3 4 5 ULTRASONIC MACHINING ULTRASONIC MACHINING USM is a process that utilizes the ultrasonic 20 kHz vibration of a tool in the machining of hard brittle non metallic materials Ultrasonic impact grinding known as Ultrasonic machining Involves an ID: 928194
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ULTRASONIC MACHINING
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Slide6ULTRASONIC MACHINING
ULTRASONIC MACHINING (USM) is a process that utilizes the ultrasonic (≥20 kHz) vibration of a tool in the machining of hard, brittle, non metallic materials.
• Ultrasonic impact grinding (known as Ultrasonic machining): Involves an
abrasive slurry and the ultrasonic vibration of a non-rotating tool
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Slide7Ultrasonic Machining (USM)
Abrasives contained in a slurry are driven at high velocity against work by a tool vibrating at low amplitude and high frequency
Tool oscillation is perpendicular to work surface
Tool is fed slowly into work
Shape of tool is formed in part
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Slide12Ultrasonic machining
The tool, which is a negative of the workpiece, is vibrated at around 20KHz with and amplitude of between 0.01 mm and 0.05 mm in abrasive slurry at the workpiece surface.
Material removal is by 2 mechanisms:
Hammering of grit against the surface by the tool.
Impact of free abrasive grit particles (erosion).
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Slide13Ultrasonic machining (USM)
In USM, high frequency electrical energy is converted in to mechanical vibrations via a transducer which are then transmitted through an energy focusing device, i.e. horn/tool assembly
This causes the tool to vibrate along its longitudinal axis at high frequency (usually ≥ 20 kHz) with an amplitude of 5–50
μ
m
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Slide14Ultrasonic machining
The tool (steel and stainless steel) oscillates in a direction perpendicular to the work surface, and is fed slowly into the work, so that the shape of the tool is formed in the part.
the action of the abrasives (boron nitride, boron carbide, aluminium oxide, silicon carbide and diamond) impinging against the work surface, that performs the cutting
Grit size ranges between 100 and 2000 µm.
The slurry in USM consists of a mixture of water/oil and abrasive particles. Concentration of abrasives in water ranges from
20% to 60%
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They are not spherical
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Slide17Ultrasonic machining (USM)
A controlled static load is applied to the tool and an abrasive slurry
(comprising a mixture of abrasive material; e.g. silicon carbide, boron carbide, etc. suspended
in water or oil) is pumped around the cutting zone.
The vibration of the tool causes the abrasive particles held in the slurry between the tool and the workpiece, to impact the workpiece surface causing material removal by microchipping
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Slide18USM- The machining system
The machining system is composed mainly the convertor (
magnetostrictor
, or piezoelectric) concentrator, tool, and slurry feeding arrangement.
The
magnetostrictor
is energized at the ultrasonic frequency and produces small-amplitude vibrations. Such a small vibration is amplified using the horn (mechanical amplifier) that holds the tool.
The abrasive slurry is pumped between the oscillating tool and the brittle
workpiece
.
A static pressure is applied in the tool-
workpiece
interface that maintains the abrasive slurry.
Slide19Basic elements of USM head
Fig. shows the basic elements of an USM set up using either a
magnetostrictive
or piezoelectric transducer with brazed and screwed tooling.
The oscillation amplitude at the face of the transducer is too small (0.001–0.1
μ
m) to achieve any reasonable cutting rate, therefore, the horn is used as an amplification device
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Slide20Ultrasonic machining
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Various horn designs with and without additional tool heads
Slide21Converter or transducer
A converter to change the electrical energy into mechanical vibrations.
Two types of converters are available for USM systems.
magnetostrictive
device,
quartz or
a lead zirconate titanate piezoelectric
transducer
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Slide24Magnetostictive transducer
Magnetostictive
transducer
The
magnetostrictor
used in USM, shown in has a high-frequency winding wound on a
magnetostrictor
core and a special polarizing winding around an armature
Magnetostictive
transducers work on the principle that if a piece of Ferro-magnetic material (like nickel alloys) is magnetized, then a change in dimension occurs.
Slide25magnetostrictive elongation
The coefficient of
magnetostriction
elongation
ε
m
is
where
Δ
l
is the incremental length of the
magnetostrictor
core and l is
the original length of the
magnetostrictor
core, both in
millimeters
.
Slide26Piezoelectric Transducer-USM
The main drawbacks of the
magnetostrictive
transducer are the high losses encountered, the low efficiency (55 percent), the consequent heat up, and the need for cooling. Higher efficiencies (90–95npercent) are possible by using piezoelectric transformers to modern USM machines.
Piezoelectric transducers utilize crystals like quartz whose dimensions alter when being subjected to electrostatic fields.
The charge is directionally proportional to the applied voltage.
Lead
zirconate
titanate
piezoelectric disks are used which convert the electrical energy from the power supply into a mechanical vibration.
To obtain high amplitude vibrations the length of the crystal must be matched to the frequency of the generator which produces resonant conditions
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Material removal mechanism of USM
1. Mechanical abrasion by localized direct hammering of the abrasive grains stuck between the vibrating tool and adjacent work surface.
2. The micro-chipping by free impacts of particles that fly across the machining gap and strike the work-piece at random locations.
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Slide36Data from various materials ultrasonically machined using 320 mesh abrasive
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Slide48Process capabilities of USM
Tolerances that can be achieved by this process range between 5 µm and 10 µm
Holes as small as 75 µm have been drilled.
Linear material removal rate, MRR, (also known as penetration rate) achieved during USM ranges from 0.025 to 25.0 mm/min, and it depends upon various parameters.
Surface finish achieved during the process varies from 0.25 µm to 0.75 µm.
Honeycomb structure machined on the back of a silicon mirror for NASA
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Slide50Ultrasonic machining
The cutting action in USM operates on the tool as well as the work. As the abrasive particles erode the work surface, they also erode the tool, thus affecting its shape.
to machine hard, brittle work materials, such as ceramics, glass, and carbides
.
Shapes obtained by USM include non-round holes, holes along a curved axis, and coining operations, in which an image pattern on the tool is imparted to a flat work surface.
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Slide51Ultrasonic machining (USM)
Ultrasonic machining (USM) is a non-conventional mechanical material removal process generally associated with low material removal rates, however its application is not limited by the electrical or chemical characteristics of the workpiece materials.
It is used for machining both conductive and non-metallic materials; preferably those with low
ductility and a hardness above 40 HRC , e.g. inorganic glasses, silicon nitride,
nickel/titanium alloys, etc
USM does not generate significant heating which might otherwise lead to the development of a thermally damaged layer/zone or residual stress.
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Slide52Ultrasonic machining-applications
UM effectively machines precise features in hard, brittle materials such as glass, engineered ceramics,
SiC
, quartz, single crystal materials, PCD, ferrite, graphite, glassy carbon, composites and
piezoceramics
.
A nearly limitless number of feature shapes-including round, square and odd-shaped thru-holes and cavities of varying depths, as well as OD-ID features-can be machined with high quality and consistency
Machining ceramic substrates for drilling holes in borosilicate glass for the sensors used in electronic industries
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Slide53Ultrasonic machining-advantages
Parts are burr-free with no residual stresses, distortion or thermal effects.
There are no changes to the metallurgical, chemical or physical properties of the
workpiece
.
Good for machining very brittle materials.
USM) is a non-thermal, non-chemical and non-electrical machining process that leaves the chemical composition, material microstructure and physical properties of the
workpiece
unchanged
UM is a mechanical material removal process that can be used for machining both conductive and non-metallic materials with
hardnesses
of greater than 40 HRC (Rockwell Hardness measured in the C scale).
The UM process can be used to machine precision micro-features, round and odd-shaped holes, blind cavities, and OD/ID features.
Multiple features can be drilled simultaneously, often reducing the total machining time significantly
The process offers good surface finish and structural integrity
The process is free from burrs and distortions.
Slide54Ultrasonic machining-disadvantages
USM has low material removal rate.
Tool wears fast in USM.
Machining area and depth is restraint in USM.
The USM process consumes higher power and has lower material-removal rates compared to traditional fabrication processes
Soft materials like lead and plastics are not suitable for machining by the USM process, since they tend to absorb the abrasive particles rather than to chip under their impact
While producing deeper holes through USM method, there is ineffective slurry circulation leading to presence of a fewer active grains under the tool face. Due to this, the bottom surfaces of blind holes tend to become slightly concave.
Slide55The economics-Limitations
Suitable for low production rates only.
Special tooling required for each job. High tooling costs.
Multiple cuts required for progressively better finish.
Tool wear is a problem requiring frequent changes.
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Slide56Rotary Ultrasonic Machining
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Slide57Rotary Ultrasonic Machining
Rotary ultrasonic machining is similar to the conventional drilling of glass and ceramic with diamond core drills, except that the rotating core drill is vibrated at an ultrasonic frequency of 20 kHz.
Rotary ultrasonic machining does not involve the flow of an abrasive slurry through a gap between the
workpiece
and the tool.
Instead, the tool contacts and cuts the
workpiece
, and a liquid coolant, usually water, is forced through the bore of the tube to cool and flush away the removed material.
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Slide58Rotary ultrasonic machining (RUM)
Rotary ultrasonic machining (RUM) is a hybrid machining process that combines the material removal mechanisms of diamond grinding with ultrasonic machining (USM), resulting in higher material removal rates (MRR) than those obtained by either diamond grinding or USM alone
In rotary ultrasonic machining, a rotating core drill with metal-bonded diamond abrasives is ultrasonically vibrated in the axial direction while the spindle is fed toward the work piece at a constant pressure.
By using abrasives bonded directly on the tools and combining simultaneous rotation and vibration, RUM provides a fast, high-quality machining method for a variety of glass and ceramic applications.
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Slide59Illustration of rotary ultrasonic machining
Rotary ultrasonic machining (RUM) is a hybrid machining process that combines the material removal mechanisms of diamond grinding and of ultrasonic machining.
The drilling tool is a core drill made of metal-bonded diamond abrasives.
The rotating tool is ultrasonically vibrated and fed toward the work piece.
Meanwhile, coolant is pumped through the hole in the middle of the drill to flush away the debris.
Important process variables are tool rotation speed, tool feed rate or pressure, vibration amplitude, abrasive grain size, and abrasive concentration.
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Slide60RUSM
The amplitude of vibration in rotary ultrasonic machining is usually about 0.025 to 0.05 mm and the longitudinal extension and contraction of the drill tip improves drilling performance by:
• Reducing the friction between the tool and the material at the point of cutting
• Eliminating seizure of the core piece by allowing greater coolant flow in the bore of the tool
• Preventing the diamond tool from loading with removed material
• Increasing drilling speed with less pressure on the tool and reduced tool wear
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Slide61RUM vs. USM
One of the major differences between USM and RUM equipment is that USM uses a soft tool, such as stainless steel, brass or mild steel, and a slurry loaded with hard abrasive particles, while in RUM the hard abrasive particles are diamond and are bonded on the tools.
Another major difference is that the RUM tool rotates and vibrates simultaneously, while the USM tool only vibrates.
These differences enable RUM to provide both speed and accuracy advantages in ceramic and glass machining operations.
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