Guide
How Are Bearing Bores in a Gearbox Housing Kept Aligned During Machining?
Bearing bores are the precision holes in a gearbox housing that locate each shaft through its bearings, so how well they line up with one another sets how the shafts run, how the gears mesh and how long the bearings last. Alignment is not a property of the cutting tool alone; it is built by the datum scheme on the drawing and by the order in which the bores are cut.
By Saola Manufacturing engineering team · Published · Updated
This guide is for gearbox, gearmotor and robot-joint engineers and buyers who write datums and bore tolerances for a multi-bore housing and want to check that a supplier can hold bore-to-bore alignment. It draws on a published gearbox maker’s factory standard for pre-machining split housings and on a machining-trade article about boring bearing seats. Both are examples of practice, not universal rules, and the bearing maker’s catalog still decides the fit class.
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How are bearing bores kept aligned during machining?
By cutting all the bores of one housing from the same datums, in one clamping where the part allows it, and, for a split housing, by pinning and bolting the halves together before the bores are bored. The drawing has to name the datums so the shop can plan that sequence.
The trade press puts the principle simply: bearings often run in pairs, misalignment can cause shaft wear, bearing failure and eccentric shaft motion, and “machining the seats without repositioning the part is the best solution” (Cutting Tool Engineering, November 2024). Every time a part is unclamped and re-clamped, the error between the two bores includes the re-location error of the fixture, so a bore cut in the second setup inherits an offset the first setup did not have. The sections below show how that principle turns into datums, sequence and inspection.
What datum scheme should the drawing use?
Pick datums that the shop can actually locate on, usually the mounting or split-plane face as primary and two locating features such as pin holes or one bore and a pin hole as secondary and tertiary, then tolerance every bore to that frame. Do not tolerance each bore from the previous one in a chain.
A bore-to-bore chain stacks tolerances: the second bore is positioned from the first, the third from the second, and the last bore carries the sum. Referencing all bores to one datum reference frame keeps the error from accumulating and tells the shop which features to locate on. How to choose the primary, secondary and tertiary datums is covered in the datum reference frame guide, and the guide to datums on castings explains the extra care a rough casting needs, because housings are often cast and the first machined faces become the datums for the rest.
For the callout itself, bores that must share an axis are usually controlled by position or by a coaxiality tolerance relative to a datum axis, rather than by concentricity, which is hard to measure; the concentricity, runout and position guide compares them. The geometric tolerancing symbols and datum rules follow ASME Y14.5 or ISO 1101, whichever the drawing cites, and fit classes are written in the ISO 286 code system. State which system the drawing uses so that both ends read the callouts the same way.
Why are split housings pinned together before the bores are bored?
Because the bore is a single hole shared by both halves, and it is only round and straight if the halves are in their working position when it is cut. Boring each half separately and joining them later leaves a step or an oval at the split line.
A published factory standard from a gearbox maker shows this sequence for its own housings: before pre-machining the bearing bores, the housing parts must be pinned and screwed to each other, with tightening torques chosen so that no clearance remains between the parts (Reintjes factory standard RN 1565, 7 November 2023). The same standard checks the clearance at the separating surfaces once they are finished, with the halves aligned but not pinned or tightened, and requires extra milling of the separating surface if the limits are exceeded. The standard covers external pre-machining of housings and refers to separate documents for general and positional tolerances; bearing fits and final bore finishing are not what it sets out.
| Step | What RN 1565 says | Why it matters for alignment |
|---|---|---|
| 1. Mill the split faces | Separating surfaces are milled; hub fronts and internal contact surfaces are pre-milled with 2 mm allowance per side | The split plane becomes the reference the halves close on |
| 2. Check clearance, halves aligned, not pinned | Limits of 0.1 mm in the hub area and 0.3 mm at the separating surfaces | Shows whether the halves seat flat before the bore is cut |
| 3. Pin and screw the halves | Torque chosen so that no clearance remains | The bore is cut with the halves in working position |
| 4. Pre-drill the bearing bores | Bores pre-drilled with a 4 mm allowance on the drilling diameter | Leaves stock for the final boring pass |
| 5. Keep the set together | Parts of one order stay connected and are shipped pinned and screwed | Mated halves are not swapped |
How do tool support and setups affect bore alignment?
A boring tool that is long for its diameter droops and chatters, so deep or distant bores are harder to hold in line, and one-setup machining only works if the part and the tool both reach every bore. Standard tools often can do it, but large parts and high depth-to-diameter bores are harder.
The Cutting Tool Engineering article describes two remedies for long boring tools: guide bushings that support the tool, fixed in the workholding in high-volume work, and, in low-volume work, machining one bore, installing a guide bushing in it and using it to support the tool in the second bore. For jobs that cannot go to a shop, line-boring devices can produce straight, accurate bores in place (Cutting Tool Engineering, November 2024). The same article warns about roundness: a roughing tool left a clover-leaf shape that the finish tool did not fully correct, and parts were scrapped even though they had passed air gauge checks. A diameter check alone does not prove a good bore.
The consequence for an RFQ is to ask three questions: can all the bores be finished in one setup, and if not, how is the second setup located; how is the tool supported on the deepest bore; and what is the longest bore span the supplier’s equipment has actually bored to the required tolerance. Be honest about limits: a very long bore line or a housing larger than the machine envelope may need line boring or a different machine, and the supplier should say so before quoting.
What should the inspection plan ask for?
Ask for bore diameter, roundness and cylindricity, position or coaxiality to the datum frame, and a measurement method that matches the tolerance, reported per bore on a first article. Ask which gauge is used and how it is set.
The Cutting Tool Engineering article says air gauges are the most accurate option but costly, and that a high-quality dial bore gauge suits most bearing seats; both should be set with a ring gauge, and setting a bore gauge with a micrometer is called a common but risky practice. It also advises checking roundness even where the drawing gives no roundness tolerance, and notes that bearing makers publish recommended surface finishes for their seats (Cutting Tool Engineering, November 2024).
- First article. Request a full report on every bore of the first housing, plus the datum features, as covered in the first article guide.
- Position to datums. Bore-to-bore alignment is verified on a coordinate measuring machine to the same datum frame the drawing names; see the CMM guide.
- Temperature and stabilization. As typical practice, measure parts after they have equalized with the room, since an aluminum housing grows with a few degrees of difference.
- Fit. The bore tolerance class and finish come from the bearing maker’s catalog and the press-fit guidance in the bearing seat guide.
What does a supplier need from the drawing to hold alignment?
A named datum frame, the sequence that matters (split faces first, then pin, then bore), the tolerance class and finish from the bearing maker, and the inspection method. With those, a CNC shop can plan fixtures and setups instead of guessing.
Housing blanks are often castings, and castings from partner foundries are machined at the lead CNC plant. Machining the split faces, the locating pin holes and the bore pre-machining is CNC milling work, with CNC turning for round housings. Where the housing is a casting in an alloy listed on the die casting or sand casting pages, it comes through partner plants that Saola manages; other housing materials are confirmed at drawing review. Send the model, the drawing with the datum frame, and say which tolerance and finish the bearing maker specified.
Frequently asked
Can the two halves of a split housing be bored separately?
They can be machined separately for the faces, but the shared bore should be cut with the halves pinned together. The Reintjes standard requires the housing parts to be pinned and screwed to each other before the bearing bores are pre-machined.
Is position or concentricity the right callout for bore alignment?
Position or a coaxiality tolerance to a datum axis is usually easier to measure and agree on than concentricity. The comparison guide explains the differences.
How should the bore diameter be measured?
With a bore gauge set against a ring gauge, or an air gauge, according to Cutting Tool Engineering. A bore gauge set with a micrometer is described there as risky.
Why check roundness if the drawing has no roundness tolerance?
Because a bore can pass a diameter check and still be clover-leaf shaped. The article cited above describes parts scrapped for that reason despite passing air gauge checks.
Who sets the bore tolerance class?
The bearing maker’s catalog recommends the housing fit class and finish for the bearing type and load; the drawing then expresses it in the ISO 286 system. Use the catalog value for your bearing and loading.
Sources
- Factory standard RN 1565, Gearbox housings: mechanical pre-machining, Reintjes GmbH, 7 November 2023
- Precision key to boring bearing pockets, C. Tate, Cutting Tool Engineering, 15 November 2024
- ISO 286-1:2010, Geometrical product specifications (GPS), ISO code system for tolerances on linear sizes, Part 1: Basis of tolerances, deviations and fits, ISO, 2010