Below is a set of procedures for installing magnetic shims in micro motors (brushed/brushless, ferrite/NdFeB), organised according to the actual workflow in a mass production workshop.
I. Pre-production Preparation (Error-proofing, Anti-magnetisation, Iron Filing Prevention)
Environment and Tooling
Non-magnetic workstation, copper hammer / nylon tool, stainless steel clamps, lint-free cloth, alcohol, gaussmeter, jig positioning plate.
Clear away iron filings and welding slag to prevent them from adhering to the magnet and causing jamming or short circuits.
Incoming Inspection of Magnets
Appearance: No chipped edges, cracks, missing pieces or rust spots.
Dimensions: Curvature radius (R), chord length, thickness and chamfers; clearance between magnet and housing ≤0.1mm.
Polarity: Clear N/S markings; consistent magnetic flux within the same batch (spot-checked with a gauss meter).
Material: Distinguish between ferrite and neodymium-iron-boron; mixing of materials is prohibited.
Housing / Stator Cleaning
Remove burrs, oil stains and old adhesive from the inner circumference (wipe with alcohol + high-pressure air).
Inspect locating grooves and reference surfaces to ensure no deformation.
II. Magnet Pre-treatment (Key to Reliable Bonding)
Magnet Cleaning
Wipe both sides with alcohol / isopropyl alcohol → Dry with a lint-free cloth → Air-dry (no water marks, no oil stains).
Adhesive Application (commonly high-temperature resistant epoxy AB adhesive / single-component thermosetting adhesive)
Adhesive application surface: Outer curved surface of the magnet (side to be bonded to the housing).
Application method: For micro-motors, commonly spot application (surface bonding) or thin spreading (full-surface bonding).
Adhesive quantity: Leave a 0.5mm margin at the edges to prevent overflow; thickness 0.05–0.1mm; avoid air bubbles.
Note: The polarity surface (inner arc) must not come into contact with adhesive to prevent interference with the air gap and magnetic field.
III. Installation and Alignment (Core error-proofing: polarity + position)
Taking ** two symmetrical magnetic tiles (common) ** as an example:
Housing positioning
Place the housing horizontally, with the positioning groove facing upwards / aligned with the reference line.
Installation of the first magnetic tile
Polarity verification: N pole facing inwards (air gap), with the curved surface flush against the inner circle of the housing.
Alignment: Align one end with the positioning groove / reference line, press gently into place, ensuring no misalignment or warping.
Installation of the second magnetic tile
Polarity: S pole facing inwards, symmetrical to the first tile at 180°.
Spacing: Ensure an even gap between the end faces of both tiles, with no tilting or height differences.
Pressure Application (Optional, Essential for Mass Production)
Apply even pressure using specialised elastic clamps or clamping rings at 0.3–0.5 MPa to ensure the adhesive layer is thin, uniform and free of air bubbles.
Do not strike with a hard object to prevent the magnetic tiles from cracking or the enamel from chipping.
IV. Curing (determines bond strength)
Pre-curing at room temperature (1–2 hours)
Maintain pressure to prevent displacement.
Heat curing (commonly used for batch production to improve efficiency)
Temperature: 80–120°C (depending on adhesive specifications); time: 30–60 minutes.
Allow to cool naturally to room temperature; rapid cooling is strictly prohibited to prevent cracking of the adhesive layer.
Inspection
After curing, gently push the magnet tiles; there should be no looseness, displacement or warping.
Clean up any excess adhesive and ensure the inner circle remains smooth.
V. Polarity and Magnetic Performance Re-inspection (to prevent reversal / weak magnetism)
Polarity Verification
Measure the inner arc with a gauss meter: the two pieces should be N/S respectively, with no reversed or mixed polarities.
Random Inspection of Magnetic Flux / Magnetic Field Strength
Consistency within the same batch: deviation ≤ ±5%.
Final Visual Inspection
No chipped edges, cracks, missing pieces or excess adhesive; the fit must be tight.
VI. Subsequent Assembly (Stator / Rotor / End Covers)
Install the rotor / armature, checking that the air gap is uniform and there is no brush contact.
Install end caps, bearings and brushes (for brushed motors); ensure smooth rotation with no binding.
Power-on test: Correct rotation direction, stable speed, no abnormal noise.
VII. Common Fixing Methods (selected by structure)
Adhesive bonding (common): Epoxy AB adhesive / thermosetting adhesive; suitable for micro-motors, offering strong hold and low cost.
Snap-fit + Adhesive (Combined): Housing with positioning slots + snap-fit + spot bonding; offers greater shock resistance.
Slot Insertion (Brushless / Embedded): Stator core with slots; magnetic plates inserted + potting; minimal magnetic leakage.
Key Points to Note (Critical for Mass Production Error Prevention)
Polarity must never be reversed: Incorrect installation will cause reverse rotation, zero torque and burnt coils.
Inadequate cleaning will cause the magnets to fall out: Oil residue or metal shavings lead to bonding failure, resulting in detachment during operation.
Adhesive quantity control: Too little will result in poor adhesion; too much will cause adhesive overflow, affecting rotor rotation.
Handle with care: The magnets are brittle and prone to chipping, ceramic flaking and cracking.
