Note: Descriptions are shown in the official language in which they were submitted.
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TITLE: STOCK DIVIDER INCL~DING A COMPUTER
CONTRO~LED GEAR ~OCATOR
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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 reguired 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 proqressively 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 re~uirement. 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 i5 properly ground, and in some cases the
hardened case is removed from one tooth surface.
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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. patent 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, inde~ing 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
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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 i5 combined with additional
structuxe 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, thi~ ha~ 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 increa~ed 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 obtaine~ 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 ~haped 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 tOa~) of the gear is
registered in the computer. The direction of rotation of the
gear is reversed from position Oa, continued until the flank
of the adjacent tooth actuates the probe. The angular position
of the gear (Ob~ at this instant is registered in the computer.
The difference in the two rotary positions, Oa-Ob 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.
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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 and Ob for any desired number of tooth spaces. Not only
the average value of the differences corresponding to Oa-Ob~
is computed, but also any variation in the location of the tooth
flan~s 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 ~ear, 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-Ob, 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.
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Brief Descri~tion of the Drawings
Figure 1 is a diagrammatic view illustrating stock
division.
Figure 2 is a diagrammatic view showing the probe tip
in stock dividing position.
Figure 3 is a simplified elevational view of the
grinder.
Figure 4 is a schematic representation of the stock
divider hardware.
Figure 5 is a flow chart representation of the stock
divider software.
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Description of the Preferred Embodiment
Referring first to Figure 1, there i5 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. Whera 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 ~pace 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 th~ tooth space
and the gear adjusted angularly until initial contact of the
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wheelwith bo~h 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 ~ 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 Figure 2 as spaced substantially equally ~rom 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 Figure 1 is achieved.
In accordance with the present invention, the gear
is rotated in one direction until the probe is actuated by one
~lank 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 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 khe 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, 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 r Ob, which is transmitted
to the computer, which is programmed to determine the angular
displacement represented by the difference between Oa and Ob.
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-Oa, and
stop. At this time the angular position of the gear is then
Ob- Ob-Oa, designated Oc, which represents a true stock
2 divided position, based only on probe determined
positions Oa and Ob 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 co~puter control
of the traversemotor drive means, together with computer control
of the rotaticn 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 traver~e 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.
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Referring now to Figure 3, there is a diagrammatic
illustration of the essential components of the grinder with
the stock divider.
`~ The base 10 has a horiæontally 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
36 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. ~his is not required, however, and
the tip may be angularly displaced from this plane by a known
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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.
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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 stro~es ~o 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.
Figure 4 shows a schematic representation of the gear
locator and stock divider hardware. It consists of the TT probe
who~e 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 Figure 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 until
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~A and oB 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 ~o 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 Figure 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
o~ 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 anydesired number of tooth ~paces, and averaging
these to provide a stock dividing operation which is based on
these average va~ues. 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
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amount of reverse rotation from the position occupied by the
` gear following completion of the last width measuring operation.
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-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 toGth
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.
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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 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 i5 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 hall tip B is known and stored in the computer.
The ball tip is centered in stoc~ dividing relation in a tooth
space at the top of the gear, the rotational position of the
gear i5 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 heli~ 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 i3 traversed beneath the wheel
with appropriate rotation, incremental wheel depth feed, and
automatic indexing as is known in the art.
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