Patent GB-1187426-A (GB)
Title: A method of preparing Three-Dimensional Surfaces
IPC: G05B19/42
Filed: 1967-07-27
Abstract: 1,187,426. Draughting machines. FORD MOTOR CO. Ltd. July 27, 1967 [Sept. 8, 1966], No.34492/67. Heading G1X. [Also in Division G3] Contoured surfaces are produced by an automatic machine tool by a method comprising preparing a three-dimensional model, measuring coordinate data for selected points on the model, preparing two-dimensional layout draughts from the measurements, recording model and draught data and computing the equations of parabolas through selected data points, and using the equations to derive an intelligence storing tape which is entered into a numerical control system for the machine tool, whereby the surface is produced. Measurement of model A clay model of a vehicle body is positioned in a bridge type measuring device and pre-selected section lines are taken off by an operative positioning scaled probes. The bridge is moved longitudinally along the body and desired points measured in the X, Y and Z co-ordinates and the values recorded. Plotting Fig. 10 The recorded point values are plotted on a twodimensional draft plate 40 in sections, the selected boundary lines of which break the contour surface into zones. The draft plate 40 is supported on a table 38 and a line 43 through the plotted values is scanned by a T. V. camera 42 positioned through servos 48, 50 by an operator observing a monitor 52 having X, Y axis display controls. The servo drives are associated with X, Y axis magnetic reading heads 42, 46. A position digitizer 54 receives the data obtained by the operator as he selects with the camera the information to be recorded and the data is registered by a card punch assembly 56. The operator employs a set of prepared numerical process sheets and a template information drawing that indentifies the lines and zones on the draft plate that are required for data reading. The punched cards are stacked in the order in which they are punched and are used in a computer. The registered data can be checked by introducing the co-ordinates into a high speed plotter. Drawing analysis The data relating to a sequence of registered points is analyzed and the equation for a parabola through a group fo four adjacent points is derived. This equation is introduced into a computer which is programmed to produce the co-ordinates of any desired number of points intermediate of the measured points. For a part, Fig. 30, having boundary lines AF to DF, AR to DR, points between the end points AR, DR and AF, DF are computed to produce section planes, at uniform X axis increments, having intercepts with the boundary lines AF, to DF, and AR to DR. The next step is the computation of the chord heights and the intersection points of the curves on planes by a chord bi-section technique. The co-ordinates of points AF, AR are connected by a chord, and the slope and mid-point MA8 determined. The equation of the perpendicular bisector HA8 is calculated and the intercept with the curve forms the mid-point A8 of a sequence of equi-spaced points. Two points measured from the model, adjacent to the mid-point A8 are found and the parabola through AF, A8, AR derived. Then by successive bi-sections of the chords, points such as A4, are derived. In an alternative method, the equations of a succession of parabolic arcs are derived each arc having points in common with the previously derived arc. For an end point, such as A8, a circle is fitted through the end point and two adjacent points. The next step is the derivation of the equations of the parabolas through points A8, P18, P28, B8, C8, D8 by the selection of successive four points along the curves. Then the chords along the base line MA8-MD8 are derived and the chord heights, such as HP18, HP28 computed. The procedure is then repeated with other surface lines, such as A4, D4. Thus the co-ordinates of a predetermined number of surface points are established. The determination of these points being made by equal X-axis increments along the base line or, as preferred to facilitate machining, by dividing the base line by equal increments such that an odd number of points along each of the arcs at the various sections is obtained. The final surface is defined numerically by the co-ordinates of the points that represent the ends of all the interpolated chord heights, such as P18, P28. For each of the computed points, except for the end points, the partial derivatives of the parabolas are found by consideration of adjacent points and the tangent vectors established by determination of the slopes. From the tangent vectors, a normal vector is established and when the radius of the cutting tool is known, the co-ordinates of the offset cutter path are calculated. The surface point information is then registered in a memory store of a computer and used to prepare a magnetic tape which is fed to a tape punch attachment producing a binary-coded punched tape containing the intelligence necessary for directing a milling machine tool. Numerical control system, Fig. 2 A tape 10, bearing binary-coded co-ordinate numerical data, is read and the data blocks fed to the appropriate buffer store 14 while the milling is operating under the data contained in an active store 16. For the X axis a train of pulses from a three-point parabolic interpolator, not shown, metered in dependence on the rate of travel are fed to a modulator 18 wherein they are added to or subtracted from a constant rate train of pulses in dependence on the programmed direction of travel. The derived pulse train is fed to a frequency divider 20 deriving reduced frequency square waves. The square wave is fed to a demodulator 22 and compared with the input from a rotary resolver 24 driven by a lead-screw of the milling machine. The D.C. output from the demodulator is amplified and controls a servovalve 26 varying the flow of hydraulic fluid to a driving motor for the lead-screw.