Gearbox Housing Machining: Bore Alignment, Seal Bores and Material Choice
A gearbox housing is a set of bearing bores that have to share an axis across a span of 150 to 250 mm, with faces that have to be square to that axis, and a seal bore that has to be smooth enough not to cut the lip. Get the bore diameters right and the gearbox still whines, leaks, or fails a 2,000-hour durability test in 400 hours.
The reason is that gear tooth contact is far more sensitive to shaft position than to shaft size. A 0.02 mm offset between two bearing bores tilts the shaft enough to shift the contact pattern toward one end of the tooth face and roughly double the load there. That is where pitting starts.
What alignment means in a housing
There are three separate geometric requirements, and drawings often collapse them into one note. The first is coaxiality between the bearing bores that carry one shaft. The second is parallelism between the axes of two different shafts — input and output, or two stages. The third is center distance between those axes, which sets backlash.
Coaxiality is the one that shows up as noise. Parallelism is the one that shows up as uneven tooth wear. Center distance is the one that shows up as backlash that is tight at one end of the housing and loose at the other, which happens when the housing is machined in two setups and the second setup is rotated slightly.
For a single-stage spur reducer with 2 mm module gears, 0.015 mm coaxiality and 0.02 mm parallelism will hold a contact pattern within 70 percent of the tooth face. For a planetary stage, tighten coaxiality to 0.010 mm, because three or four planets share the load and one that sits 0.01 mm off carries disproportionately more.
Bearing bores across a span
The hard part is not boring a Ø40 H7 hole. It is boring two of them 200 mm apart, from opposite ends of a part that is not rigid, and having them share an axis within 0.015 mm. A 3-axis mill with the part flipped between operations will typically land at 0.03 to 0.05 mm, because the refixture error alone is 0.02 mm and the part moves when the clamps release.
| Application | Bore diameter | Span | Coaxiality | Perpendicularity of end face |
|---|---|---|---|---|
| Single-stage spur reducer | Ø35 to Ø50 H7 | 150 to 200 mm | 0.020 mm | 0.020 mm |
| Two-stage helical reducer | Ø40 to Ø60 H7 | 200 to 250 mm | 0.015 mm | 0.015 mm |
| Planetary stage | Ø50 to Ø80 H7 | 100 to 150 mm | 0.010 mm | 0.010 mm |
| Worm gearbox | Ø30 to Ø45 H7 | 120 to 180 mm | 0.020 mm | 0.025 mm |
| Servo planetary, robot joint | Ø40 to Ø70 H7 | 80 to 140 mm | 0.008 mm | 0.008 mm |
Leave 0.3 mm on bore diameters for the finish pass, and bore with the part at room temperature, not straight out of a stress-relief oven. A 4140 housing at 45 °C bores 0.012 mm large on a Ø50 hole and shrinks as it cools.
See also: Harmonic Drive Housing Machining: Keeping Three Bores on One Axis and Battery Housing Machining: Gasket Grooves, Flatness and Leak Test Prep.
Face perpendicularity and the seal bore
The end face that the bearing outer ring sits against sets the axial position of the shaft, and its perpendicularity to the bore sets the tilt. A face that is 0.03 mm out of square over a 60 mm diameter tilts the bearing by about 30 arcseconds, which is enough to load the ball set unevenly and to make the seal run off-center.
Face it in the same setup as the bore, with a light finishing cut and a wiper insert. Do not grind it unless the drawing requires it; a turned face at Ra 1.6 µm with 0.01 mm squareness is better than a ground face that was set up from a different datum.
The oil seal bore is a separate problem. It needs Ra 0.8 µm or better, tolerance H8, and concentricity to the bearing bore within 0.05 mm. More important, it must have no lead — no helical tool mark that can pump oil past the lip. Cut it with a plunge or a single-point boring pass with zero feed per revolution in the axial direction, or finish it with a roller burnishing tool. A seal bore turned with a 0.1 mm/rev feed leaves a helix that acts as a screw pump, and the gearbox will leak at the seal within a few hundred hours even though every dimension is in tolerance.
Pre-hard 4140 or aluminum
4140 pre-hardened to 28 to 32 HRC is the default above 50 N·m output. Bearing seats do not fret, threads survive assembly, and the housing keeps bore alignment under load. It machines at 180 to 220 m/min with carbide, needs a rigid setup, and moves 0.1 to 0.2 mm after roughing, so leave 1.0 mm and stress-relieve at 550 °C before finishing.
6061-T6 suits small servo reducers under 20 N·m where weight matters, and it is easier to hold tolerance. Its limits are thread durability and fretting: a steel outer ring in an aluminum bore with a light press fit will rotate and polish the seat after a few thousand cycles.
Cast iron, when the customer supplies it, is stable, damped, and holds a bore — but a casting with a void at a seal bore cannot be salvaged. Hybrid housings with steel bearing inserts are common in robot joints; machine the insert bores after the inserts are installed and pinned, and expect the press fit to move the bore 0.01 to 0.02 mm.
Holding alignment in one setup
The reliable method is to bore both ends without unclamping. On a 5-axis center with a rotary table, mount the housing on its mounting face, bore one end, index the table 180 degrees, and bore the other end with the same tool and offsets. The error then comes from the table's indexing accuracy, typically 0.005 mm at 200 mm radius, not from refixturing.
The alternative is line boring with a bar supported at both ends, which is how large reducers are made. A 25 mm bar over a 300 mm span deflects 0.005 mm under its own weight, so support it with a bushing in the first bore and cut the second from the inside out.
Control the temperature either way. A 4140 housing at 22 °C bored with a bar at 30 °C produces a bore that is round and straight but 0.006 mm off-axis at the far end once the part equalizes. Let it sit four hours, measure, and correct with a light spring pass rather than a re-setup.
For production, ask for a CMM report that evaluates coaxiality as one cylindrical zone across both bores, plus parallelism between input and output axes. A report that lists roundness at each bore and nothing else will pass a bad housing. If you are sourcing robot joint reducers, specify that report in the purchase order, and request a quote with the gear data and housing drawing together.
Frequently asked questions
How tight should bearing bore alignment be?
0.020 mm coaxiality for a single-stage spur reducer over a 200 mm span, 0.010 mm for a planetary stage. Tightening below 0.008 mm rarely helps unless the bearings themselves are a precision grade.
Why does my gearbox leak at the oil seal?
Almost always a lead mark in the seal bore. A turned bore with axial feed leaves a helix that pumps oil past the lip. Plunge-cut or burnish the bore to Ra 0.8 µm and check that no tool mark spirals across the sealing band.
Can an aluminum housing replace 4140?
For small servo reducers under about 20 N·m, yes. Above that, fretting at the bearing seats and thread durability become the limit, and a steel or cast iron housing lasts longer.
Should the housing be machined before or after heat treatment?
Machine pre-hardened 4140 at 28 to 32 HRC and finish after stress relief. If the part is hardened after machining, leave 0.4 mm on bores and expect 0.1 to 0.2 mm of movement, which has to be removed by boring or grinding.
Related reading
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