, the higher the ratio, the lower will be MRR.
• For this reason soft and tough materials are recommended for USM tools.
4. Tool Shape
• Increase in tool area - decreases the machining rate; due to inadequate distribution of abrasive slurry over the entire area.
• McGeough (1988) reported that, for the same machining area, a narrow rectangular shape yields a higher machining rate than a square shape.
• Rise in static pressure - enhances MRR up to a limiting condition, beyond which no further increase occurs.
• Reason - disturbance in the tool oscillation at higher forces where lateral vibrations are expected.
• According to Kaczmarek (1976), at pressures lower than the optimum, the force pressing the grains into the material is too small and the volume removed by a particular grain diminishes.
• Measurements also showed a decrease in MRR with an increase in the hole depth.
• Reason - deeper the tool reaches, the more difficult and slower is the exchange of abrasives from underneath the tool.
Factors Affecting Dimensional Accuracy
Accuracy (oversize, conicity, out of roundness) - affected by
– Side wear of the tool
– Abrasive wear
– Inaccurate feed of the tool holder
– Form error of the tool
– Unsteady and uneven supply of abrasive slurry
Overcut
• Holes accuracy is measured through overcut (oversize).
• Hole oversize measures the difference between the hole diameter, measured at the top surface, and the tool diameter.
• Side gap between tool and hole is necessary to enable abrasive flow.
• Hence, grain size of the abrasives represents the main factor, which affects the overcut produced.
• Overcut is considered to be about 2 - 4 times greater than the mean grain size when machining glass and tungsten carbide.
• It is about 3 times greater than the mean grain size of B4C.
• However, the magnitude of overcut depends on many other process variables (type of workpiece material and the method of tool feed).
• In general, USM accuracy levels are limited to + 0.05 mm.
Conicity (non-parallel sides)
• Overcut is usually greater at the entry side than at the exit.
• Reason - cumulative abrasion effect of fresh and sharp grain particles.
• As a result, a hole conicity of ~ 0.2° arises when drilling a hole of f 20 mm and a depth of 10 mm in graphite.
The conicity may be reduced by
• Direct injection of abrasive slurry into the machining zone.
• Use of tools having negatively tapering walls.
• Use of high static pressure that produces finer abrasives, which in turn reduces the tool wear.
• Use of wear-resistant tool materials.
• Use of an undersized tool in the first cut and a final tool of the required size, which will cut faster and reduce the conicity.
Out of roundness
• Out of roundness arises by the lateral vibrations of the tool.
• Such vibrations - due to out of perpendicularity of tool face and centerline.
• Also due to misalignment in the acoustic parts of the machine.
• Typical values - ~40-140 μm for glass and 20-60 μm for graphite.
Surface Quality
• Surface finish - closely related to the machining rate in USM.
• Table shows the relationship between grit number and grit size.
Production of EDM Electrodes •
Gilmore (1995) used USM to produce graphite EDM electrodes (Fig.).
• Typical machining speeds, in graphite range from 40 to 140 mm/min.
• Surface roughness from 0.2 to 1.5 μm with an accuracy of ±10 μm.
• Small machining forces permit the manufacture of fragile graphite EDM electrodes.
[attachment=65103]
• Ultrasonic machining (USM) is the removal of hard and brittle materials using an axially oscillating tool at ultrasonic frequencies [18–20 kHz].
• During that oscillation, the abrasive slurry is continuously fed into the machining zone between a soft tool (brass or steel) and the work piece.
• The abrasive particles are, therefore, hammered into the work piece surface and cause chipping of fine particles from it.
• The oscillating tool, at amplitudes ranging from 10 to 40 μm, imposes a static pressure on the abrasive grains and feeds down as the material is removed to form the required tool shape.
Machining System (Elements of USM)
• The machining system of USM is composed mainly from the
Magnetostrictor,
Concentrator or Horn,
Tool and slurry feeding arrangement.
Magnetostrictor
• It has a high-frequency winding wound on a magnetostrictor core and a special polarizing winding around an armature.
• Magnetostriction is a property of ferromagnetic materials that causes them to change their shape or dimensions during the process of magnetization.
• Magnetostrictive materials can convert magnetic energy into kinetic energy, or the reverse, and are used to build actuators and sensors.
• The property (magnetorestriction) can be quantified by a factor called “magnetostrictive coefficient” or “coefficient of magnetostriction elongation”.
• It is the fractional change in length as the magnetization of the material increases from zero to the saturation value.
• The effect is responsible for the familiar "electric hum“ which can be heard near transformers.
• This effect is used to oscillate the USM tool at ultrasonic frequencies (18 to 20 kHz).
• The USM tool is mounted at the end of a magnetostrictor.
• Materials having high coefficient of magnetostrictive elongation are recommended to be used for a magnetostrictor.
\Factors Affecting MRR
1. Tool Oscillation or Vibration – Amplitude & Frequency.
• Amplitude of the tool oscillation has the greatest effect of all the process variables, MRR increases with a rise in the tool vibration amplitude.
• Vibration amplitude determines the velocity of the abrasive particles at the interface between the tool and work piece.
• Under such circumstances the kinetic energy rises, at larger amplitudes, which enhances the mechanical chipping action and consequently increases the MRR.
2. Abrasive Grains
• Both the grain size and the vibration amplitude have a similar effect on the removal rate.
• According to McGeough (1988), MRR rises at greater grain sizes until the size reaches the vibration amplitude, at which stage, the MRR decreases.
• When the grain size is large compared to the vibration amplitude, there is a difficulty of abrasive renewal.
• Because of its higher hardness, B4C achieves higher removal rates than silicon carbide (SiC) when machining glass.
• The MRR obtained with silicon carbide is about 15 % lower when machining glass, 33 % lower for tool steel, and about 35 % lower for sintered carbide.
• Both the grain size and the vibration amplitude have a similar effect on the removal rate.
• According to McGeough (1988), MRR rises at greater grain sizes until the size reaches the vibration amplitude, at which stage, the MRR decreases.
• When the grain size is large compared to the vibration amplitude, there is a difficulty of abrasive renewal.
• Because of its higher hardness, B4C achieves higher removal rates than silicon carbide (SiC) when machining glass.
• The MRR obtained with silicon carbide is about 15 % lower when machining glass, 33 % lower for tool steel, and about 35 % lower for sintered carbide.
• Water is commonly used as the abrasive carrying liquid for the abrasive slurry while benzene, glycerol, and oils are alternatives.
• The increase of slurry viscosity reduces the removal rate.
• The improved flow of slurry results in an enhanced machining rate.
• In practice a volumetric concentration of about 30 to 35 percent of abrasives is recommended.
• A change of concentration occurs during machining as a result of the abrasive dust settling on the machine table.
• The actual concentration should, therefore, be checked at certain time intervals.
• The increase of abrasive concentration up to 40 % enhances MRR.
• More cutting edges become available in the machining zone, which raises the chipping rate and consequently the overall removal rate.
3. Workpiece Impact Hardness
• MRR is affected by the ratio of tool hardness to workpiece hardness.
• In this regard, the higher the ratio, the lower will be MRR.
• For this reason soft and tough materials are recommended for USM tools.
4. Tool Shape
• Increase in tool area - decreases the machining rate; due to inadequate distribution of abrasive slurry over the entire area.
• McGeough (1988) reported that, for the same machining area, a narrow rectangular shape yields a higher machining rate than a square shape.
• Rise in static pressure - enhances MRR up to a limiting condition, beyond which no further increase occurs.
• Reason - disturbance in the tool oscillation at higher forces where lateral vibrations are expected.
• According to Kaczmarek (1976), at pressures lower than the optimum, the force pressing the grains into the material is too small and the volume removed by a particular grain diminishes.
• Measurements also showed a decrease in MRR with an increase in the hole depth.
• Reason - deeper the tool reaches, the more difficult and slower is the exchange of abrasives from underneath the tool.
Factors Affecting Dimensional Accuracy
Accuracy (oversize, conicity, out of roundness) - affected by
– Side wear of the tool
– Abrasive wear
– Inaccurate feed of the tool holder
– Form error of the tool
– Unsteady and uneven supply of abrasive slurry
Overcut
• Holes accuracy is measured through overcut (oversize).
• Hole oversize measures the difference between the hole diameter, measured at the top surface, and the tool diameter.
• Side gap between tool and hole is necessary to enable abrasive flow.
• Hence, grain size of the abrasives represents the main factor, which affects the overcut produced.
• Overcut is considered to be about 2 - 4 times greater than the mean grain size when machining glass and tungsten carbide.
• It is about 3 times greater than the mean grain size of B4C.
• However, the magnitude of overcut depends on many other process variables (type of workpiece material and the method of tool feed).
• In general, USM accuracy levels are limited to + 0.05 mm.
Conicity (non-parallel sides)
• Overcut is usually greater at the entry side than at the exit.
• Reason - cumulative abrasion effect of fresh and sharp grain particles.
• As a result, a hole conicity of ~ 0.2° arises when drilling a hole of f 20 mm and a depth of 10 mm in graphite.
The conicity may be reduced by
• Direct injection of abrasive slurry into the machining zone.
• Use of tools having negatively tapering walls.
• Use of high static pressure that produces finer abrasives, which in turn reduces the tool wear.
• Use of wear-resistant tool materials.
• Use of an undersized tool in the first cut and a final tool of the required size, which will cut faster and reduce the conicity.
Out of roundness
• Out of roundness arises by the lateral vibrations of the tool.
• Such vibrations - due to out of perpendicularity of tool face and centerline.
• Also due to misalignment in the acoustic parts of the machine.
• Typical values - ~40-140 μm for glass and 20-60 μm for graphite.
Surface Quality
• Surface finish - closely related to the machining rate in USM.
• Table shows the relationship between grit number and grit size.
Production of EDM Electrodes •
Gilmore (1995) used USM to produce graphite EDM electrodes (Fig.).
• Typical machining speeds, in graphite range from 40 to 140 mm/min.
• Surface roughness from 0.2 to 1.5 μm with an accuracy of ±10 μm.
• Small machining forces permit the manufacture of fragile graphite EDM electrodes.