Tool-holding spindle comprising: - a stationary part defining a cavity extending along a longitudinal axis (X-X), - a rotary part mounted in said cavity to rotate with respect to said stationary part around said longitudinal axis (X-X), said rotary part being intended to support a tool, - a first group of bearings and a second group of bearings spaced along said longitudinal axis (X-X) from the first group of bearings, each bearing (41, 42, 43, 51, 52) of the first and of the second group of bearings having an inner ring (41a, 42a, 43a, 51a, 52a) fixed to said rotary part and an outer ring (41b, 42b, 43b, 51b, 52b), - locking means movable between a locked configuration and an unlocked configuration to respectively lock and unlock said rotary part with respect to said stationary part to make the spindle operate with a fixed tool and a rotary tool, respectively, wherein the locking means comprise - a first element fixedly connected to said rotary part, - a second element fixedly connected to said stationary part, and - a third element which is positioned in an annular cavity formed between the stationary part and the rotary part and which is axially movable with respect to said first and second element, wherein in said locked configuration, said third element engages said first and second element to lock the rotary part with respect to the stationary part, and in said unlocked configuration, said third element is disengaged from said first and second element to allow the rotation of said rotary part with respect to said stationary part, characterized by - an annular part which is positioned in an annular cavity formed between the stationary part and the rotary part, which is axially movable to engage and disengage the second element, which is connected to the outer rings (41b, 42b, 43b) of said first group of bearings and which is interposed between said first group of bearings and said stationary part, and - connection means to connect and disconnect said annular part with said stationary part, wherein - in said locked configuration of the rotary part, said connection means are designed so that said annular part is disconnected from said stationary part so that the outer rings (41b, 42b, 43b) of said first group of bearings are unconstrained from said stationary part in operation with a fixed tool, - in said unlocked configuration of the rotary part, said connection means are designed so that said annular part is connected with said stationary part so that the outer rings (41b, 42b, 43b) of said first group of bearings are constrained to said stationary part in operation with a rotary tool. - a stationary part defining a cavity extending along a longitudinal axis (X-X), - a rotary part mounted in said cavity to rotate with respect to said stationary part around said longitudinal axis (X-X), said rotary part being intended to support a tool, - a first group of bearings and a second group of bearings spaced along said longitudinal axis (X-X) from the first group of bearings, each bearing (41, 42, 43, 51, 52) of the first and of the second group of bearings having an inner ring (41a, 42a, 43a, 51a, 52a) fixed to said rotary part and an outer ring (41b, 42b, 43b, 51b, 52b), - locking means movable between a locked configuration and an unlocked configuration to respectively lock and unlock said rotary part with respect to said stationary part to make the spindle operate with a fixed tool and a rotary tool, respectively, wherein the locking means comprise - a first element fixedly connected to said rotary part, - a second element fixedly connected to said stationary part, and - a third element which is positioned in an annular cavity formed between the stationary part and the rotary part and which is axially movable with respect to said first and second element, - a first element fixedly connected to said rotary part, - a second element fixedly connected to said stationary part, and - a third element which is positioned in an annular cavity formed between the stationary part and the rotary part and which is axially movable with respect to said first and second element, - an annular part which is positioned in an annular cavity formed between the stationary part and the rotary part, which is axially movable to engage and disengage the second element, which is connected to the outer rings (41b, 42b, 43b) of said first group of bearings and which is interposed between said first group of bearings and said stationary part, and - connection means to connect and disconnect said annular part with said stationary part, - in said locked configuration of the rotary part, said connection means are designed so that said annular part is disconnected from said stationary part so that the outer rings (41b, 42b, 43b) of said first group of bearings are unconstrained from said stationary part in operation with a fixed tool, - in said unlocked configuration of the rotary part, said connection means are designed so that said annular part is connected with said stationary part so that the outer rings (41b, 42b, 43b) of said first group of bearings are constrained to said stationary part in operation with a rotary tool.