Patent GB-414663-A (GB)
Title: Improvements in or relating to the optical inspection of screw threads
IPC: G02B21/00
Filed: 1933-02-07
Abstract: 414,663. Screw - thread gauges. HILGER, Ltd., A., and SMITH, C. F., 98, King's Road, Camden Road, London. Feb. 7, 1933, No. 3753. [Class 106 (ii).] Relates to apparatus for illuminating screw threads in a direction substantially tangent to the thread flank and consists of means for varying along the flank section the direction of the rays of light to suit the varying rake of the thread at different distances from the axis of the screw. The screw 25 is carried on centres which may be adjusted vertically and horizontally on a carrier which is capable of being turned through an angle about a vertical axis which cuts the axis of the screw. The projection lens 16 is also capable of slight adjustment to align the image with a drawing of a standard thread on a screen. Light from a straight filament source 22 is passed through a planoconvex lens 26 and is collimated as regards any one point in the filament but there is a range of direction as between one point and another. To select the correct direction for each horizontal lamina of the beam thus collimated, a disc 27 in front of the condenser 26 and provided with a slit may be rotated by the handle 28 the setting being read at 29. The disc is at the principal focus of a cylindrical lens 30 having a vertical axis. After passing the lens 30, the direction of the collimation in relation to the optical axis of the system will depend on the horizontal position of the corresponding point of light in the slit to left or right of this axis. In whitworth threads where the helix angle or rake of the thread is small, it may be unnecessary to incline the axis of the screw to the optical axis of the system, and more than one thread can be projected. With large rake threads, the axis must be inclined, and only one thread projected. The condenser 26 may be cylindrical. A slit illuminated by diffused light may be used as a source of light or a slit illuminated by a strip filament passing light through a supplementary condenser. If deep threads are to be inspected, a curved slit may be used. The arrangement comprising the condenser 26, slit disc 27 and lens 30 may be replaced partly or wholly by other arrangements giving either a continuously graded direction of beam, or a stepped gradation, and operating either by reflection or refraction. In one form a series of plates are stacked and ground to a lens shape. A parallel diametrical strip is then cut out and the pieces mounted in a rectangular frame which may be rocked to give an angular separation of collimated beams. A stack of thin isosceles prisms may be used, or a series of thin rectangular plates 48 may be stacked and then twisted as in Fig. 20. The one half of the stack is ground off to form a plane surface and then cemented together with or without a plane parallel glass plate cemented to the plane surface. The stepped surface may be silvered to form a mirror 50 as in Fig. 23. Such a stack of mirrors may with the assistance of three ordinary reflecting mirrors be used to adapt the invention to projectors of the usual type. The mirrors may be made adjustable as shown in Fig. 26 by mounting them on a spindle 53 and connecting the one set of ends independently to a cone pulley 55 having grooves and passing the threads from the other ends around a parallel pulley 57 to an anchored spring 59. By rotating the pulley 55 the stepped mirrors will be rotated through angles which progressively increase. In another adjustable form, the strips are attached to successive convolutions of a coiled spring one end of which is fixed to a support and the other to a ring which can rotate on the support. In another form, Fig. 31, a series of fine saw cuts are made in a polished flat metal plate one end 68 of which is clamped and the other end located in a groove 70 in a circular disc 71. When the latter is rotated and clamped the strips formed by the saw cuts will be bent backwards and forwards each strip being advanced by a small angle in front of the next. A cross saw cut 74 near the end facilitates tending. In a further form, a rectangular strip of glass is placed vertically in an electric furnace and heated to a temperature at which it can just be twisted. When cooled, a rectangular strip may be taken from the central portion to form a skew mirror. A metal strip may be similarly treated or if thin bent cold.