Magnetization Direction Selection for Sensor Magnets

Last year I reviewed 340+ sensor magnet drawings from OEM buyers across robotics, HVAC, automotive, and industrial automation. About one in five had a magnetization problem that would have caused a sample failure — not because the magnet was bad, but because the pole direction did not match the sensor sensitive axis.

Two magnets can share the same shape, grade, coating, and price, yet behave completely differently if the field points the wrong way. I once had a customer spend eight weeks qualifying a 6 × 3 mm N42 cylinder for a Hall latch, only to discover at assembly that the magnet was axially magnetized while the sensor was mounted beside the cylinder wall. The fix was not a stronger grade — it was switching to diametric magnetization. Total cost of the mistake: two sample rounds and a six-week schedule slip.

This guide is what I wish every buyer had before sending the first RFQ. It covers axial, diametric, radial, and multipole magnetization — when each one works, when it fails, and what numbers you should put on the drawing.

The First Question Is Field Direction, Not Price

Before asking "which magnet is cheapest," ask "which magnetic field component does my sensor actually read?" The answer determines everything downstream — magnetization, measurement method, marking, and packaging.

I added the "What the Sensor Reads" column because this is the detail most drawings miss. A Hall IC datasheet will tell you which component (Bx, By, or Bz) triggers the output. If you do not match that axis, no amount of grade upgrade will fix it.

Why Field Drops Faster Than You Expect

Here is a detail that surprises many first-time sensor magnet buyers: magnetic field does not drop linearly with distance. For a small disc magnet, it follows roughly an inverse-cube law beyond a few millimeters. This means doubling the air gap can cut the field at the sensor by 75–85 %, not 50 %.

The chart below shows measured field decay for a typical N35 NdFeB cylinder (Ø6 × 3 mm, axial magnetization). I measured this with a calibrated gaussmeter at our factory — it is real production data, not a simulation.

The practical takeaway: if your Hall switch needs 15 mT to trigger, this magnet works at 5 mm but has almost no margin. At 3 mm it has 62 mT — a 4× safety margin. This is why I always ask buyers to send the minimum and maximum air gap, not just the nominal.

Visual Comparison

Axial Magnetization

Axial magnetization places north and south poles on opposite flat faces. It is the most common direction for sensor magnets — roughly 60 % of the sensor magnet orders I handle are axial.

Choose axial magnetization when:

• the sensor is directly above or below the magnet face;
• a simple on/off switch point is needed;
• the magnet travels toward or away from the sensor along the cylinder axis;
• incoming inspection can use a surface field reading at the pole face;
• assembly orientation can be controlled by marking one face.

Axial magnets are the default for proximity switching magnets, Hall switch triggers, and reed switch packages. The main risk is assuming axial works when the sensor is actually mounted beside the magnet.

A real case I see often: A buyer sends a Ø8 × 4 mm N42 axial disc for a proximity sensor mounted 3 mm above the face. Surface field is 320 mT, field at 3 mm is about 78 mT — plenty of margin for most Hall switches (typical Bop 5–20 mT). But the same buyer then mounts the magnet sideways in a rotary knob, expecting the sensor beside the cylinder wall to read a clean signal. The side field of an axial magnet is much weaker and has no useful sinusoidal profile. The fix is always the same: switch to diametric.

Typical field reference for axial cylinders (N35 NdFeB, NiCuNi coating):

These are production average values from our factory — individual pieces vary ±8 % due to material batch and magnetization fixture position. If your sensor margin is tight, ask for the worst-case value, not the typical.

Diametric Magnetization

Diametric magnetization places the poles on opposite sides of a cylinder or disc. As the magnet rotates, the field direction changes smoothly — this is what makes it useful for angle sensing.

Choose diametric magnetization when:

• the sensor is mounted to the side of a rotating magnet;
• angle or rotary position information is needed;
• the sensitive axis points across the cylinder diameter;
• the design needs a strong side field rather than a face field;
• the assembly needs a defined angular orientation mark.

Diametric magnets are common in angle sensing, compact encoders, rotary knobs, and sensor modules where the magnet rotates over or beside a Hall IC.

The detail most people miss: the pole-axis angular accuracy matters more than you might expect. I measured a batch of 500 diametric Ø6 × 4 mm cylinders last quarter — the pole axis alignment was within ±2° of the datum mark for 98 % of the batch. That sounds good, but for a 12-bit angle sensor (0.088° resolution), even ±2° creates a systematic offset that the sensor calibration must absorb. If your sensor has limited calibration range, discuss this tolerance early.

Angular accuracy impact on common sensor types:

The buyer must specify the angular pole orientation relative to a mechanical datum. A diametric magnet without a datum note can pass magnetic inspection but fail assembly because the pole axis is randomly oriented.

Radial Magnetization

Radial magnetization places poles through the wall of a ring — field direction runs from inner diameter to outer diameter (or vice versa). It is more specialized and more expensive than axial or diametric because it needs a custom magnetizing fixture with internal and external conductors.

Choose radial magnetization when:

• the sensor reads field through the ring wall;
• the magnet is part of a rotating sleeve or ring assembly;
• the design requires field direction from ID to OD;
• the magnetic circuit includes a soft-iron back plate that completes the flux path.

Feasibility note: not all ring geometries can be radially magnetized. Very thin walls (< 1.5 mm) or very large OD/ID ratios can make the fixture impractical. I always ask buyers to send the ring dimensions before quoting radial magnetization — about 15 % of the requests I receive need a geometry adjustment to make radial feasible.

Multipole Magnetization

Multipole magnetization creates alternating N/S poles around a ring, strip, or disc. The sensor counts pole transitions — each N-to-S and S-to-N boundary produces one signal edge.

Choose multipole magnetization when:

• pulse count per revolution matters;
• the application is speed or RPM sensing;
• an encoder IC reads pole transitions;
• pole pitch uniformity is more important than peak surface field;
• the drawing defines pole count, pole width, index position, and sensor track.

How pole count determines encoder resolution:

With quadrature decoding (two sensors offset by 90° electrical), the effective resolution quadruples. A 32-pole ring with quadrature gives 128 edges per revolution — 2.8° resolution — often enough for mid-range industrial servo applications.

For multipole ring magnets, the drawing should include pole count, sensing diameter, pole track width, index direction, and whether the field is measured at OD, ID, or face.

Selection Matrix

How Magnetization Choice Affects Assembly Work

The same magnetization decision also changes how the part is handled on the production line.

If the production line cannot reliably control orientation, discuss a magnetic assembly instead of loose magnets. A supplier can sometimes provide a carrier-mounted magnet, marked part, or oriented packaging that reduces operator error.

Measurement Method by Magnetization

Do not use the same measurement note for all magnetization types. The wrong measurement method can approve a part that still fails the sensor.

For production approval, the measurement should match how the product uses the magnet. If the sensor reads at 2.5 mm air gap, a contact surface reading may not predict field margin well enough.

Air Gap and Alignment Matter More Than Grade Alone

If the air gap doubles, field at the sensor can fall by 75–85 %. Upgrading from N35 to N52 only gains about 20–25 % more field — it cannot compensate for poor alignment, excessive distance, or the wrong pole direction.

Use this review sequence before changing grade:

1. Confirm the sensor sensitive axis.
2. Confirm pole direction relative to that axis.
3. Confirm nominal, minimum, and maximum air gap.
4. Confirm mechanical runout or positional tolerance.
5. Confirm temperature and coating impact.
6. Then — and only then — decide whether a grade change is needed.

Grade upgrade reality check — Ø6 × 3 mm axial cylinder at 3 mm air gap:

Notice the trap: N52 has the highest field but the lowest temperature rating. I have seen buyers request N52 for an automotive HVAC sensor operating at 105 °C — the magnet would partially demagnetize on the first thermal cycle. For applications above 80 °C, always use an SH, UH, or EH suffix grade, or consider SmCo.

Temperature demagnetization coefficient: NdFeB loses approximately −0.11 %/°C of remanence. At 120 °C, that is roughly −11 % field loss compared to room temperature. If your sensor margin is already tight, this thermal loss can push the field below the switching threshold in summer conditions or during equipment warm-up.

Drawing Notes Buyers Should Add

Use direct notes on the drawing rather than relying on email text. Recommended notes:

MAGNETIZATION: DIAMETRIC, POLE AXIS ALIGNED WITH DATUM A.
MARKING: NORTH POLE SIDE MARKED WITH DOT.
MEASUREMENT: FIELD CHECK AT 2.0 MM FROM MARKED SIDE, FIXTURE-CENTERED.
ASSEMBLY: MARKED SIDE FACES SENSOR IC SENSITIVE AXIS.

For a multipole ring:

MAGNETIZATION: 24 POLES AROUND OD, ALTERNATING N/S.
SENSING TRACK: OUTER DIAMETER, CENTERED ON HEIGHT.
INDEX: FIRST NORTH POLE CENTERLINE ALIGNED TO DATUM SLOT.
FIELD MAP: SUPPLIER TO PROVIDE SAMPLE POLE MAP FOR APPROVAL.

Common Mistakes We See in RFQ Review

When to Ask for Supplier Engineering Review

Ask for engineering review before quoting when any of these are true:

• the magnet rotates and angular accuracy affects the signal;
• the air gap has a wide tolerance stack;
• the sensor reads a side field rather than a face field;
• the design needs a multipole pattern or pole map;
• the magnet is inserted into a carrier, float, piston, or overmolded part;
• the working temperature is close to the material limit;
• the drawing has no clear polarity mark or assembly datum.

This review does not need to slow the project. A 15-minute check at RFQ stage is faster than a second sample round caused by wrong pole orientation.

Quick Selector

Use this quick selector when starting a design review:

Worked Example: Same Cylinder, Different Sensor Result

Consider a 6 mm diameter x 3 mm cylinder magnet used near a Hall IC. The dimensions are identical in both cases, but the correct magnetization can be different.

This is why copying only size and grade from a working prototype can be dangerous. The part number may be correct mechanically but wrong magnetically if the pole direction was never documented.

Supplier Drawing Review Checklist

Before releasing a drawing for quotation, check whether the supplier can answer these questions without guessing.

If the drawing cannot answer these questions, production inspection will also struggle to answer them.

Sensor Position Map

The sensor position often decides the magnetization choice before price or grade is discussed. Use this map to explain the layout to a supplier when the drawing is still early.

Validation Data to Request by Magnetization Type

For buyer approval, the supplier data should match the magnetization risk. Do not request a generic magnetic report when the real question is pole-axis angle or pole count.

This data request creates a stronger approval baseline. If repeat lots drift later, the baseline makes it easier to decide whether the change is acceptable or requires engineering review.

Production Release Criteria

Before a magnetization design is released to production, define what must be true for the design to be stable.

For B2B OEM sourcing, the best magnetization choice is not only the one that works in a prototype. It is the one that can be measured, packed, assembled, and repeated without relying on memory.

Drawing Review Decision Tree

Use this decision tree when a supplier or internal engineer reviews a magnet drawing. The goal is to catch missing magnetization details before samples are ordered.

Magnetization Change-Control Checklist

Once samples are approved, control magnetization like a functional specification. These are the changes that should trigger buyer review.

This checklist is useful for repeat orders because magnetization changes are easy to describe as "process details" even when they affect product function.

FAQ

Can a supplier change magnetization direction after samples are approved?

No. Magnetization direction is a functional characteristic for sensor magnets. Any change to axial, diametric, radial, pole count, index position, or pole-axis datum should be treated as an engineering change and reapproved with samples.

Is diametric magnetization always better for rotation?

Not always. Diametric magnets are common for compact angle sensing, but pulse counting may require a multipole ring or a magnet array. The choice depends on whether the sensor reads angle, speed, position, or only a switch event.

Can a stronger grade fix the wrong magnetization?

Usually not. A higher grade may increase field strength, but it does not move the pole to the correct side or create the correct pole pattern. Confirm direction and air gap before increasing grade.

What should be marked on the part?

Mark whatever the assembly process must control: north face, pole-axis line, index position, or sensing track. If the part is too small for reliable marking, use oriented tray packing or a keyed magnetic assembly.

Practical Next Step

Send your sketch, sensor position, air gap, and required magnetic behavior to [email protected] or WhatsApp +86 18857971991. If the magnet is rotating, include the mechanical datum and the required pole orientation relative to that datum.

Related pages: diametric magnet guide, position sensing magnets, and magnetic encoder angle sensing.