Centrifugal Fan Impeller Processing on 5 Axis CNC

A centrifugal impeller does two jobs at once. It accelerates air outward and it holds its shape while doing so at speed. That second job is why the part is usually judged by runout, blade thickness consistency and balance, not just by whether the blades look right.

Open or semi-open impellers with 6 to 16 curved blades are the natural fit for 5 axis milling. The tool reaches the blade root from both sides, the shroud stays open, and you can machine the whole wheel from one billet or a hub-and-blade assembly. With a Ø400 mm rotary table we can index around the hub and cut continuous blade paths without repositioning.

Closed impellers with a full shroud are harder. The channel between hub and shroud is narrow and curved, so tool access is limited by shank diameter rather than by machine travel. A 12-blade closed wheel with 18 mm channel width leaves little room for a cutter stiff enough to hold tolerance.

This is where material choice starts to matter as much as shape. Aluminium 6061-T6 and 7075 cut fast and hold thin blade sections well. Stainless 316L and 17-4PH move more under cutting load, so blade walls below 2 mm need light radial passes and more finishing time.

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  Open impellersBest fit for 5 axis milling; easy tool access at the blade root.
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  Semi-open wheelsMachinable from solid when blade count stays under about 16.
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  Closed impellersFeasible but channel width sets the minimum cutter diameter.
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  Thin bladesUnder 2 mm walls, expect extra finishing passes and balance correction.

We rough the outside profile and the hub first, leaving 0.5–1.0 mm on blade surfaces. Roughing with a larger cutter removes most of the volume before the tool gets anywhere near a thin blade. Then stress relief, if the material and the part size call for it.

The hub bore and the mounting face are machined in the same setup as the blade roots whenever possible. That keeps the bore perpendicular to the backplate and the blade tips concentric with the axis of rotation. On a Ø400 mm rotary table the part can be turned, indexed and finished without losing the datum.

Blade finishing runs in two passes per side: a semi-finish at 0.3 mm radial stepover and a finish at 0.1–0.15 mm. Stepover is set by the surface finish target, not by the toolpath. If you need Ra 0.8–1.6 μm, a 0.1 mm stepover with a ball nose cutter is a practical starting point.

For a welded or bolted impeller, we machine the hub, blades and shroud separately, then fit them and machine the assembly. The bore is often left undersize until after assembly, so the final bore and face runout reflect the true assembled geometry.

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  RoughRemove bulk volume, leave 0.5–1.0 mm on blade surfaces.
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  Bore and faceCut in the same setup as blade roots to protect concentricity.
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  Finish0.1–0.15 mm stepover for Ra 0.8–1.6 μm.

Material drives both the machining plan and the final balance. Aluminium 6061-T6 is the default for general ventilation and low-temperature work. 7075 gives higher strength for high-speed wheels but machines with more chatter risk on thin blades.

Stainless 304 and 316L suit corrosive or wet airstreams. They cut slower, and the blade surface work-hardens if the cutter rubs instead of cutting. Sharp tools and consistent feed per tooth matter more here than on aluminium. 17-4PH is the choice when you need strength plus corrosion resistance, and it can be aged after machining.

Titanium TC4 (Ti-6Al-4V) and Inconel appear in high-temperature or high-speed applications. Both are slow. On Inconel, expect the finishing time to be several times the aluminium equivalent, and plan for a separate balance correction step.

After machining, impellers are usually anodized, bead blasted or left as machined. Anodizing adds a hard surface but changes the fit slightly, so bores that need a press fit are often masked. Bead blasting evens out tool marks on blade surfaces and helps visual inspection of the finish.

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  6061-T6General ventilation, fast cutting, good thin-blade stability.
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  316L stainlessCorrosive airstreams; watch work hardening on blade surfaces.
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  17-4PHStrength plus corrosion resistance; can be aged after machining.
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  TC4 / InconelHigh temperature and speed; plan more time and a balance pass.

An impeller that meets every dimensional callout can still vibrate. Unbalance comes from uneven blade thickness, a bore that is off-axis, or a hub face that is not square to the axis. Machining tolerance alone does not fix it.

We check bore runout and backplate squareness against the blade roots, and we measure blade tip radius at several points around the wheel. If the tips vary, the wheel will produce a once-per-revolution vibration that no amount of balancing weight will fully remove.

Balancing is done after finishing, usually by removing material from the hub side rather than adding weight. For thin aluminium wheels, the correction amount is small and easy to overshoot, so the material removal is done in light steps and rechecked.

Every impeller is inspected before shipment. Raw material certificates are checked on receipt, in-process measurements track blade thickness and bore size, and a final dimensional and visual inspection closes the job. Inspection reports are available on request.

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  RunoutBore and backplate checked against blade root datum.
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  Tip radiusMeasured at multiple points to catch once-per-rev vibration.
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  BalanceCorrected by light hub-side material removal after finishing.