Countersunk

A fastener feature, not a magnetic one

A countersunk magnet is bought because the screw has to sit below the surface. Nothing about the countersink makes the field stronger; what it does is let a magnet be fastened to a housing without a proud head, which is why the part appears in assemblies where the magnet has to hold a cover, a sensor or a bracket and the space in front of it is the working gap.

So the countersink is a mechanical decision that lands on a brittle part, and both sides of it have to be specified: the seat, and what is left of the magnet around it.

The seat is where the section is thinnest

A countersink removes material from the face, and the material it removes is the material the flux would otherwise have crossed. On a thin part the seat can take a large share of the section under the head, and the part works with less than its outline suggests.

That is why the seat depth, the head diameter and the remaining section belong on the drawing together, and why the working face has to be named: a seat on the non-working face is a different part from the same seat on the working one. Where the seat and the working face coincide, the effective pole area is the outline minus the seat.

Brittle material and a screw that can break it

A permanent magnet is brittle, and a screw is a controlled overload. The head bears on the seat, the torque goes through it, and the edge of the seat is a stress concentration in the hardest possible place. A washer-style head contact, a defined torque and a defined seating face are not details for the assembly drawing; they are the difference between a part that survives the line and one that cracks at the first tightening.

An oversized or undercut seat is not the fix either. The seat has to match the fastener the assembly actually uses, which is why the screw standard and size belong on the magnet drawing rather than in a note referring to another file.

Which route is open on this feature

In the range currently listed for this shape, sintered NdFeB is the route. Sintered NdFeB gives the flux that justifies a part small enough to be screwed down in the first place, and the seat is machined rather than formed — which is also why the feature carries a cost that a plain disc of the same size does not.

That a ferrite countersunk part is not listed here says the route is not confirmed for this feature; it does not say the geometry is impossible in ferrite. If the position can take ferrite, the question is worth asking before the drawing is fixed, because it changes both the coating question and the flux available.

Coating and the machined seat are one decision

The seat is a machined surface and it sits inside the fastener’s contact area, so protection has to cover it. Plated finishes — nickel, zinc, epoxy — are chosen against the environment the part sits in, and the seat is part of the exposure rather than a protected interior: a head that traps moisture against a plated seat is still an environment.

Because the seat is machined after the body is formed, the finish and the machining order are settled together. What governs the coating and what governs the tolerance are decided by different things, and on a countersunk part they arrive at the same drawing.

What to send for a countersunk review

• The screw standard and size, the head type, and the seat geometry — countersunk or counterbored, with the depth.
• The outline, the thickness, and the remaining section under the head.
• The working face, and the magnetization with N and S.
• The assembly torque, whether a washer is used, and the material the screw threads into.
• The environment and the temperature in service, and the annual quantity.

The table further down this page lists the parameters a countersunk drawing has to settle before the part can be quoted.