Method and device for controlling the working depth for a numerically controlled machine tool

In a numerically controlled machine tool having a workpiece holding table, at least an operating head, displacing means for mutually displacing said table and said head to selected working positions, a rotatable spindle mounted on said head, an asynchronous electric motor for rotating said spindle, said electric motor including a rotor secured to said spindle and a stator having at least an electric coil, axially moving means for axially moving said spindle on said head, a working tool removably mounted on said spindle, and a workpiece hold-down associated with said head, said numerically controlled machine tool further comprising a working depth controlling device comprising a reference member carried by said table at a predetermined sensing position, control means conditionable for enabling said axially moving means to cause said tool to sense said reference member, position transducer means enabled by said control means for defining a mutual axial distance of said tool with respect to said hold-down upon said tool sensing said reference member, an electric power supply connectable with said electric coil to induce an electric current into said rotor causing the rotor temperature to increase without any rotation thereof, electric means for temporarily connecting said electric power with said electric coil, whereby said spindle is heated while it is rotatably standing, temperature sensing means for sensing the temperature of said spindle, and conditioning means controlled by said temperature sensing means for conditioning said control means when a temperature corresponding to the running temperature of said spindle is sensed.

In a numerically controlled machine tool for drilling or milling printed circuit boards having a circuit boards holding table, at least an operating head, displacing means for mutually displacing said table and said head to selected working positions, a rotatable spindle mounted on said head, axially moving means for axially moving said spindle on said head, a working tool removably mounted on said spindle, and a circuit boards hold-down associated with said head and yieldably brought to a circuit boards blocking position, said numerically controlled machine tool for drilling or milling printed circuit boards further comprising a working depth controlling device comprising a reference member carried by said table at a predetermined sensing position, control means conditionable for enabling said axially moving means to cause said tool to sense said reference member, a reference stop associated with said reference member and adapted to stop said hold-down at said blocking position while said axial moving means are so enabled, a normally effective position transducer associated with said spindle, a second position transducer associated with said hold-down, said second position transducer being disabled by said control means upon said tool sensing said reference member to define a mutual axial distance of said tool with respect to said hold-down as difference of the position indications of said two transducers, heating means conditionable for heating said spindle while said spindle is rotatably standing, temperature sensing means for sensing the temperature of said spindle, and conditioning means controlled by said temperature sensing means for conditioning said control means when a temperature corresponding to the running temperature of said spindle is sensed.

In a numerically controlled machine tool for drilling or milling multi-layer printed circuit boards having circuit boards holding table, at least an operating head, displacing means for mutually displacing said table and said head to selected working positions, a rotatable spindle mounted on said head, axially moving means for axially moving said spindle on said head, a working tool removably mounted on said spindle, and a circuit boards hold-down associated with said head, said numerically controlled machine tool for drilling or milling multi-layer printed circuit boards further comprising a working depth controlling device comprising a reference member carried by said table at a predetermined sensing position, control means conditionable for enabling said axially moving said spindle to cause said tool to sense said reference member, at least a pair of telescopic columns for connecting said hold-down with a support of said spindle, a reinforced bracket connecting said columns, a normally effective position transducer connected with said spindle, a second position transducer connected with one of said columns, said second position transducer being disabled by said control means upon said tool sensing said reference member to define a mutual axial distance of said tool with respect to said hold-down as difference of the position indications of said two transducers, heating means conditionable for heating said spindle while said spindle is rotatably standing, temperature sensing means for sensing the temperature of said spindle, and conditioning means controlled by said temperature sensing means for conditioning said control means when a temperature corresponding to the running temperature of said spindle is sensed.