Patent GB-807994-A (GB)
Title: Improvements relating to machine tools
Inventor: BOWDEN BERTRAM VIVIAN
IPC: G05B19/21
Filed: 1954-10-29
Abstract: 807,994. Programming for machine-tools; milling. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Oct. 28, 1955 [Oct. 25, 1954], No. 31281/54. Class 83 (3). [Also in Groups XXXVIII and XL (b)] The movements of the work-slide of a vertical milling machine, and of the tool spindle and its slide, in pitch, roll and yaw, are measured by the variation in capacitance between fixed electrodes mounted on the machine frame coacting with movable electrodes mounted on the moving parts, the capacitances being included in computing circuits whereby the relative positions of the work and tool are adjusted to take into account the measured distortions. The displacements of the work and tool from datum positions are measured by photo-cells receiving light through diffractiongratings. As shown, Fig. 1, the work 4 is mounted on a horizontal slide 1 movable along an X axis between flanges 3 in a bed 2, the milling cutter 5 being mounted on a slide 7 for movement along a perpendicular Y axis in flanges 9 in a bed 8. Roll movement of slide 1 about the X axis is measured by the variation in capacity between an optical flat 12 attached to the bed 2 of the machine and a flat surface bar 15 carried by brackets 14 projecting from the side of the slide 1. Yaw movement of the slide 1 is measured by the relationship between the capacity of the condenser formed by plates 12, 15, and that of a similar condenser formed by the bar 15 and a plate 17 mounted similarly to plate 12 but displaced axially along the slide 1. Roll movement of the slide 7 about the Y axis is measured by the variation in capacity between optical flats 24, 25, Fig. 4, located in channels cut in the bed 8, and small flat plates 28, 29 on the slider 7 carrying the milling cutter 5. Pitch movement of the slide 7 is indicated by the relationship between the capacity of the condenser formed by plates 28, 24 and that of a similar condenser formed by the bar 24 and a plate 30, Fig. 5, mounted similarly to plate 28 but displaced axially along slide 7. A second small condenser plate may also be associated with the bar 25, and the four capacitances so formed may be measured and used to indicate the axial displacement of the slide 7 and tool 5. Axial displacement of the tool spindle 85, Fig. 8, is measured by inserting four insulating blocks 91 into the tube 87 carrying the bearing for the spindle, the inner surfaces of the blocks 91 being covered with conducting material 93 so that each coating 93 forms a capacitor with the spindle 85, the differential variation of capacitance of opposite capacitors being an indication of the axial displacement of the spindle in the X or the Y axis. The position of each slide along its guides is determined by a pair of diffraction gratings. As shown, in association with slide 1, a long grating 33, Figs. 1 and 2, is attached to the bed 2 and a short grating 34 is carried by the plate 15. Light from a lamp 35 passes through the two gratings into a photocell 36. Relative movement between the two gratings of a pair produces a moire fringe effect forming a series of alternate light and dark strips producing a sinusoidal output from the photocell 36, which may be analysed to determine the extent and direction of movement. The output signals may be compared with coded control signals derived from a punched tape or magnetic tape record, whereby movement of the slides 1, 7 may be effected automatically. The errors introduced by the unwanted displacements measured by the capacity systems described above are algebraically summed, and used to modify the control signals of each slide. Each condenser is located in a resonant circuit to determine the frequency of an oscillator output which is fed to a discriminator to provide a D.C. voltage proportional to frequency. The D.C. voltages are modified as required, e.g. the yawmeasurement of slide 1 is determined from the difference of the signals derived from condensers 12, 16 and 17, 16, and multiplied by a factor representing the position of slide 7, before they are combined with the control signals. The transducers measuring the error movements may alternatively be piezo crystals or strain gauges.