Automotive metal bearings are the precision components that let a vehicle's rotating assemblies turn with controlled friction, controlled play and predictable life. In practice that means races, rings, sleeves, bushings, cages and housing seats — and almost all of them start life as a CNC turned or milled part before any grinding, heat treatment or assembly takes place.
We are an IATF 16949 certified CNC machine shop in Dongguan, China, and bearing-related components are one of our recurring automotive programmes. This guide covers the machining side specifically: which bearing components are machined rather than bought standard, what tolerances actually matter, how the process route is sequenced around heat treatment, and where bearing projects typically go wrong.
Why bearings are machined rather than bought
A large share of bearings in a vehicle are catalogue items — you look up a 6205-2RS and buy it. The parts that get machined are the ones a catalogue cannot serve:
- Non-standard envelopes where the housing bore or shaft diameter is fixed by an existing assembly
- Integrated features — flanges, mounting bosses, snap-ring grooves, sensor targets machined into the ring itself
- Application-specific materials — corrosion-resistant stainless, non-magnetic grades, or bronze for dry-running duty
- Low volume — prototype builds, motorsport, restoration and low-volume specialty vehicles
- Split or segmented designs that must be assembled around a shaft
- Housing and shaft seats where the bearing is standard but the *seat* is the critical machined feature
That last category is easy to overlook and often the most important. A standard bearing pressed into a bore that was machined 0.02 mm oversize will fail the assembly regardless of how good the bearing was.
Six bearing components we machine
| Component | Typical material | Primary machining process | Critical feature |
|---|---|---|---|
| Bearing rings / races | 100Cr6, GCr15, 42CrMo | Turning, then hardening and grinding | Raceway geometry, roundness |
| Bearing housings | 6061-T6, 7075-T6, cast iron | Multi-axis milling, boring | Bore diameter, roundness, coaxiality |
| Shafts and journals | 42CrMo, 20CrMnTi, S45C | Turning, then grinding | Diameter, runout, Ra |
| Bushings and sleeves | Bronze, brass, POM, steel | Turning, boring | ID/OD concentricity, wall thickness |
| Bearing seats and inserts | 304, 316, 42CrMo | Turning, milling | Press-fit class, seat depth |
| Cages and retainers | Brass, phosphor bronze, PEEK | Milling, turning, sometimes stamping | Pocket position, pocket depth |
The tolerances that decide whether a bearing project succeeds
Bearing work lives or dies on a small number of characteristics. Everything else on the drawing is secondary.
Bore and shaft diameter to a fit class
Bore and shaft diameter to a fit class. A press fit is specified as a class, not as a value — H7/p6, H7/r6, H7/n6. The tolerance is not the same in absolute terms at 20 mm and at 120 mm, which is why the drawing must state the class and the nominal. Our CNC machining tolerance guide explains how ISO 286 classes translate into machining limits.
Roundness and cylindricity
Roundness and cylindricity. A bore can measure correct in two perpendicular directions and still be tri-lobed, which kills bearing life. Roundness is measured on a roundness tester, not with a bore gauge, and it must be called out explicitly — it is not implied by a diameter tolerance.
Coaxiality between the two bearing seats
Coaxiality between the two bearing seats. In almost every rotating assembly there are two bearings on one shaft, or two seats in one housing. If those two seats are not coaxial, the bearing is misaligned at assembly and its calculated life drops sharply. Typical requirement: coaxiality ≤ 0.02 mm, and tighter in high-speed assemblies.
Total runout on the assembled datum
Runout on the assembled datum. Total runout combines form, location and orientation error into one number, which is why it is the right callout for a rotating component. Typical requirement: ≤ 0.01 mm.
Surface roughness on the contact surfaces
Surface roughness on the contact surfaces. Ra affects both fit and fatigue. A press-fit seat machined to Ra 3.2 µm behaves differently from the same seat at Ra 0.8 µm, because the asperities collapse differently under press load. Our CNC parts finishing guide covers how Ra interacts with coating and passivation.
| Feature | Typical limit | Measurement | Inspection level |
|---|---|---|---|
| Press-fit bore / shaft | ISO 286 class (H7/p6 etc.) | Bore gauge, CMM, air gauge | 100% on critical |
| Roundness | 0.003–0.01 mm | Roundness tester | Sampling per batch |
| Coaxiality (two seats) | ≤ 0.02 mm | CMM | Sampling |
| Total runout | ≤ 0.01 mm | Bench centres + indicator, CMM | 100% on rotating assemblies |
| Contact surface Ra | 0.4–0.8 µm | Roughness tester | Batch |
| Seat depth / shoulder position | ±0.05 mm | Depth gauge, CMM | Sampling |
The process route: why grinding still exists in a CNC shop
A common misassumption is that a good 5-axis machining centre with a fine boring head can finish a bearing seat to final size in one operation. Sometimes it can. Usually it cannot, for two reasons.
Reason one — heat treatment moves the part. Bearing steels such as 100Cr6, GCr15 and 42CrMo are hardened after rough machining, and hardening introduces distortion. The rule is the same one we apply to every camshaft and gear: rough out with 0.3–0.5 mm of stock, heat treat, then finish. Our heat treated CNC parts guide covers distortion-control techniques.
Reason two — hardness limits what a cutting tool can do. Above roughly 45 HRC, carbide turning gets difficult and unpredictable on a surface that has to hold 0.005 mm. Hard turning works well in a controlled process window with CBN tooling; below that, precision grinding is more repeatable, and it is the conventional route for raceway and journal finishing.
A typical bearing component route looks like this:
- Blank preparation — bar stock cut to length, or near-net forging where volume justifies it
- Rough turning or rough milling — establish geometry, leave 0.3–0.5 mm on functional surfaces
- Stress relief — where thin walls or tight tolerances make residual stress a risk
- Heat treatment — through hardening, case carburising or induction hardening per the hardness target
- Semi-finish machining — re-establish datums that moved during heat treatment
- Precision grinding or hard turning — raceways, journals, bores to final size and Ra
- Superfinishing — where the application demands Ra below 0.2 µm
- Deburr, clean and inspect — roundness, roughness, diameter, runout
- Surface protection — passivation for stainless, oiling or plating for carbon and alloy steels
The lesson is that CNC machining and precision grinding are not competing routes; they are consecutive steps in a bearing route. Ask a supplier which grinding capacity they control, and if it is outsourced, who verifies the finished geometry.
Materials for automotive bearing components
| Material | Hardness after treatment | Use | Notes |
|---|---|---|---|
| 100Cr6 / GCr15 | 58–65 HRC | Rings, races, rolling elements | The classic bearing steel; requires dimensional stability control |
| 42CrMo4 | 28–35 HRC (Q&T) | Housings, seats, high-load rings | Good toughness; machines well in the annealed state |
| 20CrMnTi | 58–62 HRC case | Case-hardened rings, shafts | Tough core with hard surface — good for shock-loaded duty |
| S45C / 1045 | 20–30 HRC | Shafts, spacers, low-speed seats | Cheap, adequate for non-critical duty |
| SS 304 / 316 | Not hardenable | Corrosion-resistant housings, food and marine duty | Passivation mandatory after machining |
| SS 430 | Not hardenable | Light corrosion duty, magnetic applications | Better machinability than 304 |
| Bronze (QSn / phosphor) | As-cast or as-machined | Bushings, dry-running sleeves, cages | Excellent embeddability and low friction |
| Brass | As-machined | Cages, light-duty bushings | Watch RoHS restrictions on leaded grades |
| POM / PEEK | As-machined | Low-load, non-metallic bushes | PEEK for high temperature; POM for general duty |
Two selection notes worth carrying into a design review. First, dimensional stability after heat treatment is a material property, not just a process outcome — a steel with unstable retained austenite will keep moving for weeks after hardening, which is fatal on a 0.005 mm feature. Second, stainless cannot be hardened by heat treatment, so any "stainless bearing" is either a corrosion-resistant housing around a standard bearing, or a design that does not rely on hardness for wear life.
Where bearing projects go wrong
We have seen the same four failure modes repeatedly across customer bearing programmes.
Fitting problems caused by geometric error, not size error. A bore that measures in tolerance but is not round will assemble with the wrong interference in some directions and none in others. The part passes the bore gauge and fails at the press. This is why roundness must be a separate callout.
Heat treatment sequencing errors. Machining to final size and then hardening produces distortion that either requires rework or scrap. The fix costs nothing at the drawing stage: agree the stock allowance and the finish-machining step before the first cut.
Ignoring the assembly stack. Bearing life depends on the alignment of the *assembly*, not just of one part. If the housing bore and the shaft journal are both in tolerance but on opposite ends of their tolerance bands, the internal clearance in the bearing goes to zero or doubles. Both the tolerance bands and the datum scheme should be chosen with the stack in mind.
Under-specified bore finish. Ra is often omitted entirely, and the shop reasonably defaults to whatever the process produces. In a press fit, Ra determines how much of the nominal interference survives assembly.
Why a Dongguan IATF 16949 shop for bearing components
Bearing components sit at the intersection of three capabilities: precision turning, heat treatment control and precision grinding. In Dongguan, all three sit inside a short supply radius, which keeps the process route in one accountable chain rather than splitting it across three suppliers with three sets of records.
We hold IATF 16949, ISO 9001:2015 and ISO 13485 certification, run 200+ machining centres with 23+ years of precision manufacturing experience, and hold ±0.01 mm on automotive metal bearing work with tighter capability on selected features such as press-fit diameters and seat coaxiality.
A representative programme: an automotive Tier 1 supplier brought us a metal bearing for an engine auxiliary system and transmission assembly, with three problems — dimensional control on the running surfaces, batch-to-batch variation in surface quality, and a prototype cycle that had stretched to 12 days. We replaced the original route with a turning plus precision grinding combination, specified and verified roughness on the functional surfaces, and brought heat treatment and protective finishing into one controlled sequence with defined inspection gates. Prototype lead time fell from 12 days to 5, dimensional acceptance held above 99.5%, on-time delivery of serial batches held at 100%, and the customer's return rate stayed below 0.3%. Their engineering contact summarised it as: *"Their machining consistency and quality control give us great confidence in mass production."*
Automotive metal bearings come in a wide range of configurations; if your requirement is a standard envelope and a standard load rating, a catalogue bearing with a precision machined housing is usually the lower-risk answer. Automotive metal bearings come in a wide range of configurations; if your requirement is a standard envelope and a standard load rating, a catalogue bearing with a precision machined housing is usually the lower-risk answer. Related reading for bearing projects: transmission shaft machining, engine components CNC machining and automotive CNC machining.
Conclusion
Automotive metal bearings reward the buyer who specifies roundness, coaxiality and surface roughness explicitly, rather than hoping a diameter tolerance will carry the whole requirement. The dimensional control is achievable in almost any competent shop; what separates suppliers is whether they sequence heat treatment before finish machining, whether they control the grinding step rather than outsourcing it blind, and whether they can show you the capability data.
Send us your bearing drawing, the fit class and the hardness requirement, and we will return a process route and quotation within 24 hours — including a DFM note on any feature where the specified tolerance is tighter than the application needs.