527,356. Gear-cutting. PARNALL AIRCRAFT, Ltd., LE CHEMINANT, H., and CLAYTON, A. C. March 16, 1939, No. 8471. [Class 83 (iii)] In a machine for cutting toothed rings or circular racks of large diameter the work is supported on a table which is indexed by means of a helical cam engaging a series of pins spaced round the table at the required pitch, the cam having a plain circular grooved portion in which the successive pins engage while the raised edges of the groove are located between the pins to hold the table stationary during the cutting operation in conjunction with clamping mechanism. The cutter head B in the diagrammatic Fig. 1, has a spindle C driven by worm gearing D, E from a shaft 7. The work table G carries a number of accurately formed and spaced pins 48 cooperating with a cam 47 on an indexing spindle 45. The cutter slide 5 on the front of the head is vertically adjustable by a screw 6. The shaft 7, Fig. 4, carrying the worm E is splined in a shaft 8 driven from a motor 12 through a pulley 10 which serves as a fly-wheel. A separate motor 13 is connected through reduction gearing 14 and worm gearing 16 to a shaft 18 which may be connected either directly through a high-speed clutch 21, Fig. 6, or through back gearing controlled by a low-speed clutch 27 to a gear 22 connected to the main camshaft 36, Fig. 4, carrying a cam drum 37 having a cam 38, Fig. 7, engaged by a roller to reciprocate the cutter head. The camshaft is also connected to a shaft 45, Fig. 4, carrying the indexing cam 47 which consists of table-rotating strips 49, Fig. 9, and locking-strips 50 which engage between the pins 48. The drum 37 also carries a second cam track 51, Fig. 7, which actuates a spring arm 58, Fig. 11, connected to a segment 61 which actuates a nut 62. This imparts axial movement to a clamping element 64 and engages a projection 66 with a lip 67 on the table G to hold the latter stationary during the cutting. The table is carried on a spindle 69, Fig. 12, having roller bearings 70 associated with footstep bearing 71. A ring 72 secured by screws 73 carries the work 74 which is held by a number of dogs 75 secured by screws 77. When the table has rotated through a whole revolution or a predetermined fraction thereof the drive is interrupted by neutralising the gear lever 79, Fig. 15, controlling the clutches 21, 27. The shaft 80 operating the clutch yokes carries a hand lever 87 moving over a marked plate 92 the neutral position being determined by a normally retracted plunger 88 engaging a hole 89. The speed change is normally arranged so that the cutter moves slowly while actually cutting, the approach and return movements being effected at high speed. A speed selector cam 94, Fig. 18, mounted on the main cam shaft comprises side webs 95, 95a, spring-pressed pivoted elements 97,97a and trip portions 96,96a. The trip portions co-operate with a roller on the speed-change lever 79. A catch is brought into position to hold the lever 79 in neutral position when the work has been completed. The cutters may be in the form of a gang of milling cutters. For instance, a gang of three cutters may be employed two of the cutters taking roughing cuts while the third takes a finishing cut on previously roughed teeth. By employing slip pieces of suitable shape the machine may be arranged to cut helical gears and by arranging the turntable to be tiltable, both plain and spiral bevels may be formmilled.