A magnetic pole piece can pass a diameter check and still miss the functional air gap. One common cause is process order: the gap surface is finished, then narrow isolation slots release thin bridges and shift the surface that was already measured.
The safer sequence is to treat the mounting features, bridge release and final gap as three different control stages.
1. Establish a repeatable locating frame
Machine the mounting pad with a fine-pitch face mill and sharp PVD carbide inserts. Use the pad and locating bores as the frame for later operations. If bore position is important, interpolate and fine-bore it. A reamer follows a displaced pilot; it does not reliably correct location.
Rough the curved or conical gap surface now, but leave a deliberate and measurable finishing allowance.
2. Open isolation slots symmetrically
Every slot changes the stiffness around the pole face. Open opposing slots alternately and support the remaining bridges. A fine-tooth carbide slitting cutter gives more predictable side force than a long small-diameter end mill, which can bend and leave tapered walls.
Watch three signals during this stage:
- bridge thickness begins to trend from one side to the other;
- the slot burr turns toward the future gap surface;
- stock remaining on the gap becomes asymmetric.
Any of those is a reason to stop and inspect, not to add another offset.
3. Finish only after stiffness-changing features are open
Relocate from the mounting pad and datum bores after slotting and any specified stress relief. Semifinish the gap with a short variable-helix carbide tool in a hydraulic or shrink-fit holder. Use a controlled form lap only for the final small allowance.
Hand polishing is risky because it can round the functional arc or cone. It also removes material faster at exposed edges, exactly where the released bridges are least stiff.
4. Inspect the geometry that will actually work
Relate the finished gap profile to the locating bores on a CMM. Measure bridge thickness optically and record burr direction. Use a gap master, radius gauge or verified profile routine only after the slots are complete. Magnetic property testing belongs after the specified final heat treatment; machining dimensions alone do not prove flux performance.
Three drawing-based examples show how the sequence changes with material and feature layout:
- Hiperco 50 pole shoe with a lapped air gap and thin flux bridges
- ARMCO pure-iron pole piece with an air-gap cone and radial slots
- Invar 36 spacer frame with flexure slots and kinematic pads
The practical rule is simple: finish and qualify the functional surface after the operations that change local stiffness, not before them.
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