Patent GB-373998-A (GB)
Title: Improvements in method and machine for grinding worms and screws
IPC: B23F13/04
Filed: 1931-01-26
Abstract: 373,998. Gear-grinding. TRBOJEVICH, N., 71, Glendale Avenue, Highland Park, Michigan, U.S.A. Jan. 26, 1931, No. 2555. Convention date, July 7, 1930. [Class 83 (iii).] In a method of grinding worms, screws and the like the work is moved axially and correspondingly rotated to feed it in a helical path past a rotary grinding-wheel which is oscillated in a straight line path in a direction transverse to the work axis. Fig. 1 shows a transverse section 21 through a double-threaded worm having tooth curves 23 which are involutes developed from the base circle 24. The section 21 may be regarded as a spur gear element meshing with a rack element 29 which is straight sided in the case of involute teeth and has a pitch line 29 tangential to the pitch circle 25. When the section 21 is rotated and moved axially to describe the complete worm surface the imaginary rack will be moved in the direction of the worm axis 31 and also along its pitch line. The rack teeth will thus describe a prism having an axis along the line 33, Fig. 2. Formulµ are given in the Specification for determining the thickness t<1> and the pressure angle A<1> of the rack section of a grinding wheel 34 which may be oscillated along the line 33 during the helical movement of the worm blank to generate the required teeth The path of the instantaneous points of contact between the grinding wheel 34 and the worm thread are shown by the straight lines N<1>, K<1>, M<1>, H<1> in Fig. 2. From the position of these lines the amplitude of the transverse oscillation S<1> of the grinding wheel required to produce complete generation of the teeth is deduced. In a machine embodiment for carrying out the invention the worm blank is mounted in bearings on a plate 54, Fig. 8, carried by a table 42 which reciprocates in the direction of the arrows 70, 71 through the distance S<1>. The grinding wheel is mounted on a spindle 43 arranged at the helical angle D to the work axis. The work arbor 45 carries a master screw 49 which has the same lead as the worm to be ground and works in a stationary nut 51. The front of the work arbor is splined to a worm-wheel 57 operated by a worm on a spindle 62. At each reciprocation of the table a ratchet wheel 66 engages an abutment 72 to impart a small rotary movement to the work arbor, the work' being moved in the required helical path by the action of the master screw. In this way the worm is generated by a series of straight line cuts such as N<1>, K<1>, H<1>, M<1>, the generating lines produced by successive working strokes being about 4/1000" apart. In some cases the helical feed of the work may be continuous instead of intermittent and the grinding wheel may be reciprocated instead of the work table. Globoid worms may be ground in a similar way, Fig. 10, the grinding wheel 34 being arranged on an axis inclined at the helical angle D to the work axis and being reciprocated in the direction 70, 71 while the worm is progressively rotated on its axis and at the same time moved longitudinally in a circular path. Where more than one thread is to be cut, the work is indexed between successive thread cutting operations.