Cylinder

The ratio does more work than the two dimensions

A cylinder is diameter and length, and on a drawing those are the only two numbers usually given. What the part can do is set by their ratio. A part much shorter than its diameter behaves like a disc; a part much longer behaves like a rod, and the two are not made or tolerance in the same way even when the material and the route are identical.

The ratio is also what the drawing has to answer for: it decides whether the working surface is an end face or the curved surface, and that one decision moves every tolerance on the page.

Axial or diametrical, and what the length changes

An axially magnetized cylinder puts one pole on each end face with the flux running along the length. The working surfaces are the ends, the length is the flux path, and the length therefore has magnetic significance rather than being only a fit dimension.

A diametrically magnetized cylinder puts the poles at two opposed points on the circumference with the flux crossing the diameter. Here the length does not carry the flux at all — the diameter does — and increasing the length adds pole area on the curved surface without thickening the path. That is why a diametrical rod is a common choice where a long, narrow field is wanted, and why adding length to an axial cylinder and adding length to a diametrical one are two different engineering moves.

Either way the pressed orientation has to serve the magnetization. On an anisotropic grade, the direction set while the part is formed is the direction the circuit reads, so the two are decided together. The directions and what each one requires on the drawing are set out separately.

There is a length beyond which the part is not pressed to length

A cylinder that is tall relative to its diameter is the geometry where the die, the fill and the ejection matter most, and past a certain length the part is no longer formed to its final length at all: it is made longer and finished to length afterwards. That is a finishing route rather than a different shape, but it changes what the two ends are — from formed faces to machined ones — and machined ends carry dimensions, a finish and a cost that pressed ends do not. It is worth settling before the quotation rather than after the first sample, because the answer decides whether the length on the drawing is a formed dimension or a finished one.

Which routes are open on a cylinder

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

Wet pressing compacts the slurry at 30–40 % water and drains it during the press, which is what allows the particles to align as the part is formed; dry pressing takes the slurry below 10 % moisture first and compacts free-flowing powder in a single stroke. On a long part the pressed density has to hold over the whole length, which is why the wet side of that line gets reviewed where the flux per unit volume matters. Ferrite needs no coating and suits the position that cannot be protected; sintered NdFeB gives more flux from the same diameter and requires coating, since the material corrodes unprotected and the exposure of the position decides which finish applies.

The working face decides which tolerance matters

On an axial cylinder the dimensions that carry function are the two end faces and the length between them: squareness of each face to the axis, and the length itself. On a diametrical cylinder the diameter and the position of the pole axis relative to it carry the function, and the ends matter mainly for location.

Concentricity appears on both, and it is the dimension most often assumed rather than specified. There is no single tolerance band for “cylinder”, 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 decided by different things and are treated separately for that reason.

What to send for a cylinder review

• Diameter and length, and which of the two is the fit dimension.
• The magnetization — axial or diametrical — with N and S, and the surface that has to carry the field.
• How the cylinder is held: in a bore, on a shaft, bonded into a pocket, or clamped at the ends.
• Whether the length is a formed dimension or a finished one, if the part is long for its diameter.
• The flux or field the circuit needs, the operating temperature, and the annual quantity.

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