Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~27647~
Title: OPTIMIZATION METHOD AND APPARATUS
FOR DRESSING A GRINDING WHEEL
This invention relates generally to the profiling of
grinding wheels and refers more particularly to an optimization
technique for profiling a grinding wheel of the type used in
form grinding.
BACKGROI)ND AND S~5MARY OF THE INVENTION
~ Dressing" is the word used to describe the process
of profiling a grinding -~heel of the type used in a ~form
grinding~ operation. Dressing is necessary because in form
grinding the grinding wheel i required to have the reverse
form of what i5 required on the workpiece. The process of
grinding then transfers the required form unto the workpiece.
~ he process of ~form grinding~ is used in the
manufacture of a large variety of accurate and complex shapes
in the metal working industry. It is al80 used to manufacture
precision spur and helical gears.
The dressing of the grinding wheel is carried out by
means of a rapid~y rotating metallic wheel, called a dre~sing
wheel. While the grinding wheel and the dressing wheel are
rotated, the dressing wheel is guided through a predetermined
path corresponding to the desired form to be imparted to the
periphery of the grinding wheel. As the dressing wheel moves
throuqh the defined path successively in identical cycles of
motion, the grinding wheel i8 fed into it in equal increments
between cycles. Conversely, the dressing wheel ~ay be m~de to
execute subsequen~ paths with an incremPntal shift towards the
grinding wheel between successive cycle~ of ~otion.
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There are two distinct problems involved in dressing
or profiling a grinding wheel which can be described as the
"initial dress probl~m~ and the "redress problem~.
INITIAL DRESS PROBLEM
In most instances, grinding wheels are available
initially as cylindrical discs. In carrying out the dressing
operation, in which the dressing wheel executes the defined
path repeatedly in successive cycles and the grinding wheel is
gradually and incrementally fed into the path of the dressing
wheel, it is obvious that at first only the corners of the
grinding wheel will be affected. Gradually as the grinding
wheel is fed farther into the path of the dressing wheel, layer
upon layer of the grinding wheel will be removed until the
entire profile is complete. Although the incremental movement
of the grinding wheel is carried out automatically, the instant
at which the required form has been achieved on the grinding
wheel i8 a judgment call and requires a skilled operator. If
the dressin~ process is stopped too soon, the form on the
grinding wheel will be incomplete or imperfect. If the process
is stopped too late, unnecessary loss of grinding wheel material
results, with accompanying 108s of time.
REDRESS PROBLEM
After the initial dress, the grinding wheel is put
into use. As the grinding proceeds the wheel starts to wear.
Owing to unevenneYs in grinding wheel and workpiece
characteristics, uneven wear on the grinding wheel i8 almogt
always the result. Af~er grinding for a while it therefore
becomes necessary to redress the grinding wheel.
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The redress of the qrinding wheel is carried out
similarly t~ the initial dress. The high spots will be removed
first and after several cycles the proper form will be returned
to the qrinding wheel periphery. However, once a~ain, the
instant at which dressiny is complete is a judgment call.
PROPOSED SOLUTION
The proposed solution for both of these problems i5
based on the concept that since the dressing mechanism always
executes identical cycles of motion and the wheel is fed into it
in equal increments, non-uniform but increasing contact duration
will result between the grinding wheel and the dressing wheel.
In the initial dress situation, only the corners of the wheel
are first in contact. As the grinding wheel is fed into the path
of the dressing wheel, more and more contact and consequently
an increasing duration of contact is generated. In the redress
situation, initially only the high points are in contact until
gradually the entire form is in contact. However, in both
instances, once the correct profile has been duplicated on the
grinding wheel, if dressing is allowed to continue, there i5
no increase in the duration of contact.
In accordance with the present invention, means are
provided for monitoring the duration of cont~ct during each
cycle. The difference between the duration of contact in each
cycle and the duration of contact in the preceding cycle i8
determined. When this difference becomes negligible, the
dressing operation is complete.
More ~pecifically, the current drawn by the motor
used to drive the dressing wheel is monitored with respect to
time as a base. When there is no contact between the dressing
wheel and the grinding wheel, the current that is required to
keep the spindle running is constant (idle current). As the
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contact time between the dressing wheel and the grinding wheel
increases, ~he duration of the higher current draw increases
and stabilizes only when a full form condition has been reached.
A current - time curve for each cycle represents the duration
of contact. Integration yields the area under the current
- time curve. This may then be compared to the previous cycle
and if a significant difference exists, dressing is continued.
When the difference becomes negligible, dressing is stopped.
These and other objects of the invention will become
more apparent as the following description proceeds, especially
when considered with the accompanying drawings.
BRIEF DESCRIPTION OF THE D~AWINGS
Fig. 1 is a fragmentary view illustrating a grinding
wheel with its formed periphery in the space between the teeth of
a gear.
Fig. 2 is an enlarged fragmentary view of the grindinq
wheel shown in its relation~hip to the dressing ~echani~m and
indicating the defined path of the dress wheel in profiling the
periphery of the grinding wheel.
Fig. 3 ls a diagrammatic view showing the grinding
and dre~ 8 ing mechani~ms.
Fig. 4 is a fragmentary view of a grinding wheel
showing the stages in the removal of layers of grinding wheel
material during sycling of the dresser, in an initial dressing
operation.
Fig. 5 ~hows a worn wheel, illustrating also the
redressed configuration and the defined path of the dreq er
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wheel. The unevenness of the worn wheel is shown exaggerated
for clarity~
Fig. 6 illustrates the curren~-time curve in each of
a sequence of cycles of the dresser wheel.
Fig. 7 shows the hardware system.
Fig. 8 ~hows a software flow chart.
DETAILED DESCRIPTION
Referring now more particularly to the drawings, Fig.
1 shows a circular grinding wheel 10 having a peripheral profile
which has the exact reverse form as the space between two
adjacent teeth on the workpiece 12, which in this instance is
a gear, that has just been ground. In other words, it should
be understood that the grinding wheel in Fig. 1 ha~ witbdrawn
from grinding contact with the gear after completing thegrinding
process in which two adjacent flanks of the gear teeth and the
root have been ground.
The dressing of the grinding wheel, which generally
consists of abrasive~ held in a bond, is generally carried out
as shown in Figure 3. The abrasive wheel i9 profiled accurately
using a rapidly rotating metallic dressing wheel 14 which may
have embedded in it particles of diamond or other hard material3
such as Cubic Boron Nitride. While the grinding wheel and the
dressing wheel are rotating in a specified relationshlp, the
dressing wheel is made to execute motions in the direction of
the arrows along a defined path in the VW plane (Fig. 2), to
generate the desired form on the grinding wheel. The defined
or predetermined path is indicated at 16. This i~ generally
accomplished by having the motor and spindle carrying the
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dressing wheel on a mechanism 18 that has two linear axes of
motion ( v ~nd w directions) under the control of a computer
numerical control (CNC) system. As shown in Figure 3, the
dressing wheel spindle 20 is driven by an electric motor 22
carried on a base 24 reciprocable horizontally in ways 26 of
the support 28 by means of a motor and ball screw drive 30.
The support 28 is movable vertically on ways 32 hy means of the
motor and ball screw drive 34. The grinding wheel spindle 36
is dri~en by a motor 40 vertically reciprocable in ways 42 on
the column 44 by means of the motor and ball screw drive 46.
The coordinates V and W describing the motion of these
two linear axes carried out by the mechanism 18 will already
have been generated mathematically since most forms required
in metal working can be described mathematically. In gear
grinding, the form required on the two flanks of the grinding
wheel are generally described by Involutometry and the part
of the wheel that grinds the root between the two teeth is
generally a simple shape such as a radius, etc. The path 16
typically undertaken by the dressing wheel during the process
of dressing the grinding wheel in form gear grinding is shown
in Figure 2. The dressing mechanism causes the dressing wheel
to execute the defined path in repeated cycles, and the grinding
wheel is fed into the defined path nlong the Y or verticnl axis.
Conversely, the drescing mechanism 18 may be sade to execute
the defined path with incremental upward shifts towards the
grinding wheel.
Figure 4 shows the grinding wheel 10 in its initial
cylindrical fonm an~ also ~hows the dres~ed profile ~t 50. The
area between the corners and the dressed profile is the material
that is remoYed in dressing. When thegrinding wheel is gradually
and incrementally fed into the path of the dressing wheel, it
should be clear that at first only the cornersl marked
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A on Figure 4, will be removed by the dressing wheel. Gradually,
increasing laYers of grinding material will be removed in
succeeding cycles until the entire dressed profile, indicated
at 5Q, is complete. If the dressing process is stopped too
soon, the form on the grindir.g wheel will have an intermediate
shape (marked ~ in Figure 4) which would grind an unsatisfactory
gear tooth. If the process is stopped too late, the profile C
will result, and although it is a correct profile, there has
been an unnecessary loss of grinding wheel material and
accompanying loss of time.
In the past, the determination of when dressing is
complete has required the presence of a skilled human operator
who listens for the complete contact between the dressing wheel
and the grinding wheel. In the automatic unmanned factories
of tomorrow, this will not be acceptable.
In accordance with the present invention, the solution
to the problem is based on the concept that since the dressing
mechanism always executes identical cycles of motion and the
grinding wheel is fed into the path of the drescing wheel in
equal increments, non-uniform but increasing contact duration
will result between the grinding wheel and the dressing wheel.
ln the initial dress situation as illustrated in Figure 4, where
only the corners A are first in contact, as the grinding wheel
is fed into the path of the dressing wheel there iB more and
more contact and consequently an increasing duration of contact
with each cycle. In redres~ing ac shown in Figure 5, the
situation is virtually the same with the high points of the
material 52 to be removed first coming in contact and then
gradually increasing contact in subsequent cycles until the
entire form i5 in contact and the redressed profile 50 is
achieved .
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In both situations, that is initial dress and redress,
once the correct profile has been duplicated on the grinding
wheel, if grinding is allowed to continue there is no increase
in the duration of contact. Identical layers of the wheel will
be removed which is both wasteful and unnecessary.
The dressing wheel is mounted on a spindle 20 driven
by an electric motor 22. The current drawn by motor 22 is, in
accordance with this invention, monitored with respect to time
as a base during continuing and repeated cycles. The change
in current drawn is illustrated in the sequence of current-time
diagrams shown in Figure 6. When there is no contact (cycle
1), the current is constant and only that current is drawn which
is required to keep the spindle running (idle current). As
contact occurs between the dressing wheel and the grinding
wheel, the duration of the higher current draw in subsequent
cycles increases until it stabilizes at cycles 10 and 11. Full
form condition has been then accomplished. A similar
characteristic will also be observed in the case of the redress
cycle except that initial contacts are somewhat more random.
The ba9 iC concept of the optimization technique of
this invention is to integrate the current - time curve of the
dressing spindle motor 22 during the execution of each complete
dressing cycle, a8 shown in Figure 6. The integr~tion i8 done
by computer and yields the area under the current - time curve.
Thi~ is then compared in the computer to the area of the previous
cycle and if ~ difference exist~ with the value ln the present
cycle being significantly larger than the value in the next
previous cycle, dressing i~ continued. If the difference i~
zero or negligible, dres3ing can stop and the grinding wheel
is ready again for service.
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Figure 7 shows the essential hardware used in the
practice o~ this invention. The motor 22 for driving the
dressing wheel may be either an AC or a DC motor. The power
to drive the motor 22 comes from the spindle drive 60 which must
of course be compatible with the motor. The spindle drive puts
out a voltage which, for monitoring purposes, is proportional
to the current being drawn by the motor. This voltage signal
is then fed to a computer, in this instance a CNC system.
Specifically, the voltage signal is read through an analog to
digital converter by the CNC system controlling the machine.
The software in the system is shown in Figure 8.
Fiqure 8 shows a very high level flow chart. The voltage from
the spindle drive is measured and integrated to obtain the area
under the current - time curve for one dress cycle. This is
then compared with the previous value, that is, the area under
the curve for the previous cycle. If there is a difference,
the present value is stored as a previou~ value in the computer
and the dres3ing cycle is continued with a new present value
being obtained. If no difference, or only a negligible
difference, is detected, the previous value is ~et to zero to
allow this algorithm to be carried out the next time and the
dres~ing operation i9 ended.
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