Patent GB-762271-A (GB)
Title: Machine tool
Inventor: CAMPBELL PAUL JOHN
IPC: B23Q35/12
Filed: 1952-04-17
Abstract: 762,271. Copy-milling. PRATT & WHITNEY CO., Inc. April 17, 1962, No. 9672/52. Class 83 (3). [Also in Group XXXVIII] In a feeler-controlled copying- machine the work-table is movable in a plane in two directions normal to each other, and the feeler and cutting-tool are movable together, in a direction normal to the plane, under the influence of opposed thrust forces, the resultant of which is a function of the distance between the feeler and a pattern on the work-table. The opposed thrust forces are derived from magnetic clutches in an electronic control circuit similar to that shown in Specification 699,452. In a copy milling machine the work and the pattern are fixed to a table 50 movable longitudinally on a cross-slide 52, movable on a vertically-adjustable knee 24. Both tool and feeler are reciprocated together towards and away from the work and pattern, respectively, by a continuously-running motor 66 acting through two opposing slipping clutches normally energized equally. " In " or " out " feed is then controlled by varying the energization of the clutches differentially under the control of an electric-circuit including an electric spark between the feeler and pattern. Variations from the normal distance between the feeler and pattern vary the spark-current from normal, these current variations being amplified and controlling the clutches as described in Specification 699,452. Scanning may be effected manually through handles 374, 376 or automatically. The cutter 58 is secured by a collet 126 in a spindle 74 rotatably mounted in a quill 72 slidable in a housing 84. The upper end of spindle 74 is splined at 134 to an intermediate shaft 138. The motor 80, Fig. 1, drives the cutter through a pulley 170, belt 174, pulley 140 and the intermediate shaft 138. The quill is reciprocated towards or away from the work by the differential energization of two slipping clutches 68, 70, the driving elements 190, 194 of which are continuously driven by the motor 66 in opposite directions. The driven elements of the clutches are connected through similar gear trains to respective pinions 228 (not shown) engaging a rack on the quill. When energization of the clutches is varied differentially by a control signal the appropriate one of the two clutches drives the quill to adjust the tool and the feeler. Keyed to the quill 72 at 276 is an arm 78 guided by a rod 266. At its outer end the arm is bifurcated to support the feeler assembly. The feeler 60 is made of brass, steel or low-fusing non-shrinking lead or bismuth alloys and the pattern may be moist plaster of Paris, a casting or modelling of wax covered with an electrically-conductive paint or of metal. The feeler 60 is secured in, but insulated from, a tube 290 freely reciprocable in bearings 304, 306 in the casing 88. At its upper end the tube 290 carries a micrometer 320 for axially adjusting a sleeve 300 flanged at 302 to engage the ends of arm 78. The feeler assembly merely rests on the arm 78 so that if the feeler contacts the pattern it may rise and damage is prevented. Current is led to the feeler by leads 588, 606 and respective series resistors 586, 604 to a common contact point engaging the top of the feeler. The table 50 is reciprocated by a traverse motor which is reversed at each end of the table travel by stops 430, 432 opening or closing a micro-switch 436 on the cross-slide. At each actuation of the motor reversing switch a further switch is actuated to cause increased conduction in one or the other of two triodes for a duration dependent on the time constants of associated resistor - condenser combinations. During these short time intervals a sensitive relay is actuated, which, if the machine is set for automatic feed, disconnects the table traverse motor and switches in the cross-slide feed motor, the direction of which is controlled by a manually set switch. The work and the pattern are thus scanned in a series of parallel lines, the distance between alternate pairs of these lines being determined by the settings of the variable resistors. The cross-slide feed may alternatively be effected at will by moving a switch so that the feed motor is connected up continuously. Should the feeler approach the pattern too closely, the resultant current through the two clutches increases sufficiently to operate a series-connected relay which disconnects the scanning feed motors. Should the power supply fail a no-volt release relay drops off and emits an earth (i.e. positive) pulse to energize the outfeed clutch for the short time taken for the moving elements to come to rest. Limiting depth of cut.-The tube 290 of the feeler assembly carries a contact 342 which, when the cutter has reached a predetermined depth in the work, approaches near enough to a rod 326 secured to the casing 88, to cause a spark to pass. This current functions similarly to an increase in the current of the spark between the feeler and pattern to energize the " out " feed clutch and prevent further inward feed movement of the feeler and tool. Specifications 762,341 and 762,342, [Group XXXVIII], also are referred to.