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Instrument for finding the centre of a circular hole and for providing a basis or datum for measurements or positions in relation to such centre
Instrument for finding the centre of a circular hole and for providing a basis or datum for measurements or positions in relation to such centre
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机译：查找圆孔中心并为测量或相对于该中心位置提供基准或基准的仪器
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摘要
640,339. Tools for setting out work. DRURY, K. G. June 29, 1948, No. 17397. [Class 61(ii)] [Also in Group XX] An instrument for finding the centre of a circular or part circular hole or other datum in relation to such centre comprises a bridge arm 1, adjustable in length, one end 5 of which is adapted to contact with one side of the hole whilst the other end passes at right angles from the centre of a chordal member 2, a slide 6 mounted on the bridge arm 1 and a second arm 7 slidably mounted on the slide with its axis intersecting the axis of the bridge arm and extending at right angles from the centre of a second chordal member 8. When the instrument is adjusted so that each chordal member makes contact with the circular hole, the centre of the hole lies at the intersection of the axes of the arms 1 and 7. The bridge arm is adjusted in length by means of the screw 4. The chordal members 2, 8 are provided with cylindrical rollers 3 by means of which contact with a cylindrical hole is established to ensure that the plane of the instrument is at right angles to the axis of the hole. As shown, the point of intersection of the two axes at the centre of the hole is indicated by a pin 10 and a datum plate 11, rotatable about 'the axis of the centre pin over a dial plate 9 and having faces 12 passing radially from the centre pin for use with a height gauge may be provided. In a modification the centre pin indicator is carried on a block provided with datum faces and adjustably mounted radially of the dial plate by means of a vernier screw. The centre of the circular hole may be located by means of a hole situated at the centre of the datum plate and ground to size for use with mandrils and similar instruments.
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