Inductive sensors trusted to keep CNC high-speed spindles spinning

17/09/24 09:00

Machine manufacturers need to be confident that the sensor is reliable to prevent errors that could damage tools and workpieces or cause spindle malfunctions.

A miniature M5 inductive sensor was selected to detect the position of a slotted disc mounted on the motor shaft because of the sensor's exceptional repeatability and positional accuracy better than 0.01 mm.

Multiple metal-cutting operations

Modern machining centres execute multiple metal-cutting operations within a single machine, often deploying a separate tool for each operation. Automated tool changes remove labour and reduce the non-productive time needed to change tools, improve efficiency and machine utilisation, enhancing their flexibility to cope with a wide range of tasks, various materials, batch sizes, work-pieces, tools and cutting speeds.

After completing a machining operation, the machine's main spindle comes to rest, a sensor confirms that it has reached the null position. This is the point at which the tool can be disengaged completely, and it is safe to initiate the tool-change sequence.

Exceptional accuracy and repeatability are required to sense the position of a shaft previously rotating at up to 14,000 rpm. A robust and reliable solution to detect the null position is needed to prevent damage from occurring.

Space constraints require the sensor to be no larger than 5mm in diameter but capable of withstanding mechanical shock and high vibration levels. The sensor is also likely to be covered in oil due to its location, which detects the position of a slotted disc mounted on the drive motor shaft.

Reliability in demanding environments

Contrinex inductive sensors are renowned for exceptional long-term reliability and repeatability in demanding environments, and this is also true of the sensors in its extensive miniature range. An M5 embeddable inductive sensor was selected, constructed from V2A / AISI 303 stainless steel with a PPE sensing face and hermetically sealed cable entry.

The 25mm-long sensor is embedded directly in the wall of a rotary-encoder housing fitted to the drive-motor shaft. Its sensing distance of 0.8mm eliminates the risk of accidental collision with the spinning disc. The required angular resolution translates to a positional accuracy of better than 0.01mm.

The sensor's vacuum-encapsulated electronics ensure excellent resistance to shock and vibration, and no additional electrical or mechanical protection is required. Fit-and-forget operation keeps maintenance requirements to a minimum, overcoming the mechanical fragility of alternative optical-sensing encoder technology. The sensors are configured with an industry-standard PNP normally-open interface, connected to the customer's control system via a flexible, oil-resistant PUR sheathed cable.

Contrinex Miniature inductive sensors, therefore, provide a highly effective, reliable and affordable solution.