Disc

The simplest geometry, and the two questions underneath it

A disc is specified by diameter and thickness, and most disc drawings stop there. Two questions sit underneath those two numbers and both change what the part can do: which way the flux leaves it, and how thin the section is where it has to survive handling. Neither is answered by the diameter.

Axial is the default — diametrical is a commitment

Most discs are magnetized axially: one pole on each flat face, flux along the thickness, working surfaces the two faces. That arrangement costs a magnetizing fixture and nothing else.

A diametrically magnetized disc is a different part wearing the same three numbers. The poles sit at two opposed points on the circumference, the flux crosses the diameter, and the curved surface becomes the working one; the field no longer runs through the thickness but across the smallest width of the part. That also commits the pressed orientation. On an anisotropic grade the direction established while the part is formed has to be the direction the circuit reads, so orientation and magnetization are settled together rather than one after the other. A disc marked “diametrical” after an axial tool has been cut is a new tool, not a new note.

Thin discs fail in the assembly, not on the drawing

A disc that is thin relative to its diameter is the shape most often lost at the point of use rather than in manufacture: it bends while it is separated from a stack, it chips at the rim, it cracks when it is pressed into a seat, or it takes the load of whatever retains it. A permanent magnet is brittle and a thin section has less material to distribute that load over.

So the thickness has to be read against the way the part is held, not only against the flux. Bonded, potted, clamped and press-fitted are four different loads on the same disc, and the section that suits one does not necessarily suit the next. Where the assembly clamps the face, the clamp force is a specification item; where the part is only held by the fit, the fit is.

A multipole face on a disc

Sensor rotors and small couplings often use a disc with several pole pairs around one flat face. The pattern turns a two-pole part into a periodic field, and it turns the drawing into a pattern drawing: pole count, pitch, the track diameter the pattern sits on, and the datum the positions are dimensioned from all have to be on it, because the fixture is built from those and not from the outline. Each direction and pattern carries its own drawing requirements, which is worth reading before the drawing is issued rather than after.

Which routes are open on a disc

Three are worked here: wet-pressed ferrite, dry-pressed ferrite and sintered NdFeB.

The two ferrite routes are not interchangeable by preference. Wet pressing compacts a slurry at 30–40 % water and drains that water during the press, which is what lets the particles align as the part forms; dry pressing takes the same slurry below 10 % moisture first and compacts free-flowing powder in one stroke. Where flux per unit volume matters on a thin disc, the wet side is the one that has to be reviewed. Ferrite also carries the corrosion and cost argument: it needs no coating. Sintered NdFeB gives far more flux from the same diameter and brings coating with it, because the material corrodes unprotected and the exposure of the position decides which finish applies.

Tolerance lands on the faces and the diameter

On a disc the dimensions that carry function are usually the diameter, the thickness, and the parallelism of the two faces — a disc that is parallel but oversize is a different problem from one that is on diameter and wedged. On a diametrical disc the pole axis also has to hold its position relative to the diameter, which is a fixture and finishing question rather than a turning one.

There is no single tolerance band for “disc”, and none is quoted here. What a part holds follows its own dimensions and the route it is made on, and the band is agreed against the numbers on the drawing. What governs tolerance and what governs the coating are treated separately, because they are decided by different things.

What to send for a disc review

• Diameter, thickness, and the datum the two are taken from.
• The magnetization — axial, diametrical, or a multipole pattern — with N and S, and the pole count and track diameter where the pattern applies.
• How the disc is retained, and whether the face is clamped, bonded or press-fitted.
• The flux or field the circuit needs, the operating temperature, and the annual quantity.
• Magnetized, or supplied unmagnetized, and whether the field is applied with the assembly in place.

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