Polishing Internal Gears: A Mass-Production Method Using Generating Grinding Kinematics (Nidec Machine Tool x RWTH Aachen), Explained with Figures

1. 1Gear cutting
   Cut the teeth by hobbing, shaping or skiving
2. 2Heat treat
   Hardening makes it hard, but distorts the shape
3. 3Generating grinding
   Corrects the distortion. Accuracy ISO grade 3–5, roughness Ra 0.35 µm or less
4. 4Polishing (this article)
   Swap the wheel to remove only the peaks. Ra 0.1–0.05 µm

The flow of making an internal gear, and the step added here. Grinding and polishing values are from the paper^[1].

Why Internal Gears Are Hard to Finish

Planetary gear mechanisms can achieve large reduction ratios while staying small and light, so they are widely used in hybrid and electric vehicle drive units, transmissions and robot joints. The outermost part, the internal gear, can cause noise and vibration even from slight distortion. In electric vehicles, which have no engine sound, that noise has become especially easy to hear^[1][2].

Internal gears distort after hardening, so we would like to correct them by grinding, as with external gears. But the tool has to enter the narrow space inside the gear, so the large threaded wheels used for external gears do not fit. According to a 2009 Mitsubishi Heavy Industries (MHI) document, the industry had coped by improving cutting accuracy before hardening and had also tried honing with electroplated wheels, but high cost and low productivity meant none was a decisive answer^[6][5].

So MHI (now carried on by Nidec Machine Tool) studied the machining theory with Prof. Hiroshi Gunbara of the National Institute of Technology, Matsue College, and in 2009 launched the world's first mass-production internal gear grinding machine, the ZI20A. Its distinguishing feature is a barrel-shaped threaded wheel, thick in the middle. The company says it raises sliding speed more than 4 times over honing and cuts tooling cost per part to about 1/6^[5][6]. In 2024 the ZI25A, which can grind up to 250 mm outside diameter, was added as well^[4].

Grinding alone keeps profile and helix deviations within ISO IT3 and can also correct pitch errors caused by heat treatment. However, flank roughness is limited to about Ra 0.35 µm, and going smoother needed a different approach^[1]. In a comparison of wheels, a corundum wheel gave life and quality similar to a cBN wheel, with 1.5 times the productivity^[7].

Why Polish the Tooth Flanks?

Magnified, a ground flank shows fine peaks and valleys. When gears mesh, a thin oil film forms between the flanks, but sharp peaks break through the oil film and hit the mating flank, causing friction and tiny scratches.

For external gears, the Swiss grinding machine maker Reishauer developed polish grinding in 2012. A single threaded wheel is divided into a hard grinding section (grit 80, average grain size 185 µm) and a resilient resin polishing section (grit 800, average 7 µm), and the process moves from grinding to polishing on the same machine with the same clamping. Roughness of about Ra 0.3 µm from continuous generating grinding is brought down, as a guide, to Ra 0.1 µm. Vibratory finishing (polishing in vibrating abrasive media) can also polish tooth flanks, but it needs a separate machine and transport^[8].

What Smoothing Improves

• Longer life: In NASA tests, aerospace ground gears were superfinished by vibratory finishing (removing about 2.5 µm from the flank), and surface fatigue life rose about 4 times. The conditions were AISI 9310 material, 28 teeth and 1.71 GPa Hertz stress^[9].
• Small cracks are slower to grow: The paper gives reducing the risk of surface microcracks growing as a main reason for polishing in trucks, buses and agricultural machinery^[1].
• Less friction, higher efficiency: In drivetrains that run at high speed, such as in electric vehicles, improving efficiency and noise (NVH) is especially important^[1][8]. A superfinishing vendor cites a 2007 study by GM and Ohio State University reporting that superfinished spur gears were about 17% more efficient than ground gears (this is the vendor's account, and it is more natural to read it as a difference in loss rather than in efficiency itself)^[10].

The Nidec and RWTH Aachen Study

Mr. Fujimura of Nidec Machine Tool and Ms. Löhrer of the Laboratory for Machine Tools and Production Engineering (WZL) at RWTH Aachen University kept the motion of the ZI20A grinding machine as is and replaced only the wheel with a resin-bonded polishing wheel, and investigated the conditions for polishing an internal gear^[1].

Test conditions^[1].

Result 1: Infeed sets the roughness

At an infeed of 0.1 mm or more, roughness fell below the target of Ra 0.1 µm, and Ra 0.05 µm was reached at 0.15 mm. The measurement record of the gear finished at 0.1 mm was almost unchanged from before polishing, and it was also confirmed that polishing removed only the peaks of the roughness profile.^[1].

Result 2: Spinning faster does not worsen roughness

Roughness does not change in a straight line with speed: it is finest at low speed, gets coarser as speed rises, then gets finer again above 8,000 rpm. Since productivity is set by speed, running at the top speed of 11,945 rpm gives about the same roughness as slow running in about 1/8 of the time.^[1].

Result 3: Left and right flanks respond differently

Raising the axial feed from 0.02 to 0.04 mm lowered Ra on the left flank by 8% (0.12 to 0.11 µm) and raised it on the right flank by 33% (0.06 to 0.08 µm). The paper suggests that finishing the two flanks separately with different feeds is one option. Under every condition, no grinding burn appeared^[1].

Conditions the paper recommendsRadial infeed 0.1–0.15 mm, wheel speed 11,945 rpm, axial feed 0.02–0.04 mm per revolution. This combination is expected to give a flank smoother than Ra 0.1 µm in the shortest time. But this is a first test: confirmation and repeat tests under the same conditions, tests with other wheels, and detailed evaluation by Rpk and Rvk are still to come, the paper says^[1].

Roughness Scale and a Comparison of Internal Gear Finishing Methods

Sources: paper^[1], ZI25A document^[4], honing paper^[11], SkiveFinishing abstract^[12], REM^[10], NASA^[9].

What it means on the shop floor

FAQ

How is polishing different from grinding?

Both cut with a wheel, but grinding uses coarse grit and a hard binder to create the shape, while polishing uses fine grit (here grit 800) and a resilient resin binder to remove only the surface peaks. Polishing does not change the shape^[8][1].

How much material is removed?

A radial infeed of 0.1–0.15 mm set on the machine is recommended. For gears finished at 0.1 mm infeed, the increase in dimension between balls was 0.004 mm, within the scatter of grinding^[1].

Is a shinier, mirror-like surface always better?

Not necessarily. In polish grinding of external gears, leaving the valleys that hold oil is said to be important. Even at the same Ra, a surface with high peaks and shallow valleys performs very differently from one with low peaks and deep valleys^[8].

Which is better, compared with honing?

Honing's strengths are that it generates almost no heat and leaves a large compressive residual stress at the surface^[11]. Generating grinding and polishing are strong in the ability and speed to correct heat-treatment distortion and in fine roughness. Choose by gear size, amount of distortion and required roughness.

What kinds of internal gears can it be used for?

The paper's tests used an automotive ring gear with module 1.25 and 85 teeth. The ZI25A grinding machine handles module 1–3 and outside diameters up to 250 mm^[1][4].

For gear basics and accuracy grades, see Gear study; for machining methods such as grinding and honing, see Gear machining methods, and for how changing the flank shape from tooth to tooth cancels noise, see the MicroForm article.