Anemometer Cup Rotor Machining: Geometry, Balance and Finish

Wind speed becomes rotation rate inside any cup anemometer. Drag on the concave face of each cup exceeds the drag on the convex back, so the rotor turns at a rate that is roughly linear with wind speed over the useful range. The calibration factor, the slope between rotation rate and wind speed, is determined by cup geometry and by the friction in the bearing below it.

This is why rotor machining matters. If cup radius varies by 0.1 mm across a batch, the effective drag area changes and the slope shifts. Two rotors that look identical on a bench can read 2 percent apart in the field. Instrument makers usually absorb that spread in a calibration curve, but only up to a point. Once the geometry drifts past the curve, the sensor has to be reclassified or scrapped.

Three features set the aerodynamic behavior of the rotor: cup radius and depth, the distance from the hub axis to each cup center, and the angular spacing between arms. Radius and depth control drag area. Arm length controls the torque arm. Angular spacing controls how evenly the load is distributed through one revolution.

The hub bore and spigot are mechanical, not aerodynamic, but they matter just as much. A bore that is 0.02 mm loose lets the rotor tilt slightly on the shaft. Tilt changes the effective radius of one or two cups, which shows up as a once-per-revolution wobble in the rotation signal. On a data logger this looks like noise. It is actually geometry.

For a 100 mm radius rotor, we hold cup center position to ±0.05 mm and arm-to-arm angle to ±0.1°. On smaller instrument rotors the angular tolerance tightens because the same angular error becomes a smaller linear error, and the calibration curve is steeper. Where the cup meets the arm, we leave a fillet rather than a sharp corner. A sharp transition creates a stress riser under vibration and a small wake that adds drag.

• 1
  Cup radius and depthSets drag area and the linear range of the curve.
• 2
  Cup center radiusSets torque arm; variation shifts the calibration slope.
• 3
  Arm-to-arm angleAffects load evenness and once-per-rev wobble.
• 4
  Hub bore fitLoose fit lets the rotor tilt and adds signal noise.

Most meteorological rotors are machined from 6061-T6 aluminum and then anodized. The alloy is light, machines cleanly, and holds thin cup walls without chattering. Type II anodizing gives enough corrosion protection for inland and most coastal sites. Hard anodizing adds wear resistance on the cup rims where handling damage tends to start.

Stainless is the answer when the sensor sits in salt spray or in a chemical plant. We often machine a 316L hub with 6061 cups, hard-anodized and sealed. This keeps the rotating mass low while the hub takes the bearing load and the thread wear. A full stainless rotor is heavier, which raises bearing friction and lowers the threshold wind speed at which the rotor starts turning.

Titanium and 17-4PH appear in high-end or defense instruments. Both cost more and take longer to machine. We would only suggest them when the customer has a specific corrosion or strength requirement that aluminum and stainless cannot meet. For most anemometer cup rotor machining work, the aluminum and stainless combination is the practical choice.

• 1
  6061-T6 + Type II anodizeDefault for meteorological rotors; light and stable.
• 2
  6061 cups + 316L hubCoastal and salt-spray sites; low mass, hard threads.
• 3
  17-4PH or titaniumOnly for specific corrosion or strength demands.

The cup interior is a curved surface that blends into the arm. On a three-axis machine this needs multiple setups and hand blending, and the blend line shows up as a small ridge. That ridge changes drag and adds mass where you do not want it. Five-axis machining lets the tool stay normal to the contoured surface for the whole cup, so the interior comes off the machine with the intended form.

We machine the hub bore and spigot first, then use that bore as the locating datum for the cup and arm features. Doing as much as possible in one clamping keeps the cup centers concentric to the bore. When cup features and the bore are cut in separate setups, the stack-up between them is usually the largest single error in the rotor.

Our 16 simultaneous 5-axis centers and Ø400 mm rotary tables handle rotors from small instrument sizes up to a 4,000 mm maximum processing size for larger assemblies. Roughing removes most of the stock, then a light finishing pass cuts the cup wall to its final thickness. Wall thickness matters because it sets cup mass. A cup that is 0.1 mm thick on one side and 0.15 mm on the other will pull the rotor off balance.

Static balance is the minimum. We mount the finished rotor on a mandrel and check the residual imbalance, then remove material from the arm undersides or the hub web, never from the cup surfaces. Removing material from a cup changes its drag area and undoes the aerodynamic work.

Runout is checked on the cup rim and on the hub spigot. Cup rim runout above about 0.05 mm total indicator reading usually points to a clamping or setup issue rather than a toolpath issue, and it is worth investigating before the batch runs. We also verify cup center radius on a coordinate measuring machine and compare arm-to-arm angles against the drawing.

Every rotor gets a 100% inspection before shipment. That covers raw material certification, in-process checks after each critical operation, and a final dimensional and balance report. Reports are available on request. Our tolerance capability is ±0.005 mm on critical features, and surface finish on cup interiors typically lands in the Ra 0.8–1.6 μm band, which is smooth enough to avoid surface drag without adding polishing steps that would alter mass.