CNC ultrasonic processing: how it cuts hard, brittle parts

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  Key takeaways
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  What actually removes material in CNC ultrasonic processing
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  Rotary ultrasonic machining on a CNC platform
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  Which materials suit CNC ultrasonic processing
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  Tolerances, surface finish, and where the process stops
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  Design rules and common mistakes
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  How to inspect and qualify ultrasonic-machined parts
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  When to choose CNC ultrasonic processing over other methods

In CNC ultrasonic processing the cutting edge never touches the part. A tool vibrates along its axis at 20–40 kHz with an amplitude of roughly 5–50 μm, and a slurry of water and abrasive grit flows through the gap. The vibration drives the grit into the surface, and thousands of tiny fractures chip the material away.

That distinction matters on the shop floor. There is no built-up edge, no thermal softening zone, and almost no cutting force pushing the part sideways. The tool only has to be harder than the abrasive, not harder than the workpiece. A soft steel tool can cut a hardened ceramic as long as the grit is boron carbide or diamond.

Removal is slow. Typical rates sit between 0.5 and 10 mm³/min depending on grit size, amplitude, and how deep the tool has to reach. Deep holes and pockets get slower because the slurry has to travel further and fresh grit struggles to reach the cutting zone.

The vibration also helps the slurry. Cavitation keeps the abrasive suspended instead of letting it settle, so grit keeps circulating through the gap. Without that agitation the slurry would pack into the cut and removal would stop.

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  Frequency20–40 kHz, set by the transducer and generator
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  Amplitude5–50 μm peak, adjusted for grit size and material
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  AbrasiveBoron carbide for most work, diamond for carbide and sapphire
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  Grit sizeCoarse grit cuts faster, fine grit holds tighter tolerance

Two machine layouts are common. A stand-alone ultrasonic machine sinks a formed tool into the part, much like a die-sinker EDM, and the tool shape becomes the hole shape. It is simple and cheap to run, but every new geometry needs a new tool.

Rotary ultrasonic machining mounts an ultrasonically driven spindle on a CNC platform. The tool spins while it vibrates, and the spindle follows a programmed path. That combination cuts holes, slots, and pockets with standard tools instead of a custom form tool, which shortens setup for small batches.

In rotary mode the diamond grit is usually bonded into the tool surface rather than carried in a slurry. The vibration keeps the bond from loading up, and the rotation spreads wear evenly around the tool. Tool life on ceramics is often measured in meters of cut, not in parts.

The CNC side handles positioning, feed, and depth control. The ultrasonic side handles the actual fracture. Neither one alone would hold the geometry that hard, brittle materials demand.

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  Stand-aloneFormed tool, no rotation, best for through holes and simple cavities
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  Rotary on CNCRotating vibrating spindle, best for slots, pockets, and profiles
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  Hybrid with diamond grindingDiamond plated tool plus vibration, common on glass and ceramics

The process earns its keep on materials that crack rather than deform. Alumina, zirconia, silicon carbide, silicon nitride, fused silica, borosilicate glass, quartz, sapphire, and tungsten carbide all machine cleanly when the grit and amplitude are set correctly.

Composites are a mixed case. Carbon fibre and glass fibre laminates cut without the delamination that a router can cause, but the resin matrix is soft and the fibre is hard, so the two phases remove at different rates. Expect to tune feed and grit per laminate.

Metals are almost always a poor choice. Aluminium, brass, and mild steel smear and load the tool instead of fracturing, and a standard end mill removes them 20 to 100 times faster. Hardened tool steel above 55 HRC is the one metal case where ultrasonic assistance can pay off, mainly to avoid the heat a grinding wheel would put into the part.

Soft plastics and rubber are worse still. They absorb the vibration, the grit embeds in the surface, and the finish comes out ragged. If someone proposes ultrasonic for POM or PEEK, they usually mean a different process.

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  Clean cutAlumina, zirconia, quartz, sapphire, glass, silicon carbide
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  Workable with tuningCarbon fibre and glass fibre laminates
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  Hardened metal onlyTool steel above 55 HRC, where grinding heat is a problem
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  AvoidAluminium, brass, mild steel, POM, PEEK, rubber

On a stable setup, CNC ultrasonic processing holds ±0.005 mm on hole diameter and slot width. That is a machine and fixture number, not a process limit. A loose fixture or a worn tool will lose that accuracy long before the vibration does.

Surface finish depends on grit size. Coarse grit leaves a matte, fractured surface around Ra 1.6–3.2 μm. Stepping down to fine grit gets Ra 0.8–1.6 μm, and a final fine-grit pass with low amplitude can reach Ra 0.2–0.8 μm on glass and ceramics.

Aspect ratio is the real boundary. As the hole gets deeper, slurry exchange and chip removal get harder. Beyond roughly 5:1 depth-to-diameter, removal rate drops sharply and tool wear climbs. Past 10:1 the process becomes uneconomic for most parts.

Corner radius is another limit. The tool radius sets the smallest internal corner you can cut. Sharp internal corners are not possible; specify a radius and the drawing will match what the machine can actually produce.

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  Diameter and width±0.005 mm on a rigid setup
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  Finish rangeRa 0.2–0.8 μm with a fine-grit finishing pass
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  Aspect ratioBest below 5:1, hard to justify past 10:1
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  CornersSmallest internal radius equals the tool radius

The most frequent mistake is specifying a square internal corner. Ultrasonic tools are round, so every internal corner carries a radius. Adding that radius to the drawing before quoting avoids a redesign after the first article.

The second is asking for a deep, small-diameter hole in one pass. Split the depth into steps, or add an entry chamfer so the tool does not chip the edge on breakthrough. Support the exit side of a through hole with a backing plate when the material is thin.

Fixturing matters more than people expect. The vibration is small, but the part still needs to sit flat and stay put. Wax mounting, vacuum chucks, and epoxy fixtures all work; a single clamp on one side usually does not.

Finally, keep the wall count reasonable. Thin walls in ceramic vibrate along with the tool, and the fracture pattern becomes uneven. A wall at least as thick as the tool diameter machines far more predictably.

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  Add corner radiiEvery internal corner is at least the tool radius
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  Step deep holesReduce depth per pass to keep slurry moving
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  Support thin sectionsBacking plate on through holes, flat mounting for plates
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  Watch wall thicknessKeep walls at least one tool diameter thick

Ceramic and glass parts cannot be checked the same way as metal. A visual pass finds edge chipping, but subsurface cracks need a different method. Dye penetrant works on non-porous ceramics, and for critical glass parts a polariscope reveals residual stress around the cut.

Dimensional checks use the same tools as any other precision part: CMM, optical comparators, and air gauges for small holes. The difference is handling. Fractured edges chip easily, so parts should be supported on soft jaws and measured without point loading.

For a production run, qualify the first article against a full dimensional report, then set the sampling plan. If the tool wears predictably, in-process checks on hole diameter are usually enough. If wear is erratic, tighten the sampling.

We run 100% inspection before shipment on ultrasonic work, with raw material checks, in-process monitoring, and a final dimensional report available on request. That covers the fracture surface as well as the nominal dimensions.

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  SurfaceVisual for chips, dye penetrant for surface cracks
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  SubsurfacePolariscope on glass, acoustic methods on thick ceramic
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  DimensionCMM and optical comparators, soft jaws for handling
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  DocumentationFirst article report, then a sampling plan based on wear