A stator housing is a thin tube with a precision bore, a mounting interface and — increasingly — a cooling jacket inside the wall. Machining of stator housings is therefore a distortion problem before it is a tolerance problem: the wall is often 3–5 mm of aluminum, and every clamping force, cutting force and heat input leaves a mark that the bore remembers.

 

The bore has to accept a laminated stator stack with an interference fit, hold it without crushing the laminations, and stay round enough that the air gap between stator and rotor is uniform. This article works through the sequence that achieves that, and the checks that show whether it did.

 

 

 

Three requirements drive every machining decision on a stator housing.

 

Stack fit. The stator stack is pressed or shrunk into the bore. A common specification is an H7 bore with a stack OD tolerance that gives 0.02–0.06 mm interference on a 150 mm bore. Too little and the stack spins under torque; too much and the laminations compress, which changes the magnetic circuit and can crack the insulation.

 

Roundness over the stack length. The air gap is typically 0.5–1.0 mm. A bore that is 0.05 mm out of round makes the gap vary by 10%, which shows up as torque ripple and audible noise.

 

Thermal contact. The housing is the heat path from the laminations to the cooling air or jacket. A bore that touches the stack only at three high points has a poor thermal path, even when it measures within tolerance at those points.

Feature	Typical callout	Consequence if missed
Bore diameter	H7	Stack loose or over-compressed
Bore roundness	0.01–0.02 mm	Air gap variation, noise, torque ripple
Cylindricity over stack length	0.02–0.03 mm	Stack skewed, rotor drag
Bore roughness	Ra 1.6 µm	Reduced contact area, thermal resistance
Bore to mounting face perpendicularity	0.03 mm	Rotor misalignment, bearing preload
Cooling jacket leak integrity	No leak at test pressure	Coolant into windings

 

Thin-wall aluminum distorts for four reasons, and only one of them is the cutting tool.

 

 

A three-jaw chuck on a 4 mm wall will close the bore into a triangle. The part measures round when released only if the clamping stress stayed below yield; in practice a part clamped hard enough to resist cutting forces springs back to a shape that is 0.02–0.05 mm out of round, and it springs back differently on each part.

 

The fix is to distribute the clamping load: pie jaws machined to the part diameter, a collet with full-circumference contact, or an expanding mandrel that works from the bore. The last option is often the best for a thin housing because it puts the clamping force where the finishing cut will remove it.

 

 

A boring bar with a long overhang deflects under cutting force, and the deflection is largest at the middle of the bore. The result is a barrel-shaped bore that is tight at both ends and large in the middle, or the reverse, depending on the bar's stiffness. A 0.1 mm error in bore size across the length is normal for a bar that is too slender, and no amount of measuring will fix it. Machining of stator housings with a rigid bar and a short overhang is cheaper than machining the bore twice. Machining of stator housings with a rigid bar and a short overhang is cheaper than machining them twice.

 

 

Aluminum expands about 23 µm/m·K. A 200 mm housing that warms by 10 °C during roughing grows 0.046 mm. If the finishing cut is taken while the part is warm and the part is measured after it cools, the bore will be undersize by that amount. Temperature soak before finishing, or measurement correction, is part of the process rather than an afterthought. Chuangling soaks thin-wall housings to shop temperature before the finishing pass for the same reason.

 

 

A housing machined from extruded tube carries the extrusion stress; a housing machined from a casting carries the casting stress. Both release as material is removed, and both move the bore after the final cut. A stress-relief cycle between roughing and finishing, or a rough machine–age–finish sequence, is the standard answer.

Distortion source	Measured symptom	Countermeasure
Three-jaw clamping	Tri-lobe, 0.02–0.05 mm	Pie jaws, collet, or bore-mounted mandrel
Slender boring bar	Barrel or hourglass bore profile	Larger bar, shorter overhang, two-direction boring
Cutting heat	Bore size drifts with cycle time	Coolant flood, temperature soak, warm-up parts
Blank residual stress	Bore moves hours after machining	Stress relief between roughing and finishing
Weld heat from the jacket	Local ovality near the weld	Weld before finish boring, stress relieve after welding

 

The jacket is what makes the sequence non-obvious. Welding a cooling jacket into a thin aluminum wall puts more heat into the part than any machining operation, so the weld has to happen before the bore is finished.

 

1. Rough machine the bore, the mounting face and the jacket seat, leaving 1.0–1.5 mm on the bore.

2. Weld or braze the jacket, then stress relieve.

3. Pressure test the jacket at the specified pressure, before any finish machining, so a leaking weld does not scrap a finished bore.

4. Semi-finish the bore to 0.2–0.3 mm allowance, establishing the mounting face as the datum.

5. Finish bore and finish face in one clamping, with the part at stable temperature.

6. Drill and tap mounting holes and terminal box holes after the bore is finished.

7. Final leak test and dimensional inspection with the part free, not clamped.

 

Step 5 is where the geometry is decided. Turning the face and boring in the same clamping is the only reliable way to hold perpendicularity between the bore axis and the mounting face on a thin part, because a second clamping reintroduces the distortion the first one avoided.

Stack Fit: Press or Shrink

Both methods work; they fail in different ways.

Measuring Roundness Honestly