CNC Bearing Fits: Shaft and Housing Tolerances Guide

Bearing performance depends on more than the bearing part number. The shaft seat, housing bore, shoulder, fillet, surface finish and geometric accuracy all influence internal clearance, running temperature, noise, service life and assembly reliability.

This guide explains how to select and machine CNC bearing fits for shafts and housings. It covers clearance, transition and interference fits, common ISO fit classes, tolerance stack-ups, inspection methods and practical design rules for prototype and low-volume assemblies.

What Is a Bearing Fit?

A bearing fit is the dimensional relationship between a bearing ring and its mating shaft or housing. The fit determines whether the ring slides freely, locates with light resistance or is held by interference.

The correct fit prevents unwanted ring movement without creating excessive stress or reducing the bearing's internal clearance too far.

Why Bearing Fits Matter

A seat that is too loose can allow the ring to creep, fret or rotate relative to the shaft or housing. A seat that is too tight can expand or compress the ring, increase preload and create heat.

Fit selection therefore affects accuracy, stiffness, noise, fatigue life, serviceability and the risk of early failure.

Shaft Fit vs Housing Fit

The inner ring normally mates with the shaft, while the outer ring mates with the housing bore. Each interface may require a different fit because the load direction and assembly method are different.

Do not apply one generic tolerance to both seats. Review which ring rotates relative to the load and which ring must move for adjustment or thermal expansion.

Clearance, Transition and Interference Fits

The Rotating-Load Rule

A bearing ring subjected to a load that rotates relative to the ring usually needs an interference fit. The interference prevents the loaded zone from walking around the seat.

A ring carrying a stationary load may use a looser fit, but shock, vibration, imbalance and variable load direction can justify a tighter selection.

Loads That Change Direction

Reversing, oscillating or indeterminate loads can expose different parts of a ring to the loaded zone. In these cases, the simple stationary-load rule may not be sufficient.

Use the bearing manufacturer's application tables and account for maximum radial load, axial load, moment and shock.

Common ISO Shaft Fit Classes

Metric bearing seats are often specified with ISO tolerance zones such as g6, h6, j6, k6, m6 or n6. The letter defines the tolerance zone position and the number defines the tolerance grade.

Moving from g6 toward n6 generally changes a shaft seat from clearance toward greater interference, but the actual limits depend on nominal diameter.

Common ISO Housing Fit Classes

Housing bores are often specified with zones such as G7, H7, J7, K7, M7 or N7. H7 is a common baseline, but it is not automatically correct for every outer ring.

Load direction, housing material, wall thickness, temperature and the need for axial movement determine the appropriate zone.

What H7, h6, g6 and k6 Mean

An H7 hole has its lower deviation at the basic size, while an h6 shaft has its upper deviation at the basic size. A g6 shaft lies below the basic size and a k6 shaft is shifted toward interference.

The letter and grade must always be paired with a nominal diameter. Writing only H7 or k6 without the basic size is incomplete.

Do Not Guess the Fit from a Letter Alone

The numerical limits for a tolerance class change with the diameter range. A 10 mm k6 seat and a 100 mm k6 seat do not have the same micrometer limits.

Use the applicable ISO table, bearing catalog or controlled engineering standard to calculate the upper and lower limits shown on the manufacturing drawing.

Bearing Internal Clearance

An interference fit stretches the inner ring or compresses the outer ring. This reduces the bearing's remaining internal clearance after installation.

If the installed fit, temperature gradient or preload consumes too much clearance, the bearing may run hot or bind. Bearings with increased initial clearance may be required for tight fits or high temperatures.

Radial Clearance vs Preload

Radial clearance permits rolling elements to move relative to the raceways. Preload intentionally removes clearance to improve stiffness and running accuracy.

Fit is only one part of the final clearance or preload. Spacer length, locknut force, housing alignment and temperature also contribute.

Temperature Effects on Bearing Fits

Steel, aluminum, stainless steel and polymer housings expand at different rates. An aluminum housing can loosen an outer-ring fit as temperature rises, while a cold-start condition may create greater interference.

Calculate the operating fit at the maximum and minimum service temperatures, not only at room temperature.

Aluminum Bearing Housings

Aluminum is lightweight and easy to machine but has a higher thermal expansion coefficient than bearing steel. Thin aluminum bores can also grow during machining, anodizing or assembly.

Designers may use greater room-temperature interference, a steel sleeve or a retaining feature when the operating temperature would otherwise loosen the bearing.

Steel and Stainless Steel Housings

Steel housings provide stiffness and thermal expansion closer to the bearing ring. Stainless grades improve corrosion resistance but may be more difficult to machine consistently.

Material condition, heat treatment and stress relief should be finalized before the precision bearing bore is finished.

Thin-Wall Housing Distortion

A thin housing can become oval under chucking force, bolting, interference assembly or external load. A bore that gauges correctly in the free state may change after installation.

Increase wall support, control clamping, machine critical bores in the assembled condition or inspect the housing under representative mounting conditions.

Split Bearing Housings

Split housings can distort when the cap bolts are tightened. The bearing seat should be machined with the mating halves assembled and torqued using the defined fasteners.

Identify the match-marked halves and preserve the assembly orientation through inspection and production.

Shaft Material and Hardness

Soft shafts can fret, wear or yield beneath a loaded inner ring. Hardened bearing seats improve durability but add heat-treatment and grinding considerations.

Specify material, heat treatment, case depth and final hardness only where the application needs them.

Shaft and Housing Roundness

A tight size tolerance does not guarantee a round seat. Lobing or ovality changes local interference and can distort the bearing raceway.

Roundness should be controlled separately when noise, speed, stiffness or bearing life makes form important.

Cylindricity of Bearing Seats

Cylindricity controls the complete three-dimensional form of a shaft or bore. It can be more useful than separate roundness and straightness controls for long bearing seats.

Choose a value consistent with bearing width, speed and required running accuracy rather than applying an unnecessarily small tolerance to every design.

Coaxial Bearing Seats

Two bearing seats on the same shaft or housing must share a functional axis. Misalignment creates edge loading, friction and heat.

Use a datum system that represents the assembly and control runout or position of the secondary seat relative to the primary bearing datum.

Total Runout

Total runout controls cumulative circular variation along a rotating surface relative to a datum axis. It is useful for bearing journals, shoulders and seal surfaces that must run together.

The datum feature must be long and stable enough to establish the intended axis during inspection.

Bearing Shoulder Squareness

The shaft or housing shoulder locates the ring axially. If the shoulder is not square to the bearing axis, the ring may tilt and load the raceway unevenly.

Control face runout or perpendicularity relative to the bearing seat datum when axial accuracy is important.

Shoulder Height

The shoulder must contact the bearing ring face without touching the cage, seal or shield. It also needs enough height to transmit the axial locating load.

Use the bearing catalog's minimum and maximum abutment dimensions for the selected bearing.

Corner Radius and Bearing Chamfer

Bearings include a corner chamfer or radius between the ring face and bore or outside diameter. The seat fillet must be smaller than the permissible bearing corner dimension.

A fillet that is too large prevents the bearing from seating against the shoulder, while a sharp corner increases stress concentration in the shaft.

Relief Grooves

A relief groove can provide clearance for grinding wheels or turning tools at a shoulder. It also allows the bearing ring to seat fully against a flat face.

Balance tool access with shaft strength and fatigue requirements. Avoid deep grooves in highly stressed journals.

Surface Finish for Bearing Seats

Bearing seats usually need a smooth, controlled finish so high peaks do not collapse during assembly and reduce effective interference. Excessive roughness can also encourage fretting.

The required roughness depends on diameter, load, fit and bearing guidance. Specify a measurable Ra value where it is functionally necessary.

Waviness and Chatter

A low Ra value can still hide periodic waviness or chatter. These errors may affect high-speed rotation, noise and local contact pressure.

Use stable tooling, adequate support and appropriate measurement when the application is sensitive to surface wavelength.

CNC Turning Bearing Journals

Turning efficiently produces coaxial shaft seats and shoulders. A finishing pass with stable tool engagement can hold close diameter, roundness and surface finish on suitable machines.

Long slender shafts may need a tailstock, steady rest, soft jaws or process sequencing to control deflection.

CNC Boring Bearing Housings

Boring establishes a true cylindrical housing seat and can finish multiple coaxial bores in one setup. Tool overhang, insert geometry and interrupted cuts affect accuracy.

For split or flexible housings, machine the bore in the same clamped condition used during assembly.

Reaming Bearing Bores

Reaming can size smaller bores economically, but it tends to follow an existing hole and may not correct poor location or straightness. Allowance, tool condition and lubrication affect the result.

Use boring or interpolation before reaming when position and form must be established accurately.

Circular Interpolation

A machining center can finish a housing bore by circular interpolation. This is useful for prototypes, large bores and parts that already require milling setups.

Machine geometry, backlash, tool deflection and entry strategy influence roundness. A calibration cut is often appropriate for close fits.

Grinding Bearing Seats

Grinding can achieve tight size, form and surface-finish requirements on hardened components. It is common for high-speed, high-accuracy or production bearing journals.

Grinding allowance, heat treatment distortion and wheel-access relief should be planned before rough machining.

Honing and Lapping

Honing improves bore size, roundness and surface texture, while lapping supports extremely fine finishing in specialized applications. These processes add cost and are not needed for every bearing seat.

Use them when functional requirements exceed economical CNC turning or boring capability.

Machining Sequence

1. Rough-machine the part and leave finishing allowance.

2. Complete stress relief or heat treatment where required.

3. Establish functional datums in a stable setup.

4. Finish the bearing seat and shoulder together where possible.

5. Deburr without rolling material onto the seating surface.

6. Clean and inspect under controlled temperature.

Allowing for Heat Treatment

Carburizing, nitriding, through hardening and stress relief can change diameter, roundness and straightness. Precision seats are often left with stock for final grinding after treatment.

Coordinate the heat treater's expected growth or distortion with the machining plan.

Allowing for Anodizing and Plating

Anodizing, hard anodizing, electroless nickel and other coatings change the bearing-seat dimension. Coating thickness may build on each surface and can be nonuniform in deep bores.

State whether the drawing limit applies before or after finishing, and define masking when the seat must remain uncoated.

Bearing Retaining Compounds

A retaining compound can fill small gaps, resist fretting and support repair of slightly loose seats. It should not be used to hide a major dimensional error without engineering approval.

Gap capability, temperature, fluid compatibility, cure time and disassembly needs must be reviewed.

Snap Rings, Locknuts and End Plates

Axial retention is separate from radial fit. Snap rings, locknuts, shoulders and end plates prevent the bearing from moving along the axis.

Provide tool access and check that retention loads do not pass through seals, shields or unsupported ring features.

Press Assembly

Apply installation force only through the ring being fitted. Pressing an inner ring onto a shaft through the outer ring can transmit damaging force through the rolling elements.

Use a square driver, controlled force and clean supports. Monitor force versus displacement for critical production assemblies.

Thermal Assembly

Heating a bearing or housing, or cooling a shaft, can reduce the force required for interference assembly. Induction heaters and controlled ovens provide more uniform temperature than open flames.

Respect the bearing manufacturer's temperature limits, especially for sealed bearings, lubricants and heat-treated rings.

Assembly Lead-In Chamfers

A small chamfer helps start the bearing squarely and protects the ring edge. The chamfer should not remove too much of the effective bearing-seat length.

Remove burrs at oil holes, keyways, grooves and cross holes before installation.

Measuring Shaft Bearing Seats

A calibrated micrometer is commonly used for shaft journals. Measure at several axial locations and angular positions to detect taper and ovality.

Allow the part and instrument to stabilize near the reference temperature before evaluating micrometer-level limits.

Measuring Housing Bores

Dial bore gauges, three-point bore micrometers and air gauges can measure housing bores. The chosen tool must resolve the tolerance and reach the full functional depth.

Measure in multiple planes and directions, especially for thin or split housings.

CMM Inspection

A coordinate measuring machine evaluates bore location, axis alignment and shoulder geometry. Sampling strategy and fitting method affect the reported size and form.

Use direct diameter gauges in addition to CMM data when the fit itself is critical and measurement uncertainty is tight.

Gauge R&R and Measurement Uncertainty

A tolerance is only useful when the inspection system can distinguish acceptable from unacceptable parts. Gauge repeatability, operator technique, temperature and calibration all contribute uncertainty.

For close bearing fits, validate the measurement process before relying on production data.

Common Bearing Seat Problems

• Loose ring creep and fretting marks

• Excessive interference and lost internal clearance

• Housing ovality after bolting

• Shaft taper from tool deflection

• Bearing not seated because the fillet is too large

• Coating buildup that closes the fit

• Misaligned seats that create edge loading

• Burrs that damage the ring during assembly

• Measurement disagreement caused by temperature

Reducing Bearing Seat Machining Cost

• Select a catalog bearing before detailing the seat.

• Use standard ISO fits supported by supplier recommendations.

• Apply tight tolerances only to functional bearing features.

• Finish coaxial seats in one setup where practical.

• Provide tool access for shoulders and reliefs.

• Avoid grinding unless speed, hardness or accuracy requires it.

• Define the coating sequence before requesting quotations.

• Use accessible inspection points and realistic reporting requirements.

Bearing Fit Drawing Checklist

• Is the exact bearing part number identified?

• Are shaft and housing limits based on load direction?

• Is installed internal clearance or preload considered?

• Are material and operating temperature defined?

• Are roundness, cylindricity and coaxiality adequate?

• Is the shoulder square and high enough?

• Does the seat fillet clear the bearing corner?

• Are surface finish and coating conditions clear?

• Can the bearing be assembled without loading the rolling elements?

• Is the inspection method capable of verifying the tolerance?

Information to Send for a CNC Quote

• 3D CAD model and controlled 2D drawing

• Bearing manufacturer and complete part number

• Shaft and housing materials

• Heat treatment, hardness and surface finish

• Operating speed, load and temperature range

• Required fit limits and geometric tolerances

• Coating or masking requirements

• Inspection report and traceability needs

• Prototype and production quantities

Frequently Asked Questions

Is H7 always the correct bearing housing tolerance?

No. H7 is common for some stationary outer-ring applications, but rotating loads, shock, thin housings, aluminum expansion and axial movement requirements can call for a different fit.

What shaft tolerance should I use for a bearing?

The answer depends on bearing type, diameter, load magnitude, which ring rotates relative to the load, speed and assembly method. Manufacturer fit tables may recommend zones such as g6, h6, j6, k6 or m6 for different conditions.

Can a bearing seat be CNC machined without grinding?

Yes. Modern CNC turning and boring can produce many prototype and low-volume bearing seats. Grinding becomes useful for hardened parts, very tight form tolerances, fine finishes or high-speed applications.

Should an aluminum bearing bore be anodized?

It depends on wear, corrosion, electrical and dimensional needs. If anodized, the final bore size and coating buildup must be controlled. Critical seats are often masked or finish-machined according to the approved process.

Why did a correct bearing fit become loose in service?

Possible causes include thermal expansion, housing deformation, fretting, overload, insufficient interference, material creep or an incorrect load assumption. Evaluate the operating condition rather than only the room-temperature dimensions.

Final Design Recommendations

Start with the selected bearing, real load direction and operating temperature. Choose shaft and housing fits from the bearing manufacturer's guidance, then convert the fit class into explicit dimensional limits for the nominal size.

Control the features that make the fit work: roundness, coaxiality, shoulder squareness, fillet clearance, surface finish and coating sequence. Confirm critical assemblies with capable inspection and production-intent assembly trials.

Request a CNC Bearing Seat Quote

Aluprototype provides precision CNC turning, milling, boring and inspection for prototype and low-volume shafts, housings and bearing components. Our engineering team can review fit limits, datum structure, material, coating and assembly requirements before manufacturing.

Explore our CNC machining capabilities, read our CNC prototype machining guide, or request a quote with your CAD model, drawing and quantity.