Spindle housing: the bored cast-iron or steel body that holds a machine-tool spindle's bearings or receives a spindle cartridge, specified by bore fit, bore geometry, thermal behaviour and cooling passages
The bored casting a spindle runs in · bearing-seat fits measured in microns, roundness and coaxiality of the bores, the housing's own thermal expansion, and the coolant passages and mounting face it carries
Every spindle, whatever drives it, ends up as a shaft and bearing set sitting in a housing bore, or as a cartridge slid into one. The housing is the part a machine builder or rebuilder machines, inspects and pays for separately: a ribbed iron casting or a steel weldment with precision bores, a flanged front face, coolant grooves and lubrication ports. What decides whether the bearings inside will hold their accuracy is a handful of microns: how much clearance the bore leaves each bearing, how round and coaxial the bores are, and how the casting grows when it warms or is cooled.
Spindle housings (spindle head bodies, quill and cartridge housings) as machined castings or weldments: bearing-seat bores and their fits, bore geometry and surface finish, housing material and thermal expansion, coolant jackets or grooves, lubrication and purge ports, and the mounting face or bore that receives a cartridge. Complete spindle units are covered elsewhere: motorized router and engraving spindles on the Spindle motor page; liquid-cooled motor spindles for machining centres on the High-speed spindle page; direct-drive units on the Spindle assembly (direct-drive spindle unit) page; belt- and gear-driven units on the Main spindle unit with external belt or gear drive page; lathe workpiece spindles on the Built-in motor rotary spindle for CNC lathes page; tilting heads on the Rotary spindle head page. Aluminium bearing housings for gearboxes and motors, which carry ordinary rolling bearings rather than super-precision spindle sets, are covered on the Aluminium CNC-machined bearing housing page.
CNFX editorial research · Sources checked October 13, 2026 · 3 sources
Bearing-seat bores: why the fixed end sits nearly line-to-line and the free end gets 8–12 µm of clearance in an 80–120 mm bore
NSK's super-precision bearing catalogue gives housing bore tolerances and target clearances by bearing type and position, for machine tool spindle bearings running below 800 000 dmn in standard conditions. The fixed (locating) end keeps only a few microns of clearance so the angular contact set cannot shift radially; the free end is bored larger so the outer rings can slide as the shaft grows; a thrust bearing used beside radial bearings gets a deliberately loose bore so it carries no radial load. [1]
NSK adds that when a ring is mounted with interference, the out-of-roundness of the housing is transferred to the raceway and affects running accuracy, and that with multiple angular contact bearings in combination the cylindricity of the bore affects how the preload is shared between them. [1] Its Table 8.6 therefore sets geometry limits on the housing bore itself:
For an 80–120 mm housing bore with P4, P3 or P2 bearings: out-of-roundness 1.2 µm, cylindricity 2 µm, runout 3 µm, coaxiality 6 µm and Ra 0.8 µm; with P5 bearings the limits are 2, 2, 5 and 10 µm and the same Ra. For 120–180 mm with P4 to P2 bearings: 1.7, 2.5, 4 and 8 µm, Ra 0.8 µm. [1] Editorial calculation: the 6 µm coaxiality limit between front and rear bores at 80–120 mm is the same size as the whole 0.002–0.006 mm clearance band at the fixed end, which is why the two bores are best finished in one setting and inspected together rather than as separate features. [1]
The image above shows the kind of part these figures apply to: a ribbed, cast housing with a machined front flange and bolt circle around its bearing bore, next to a separate cylindrical spindle cartridge.
Bearing in the housing (NSK Table 8.2)
Housing bore tolerance, 80–120 mm
Target fit, 80–120 mm
Target fit, 120–180 mm
Angular contact ball bearings, fixed end
−0.003 / +0.003 mm
0.002–0.006 mm clearance
0.003–0.009 mm clearance (bore −0.004 / +0.004 mm)
Angular contact ball bearings, free end
+0.006 / +0.010 mm
0.008–0.012 mm clearance
0.011–0.017 mm clearance (bore +0.008 / +0.014 mm)
Angular contact thrust ball bearings used with radial bearings
−0.007 / 0 mm
0.033–0.047 mm clearance
0.042–0.059 mm (120–150 mm), 0.039–0.059 mm (150–180 mm)
Cylindrical or tapered roller bearings (outer ring inserted separately)
−0.007 / 0 mm
0.002 mm clearance to 0.002 mm interference
0.003 mm clearance to 0.003 mm interference (bore −0.008 / 0 mm)
Send the housing drawing with each bore diameter, the bearing type, precision class and position (fixed or free) in each bore, and the fit or clearance required; ask for a measurement report covering bore diameter, roundness, cylindricity, coaxiality of front and rear bores and surface roughness.
02
Cast iron at about 10 × 10⁻⁶/°C, cooled housings, and how cooling the bore can squeeze a bearing's clearance shut
A Toshiba Machine patent (US 6,158,895) describes the spindle head body, the spindle and the inner collar as made of materials with nearly the same thermal expansion coefficient, about 10 × 10⁻⁶ per °C, for example cast iron or a structural carbon or chromium-molybdenum steel, with the bearings' outer rings fitted in the housing bore with a designed clearance. [3]
The same patent describes the problem with an oil jacket on the bearing case: cooling from the outside makes the housing colder than the outer rings and the outer rings colder than the inner rings, so the bearing's radial clearance can be closed and heat generation can rise; to avoid this, bearings had to be assembled with deliberate looseness, at a cost in running accuracy and stiffness. [3] Its answer in one embodiment is a coolant housing of a higher-expansion stainless steel, about 16 × 10⁻⁶/°C, around outer races of about 10 × 10⁻⁶/°C. In its worked example, outer races of 150 mm outside diameter in a coolant housing with a zero-clearance 150 mm bore, a radial clearance of more than 4.5 µm is given as allowing the bearing pair to work smoothly. [3]
Editorial calculation: 4.5 µm equals 150 mm × (16 − 10) × 10⁻⁶/°C × 5 °C, so the patent's figure corresponds to a 5 °C temperature change; the patent text reviewed does not state the temperature itself. [3] For comparison, the same 150 mm bore in NSK's 120–180 mm band has a fixed-end target clearance of only 0.003–0.009 mm. [1] As an example value only, a cast-iron housing (10 × 10⁻⁶/°C) held 5 °C colder than steel outer rings of the same coefficient would shrink onto them by 150 × 10 × 10⁻⁶ × 5 = 7.5 µm, more than the 3 µm minimum of that band.
Setco's spindle manual describes the two cooling layouts machined into housings: a cooling jacket and/or a continuous serpentine groove, used for motor cooling around the stator, for bearing cooling around the front bearings, and on clustered multi-spindle heads to hold the centre-to-centre distances between spindles. [2]
Motor and bearing cooling ports connected separately unless the drawing says otherwise; flow not stopped and restarted while the spindle rotates [2]
State the housing material and its expansion coefficient, whether the housing is cooled and where (motor, front bearings, whole body), the coolant temperature band and pressure, and how the bearing clearances were chosen for the cooled condition.
03
The cartridge bore and mounting face: a sliding fit, a flat and clean seat, and bolts that must not distort the casting
Where the housing receives a ready-made cartridge rather than loose bearings, Setco's instructions are explicit: never force a spindle into position, use a running or sliding fit, and never force a cartridge spindle into a housing. [2]
The mounting surface the spindle bolts to must be of the highest quality in cleanliness, strength, flatness, surface finish and vibration damping, and the mounting bolts must not be over-tightened: Setco states that over-tightening distorts the spindle housing, causing immediate spindle failure and voiding the warranty. Mounting hole positions, sizes and bolt specifications come from the spindle outline drawing; Setco does not supply the bolts. [2]
The housing also carries the service ports. Setco describes air/oil lubrication entering through inlet ports machined into the housing, with the oil carried by air at about 30 psi (2.1 bar), separate air-purge inlets where the housing allows them, and cooling ports tagged as motor or bearing coolant inlet and outlet. [2] During run-in, bearing temperature is checked with a pyrometer around the front and rear of the housing; if it reaches 140 °F (60 °C), or rises more than 5 °F in one minute, the spindle is stopped and allowed to cool. [2]
Editorial note: these handling rules and the micron-level bore limits above describe the same requirement from two sides; a housing bore that is out of round, or a flange that is pulled down unevenly, transfers its distortion to the bearings. [2][1]
For a cartridge housing, give the cartridge outside diameter and the fit wanted, the flange or face flatness, bolt size and torque, and the positions of lubrication, purge and coolant ports to match the cartridge drawing.
04
Housing details to send
Paste it into your enquiry below or into an email to a maker, then replace every bracket.
Bores: [diameters; bearing type and class per bore; fixed or free end; fit or clearance]
Geometry: [roundness; cylindricity; runout; coaxiality; Ra]
Material: [cast iron grade or steel; expansion coefficient]
Cooling: [jacket or groove; motor, bearing or whole body; coolant temperature and pressure]
Ports: [air/oil; purge; coolant]
Cartridge seat: [cartridge OD; sliding fit; face flatness; bolt pattern and torque]
Inspection: [measurement report for each bore]
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Compiled and source-reviewed by the CNFX editorial team.
Each manufacturer above links to the page on that company's own website where the product is listed, with the date we checked it. How we source and review
· Page last updated 13 October 2026
Buyer questions
What fit should a spindle housing bore have for angular contact bearings?
NSK recommends a 0.002–0.006 mm clearance at the fixed end and 0.008–0.012 mm at the free end for an 80–120 mm housing bore, for machine tool spindle bearings below 800 000 dmn. [1]
How round does a spindle housing bore need to be?
For an 80–120 mm bore holding P4, P3 or P2 bearings, NSK gives 1.2 µm out-of-roundness, 2 µm cylindricity, 3 µm runout, 6 µm coaxiality and Ra 0.8 µm. [1]
Can cooling the spindle housing cause problems?
A Toshiba Machine patent explains that cooling the bearing case from outside can make it colder than the outer rings and close the bearing's radial clearance, and proposes a higher-expansion coolant housing to prevent it. [3]
Should a spindle cartridge be pressed into its housing?
No. Setco's manual says to use a running or sliding fit and never to force a cartridge spindle into a housing, and warns that over-tightened mounting bolts distort the housing. [2]
Sources and what still needs a direct check
Sources checked 2026-10-13. Three sources from three publishers: NSK's Super Precision Bearings catalogue (E1254) for housing fits and bore geometry, Setco's Use & Care Manual for cartridge mounting, housing cooling and service ports, and Toshiba Machine's US patent 6,158,895 for housing materials and the effect of cooling on bearing clearance.
