1,074,982. Grinding; gear grinding. GLEASON WORKS. Jan. 26, 1965, No. 3374/65. Headings B3D and B3K. A method of dressing a surface of revolution of a grinding wheel for grinding spiral bevel or hypoid gears by swinging the wheel in a helical path relative to the gear through a tooth space of the gear with the axis of helical motion inclined to the rotation axis of the wheel to cause the surface of revolution of the wheel to produce a helicoidal tooth surfaco on the gear, comprises effecting a relative swinging motion between the rotating wheel and a wheel dressing tool through a helical path along and about the axis of helical motion that substantially duplicates the first-mentioned helical path and with the wheel in contact with a surface of the tool which represents the helicoidal tooth surface in profile shape. Apparatus for carrying out the method may be similar to that described in Specification 1,074,981 having a base 10, Fig. 1, on which a cradle 11 is rotatable about an horizontal axis 12. The cradle carries an annular grinding wheel W, the spindle 13 of which is rotatable about an axis 14 which is adjustable in the cradle so as to be either coincident or parallel, or inclined and either intersecting or offset relative to the axis 12. A gear G to be ground has its spindle 15 rotatable about an horizontal axis 16 in a work-head 17 vertically adjustable on a column 18 horizontally movable on ways 19 on a plate 21 which, in turn, is horizontally movable along ways 22 in a base 23 adjustable about a vertical axis 24 in ways 25 on a base 26, the latter being movable along ways 27 in the frame 10. The work-head 17 also carries a dressing tool D having a body 97, 103 adjustably mounted in a spherical mount 98 which is adjustable along a slide 100 radially of a bar 101 secured to the work-head, the bar having an axis 16<SP>1</SP> which also represents the axis of an imaginary gear G<SP>1</SP>, the dressing tool D representing a tooth slot of that gear. The column 18 is formed with a cylinder 93 containing a piston 92 secured to the slide 21 whereby the column can be moved from the position shown during gear grinding to a position in which the dressing tool and imaginary gear G<SP>1</SP> are in the position of the gear. The tooth surfaces of the gear are helicoidal, i.e. they have straight line generatrices and intersect planes perpendicular to the helix axis in curves which are involutes of a circle. Each tooth slot has a concave face 73, Figs. 2 and 3 (not shown), a convex face 74 and a bottom 75, the face plane 71 of the gear being perpendicular to the axis 12. The surfaces 73, 74 have line elements 86, 88 having normals 76, 77 which pass through a mean 72 and are tangent to respective involute base circles 78, 79. The grinding wheel has its axis inclined and offset to the axis 12 so that it intersects both normals 76 and 77 at points 82 and 83 and thus has line contacts with the surfaces 73, 74 in the planes tangent to the respective base circles. Thus the profile of the grinding wheel can be produced by replacing the gear G to be ground with the dressing tool which has straight surfaces in normal planes parallel to the axis 12 and occupies the same relation and has the same relative motion as the gear during the grinding operation. The dressing tool may comprise a block 97, Fig. 6 (not shown), having a bit 102 formed by moulding with diamond or other abrasive and having helicoidal concave and convex surfaces 73<SP>1</SP>, 74<SP>1</SP> and a helicoidal bottom 75<SP>1</SP>, which dresses the tip of the grinding wheel, corresponding to the tooth slots to be ground on the gear. In another embodiment, the dressing tool has a body 103, Fig. 8 (not shown), having a pair of dressing bits 104, 105 having straight wheel engaging portions 106 lying in parallel planes tangent to the base cylinders 78, 79 of the helicoidal tooth surfaces of the imaginary gear G<SP>1</SP> which the dresser represents. The bit 104 is secured to the holder 103 by screws 109, 111 and the bit 105 is relatively adjustable to the bit 104 by shims 112, 113 and is secured in adjusted position by screws 117, 118. Each bit has a portion 115, 116 for dressing the tip of the wheel. In a further embodiment, the dressing tool comprises dresser wheels 119, 121 of the abrading or crushing type having their axes 122, 123 lying in planes 124, 125 tangent to the base circles 78, 79 of the tooth surfaces. The flanges of the wheel may dress the tip of the wheel. During the dressing of the wheel, the base 26 is fed along ways 27 in the direction of the axis 12 while the cradle 11 is swung about the axis.