Presetting devices are used in machining centers where various exchangeable tools can be inserted. The tools are measured and/or set in a presetting device before use. To this end, the connecting zone of the tools, in particular a tapered trunnion, is put into a corresponding, and in particular also tapered, receiving recess. In doing this, the tool's axis has to be oriented in a defined direction and has to be enabled to be rotated about the tool's axis for measuring purposes.
From U.S. Pat. No. 4,532,716, a presetting device having an air bearing is known. While rotatability of the tools is ensured by an air bearing, rotating may be facilitated or made more difficult by building up or reducing the air suspension. Orientation in rotation is effected when the air suspension is built up. Having attained the desired rotatational orientation, the supply of pressurized air to the air suspension is switched off. In this way, the tool assumes a supported position in which it is rotatable only with high forces applied, and in which it is supported within its carrier at a precise level which can be defined and determined. In this supported position, the tool can be measured precisely and may be set without the risk of undesired rotation while being continuously measured and monitored.
In order to build up the air suspension, a source of pressurized air is connected to an annular recess in a conical bearing surface via an On/Off valve and a pressure regulator. In this way, pressurized air may be led between the bearing surface and a supporting surface of the tool assigned to each other which are rotationally symmetric about the bearing and the tool's axis. Without supply of air under pressure, these supporting surfaces are pressed together by the weight of the tool, thus ensuring a desirable braking friction against rotation. Since the surfaces of the bearing surface of the carrier and tlie supporting surface of the tool, which are effective by friction and engage each other in a state without pressurized air, are tapered or conical, they engage each other in a centered manner, thus ensuring that the tool's axis extends always the same way.
When pressurized air is introduced under sufficient pressure, an air cushion is created between the two supporting surfaces which renders the frictional force neglectibly small. Since the required pressure depends on the weight of the carrier and the tool, a pressure regulator is provided. Thus, the pressure can be adjusted by the regulator in such a manner that the desired characteristics of rotation are attained. If the pressure is too high, undesirable vibrations could occur; if it is too low, the resistance against rotation is too high.
Now, it has been found that the tool carrier and the tools are often not supported by an appropriate pressure so that measuring and setting is done under bad supporting conditions which, in turn, involve a higher expenditure of energy and, particularly, lower precision as well as, in some cases, even damages caused by vibrations.