ULTRASONIC MACHINING (USM): USM working principle, USM Machine, PROCESS VARIABLES, Abrasive material, Applications of USM, Advantage of USM, Disadvantages of USM

ULTRASONIC MACHINING (USM)

USM is mechanical material removal process or an abrasive process used to erode holes or cavities on hard or brittle workpiece by using shaped tools, high frequency mechanical motion and an abrasive slurry. USM offers a solution to the expanding need for machining brittle materials such as single crystals, glasses and polycrystalline ceramics, and increasing complex operations to provide intricate shapes and workpiece profiles. It is therefore used extensively in machining hard and brittle materials that are difficult to machine by traditional manufacturing processes. Ultrasonic Machining is a non-traditional process, in which abrasives contained in a slurry are driven against the work by a tool oscillating at low amplitude (25-100 μm) and high frequency (15-30 KHz):

The process was first developed in 1950s and was originally used for finishing EDM surfaces.

The basic process is that a ductile and tough tool is pushed against the work with a constant force. A constant stream of abrasive slurry passes between the tool and the work (gap is 25-40 μm) to provide abrasives and carry away chips. The majority of the cutting action comes from an ultrasonic (cyclic) force applied.

The basic components to the cutting action are believed to be,

brittle fracture caused by impact of abrasive grains due to the tool vibration;

cavitation induced erosion;

chemical erosion caused by slurry.

USM working principle

USM working principle

Material removal primarily occurs due to the indentation of the hard abrasive grits on the brittle work material.

Other than this brittle failure of the work material due to indentation some material removal may occur due to free flowing impact of the abrasives against the work material and related solid-solid impact erosion,

Tool’s vibration – indentation by the abrasive grits.

During indentation, due to Hertzian contact stresses, cracks would develop just below the contact site, then as indentation progresses the cracks would propagate due to increase in stress and ultimately lead to brittle fracture of the work material under each individual interaction site between the abrasive grits and the workpiece.

The tool material should be such that indentation by the abrasive grits does not lead to brittle failure.

Thus the tools are made of tough, strong and ductile materials like steel, stainless steel and other ductile metallic alloys.

USM Machine

USM Machine
USM Machine

The basic mechanical structure of an USM is very similar to a drill press.

However, it has additional features to carry out USM of brittle work material. The work piece is mounted on a vice, which can be located at the desired position under the tool using a 2 axis table. The table can further be lowered or raised to accommodate work of different thickness.

The typical elements of an USM are

Slurry delivery and return system

Feed mechanism to provide a downward feed force on the tool during machining

The transducer, which generates the ultrasonic vibration

The horn or concentrator, which mechanically amplifies the vibration to the required amplitude of 15 – 50 μm and accommodates the tool at its tip.

Working of horn as mechanical amplifier of amplitude of vibration

Working of horn as mechanical amplifier of amplitude of vibration
mechanical amplifier

The ultrasonic vibrations are produced by the transducer. The transducer is driven by suitable signal generator followed by power amplifier. The transducer for USM works on the following principle

Piezoelectric effect

Magnetostrictive effect

Electrostrictive effect

Magnetostrictive transducers are most popular and robust amongst all. Figure shows a typical magnetostrictive transducer along with horn. The horn or concentrator is a wave guide, which amplifies and concentrates the vibration to the tool from the transducer.

The horn or concentrator can be of different shape like

Tapered or conical

Exponential

Stepped

Machining of tapered or stepped horn is much easier as compared to the exponential one. Figure shows different horns used in USM

horn or concentrator
Horn or concentrator

PROCESS VARIABLES

Amplitude of vibration (ao) – 15 – 50 μm

Frequency of vibration (f) – 19 – 25 kHz

Feed force (F) – related to tool dimensions

Feed pressure (p)

Abrasive size – 15 μm – 150 μm

Abrasive material – Al2O3, SiC, B4C, Boronsilicarbide, Diamond

Flow strength of work material

Flow strength of the tool material

Contact area of the tool – A

Volume concentration of abrasive in water slurry – C

Applications of USM

Used for machining hard and brittle metallic alloys, semiconductors, glass, ceramics, carbides etc.

Used for machining round, square, irregular shaped holes and surface impressions.

Machining, wire drawing, punching or small blanking dies.

Advantage of USM

USM process is a non-thermal, non-chemical, creates no changes in the microstructures, chemical or physical properties of the workpiece and offers virtually stress free machined surfaces.

The main advantages are

Any materials can be machined regardless of their electrical conductivity

Especially suitable for machining of brittle materials

Machined parts by USM possess better surface finish and higher structural integrity.

USM does not produce thermal, electrical and chemical abnormal surface

Some disadvantages of USM

USM has higher power consumption and lower material-removal rates than traditional fabrication processes.

Tool wears fast in USM.

Machining area and depth is restraint in USM.

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