Stock divider including a computer controlled gear locator

A grinder for form grinding gear teeth in which a work gear is positioned rotationally by a computer controlled drive. A sensitive probe in known angular relation to a grinding wheel about the axis of the gear is positioned at random in any tooth space, and senses the angular positions of the gear when the gear is driven in opposite directions into probe actuating positions of adjacent tooth surfaces. The computer then controls the gear drive to rotate it reversely by one half of such angular movement from the last position.

Grinding of cylindrical (spur or helical) gears is a precision operation 
and is employed generally in aviation and instrument gears where the 
utmost in accuracy of the shape and location of the gear teeth is 
required. In production form gear grinding in general terms, the periphery 
of a grinding wheel is trimmed to the cross-sectional shape of a tooth 
space between two adjacent gear teeth. The wheel is rotated to produce the 
required surface speed of the grinding surface, the periphery of the wheel 
is introduced into a tooth space, and, by relative axial traverse, is 
caused to grind both tooth flanks simultaneously progressively from end to 
end. 
If the gear is a spur gear, the grinding wheel is set with its axis 
perpendicular to the axis of the gear, and the wheel is fixed against 
rotation during axial traverse. If the gear is a helical gear, the 
grinding wheel is set at the required helix angle, and the gear is given a 
controlled rotation related to the axial advance to generate the helix. 
Finish form grinding of case hardened gears poses a further problem. It is 
always desirable in form grinding gear teeth to provide for removal of 
equal amounts of material from opposite sides of the tooth space, as a 
matter of economy. However, when the gear teeth are case hardened, this 
becomes a more stringent requirement. If the grinding wheel is not 
precisely centered in the tooth space, more material than necessary is 
removed at one side to insure that the other side of the tooth space is 
properly ground, and in some cases the hardened case is removed from one 
tooth surface. 
The centering of the grinding wheel in a tooth space is referred to as 
"stock dividing", and in the past this has been left to the operator to 
determine by sight and/or sound to provide simultaneous initial contact 
between the wheel and both sides of a tooth space. 
In the past, relating the rotation of the gear to the relative axial 
traverse between the gear and grinding wheel was produced mechanically by 
an accurately ground lead bar and nut mechanism, or a so-called sine bar 
and follower mechanism, such as disclosed in prior U.S. Pat. No. 
3,440,769. This patent incidentally shows means for adjusting the sine bar 
to accomplish the finely controlled rotation of the gear to perform the 
stock dividing action. 
It was also customary in the past to provide index rotation of the work 
gear mechanically, by employing accurately ground index discs with equally 
spaced notches equal to the number of teeth on the gear. The disc was 
connected to rotate with the gear, and the gear could be rotated to 
advance the disc by precisely one tooth indexed space and to be fixed in 
position by a dog or finger fitting within a notch. 
There is now available electrical motor means employing computer numerical 
control (CNC) for rotating the gear to a predetermined programmed or 
computed position which is accurate to a fraction of a second of arc, and 
to rotate the gear from a predetermined position to any other position 
with corresponding accuracy. Thus, indexing may be computer controlled 
with an accuracy not heretofore obtainable. 
In addition, the rotation of the gear in timed relation to axial transverse 
between the gear and a grinding wheel may be controlled by such axial 
traverse. Traverse of the gear or wheel in a direction parallel to the 
axis of the gear is sensed by electrical pick-up means which, through 
proper programming of the computer, operates the motor drive to rotate the 
gear in timed relation to axial traverse to generate the desired helix 
angle. More specifically, the instantaneous angular position of the gear 
is thus related with utmost precision to the relative position of the gear 
and grinding wheel as regards relative traverse in a direction parallel to 
the gear axis. 
Thus, the indexing of the gear, and the timed rotation thereof related to 
relative traverse is accomplished by computer numerical control, and the 
mechanically operable index disc and the lead or sine bar are eliminated. 
In accordance with the present invention, the computer numerical control of 
gear rotation is combined with additional structure to provide an 
automatic gear grinding machine capable of stock division with an accuracy 
and speed not heretofore attainable. 
There is thus provided a novel CNC gear grinding machine characterized by 
reduction of set up time, as well as improved accuracy in stock dividing. 
Since the work gear already has teeth cut thereon, it is necessary to 
locate the grinding wheel in a tooth space such that equal amounts of 
stock will be removed by opposite sides of the wheel. In the past, this 
has been done by the operator in a manual fashion, and the time required 
to do this has been considered a part of the set up time when each work 
gear is mounted on the machine. 
The present invention provides modification of a CNC gear grinder, as above 
described, to provide stock dividing which is automatic, thus resulting in 
a reduced set up time and correspondingly increased productivity. In 
addition, the stock dividing operating is more accurate than heretofore 
possible manually, thus actually reducing the average grinding time. 
Finally, the operation provides for the first time the capability of 
sensing the circumferential angular width of any desired number of tooth 
spaces, so that the average of such widths can be obtained and an average 
proper stock dividing location of the gear for all indexed positions 
thereof thereby determined. 
The foregoing is accomplished by providing a sensitive probe accurately 
located with reference to the location of the grinding wheel movable into 
and out of a tooth space of the gear. In a simple case, the center of the 
probe has the same angular position as the wheel circumferentially of the 
gear. The probe tip may be of the contacting or non-contacting type, and 
excellent results have been obtained using a touch trigger (TT) probe. 
A probe having a ball shaped tip of a size substantially smaller than the 
space between tooth surfaces at opposite sides of a tooth space, is thus 
positioned in the tooth space. The gear is rotated in one direction until 
one tooth flank activates the probe, and the angular position (Oa.degree.) 
of the gear is registered in the computer. The direction of rotation of 
the gear is reversed from position Oa.degree., continued until the flank 
of the adjacent tooth actuates the probe. The angular position of the gear 
(Ob.degree.) at this instant is registered in the computer. The difference 
in the two rotary positions, Oa.degree.-Ob.degree. is computed, and the 
gear reversely driven under computer control through an arc equal to one 
half of this value, at which time the center of the ball tip and hence the 
grinding wheel are centered with respect to the tooth space, and a precise 
stock divided condition is achieved. 
It will be apparent that the location of the probe may be circumferentially 
spaced from the location of the grinding wheel, provided that this 
information is registered in the computer. Similarly, the location of the 
probe axially of the gear is supplied to the computer, so that if the gear 
is a helical gear, the ball may be helically aligned with the wheel. 
In order to make a more precise stock division, a number of determinations 
may be made of the values corresponding to Oa.degree. and Ob.degree. for 
any desired number of tooth spaces. Not only the average value of the 
differences corresponding to Oa.degree.-Ob.degree. is computed, but also 
any variation in the location of the tooth flanks from the theoretical, 
taking into account the highly accurate indexing accomplished by the 
computer controlled gear positioner. 
Grinding is then initiated at the last tooth space probed, and all further 
indexing of the gear, to grind the other teeth, is done on the basis of 
the precise proper angular location of the gear teeth based on average 
values of differences corresponding to Oa.degree.-Ob.degree., as well as 
possible variations in tooth-to-tooth spacing. 
If the probe cannot conveniently be mounted in the plane of the grinding 
wheel, or in helical alignment therewith, then the angular displacement 
between the wheel and the probe will be registered in the computer. Since 
this is a constant value, all stock dividing measurements on the probe 
will be corrected by the constant angular displacement between the probe 
and wheel, but this will be accomplished automatically by the accurate 
indexing which the computer will perform.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, there is illustrated the position of a grinding 
wheel W in a tooth space at a gear G between two gear teeth Ta and Tb. The 
condition of the unground teeth is illustrated in full lines, and the 
position of the trimmed peripheral portion of the wheel is indicated by 
the dotted lines of Wa and Wb. The material of the gear teeth between full 
lines and the dotted lines Wa and Wb represents the stock which will be 
removed from the flanks of the tooth at a single pass of relative traverse 
between the gear G and wheel W in a direction parallel to the axis of the 
gear. Where the depth of the space between the full lines representing the 
underground profiles of the flanks of teeth Ta and Tb, and the dotted 
lines Wa and Wb, respectively is equal at both sides of the tooth space, 
proper stock division is achieved. 
It will be understood that if the gear is a helical gear, the grinding 
wheel will be set around at the helix angle of the gear, and that in 
addition, relative traverse between the gear and wheel axially of the gear 
will be accompanied by relative rotation between the gear and wheel in 
timed relation to traverse to generate the required helix, conveniently by 
rotation of the gear. 
Furthermore, it will be apparent that each tooth space is ground 
separately, so that after each pass, the gear will be indexed in rotation 
to bring another tooth space into alignment with the wheel. 
In the past, stock dividing has been essentially a manual operation during 
step up for each gear. The wheel while rotating was moved radially of the 
gear into the tooth space and the gear adjusted angularly until initial 
contact of the wheel with both tooth surfaces, occurred simultaneously. 
Initial contact was noted by the operator visually from sparks, or by 
sound. Once equal stock division was obtained, the gear was rigidly 
connected to index mechanism, and the gear was indexed after grinding each 
tooth space. 
The automatic stock dividing of the present invention is accomplished by 
inserting the ball tip B of a sensitive probe into the space between 
confronting tooth flanks Fa and Fb. The initial location of the tip B is 
immaterial, but it is illustrated in FIG. 2 as spaced substantially 
equally from both flanks Fa and Fb, or centered on the center line C of 
the tooth space. 
The probe tip B may be in the plane of the grinding wheel, or angularly 
spaced about the axis of the gear G by a known amount. If the gear is 
helical, the tip B is in helical alignment with the grinding wheel. 
Accordingly, when the gear is adjusted into a position such that the ball 
tip B is spaced equally from the tooth flanks Fa and Fb, proper stock 
division as illustrated in FIG. 1 is achieved. 
In accordance with the present invention, the gear is rotated in one 
direction until the probe is actuated by one flank of a tooth, as for 
example, the flank Fa. The probe may be actuated by proximity to the tooth 
flank, or by contact therewith, and such probes are readily available, one 
such being referred to as touch trigger (TT). Actuation of the probe 
signals the instantaneous angular position of the gear Oa.degree. when the 
probe tip is actuated and this position is transmitted to and registered 
in the computer, which is herein considered to be a numerical controlled 
computer (CNC). The computer is connected to control both motor means 
rotating the gear to precisely determined successive positions, and motor 
means for providing relative traverse between the gear and wheel axially 
of the gear into a succession of precisely determined relative positions. 
Actuation of the probe not only stores the instantaneous position of the 
gear, Oa.degree., but by computer control, also reverses the direction of 
gear rotation, which continues until the probe is actuated by the other 
tooth flank Fb. This determines a second gear position, Ob.degree., which 
is transmitted to the computer, which is programmed to determine the 
angular displacement represented by the difference between Oa.degree. and 
Ob.degree.. In the simplest case, the computer determines one half of this 
difference, and again reverses the direction of rotation of the gear and 
controls its motor drive to cause the gear to move through an angular 
distance of one half the arc Ob.degree.-Oa.degree., and stop. At this time 
the angular position of the gear is then 
Ob.degree.-(Ob.degree.-Oa.degree.)/2, designated Oc.degree., which 
represents a true stock divided position, based only on probe determined 
positions Oa.degree. and Ob.degree. in a single tooth space. 
The grinding wheel will now be presented to the tooth space between flanks 
Fa and Fb, and fed to a proper depth and thereafter relative traverse is 
provided by computer control of the traverse motor drive means, together 
with computer control of the rotation of the gear by the rotary drive 
means, if the gear is a helical gear. 
The grinding of the tooth flanks Fa and Fb is completed by one or more 
successive passes or traverse strokes, and radial feed between successive 
traverse strokes, as well as final depth of feed, is preferably 
accomplished by a feed motor drive controlled by the computer. 
Referring now to FIG. 3, there is a diagrammatic illustration of the 
essential components of the grinder with the stock divider. 
The base 10 has a horizontally movable slide or table 12 therein. On the 
slide is mounted a headstock 14 in which is mounted a motor 16 having a 
drive shaft 18 connected in driving relation to the shaft of a work gear 
20, here shown a helical. The shaft 18 has sensing means 22 responsive to 
the angular position of shaft 18 and gear 20. 
The slide 12 is traversed by a motor 24 through a traverse drive 26, which 
includes means (not shown) sensing the instantaneous position of table 
axially of the gear 20. The CNC system thus senses the instantaneous 
angular position of the gear 20 as well as its axial position, and is 
programmed to relate the two, thus providing for helical advance of teeth 
and tooth spaces of the gear. For a spur gear, of course, motor 16 holds 
the gear against rotation as table 12 is traversed. 
The grinding wheel 30 is vertically adjustable on a head 32 and is 
adjustable about a vertical axis radial of the gear to align the plane of 
the wheel with the tooth space being ground. The wheel is driven at 
grinding speed by a motor 34. 
The head 32 also carries a vertically adjustable probe 26 having a probe 
tip 38, which may be spherical and have a sensitive tip operated by actual 
contact with a tooth surface or by close approach to a tooth surface. 
Either type of tip may be broadly referred to as being actuated by 
proximity of the tip to a tooth surface. In a simple case, the tip 38 may 
be in the plane containing the vertical axis of adjustment of the wheel 30 
and the axis of the gear. This is not required, however, and the tip may 
be angularly displaced from this plane by a known amount. This 
displacement is programmed into the CNC system, so that when the tip is 
centered between the tooth flanks, the wheel 30 will be similarly centered 
when brought into operating position. 
The grinding operation is carried out by locating the gear in rotation for 
accurate stock division as above described followed by feeding the head 
into position to insert the wheel into stock-dividing position in a tooth 
space. Thereafter the table 12 is traversed in one or more strokes, while 
the head is fed incrementally between strokes to full depth. The head is 
then moved radially of the gear to withdraw the wheel from a tooth space, 
the gear indexed and the grinding operation repeated until all teeth have 
been around. 
The CNC control system for operating motor 16 both for stock division, lead 
control, and indexing is commercially available as is the touch trigger 
(TT) tip. 
FIG. 4 shows a schematic representation of the gear locator and stock 
divider hardware. It consists of the TT probe whose output is read by the 
numerical control computer after being processed by the signal 
conditioning electronics. The computer also controls the angular position 
of the gear for stock dividing and grinding purposes through the headstock 
servo control and the table, for gear locating and other purposes, through 
the table servo control. 
Referring to FIG. 5, which shows a flow chart of the software, when the 
gear location and stock divide function is initiated the probe is moved to 
a specified location to start the process. This starting position is 
determined automatically by the computer based on the dimensions of the 
gear that have already been communicated to it. It then moves the table 
till the TT probe senses the gear face and this position is registered in 
the computer. 
Then the probe is inserted in the space between two consecutive teeth on 
the gear. The gear is then rotated in clockwise and counter-clockwise 
directions to sense the left (Fa) and right (Fb) flanks of a tooth space. 
The angular position O.degree.A and O.degree.B are recorded and the 
average computed. If stock dividing is to be accomplished based on only 
one tooth space the "done" block is satisfied with a "Y" answer and the 
machine automatically moves to the grind position and stops for the 
grinding cycle to be initiated. If the stock dividing has to be 
accomplished on the basis of more than one tooth space, then, as FIG. 5 
illustrates, the gear is moved to the next probing position and the loop 
is followed through again. 
It will be recalled that index rotation of the gear between grinding of 
successive tooth spaces is in this grinder accomplished by the rotary 
drive motor controlled by the computer, so that having determined in 
effect the center line of one tooth space, indexing provides for grinding 
of all tooth spaces to accurately indexed and stock divided positions. 
However, there remains the possibility that variations may exist in the 
angular width of the several tooth spaces. Accordingly, the computer may 
be programmed to determine the angular width of any desired number of 
tooth spaces, and averaging these to provide a stock dividing operation 
which is based on these average values. This is readily accomplished by 
programming the computer to withdraw the probe from the tooth spaces 
following each operation, indexing the gear to one or more additional 
selected positions, reinserting the probe, and determining additional 
values of angular width of additional spaces. The average of these values 
is used to control the amount of reverse rotation from the position 
occupied by the gear following completion of the last width measuring 
operation. 
There still remains the possibility of error due to minor variations in 
tooth-to-tooth spacing. This may be determined and taken into account in 
the stock dividing operation, by programming the computer to compare the 
values of the successive angular positions occupied by the gear at the 
initial and/or second (and preferably the second) actuation of the probe 
in successive tooth spaces with the initial and/or second actuation 
thereof, respectively, in the initial tooth space. If tooth-to-tooth 
spacing is accurate, these successive values will be equal to the initial 
gear position plus an angular increment equal to tooth-to-tooth index 
rotation times the number of tooth spaces from the originally tested tooth 
space. The average of the deviations from the gear position determined at 
the initial operation is then applied as a correction in the amount of 
reverse rotation from the position occupied at the second probe actuation 
in the final gear tooth space checked. 
From the foregoing, it will be seen that the simplest operation is to 
determine by probing a single tooth space, the accurate stock divided 
position of this tooth space of the gear relative to the gear wheel. The 
center line of this tooth space is then centered with respect to the 
wheel, and successive tooth spaces are ground with the gear indexed 
accurately from this initial stock divided position. 
A first modification is to determine and use the average effective widths 
of a plurality or all of the tooth space to determine angular reverse 
movement of the gear into initial stock divided position from the position 
occupied by the gear at the conclusion of the last probe operation. 
A second modification is to determine a corrected theoretical position of 
the gear from which to apply the corrected average one half effective 
tooth space width, by computing average indexed tooth positions from an 
initial position, and basing on the reverse rotation of the gear from its 
final probed position on this averaged computed position rather than its 
actual position. 
Thus, in a simple case, the axis of the gear is horizontal, the axis about 
which the wheel is adjustable is vertical and intersects the gear axis. 
The center of the sensitive tip B occupying the vertical plane contains 
both the gear axis and wheel axis. If the gear is a spur gear, stock 
division by the mechanism described is effective to insure proper stock 
division between the wheel and gear as the gear is traversed to pass 
beneath the wheel. If the gear is helical, the wheel is set around at the 
proper helix angle. The distance between the vertical axis of adjustment 
of the wheel and the center of the ball tip B is known and stored in the 
computer. The ball tip is centered in stock dividing relation in a tooth 
space at the top of the gear, the rotational position of the gear is 
observed, and the tip B is withdrawn, by raising it or by traverse of the 
gear toward the wheel. The correction to the rotational position of the 
gear required by the helix angle of the gear and the distance between the 
center of the top and the vertical axis of adjustment of the wheel is 
computed in the CNC control and made by rotation of the gear without 
horizontal traverse. Thereafter, the gear is traversed beneath the wheel 
with appropriate rotation, incremental wheel depth feed, and automatic 
indexing as is known in the art.