Patent GB-556408-A (GB)
Title: Improvements in and relating to the cutting of bevel gears
IPC: B23F1/06
Filed: 1942-11-25
Abstract: 556,408. Gear-cutting. LICHTWITZ, O. Nov. 25, 1942, No. 16731. [Class 83 (iii)] In a method of and apparatus for form milling, particularly roughing straight bevel gears by rotary disc cutters of constant profile, the cutter which at the pitch line has a thickness which is a predetermined fraction of the circular pitch at the large end of the gear is moved along a path which is offset from the blank bone by a predetermined fraction of the said circular pitch, the blank being rotated between the cutting of opposite flanks by an amount which is a predetermined fraction of the indexing movement corresponding to the number of teeth to be cut. In cutting a bevel gear of face width f Fig. 2, (usually not more than one third of the cone distance c), the cutter is fed along a line approximating to the theoretical root line and may be 2/3 the width of gap at the large end i.e. 1/3 of circular pitch. Such a cutter must pass at least twice through each gap in order to produce the correct width at the large end. Considering a central cut such as that shown in Fig. 2 if T, t, are the widths of cut at the large and small ends respectively for a diametrical pitch of unity then one side of the cut tapers to a point G where k for pitch P the distance GH = - <SP>p</SP> If the line EF is to pass through the cone apex the cutter must be offset in the crown wheel plane by the distance OG, Fig. 2, equal to If further the line IO subtending an angle X with the central gap line BO represents the correct position of the tooth side then the line IO must be brought to the position of EF by a rotation of the blank corresponding to a rotation of X - 0 in the crown wheel plane. This rotation, which is X<SP>1</SP>-Ï<SP>1</SP>, Fig. 4 is shown to be 2#. where n is the number of teeth. Since -- n is the normal indexing movement=i we have the rotational adjustment Also the set-over where P<SP>1</SP> is the circular pitch. In these formulae k is a variable but it is shown that with slight error k may be replaced by an arbitrary constant K which for greater simplification of the rotary adjustment r may be # - The expressions for s and r then become 12. The maximum set-over which gives a finished tooth is shown to be 1/12 of the circular pitch, when the cutter thickness is 1/3 the circular pitch at the large end. One apparatus for carrying out the invention by manual adjustments on an ordinarv milling machine has a number of work spindles 1 angularly adiustable to the cone angle in a housing 3 and indexed from a control 12. The set-over is given by a graduated hand control 9 which may have 3 sets of graduations corresponding to circular pitch, diametrical pitch, and module. By adjustment of the table 7 the cutters are placed in a central position. The first set-over is then made by the handle 8 and the blanks adjusted through 1/6 of the indexing movement. One set of flanks is then cut with normal indexing. The last indexing movement is combined with the angular resetting movement for cutting the opposite flanks, the set-over being also readjusted by the hand-wheel 8. In an automatic machine embodiment. Figs. 9 and 9A, the cutter 3 is driven from a shaft 1 through pulleys 4, 9 and bevel gearing 10, 14. The guide 138 of the cutter slide is hinged at 139 and is set to the required angle by a pinion 140. A shaft 19 carries a loose constant-speed pulley 20 and also a pulley 18 driven at a variable speed from the shaft 8. The two pulleys are selectively clutched to shaft 19 by a clutch 21. The shaft 19 is connected through a clutch 22 to a shaft 24 which through bevel gearing 26 drives the cutter feed screw 27. The clutch 21 is actuated by axial movement imparted to a shaft 30 when a member 29 actuated by screw 28 coacts with abutments 31, 32. Quick movements are imparted to the shaft 30 by a lever 37 coacting with a spring plunger 39. A portion 41 of this lever coacts with detents 42, 43 the former of which prevents the clutch 21 engaging the forward drive after the reverse drive is disengaged. A pulley 44 free on a shaft 45 can be clutched thereto by a plunger 48 normally held back by a stop 50 associated with latching mechanism The detent 41, Fig. 9A, is holding the stop 50 in inoperative position in which the plunger 48 can engage the wheel 44 which thereupon makes one revolution before the plunger disengages. Shortly before this movement is completed a cam 60 removes detent 32 so that the clutch 21 can now engage pulley 18 to give the forward cutter movement. At the end of this movement the clutch is reversed. The one-revolution shaft 45 acts through a mutilated gear 64 to impart one revolution to a shaft 66 connected bv reversing bevel gearing to two-revolution shaft 75 connected by bevel gearing 78 to a shaft 80 connected through change gears 84 to a spindle 91, operating the cross feed of the cutter. By means of clutches controlled selectively by a stirrup 96, Fig. 9A, this adiustment may alternatively be effected by a hand control 93. The shaft 45 at each movement imparts a half revolution to a shaft 99 connected to a shaft 102 through mutilated gearing 100, 101 arranged, on the first half revolution of gear 100, to rotate the gear 101 through 1/3 revolution and on the next half revolution to rotate the gear 101 through 2/3 revolution. Movement of shaft 102 is transmitted through change gears 114, shaft 116, and bevel gearing to a worm 120 operating the work-spindle 122. The work-spindle mav also be rotated from a hand-wheel 125 after disengaging the power drive. The shaft 99 carries a cam 129 which controls the clutch 76 in the setting-over train After all the teeth have been cut a pin 147 on a wheel driven from shaft 102 may be arraneed to disengage the main clutch. In operation. after setting the cutter central, the first set-over is made by imparting one revolution to the hand wheel and the stirrup 96 is moved to the automatic position. The indexing train is arranged so that one revolution of shaft 102 effects the full indexing movement. At the end of the first cutter stroke the cutter is quickly reversed by the load-and-fire mechanism and at the end of the return movement shaft 45 makes one revolution. The consequent two revolutions of shaft 75 change the set-over to that required for the opposite flank and the one-third revolution of shaft 102 makes the necessary rotary adjustment of the work. Clutch 76 is reversed and cam 60 causes the next forward movement. At the end of the following return the opposite set-over is effected and the shaft 102 is rotated 2/3 revolution which movement added to the previous 1/3 revolution positions the blank for cutting the first flank of the next tooth. The cycle is then automatically repeated. For roughing fine pitch bevels a single central cut may be sufficient. In this case clutch 111 is coupled direct to wheel 97 so that a full indexing movement occurs at each revolution of shaft 45. The setover mechanism is disengaged by stirrup 96. The machine may be used in a similar way for roughing out spur gears.