Analysis of the Application of the Rotary Magnetic Coding System in CNC Machine Tools

A rotary magnetic coding system replaces the toothed timing belt or gear pair that older rotary axes used for position reference. A thin ferromagnetic track is deposited or bonded onto a drum or ring, then magnetized in alternating north and south poles at a fixed pitch. A read head sitting a fraction of a millimeter above the track senses the field pattern and converts it into a sine and cosine pair. The controller interpolates those two signals into an absolute angle, so the axis knows where it is the moment power comes up.

The drum itself is usually steel or aluminum with a nickel-iron coating. Pole pitch runs from roughly 0.2 mm to 2 mm depending on the resolution the builder wants. Finer pitch means more counts per revolution but a smaller air gap budget. Most industrial heads tolerate 0.1–0.5 mm of gap and still hold signal.

There is no glass disc, no laser diode, and no optics to fog up. That single fact explains most of the applications where this feedback wins. Coolant mist, fine chips, and vibration do not degrade a magnetic field the way they degrade an optical path.

• 1
  Absolute outputPosition is known at power-up; no homing move.
• 2
  Forgiving gap0.1–0.5 mm air gap is normal.
• 3
  Sealed read headIP67 heads are common on rotary axes.

On a 4-axis mill or a mill-turn center, the rotary axis often sits close to the cut. Swarf lands on the table, coolant splashes upward, and the operator rarely wipes the encoder. That environment is where an optical encoder loses counts and a magnetic ring keeps working.

Tilting heads on a 5-axis machine are a second fit. The head has to report its tilt angle continuously while the spindle runs, and any interruption in feedback shows up as a step in the surface finish. Magnetic rings hold up under the acceleration these heads see.

There is also the practical matter of size. A magnetic ring can be built into a hollow shaft or wrapped around a spindle nose where a glass scale would not fit. For compact rotary tables and direct-drive fixtures, that geometry advantage often decides the design.

• 1
  4-axis rotary tablesChips and coolant near the ring.
• 2
  5-axis tilt headsContinuous feedback under acceleration.
• 3
  Direct-drive fixturesRing fits inside the housing.

Magnetic feedback is not as fine as a good optical encoder. A typical industrial ring resolves to a few arc-seconds; a high-end glass scale can go below one. If a part needs angular position held to a couple of arc-seconds over a full revolution, magnetic is the wrong answer and the extra cost of optics is justified.

The second limit is magnetic interference. A large servo motor, a welding fixture, or a demagnetizing station close to the ring can shift the field pattern and push the reading off. Distance and shielding solve it, but the layout has to be planned before the machine is assembled.

Thermal drift matters too. The ring and the housing expand at different rates, and the read head sees that as a slow offset. Builders compensate in the controller, but a shop running unattended lights-out cycles should check drift over a full shift, not over ten minutes.

None of these limits is exotic. They are the same questions any feedback choice raises. The point is to pick the system that matches the tolerance the part actually needs.

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  ResolutionA few arc-seconds, not sub-arc-second.
• 2
  Stray fieldsKeep motors and welders away.
• 3
  Thermal driftCheck over a full shift.

Setup time drops. An absolute magnetic ring reports its angle the moment the drive powers up, so the operator skips the homing move on that axis. On a machine with several rotary axes, that is a few minutes per job, and it removes a step where a crash can happen.

Five-axis toolpaths get smoother. The control reads position continuously and can adjust the tilt as the tool moves along a curved surface. A discontinuous reading produces a visible mark, and operators usually catch it on the finish pass rather than on the roughing pass.

Maintenance is simpler. There is no glass to scratch and no LED to age. When a read head fails, it is a swap, not a rebuild of the axis. That matters for shops running the machine around the clock.

One caution: the ring must be protected during service. A dropped wrench or a strong magnet set next to the drum can rewrite the pattern. Keep the area clean when the covers are off.

• 1
  No homingAbsolute reading at power-up.
• 2
  Smoother tiltContinuous feedback on curved cuts.
• 3
  Swap not rebuildRead head is a service item.