Method of grinding a workpiece having plural cylindrical portions with plural grinding wheels

A method of grinding plural journal portions of a workpiece with plural grinding wheels. In the method, a pair of work rests are used to support two selected journal portions, and the selected two journal portions have a larger initial diameter as compared with the rest of the journal portions. Initially, the selected journal portions are ground without using the work rests. When the diameter of the selected journal portions becomes substantially equal to the diameters of the rest of the journal portions, the work rests are advanced to support the selected journal portions. After that, the grinding wheels are again advanced to grind all the journal portions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of grinding a workpiece having plural 
cylindrical portions to be ground, and more specifically, relates to a 
method of grinding a workpiece having plural cylindrical portions with 
plural grinding wheels, wherein the plural cylindrical portions of the 
workpiece are ground using a work rest for supporting one or more 
cylindrical portions. 
2. Prior Art of the Invention 
In general, a grinding machine having plural grinding wheels 1A-1F is used 
to simultaneously grind plural journal portions 2A-2F of a workpiece 2, as 
shown in FIG. 1. In such grinding machine, the workpiece 2 is deformed, as 
shown in FIG. 1, when all of the grinding wheels 1A-1F contact the journal 
portions 2A-2F of the workpiece 2, because of the large grinding force 
acting upon the workpiece 2. Therefore, in such grinding machine, work 
rests 3 are installed to support one or more journal portions, for 
example, the two journal portions, 2B and 2E. 
However, in such grinding machine, the grinding accuracy is deteriorated 
when the journal portions 2B and 2E to be supported by the work rests 3 
have been machined inaccurately in previous machining such as the cutting 
operation using a lathe. Namely, if the journal portions 2B and 2E have 
distorted outer surfaces, the positions of the center of the journal 
portions 2B and 2E irregularly changes, because of the contact between the 
distorted outer surfaces of the journal portions 2B and 2E and the work 
rests 3 during rotation. 
SUMMARY OF THE INVENTION 
Accordingly, it is an objective of the present invention to provide an 
improved method of grinding a workpiece having plural cylindrical portions 
to be simultaneously ground with plural grinding wheels, wherein the 
cylindrical portions are accurately ground without being affected by the 
initial accuracy of the cylindrical portions. 
Briefly, the present invention provides a method of grinding plural 
cylindrical portions of a workpiece with plural grinding wheels. In this 
method, a workpiece having plural cylindrical portions previously machined 
with some cylindrical portions having a larger diameter compared to the 
rest of the cylindrical portions can be accurately ground despite the 
previous inaccurate machining of the cylindrical portions. A work rest 
having a contact shoe engagable with selected cylindrical portion is also 
used in this method. Initially, the contact shoe is positioned at a 
position away from the selected cylindrical portion. Then the grinding 
wheels is advanced in a first grinding step to partially grind the 
selected cylindrical portion without using the work rest. The contact shoe 
is then advanced to support the selected cylindrical portion, which 
already has been partially ground, and the grinding wheels are further 
advanced in a second grinding step to grind all of the cylindrical 
portions to a predetermined dimension. 
Since the cylindrical portion having a larger diameter is initially ground 
without engaging the work rest, the selected cylindrical portion can be 
ground very accurately regardless of the initial accuracy of the selected 
cylindrical portion. Further, all the cylindrical portions are accurately 
ground, because the selected cylindrical portion supported by the work 
rest has been ground to have an accurate outer surface. Therefore, all of 
the cylindrical portions can be accurately ground regardless of the 
initial accuracy of the cylindrical portions.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
A preferred embodiment of the present invention will be now described with 
reference to drawings. In FIG. 2, numeral 10 indicates a bed of a grinding 
machine on which a spindle head 11 and a tail stock 12 are mounted. The 
spindle head 11 is provided with a main spindle 111 having a center 112, 
and the main spindle 111 is rotated by a motor 113. The tail stock 12 is 
also provided with a center 121. Supported between the centers 112 and 121 
is a workpiece W having plural cylindrical journal portions Wa-Wf which 
are separately formed in the axial direction of the workpiece W. 
A wheel head 14 is mounted on the bed 10 for movement in a direction 
perpendicular to the rotational axis of the main spindle 111. The wheel 
head 14 is moved by a servomotor 15 through a not-shown feed mechanism, 
and supports plural grinding wheels Ga-Gf through a wheel spindle 141. The 
wheel spindle 141 is rotated by a motor 142 through pulleys and belts. 
A pair of work rests 16 are also mounted on the bed 10 at locations 
corresponding to two selected journal portions Wb and We. Each of the work 
rests 16 is provided with a side contact shoe 161, which is engagable with 
each side portion of the journal portions Wb and We, and a lower contact 
shoe 162, which is engagable with each lower portion of the journal 
portions Wb and We, as shown in FIG. 3. The side contact shoe 161 and 
lower contact shoe 162 are moved back and forth, with respect to the 
rotational axis of the main spindle 111, by servomotors 163 and 164, 
respectively. 
Further, the grinding machine is provided with a measuring device 17 for 
measuring the diameter of a selected journal portion, for example, journal 
portion Wa, and outputs signals when the diameter of the journal portion 
Wa reaches plural reference diameters at which the feed rate of the wheel 
head 14 will be changed. The output signals from the measuring device 17 
are lead to a numerical controller 18. The numerical controller 18 is 
provided with a memory (not shown) in which numerical control data is 
stored. The servomotors 15, 163 and 164 are driven in accordance with the 
numerical control data and the signals from the measuring device 17. 
The grinding method according to the present invention will be now 
described with reference to FIGS. 4, 5 and 6. 
For illustration, in a previous machining operation, such as a lathe 
cutting operation, the workpiece W has been machined, resulting in the 
journal portions Wb and We having a larger diameter than the other journal 
portions Wa, Wc, Wd and Wf, as shown in FIG. 2. It is preferred that a 
size difference between the selected journal portions Wb and We, and other 
journal portions Wa, Wc, Wd and Wf be adjusted to be larger than the sum 
of the amount of distortion of the outer surfaces of the journal portions 
after cutting operation and the amount of deflection of the rotational 
centers of the journal portions from the rotational axis of the workpiece 
W. 
After the workpiece W is loaded onto the grinding machine and supported 
between the spindle head 11 and the tail stock 12, the motor 113 is 
activated to rotate the main spindle 111. Since the rotation of the main 
spindle 111 is transmitted to the workpiece W through a rotational torque 
transmission mechanism (not shown), the workpiece W is rotated at a 
predetermined speed. 
After that, a grinding cycle shown in FIG. 4 is started. The wheel head 14 
is first advanced at a rapid feed rate. At the same time, the side contact 
shoes 161 and lower contact shoes 162 of the work rests 16 are advanced at 
a rapid feed rate. However, the contact shoes 161 and 162 do not contact 
the respective journal portions Wb and We, even after the rapid advance of 
the contact shoes 161 and 162. 
The wheel head 14 is further advanced by an amount DO at an air-cut feed 
rate F0. After that, a first grinding step is carried out. Namely, the 
wheel head 14 is advanced by an amount D1 at a feed rate FI for first 
rough grinding. By the movement in the first grinding step, only the 
selected journal portions Wb and We are ground by the grinding wheels Gb 
and Ge. The amount D1 is adjusted so that the advance movement is stopped 
when the diameters of the selected journal portions Wb and We become 
substantially equal to the initial diameters of the other journal portions 
Wa, Wc, Wd and Wf. After that, the wheel head 14 is temporarily stopped 
for a predetermined time, and then retracted by a small distance in a 
back-up step. After the above operation, the selected journal portions Wb 
and We have diameters similar to those of other journal portions Wa, Wc, 
We and Wf. 
Since the selected journal portions Wb and We are ground without using the 
work rests 16, the selected journal portions Wb and We can be ground 
accurately without being affected by the initial accuracy, namely, the 
machining accuracy of the previous machining operation. Since only two 
journal portions are ground in the above-mentioned first grinding step, 
the amount of defection of the workpiece W due to the grinding force is 
relatively small as compared with the case where all of the grinding 
wheels Ga - Gf contact all the journal portions Wa -Wf. Therefore, the 
journal portions Wb and We can be ground accurately. 
After the back-up step, a second grinding step is carried out. In the 
second grinding step, the contact shoes 161 and 162 are firstly advanced 
toward the rotational axis of the workpiece W by a predetermined amount. 
In the course of this operation, the contact shoes 161 and 162 contact the 
selected journal portions Wb and We. After the advance movement of the 
contact shoes 161 and 162, the selected journal portions Wb and We are 
bent toward the grinding heels Gb and Ge by a predetermined amount. After 
that, the wheel head 14 is advanced at a feed rate F2 for second rough 
grinding until a first signal is output from the measuring device 17, and 
the contact shoes 161 and 162 are advanced again by a predetermined 
amount. After that, the wheel head 14 is advance at a feed rate F3 for 
finish grinding until a second signal is output from the measuring device 
17, and then retracted to the retracted position after a spark-out 
grinding of a predetermined period of time. The contact shoes 161 and 162 
are also retracted to their respective retracted positions. 
Since all the journal portions Wa - Wf are around under the condition that 
the contacts shoes 161 and 162 contact the journal portions Wb and We 
which have been previously ground to have accurate cylindrical surfaces, 
it becomes possible to prevent the journal portions Wb and We from 
irregularly changing their position with respect to the rotational axis of 
the main spindle 111. Therefore, all the journal portions Wa - Wf can be 
accurately ground to a desired final diameter. 
Although, the grinding machine of the above embodiment is provided with a 
pair of work rests, the present invention can be applied to grinding 
machines which have a single work rest, or three or more work rests. The 
number of the work rests may be changed depending on the number of journal 
portions to be ground. 
In the above-described embodiment, the selected journal portions are ground 
in the first grinding step to have a dimension substantially equal to the 
initial dimension of the rest of the journal portions. However, the 
grinding cycle may be modified such that the advance movement of the wheel 
head is stopped and the advance movement of the contact shoes is started 
when the diameter of the selected journal portions reaches a predetermined 
diameter larger than the initial diameter of the rest of the journal 
portions. 
Further, the grinding cycle may be modified to start the advance movement 
of the contact shoes without stopping the advance movement of the wheel 
head. In this case, the first rough grinding and second rough grinding are 
carried out continuously. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the present 
invention may be practiced otherwise than as specifically described 
herein.