Bonding & Mounting Magnets

Three properties make magnets uniquely awkward to attach:

• They are brittle ceramics-like materials. Sintered NdFeB has essentially no ductility. Press fits, screws through the part, and point loads that would be routine on a steel component crack magnets. All mechanical retention must load the magnet gently and in compression.
• You bond to the coating, not the magnet. The adhesive joint is only as strong as the coating's adhesion to the NdFeB substrate beneath it. A perfect epoxy bond to nickel plating fails if the plating itself peels.
• They generate their own loads. Attractive forces during assembly slam parts together, shift them during cure, and add service loads (including cyclic loads in motors and couplings) that the joint must survive on top of external forces.

Four adhesive families cover nearly all magnet bonding. Typical properties — always verify against the specific product datasheet:

Two supporting players worth knowing: anaerobic retaining compounds for cylindrical magnet-in-bore joints (they cure in the confined metal-adjacent gap and excel in shear), and silicones/flexible adhesives where large thermal expansion mismatch or vibration isolation matters more than strength.

Adhesion is won or lost in preparation. The magnet's coating determines the approach:

Preparation sequence (all coatings)

• Degrease with isopropyl alcohol or acetone on a clean lint-free wipe; wipe in one direction.
• Abrade the bond area only, if applicable — do not break through plating to bare NdFeB.
• Re-clean to remove abrasion debris; let solvent flash off completely.
• Bond within a few hours of prep; prepared surfaces re-contaminate.
• Prepare the mating part too — the joint has two surfaces, and the painted, anodized, or oily steel side fails just as often as the magnet side.

Adhesives are strong in shear and weak in peel and cleavage. Good magnet joints are designed so service loads shear the bondline:

• Load the joint in shear. A magnet on the inside wall of a rotor sleeve (centrifugal load presses it into the bond) beats a magnet on the outside of a hub (centrifugal load peels it off).
• Control the bondline thickness. Most structural adhesives peak around 0.05–0.25 mm bondline. Magnetic attraction squeezes joints to near-zero — starved joints are weak. Use adhesives with glass-bead spacers, add shims, or design a controlled recess depth.
• Maximize bonded area in a pocket. A shallow pocket or recess adds side-wall bond area, locates the magnet, and converts peel loads into shear — the single highest-value design change for bonded magnets.
• Radius the pocket corners. Sharp internal corners concentrate stress in both the adhesive and the brittle magnet.
• Leave a fillet. A small external adhesive fillet around the magnet perimeter significantly improves peel resistance. Don't wipe it away.

Mechanical retention supplements or replaces adhesive — with rules dictated by the material's brittleness:

• The adhesive is usually the thermal weak link. An N42SH magnet is good to 150 °C, but a commodity CA fails well below that. Match the adhesive's continuous service rating — not its short-term peak — to the application, with margin.
• Strength falls as temperature approaches Tg. Structural adhesives lose a large fraction of room-temperature strength near their glass transition. Check the datasheet's strength-vs-temperature curve at your actual operating point, not the headline number.
• CTE mismatch works the joint. NdFeB expands very little (and anisotropically); aluminum expands roughly 4–5× more. Every thermal cycle shears the bondline. For wide temperature swings on aluminum, favor toughened (flexibilized) adhesives over rigid brittle ones, and keep bondlines toward the thicker end of the allowed range.
• Moisture and chemicals attack over time. Adhesive joints lose strength with humidity aging; qualify with 85 °C/85% RH or autoclave/sterilization cycling if your product sees it.
• Cure temperature vs. magnetized parts. Heat-cured epoxies (often 80–150 °C) can approach or exceed a magnetized magnet's rating. Either cure below the magnet's max operating temperature, verify flux after cure, or bond unmagnetized and magnetize after assembly.

• Control the approach. Strong magnets accelerate toward steel and each other from surprising distances. Fixtures should bring the magnet in on a controlled slide or pivot — never free-handed above the target for larger parts.
• Prevent cure-time walking. Magnetic side-loads slide parts during cure. Fixture with hard locating features (the pocket again), not friction; verify position after fixturing, before cure.
• Sequence multi-magnet assemblies. Each added magnet changes the field the next one sees. Define placement order and shield or space in-process assemblies so finished bonds aren't loaded while green.
• Use non-magnetic tools and jigs — aluminum, brass, 300-series stainless, plastics — near the bond station.
• Protect people. Pinch injuries are the most common magnet incident in assembly. Gloves, spacers between stored magnets, and keeping pacemaker wearers clear of strong-field stations belong in the work instruction.
• Bond-then-magnetize when possible. Assembling unmagnetized parts eliminates nearly all of the above; a post-assembly magnetizing pulse finishes the job. Ask whether your geometry allows it.

Adhesive product recommendations change with formulations and availability — treat the ranges in this guide as selection direction, and validate the specific adhesive on your actual parts and process. For magnet-side questions — coating selection for bondability, countersunk and pot-mounted options, unmagnetized supply — talk to our engineering team.