5 Axis Machining Aerospace Impeller Parts

An impeller is a set of twisted blades on a hub, sometimes with an outer shroud. The blade surface is ruled or freeform, and it twists as it rises from root to tip. A three-axis machine can only reach the blade from directly above, so it has to leave the tool at a fixed angle. That works for shallow, low-twist blades. Once the twist passes roughly 20°, the shank starts rubbing the neighbouring blade and the finish goes uneven.

Five-axis work solves this by tilting the tool or the part. The cutting point stays normal to the blade surface from root to tip, so the flute cuts instead of rubbing. Tool overhang drops, which cuts chatter on thin blades. On a 16-blade impeller with a 0.8 mm tip thickness, that difference shows up directly in the surface finish and in how long the blade lasts in service.

The same setup also reaches the fillet where the blade meets the hub. That root radius is where fatigue cracks start. A constant-radius fillet blended into both surfaces is hard to cut in three axes and straightforward in five, because the tool can roll around the corner without changing its contact point.

• 1
  Under 20° twistThree-axis with a ball nose mill is usually enough.
• 2
  20–45° twistFive-axis indexing or simultaneous cutting pays off.
• 3
  Closed or shroudedSimultaneous five-axis is the only practical route.

There are two ways to cut a blade in five axes: point milling and flank milling. Point milling uses the ball tip of a small cutter and steps over in fine passes. It handles any surface and any tool reach, but the cycle runs long. Flank milling lays the side of the cutter along the blade surface and removes the whole face in one or two passes. Cycle time can drop by half or more, and the finish is better. Not every blade qualifies. The surface has to be close to a ruled surface, and the tool must reach the full flank without colliding with the hub or the next blade.

For a typical 300 mm diameter aluminium impeller with 20 blades, point milling with a Ø6 mm ball nose at 0.3 mm stepover is a long job. Flank milling the same blade with a Ø12 mm tapered barrel cutter can take a fraction of that. The trade-off is programming time and tool cost. Barrel and tapered cutters are not cheap, and the CAM setup takes longer because the programmer has to verify every contact point.

Roughing matters as much as finishing. On titanium and Inconel impellers, we leave 0.3–0.5 mm of stock for the finishing pass and control the radial engagement so the cutter does not rub. A trochoidal path with a constant chip load keeps heat out of the blade, which is where thin-wall deflection comes from. On 5 axis machining aerospace parts of this shape, heat and deflection are the same problem.

An impeller starts stiff and ends flexible. The hub is solid until the last operation, and the blades get thinner with every pass. That means the fixture has to hold the part without touching the surfaces being cut. For an open impeller, a three-jaw chuck or a collet on the hub bore usually works, with a tailstock support if the part is long. The blade tips stay free.

A shrouded impeller is harder. The shroud closes the flow path, so the cutter has to enter through the gap between blades. We often split the part into a hub-and-blade body plus a shroud ring, machine both, then join them. That is a design decision, not a machining one, and it should be made before drawings are frozen. Trying to cut a closed impeller from one billet is possible for some geometries, but the tool has to be long and thin, which brings back the chatter problem.

For thin blades, we sometimes add a sacrificial web or a wax fill. The wax supports the blade during finishing and is melted out afterwards. It adds a cleaning step and a verification step. It also lets us run a more aggressive finishing pass without the blade ringing.

• 1
  Open impellerChuck or collet on the hub bore, tips free.
• 2
  Shrouded impellerSplit body and shroud, or cut through the blade gap.
• 3
  Thin bladesSacrificial web or wax fill to control vibration.

Blade profile tolerance drives performance. On a machined impeller for aerospace use, we hold ±0.005 mm on critical diameters and blade thickness where the drawing calls for it. That is a tight number and it needs the right machine, a warm shop and a probe check before the finishing pass. Not every feature needs it. The hub bore and the blade root do. The outer tip usually does not.

Balance is a separate requirement. A machined impeller that is geometrically correct can still vibrate if the mass is uneven. We check blade-to-blade thickness variation and note it on the inspection report. If the drawing specifies a balance grade, the part goes to a balancing step after machining. That step is outside the machining scope, but the machining has to leave enough material for it.

Surface finish on the flow path affects both efficiency and fatigue life. A finish of Ra 0.8–1.6 μm is a normal target for the blade surface. Tighter, down to Ra 0.2–0.8 μm, is possible with a slow finishing pass or a polishing step. As-machined surfaces at Ra 1.6–3.2 μm are acceptable for many non-critical impellers, such as cooling fans and low-pressure blowers.

Aluminium is the easy case. 6061-T6 and 7075 cut fast, hold a good finish and do not work-harden much. Most machined impellers at prototype and low-volume stage are aluminium, because the cycle is short and the design can still change. For a 5 axis machining aerospace project, aluminium is often the right first article even when the production part will be titanium.

Titanium and Inconel are the hard case. Ti-6Al-4V (TC4) has low thermal conductivity, so heat stays at the cutting edge and tool life drops fast. Inconel is worse. Both need lower cutting speeds, higher coolant pressure and a rigid setup. On thin blades, the cutting force has to be kept low, which usually means smaller stepovers and longer cycles. A titanium impeller can take three to five times the cycle time of the same part in aluminium.

Stainless steels such as 17-4PH (SUS630) sit in the middle. They hold strength at temperature and machine better than titanium, which makes them common for compressor and pump impellers. Copper alloys, including beryllium copper, are used where thermal conductivity matters. Each material changes the tool, the speed and the finishing pass, so the material call should be made before the CAM work starts.

• 1
  Aluminium 6061-T6, 7075Fast cycle, good finish, best for prototypes.
• 2
  Titanium TC4 (Ti-6Al-4V)Low speeds, high coolant pressure, 3–5× cycle time.
• 3
  InconelLowest tool life; rigid setup and small stepovers.
• 4
  17-4PH (SUS630)Good strength at temperature, moderate machinability.

A machined impeller is checked in stages. Raw material comes in with a certificate. The first setup is probed on the machine to confirm the datum. In-process checks catch drift before the finishing pass, when there is still stock to correct. Final inspection covers blade profile, blade thickness, hub bore, runout and surface finish. Reports are available on request.

For the drawing, be specific about which features carry the tight tolerance. Marking the whole part ±0.005 mm makes it expensive without improving function. Mark the hub bore, the blade root and the blade thickness. Leave the non-critical outer surfaces at a general tolerance. If balance is required, state the grade and the correction method. If the impeller will be balanced after machining, say so, so we leave stock in the right place.

GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, with a maximum processing size of 4,000 mm. Tolerances go to ±0.005 mm (±0.0002 in) and finishes to Ra 0.2–0.8 μm. We work from one prototype to 10,000+ part runs, with no minimum order quantity. Uploads are confidential and an NDA is available on request.