Ultrasonic Machining (USM) Process: Principle, Machine Parts, Parameters and Applications

Ultrasonic machining (USM) is a non-traditional machining process in which a shaped tool vibrates at about 20 kHz (typically 19 to 25 kHz) with an amplitude of roughly 10 to 50 microns, hammering abrasive grains in a water slurry against the workpiece. Each impact chips out a tiny crater by brittle fracture, so the USM process suits hard, brittle materials such as glass, ceramics, quartz and tungsten carbide, whether or not they conduct electricity.

The tool never cuts the work directly. It is the loose abrasive that does the machining, which is why the process is also called ultrasonic impact grinding. The hole or cavity produced is a slightly oversized copy of the tool face.

What is the working principle of ultrasonic machining?

The tool, made of a tough, ductile metal such as mild steel or stainless steel, is held a few tens of microns above the work and pressed down with a small static feed force. A slurry of abrasive grains (boron carbide, silicon carbide, alumina or diamond) in water fills the gap. As the tool face moves down in each vibration cycle, it drives the grains into the work surface.

Under each grain, contact stresses start small cracks just below the surface. The cracks grow and meet, and a roughly hemispherical chip breaks away. At 20,000 cycles per second, millions of these micro-chips add up to steady material removal. The slurry also carries the debris out and brings fresh, sharp grains in.

Two consequences follow from this mechanism:

• Brittle work cuts well, ductile work does not. A soft metal like copper just dents under the grains instead of chipping, so USM is a poor choice for it.
• The tool wears too. Grains indent the tool as well, which is why the tool is made from a ductile metal that deforms rather than fractures.

Because the material is removed mechanically and cold, there is no heat-affected zone, no recast layer and no chemical change in the work surface.

Main parts of an ultrasonic machine

The structure looks like a drill press with a vibrating head. The work sits in a vice on a two-axis table, and the head feeds down under a controlled force. For a labelled diagram of this set-up, see our page on ultrasonic machining construction and working.

Piezoelectric vs magnetostrictive transducers

A magnetostrictive transducer is a stack of nickel or nickel-alloy laminations wound with a coil. An alternating magnetic field makes the stack lengthen and shorten slightly. It is rugged and was the standard choice in older machines, but it converts energy twice (electrical to magnetic to mechanical), loses more as heat and usually needs water cooling.

A piezoelectric transducer uses ceramic crystals (such as lead zirconate titanate) that change thickness when a voltage is applied. It converts electrical energy to vibration far more efficiently and runs cooler, which is why most modern USM and ultrasonic-assisted machines use it.

How the USM process runs, step by step

1. The tool is made to the shape of the required hole or cavity and fixed to the horn tip.
2. The work is clamped and the tool brought down until it is just above the surface.
3. Slurry flow starts, flooding the gap between tool and work.
4. The generator is switched on and tuned so that the transducer, horn and tool vibrate at resonance.
5. The feed mechanism applies a steady downward force. The tool sinks into the work as grains chip the surface away.
6. For deeper cavities the tool may be withdrawn now and then so fresh slurry can reach the bottom.
7. Roughing is done with coarse grit, and a finishing pass with fine grit if a better surface is needed.

Process parameters and their effect on MRR and finish

The standard teaching model of USM (developed from M.C. Shaw’s analysis and set out in the IIT Kharagpur NPTEL notes) treats each grain impact as removing a hemispherical crater and counts how many impacts happen per second. It gives this proportionality for material removal rate (MRR):

MRR ∝ c^1/4 · (F · a)^3/4 · A^1/4 · d_g · f / σ_w^3/4

where c = abrasive concentration in the slurry, F = static feed force, a = vibration amplitude, A = tool face area, d_g = grit diameter, f = frequency and σ_w = flow strength of the work material. In plain words:

The model is best used for trends. The NPTEL notes themselves point out that it does not correctly predict how MRR changes when amplitude, force and frequency are varied over wide ranges; real curves flatten and then fall.

Worked example: predicting MRR changes

These three problems follow the NPTEL lesson. Glass is being machined at an MRR of 6 mm³/min with Al2O3 grit of 150 micron mean diameter at 20 kHz. Keep every other parameter fixed unless stated.

(a) Grit changed to 100 microns. MRR ∝ d_g, so new MRR = 6 × (100 / 150) = 4 mm³/min.

(b) Frequency raised from 20 kHz to 25 kHz (original 150 micron grit). MRR ∝ f, so new MRR = 6 × (25 / 20) = 7.5 mm³/min.

(c) Feed force raised by 50 percent and concentration reduced by 70 percent. MRR ∝ F^3/4 c^1/4. The new force is 1.5F and the new concentration is 0.3c, so:

new MRR = 6 × 1.5^0.75 × 0.3^0.25 = 6 × 1.355 × 0.740 = 6.02 mm³/min, practically unchanged. The gain from extra force is cancelled by the thinner slurry.

Exam tip: in these ratio questions, write the proportionality first, cancel everything that stays constant, and only then put in numbers.

Which materials suit ultrasonic machining?

USM works best on materials that are hard and brittle. Among metals, only hard grades (roughly 45 HRC and above) respond well; brittle non-metals respond best of all.

• Glass, quartz and fused silica
• Technical ceramics: alumina, zirconia, silicon carbide, silicon nitride
• Tungsten carbide and hardened tool steels
• Semiconductors such as silicon and germanium
• Ferrites, sapphire, ruby, graphite and precious stones

Soft, ductile metals such as copper, aluminium and mild steel are poor candidates: they absorb the impacts plastically and the MRR is very low.

Advantages of USM

• Machines non-conductive materials (glass, ceramics) that EDM and ECM cannot touch.
• No heat-affected zone, no thermal cracks, no recast layer and no residual chemical change.
• Low cutting forces, so thin and fragile parts survive.
• Produces non-round holes and shaped cavities (square, hexagonal, irregular) in one pass, since the cavity copies the tool.
• Good finish with fine grit; reported values range from about 0.1 to 2.5 microns Ra depending on grit and work material.
• Operation is simple and does not need skilled operators once the tool is made.

Limitations of USM

• Low MRR. Removal is typically a few cubic millimetres per minute on hard ceramics, so USM is not a bulk-removal process.
• Tool wear. The tool wears along with the work, especially at the edges, which rounds corners and spoils accuracy on deep cavities. Harder abrasives such as B4C raise tool wear.
• Limited depth. Slurry struggles to reach the bottom of a deep hole, so practical depths are only a few times the hole diameter (a depth-to-diameter ratio of about 3:1 is often quoted).
• Taper and overcut. Side wear of the tool and grains trapped along the walls make holes slightly oversize and tapered.
• Poor on ductile metals. See the materials list above.
• Tool cost and set-up. Each shape needs its own tool, and the tool-horn assembly must be tuned to resonance.

Applications of ultrasonic machining

• Drilling round and shaped holes in glass, ceramics and quartz for optics, electronics and laboratory ware
• Machining tungsten carbide wire-drawing dies, punching dies and small blanking dies
• Cutting and slotting silicon and germanium for semiconductor devices
• Engraving and making cavities in hard stones and jewellery
• Machining ceramic components for aerospace and medical use
• Drilling and shaping ferrite components

Rotary ultrasonic machining and ultrasonic milling

Rotary ultrasonic machining (RUM) is a hybrid of USM and diamond grinding. Instead of a plain tool and loose slurry, it uses a rotating tool with diamond grit bonded to it, usually a hollow core drill or an end-mill style tool, which also vibrates ultrasonically along its axis. Coolant is pumped through the tool to wash out debris. RUM removes material faster than either conventional USM or diamond grinding alone, gives deeper holes, and allows milling of slots and pockets in ceramics and glass. This is what most people mean by “ultrasonic milling”.

USM vs AJM vs EDM

For a wider view of where these fit among cutting and forming methods, see material removal techniques in manufacturing. The process parameter ranges above follow the NPTEL (IIT Kharagpur) lesson on USM; USM is also part of the AICTE model curriculum for non-traditional machining in B.Tech mechanical and production engineering.

FAQs

What frequency and amplitude are used in ultrasonic machining?

The tool usually vibrates at about 20 kHz, within a range of roughly 19 to 25 kHz, with an amplitude of about 10 to 50 microns at the tool tip. The horn amplifies the much smaller motion of the transducer up to this level.

Why is USM used for brittle materials and not ductile ones?

USM removes material by making tiny cracks that break out chips. Brittle materials such as glass and ceramics fracture this way easily. Ductile metals deform plastically under the grains instead of chipping, so MRR is very low.

Which abrasives are used in the USM slurry?

Boron carbide, silicon carbide, aluminium oxide and diamond, mixed with water. Boron carbide and diamond cut fastest but cost more and wear the tool faster; alumina and silicon carbide are cheaper for softer brittle work such as glass.

Does the tool in USM need to be harder than the workpiece?

No. The abrasive does the cutting, and the tool is deliberately made of a tough, ductile metal such as mild or stainless steel so that it deforms rather than fractures when grains strike it.

What is the difference between USM and rotary ultrasonic machining?

Conventional USM uses a non-rotating tool and loose abrasive slurry. Rotary ultrasonic machining uses a rotating tool with diamond grit bonded to it, vibrating ultrasonically and cooled by coolant, which gives higher MRR and deeper holes.

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