Robot Microphone Array Casings CNC Milling
A microphone array casing is a mechanical part that decides how sound reaches the MEMS microphones inside. Written for robotics and hardware engineers, this page explains what milling can hold, where the geometry stops behaving, and how to tell when another process fits better.
What the casing does to the acoustic path
A microphone array does not capture sound; it captures phase differences between microphones. The beamforming algorithm assumes each MEMS port sees the same acoustic environment except for the time delay caused by the angle of arrival. That assumption holds only if every hole in the casing is the same size, the same depth, and free of burrs. A single hole that is 0.05 mm wider than its neighbors shifts the phase at that channel and pulls the beam off axis.
So the casing is not a cover. It is the front end of the signal chain. Every feature between the outside air and the microphone port matters: hole diameter, hole length, hole spacing, the cavity behind the plate, and how the part couples to the robot frame.
This is why robot microphone array casings CNC milling is usually chosen over molding or casting for the first production runs. Milling gives us control over each of those features, one at a time, and lets us measure them before the part ships. If the acoustic model changes, we re-cut the toolpath instead of re-cutting a mold.
The trade-off is cycle time and unit cost at volume. Below roughly 2,000 parts per year, milling is normally the cheaper route once tooling and lead time are counted. Above that, die casting or injection molding starts to win on piece price, and milling moves to the mold insert or the secondary acoustic features.
- 1Hole geometryDiameter and length set the Helmholtz resonance of each port.
- 2Hole spacingCenter distance controls diffraction between neighboring channels.
- 3Cavity volumeThe air behind the plate loads the microphone diaphragm.
Acoustic holes: diameter, depth and burr control
Most robot microphone array plates use holes between 0.3 mm and 0.8 mm in diameter. Below 0.3 mm, chip evacuation becomes unreliable and the drill walks. Above 0.8 mm, the port starts to act as a short waveguide and the high-frequency response rolls off earlier than the simulation predicts.
The length of the hole is the plate thickness at that point. A 0.5 mm hole through a 3 mm wall has a length-to-diameter ratio of 6:1. That is drillable with a carbide micro-drill and a peck cycle, but it needs a rigid setup. Ratios above 8:1 usually mean the hole should be reamed or EDM-drilled, and the cost goes up.
Burrs are the quiet failure mode. A burr inside a 0.5 mm hole narrows the effective diameter, adds a whistle at high frequency, and is hard to see under normal lighting. We inspect these holes under magnification and deburr with a controlled back-chamfer. Entry and exit chamfers of 0.05–0.10 mm keep the airflow clean without changing the acoustic length much.
If your design calls for hundreds of holes on a curved shell, group them into a local flat or a shallow spherical pad. Five-axis milling can drill on a curved surface, but a small flat around each hole keeps the drill perpendicular and the depth consistent across the array.
- 1Target range0.3–0.8 mm diameter, L/D below 8:1 where possible.
- 2Deburr0.05–0.10 mm back-chamfer, inspected under magnification.
- 3AvoidHoles drilled at an angle to the local surface.
Wall thickness, stiffness and vibration isolation
A thin acoustic plate is a diaphragm. If the plate is too soft, motor vibration and speaker output couple straight into the microphone and show up as low-frequency noise in the recording. If it is too stiff, the part becomes heavy and the mounting points transmit more energy into the shell.
For aluminum, wall thickness behind the acoustic zone typically lands between 1.5 mm and 3.0 mm. That range keeps the plate stiff enough to hold hole geometry after machining and light enough for a robot head or chest module. Stainless and titanium can go thinner, but machining time rises and the part may need stress relief before finishing.
Vibration isolation is usually handled at the interface, not in the casing itself. A milled groove for a silicone or EPDM gasket, or a set of counterbored pockets for rubber isolators, gives the designer a place to break the structure-borne path. These features are simple to mill and easy to hold to ±0.05 mm, which is enough for a compression gasket.
Keep the mounting bosses away from the acoustic plate where possible. A screw boss sitting 10 mm behind a microphone port transfers frame vibration directly into the cavity.
- 1Aluminum walls1.5–3.0 mm behind the acoustic zone.
- 2Gasket groovesHeld to ±0.05 mm for consistent compression.
- 3Boss placementKeep screw bosses out of the cavity behind ports.
EMI shielding and grounding paths
MEMS microphones are sensitive to electromagnetic interference, especially when the array sits close to motor drivers or a switching regulator. An aluminum casing can act as part of the Faraday cage, but only if the grounding path is continuous from the plate to the PCB ground plane.
That means the design needs a defined ground path. Common solutions are a milled groove for a conductive gasket, a machined boss that contacts a ground pad on the PCB, or a threaded hole for a grounding screw. All three are straightforward on a mill and can be held to tight flatness so the contact is reliable after anodizing.
Finishing matters here. Standard anodizing is an insulator. If the housing must conduct, specify a masked area, a conductive anodize, or a chromate conversion coating on the contact surfaces. We usually mark these zones on the drawing so the finishing shop knows not to seal them.
For plastic or carbon fiber covers, a conductive coating or a thin metal liner is needed. That is a different process chain, and it is worth deciding early because it changes the wall thickness budget.
- 1Ground pathConductive gasket groove or machined contact boss.
- 2FinishingMask contact zones; standard anodize insulates.
- 3Plastic coversNeed conductive coating or metal liner.
Why five-axis milling fits this part
A microphone array casing usually has features on five or six faces: the outer cosmetic surface, the acoustic hole field, the internal cavity, the PCB mounting plane, the gasket groove, and the cable exit. Doing this on a three-axis machine means multiple setups, and every setup adds a datum shift.
Five-axis machining lets us cut the cavity, the acoustic holes, and the mounting plane in one or two setups. That keeps the hole field concentric with the PCB pads and the gasket groove consistent around the perimeter. On a robot head shell with a curved front, it also lets the drill follow the surface normal at each hole.
For tight-tolerance work, we hold ±0.005 mm on critical features and Ra 0.8–1.6 μm on sealing faces. Cosmetic surfaces can be bead blasted or brushed after machining. The point is not to chase the tightest number on every dimension; it is to put the tight tolerance where the acoustics and the seal need it.
Where five-axis does not help: very large, flat plates with a simple hole pattern. A three-axis mill with a good fixture will do that faster and cheaper.
- 1One-setup benefitCavity, holes and PCB plane stay concentric.
- 2Curved shellsDrill follows surface normal at each port.
- 3Not always neededFlat plates with simple hole fields suit 3-axis.
Casing process options at a glance
Read this as a starting filter, not a final decision.
| Process | Best for | Watch out for |
|---|---|---|
| 5-axis CNC milling | Prototypes to ~2,000 parts; tight hole fields | Higher piece price at volume |
| 3-axis CNC milling | Flat plates, simple hole patterns | Multiple setups shift datums |
| Die casting | High volume, complex thin walls | Porosity and draft on acoustic holes |
| Injection molding | Lightweight plastic covers | Needs conductive coating for EMI |
| Milled insert + cast body | Volume with critical acoustic zone | Two-process coordination |
When to mill and when to mold
If the hole field and the sealing face carry the acoustic performance, mill the part; once annual volume passes roughly 2,000 pieces and the acoustic zone can be held as a separate insert, move the body to casting or molding and keep milling only the critical features.
Common questions
What tolerance can you hold on acoustic hole diameter?
On a 0.5 mm hole we normally hold ±0.02 mm on diameter and ±0.03 mm on position, which is enough for beamforming up to the mid-kilohertz range.
If your model needs tighter, tell us the frequency band and we will quote a reamed or EDM-drilled option.
Can you machine a curved microphone array shell?
Yes. Five-axis machining lets us cut the outer curvature and drill each acoustic port along the local surface normal.
We usually add a small flat or shallow spherical pad around each hole so the drill entry stays clean and the depth stays consistent across the array.
Do you deburr the inside of the acoustic holes?
Yes. Every acoustic hole is inspected under magnification and given a controlled back-chamfer, typically 0.05–0.10 mm.
Burrs are the most common cause of unexplained high-frequency noise in a finished array, so this step is not optional.
Which aluminum grade is best for an acoustic casing?
6061-T6 is the usual choice: stable, machinable, and good for anodizing. 7075 gives higher stiffness if the plate must be thin.
For conductive grounding paths, specify a masked or conductive anodize, or leave the contact zones in chromate conversion coating.
What is the minimum order quantity?
There is no minimum order quantity. We run from one prototype to 10,000+ part runs.
For a first article we quote and return a DFM analysis within 12 hours, and production can start within 24 hours after approval.
Can you help with the acoustic design, not just the machining?
We are a machining supplier, so we work from your acoustic model rather than building one for you.
What we do offer is DFM feedback: hole diameter versus depth, wall thickness, gasket groove geometry, and whether the part should be milled or cast.
Send us your array casing drawing
Upload a STEP file and we will return a quotation plus a DFM analysis within 12 hours, with 100% inspection before shipment.
12-hour quote100% inspectionNDA on request
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