Method and machine for grinding

A machine for grinding a cylindrical surface and the end surface of a shoulder portion, both formed on a workpiece, using an angular grinding wheel. The end surface is perpendicular to the cylindrical surface. The grinding wheel has a first outer surface parallel to the axis of the workpiece and a second outer surface perpendicular to the first outer surface. The first outer surface has a cylinder-grinding parallel surface and a cylinder-grinding tilted surface. The parallel surface has a generatrix parallel to the generatrix of the cylindrical surface to be ground. The tilted surface is continuous with the parallel surface and has a generatrix tilted away from the generatrix of the cylindrical surface. The second outer surface has a shoulder-grinding parallel surface and a shoulder-grinding tilted surface continuous with this parallel surface. The shoulder-grinding parallel surface has a generatrix parallel to the end surface of the shoulder portion. The shoulder-grinding tilted surface has a generatrix tilted away from the end surface of the shoulder portion. The grinding wheel is fed into the shoulder portion from a given position located radially inside the end surface. Then, the wheel is moved radially outwardly to grind the end surface. Subsequently, the wheel is moved relative to the workpiece along the generatrix of the cylindrical surface away from the end surface to grind the cylindrical surface.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method and a machine for grinding 
cylindrical surfaces of workpieces with a circular grinding wheel. 
2. Description of the Prior Art 
A conventional angular grinding wheel is shown in FIG. 1, where a workpiece 
W has a cylindrical surface and a shoulder portion to be ground. The 
grinding wheel has a cylinder-grinding surface 1 and a shoulder-grinding 
surface 2 perpendicular to the cylinder-grinding surface 1 whose 
generatrix is parallel to the generatrix of the cylindrical surface to be 
ground. The shoulder-grinding surface 2 grinds the shoulder portion of the 
workpiece. Where the cylindrical surface is ground with this angular 
grinding wheel, the wheel is moved toward the central line of the 
workpiece W in a direction intersecting the cylindrical surface so that 
the wheel may be fed into the workpiece. Then, the wheel is moved relative 
to the workpiece along the generatrix of the cylindrical surface. As a 
result, the cylindrical surface of the workpiece W is machined with the 
cylinder-grinding surface 1 of the angular grinding wheel by traverse 
grinding. 
Since the cylinder-grinding surface 1 of this grinding wheel is completely 
parallel to the generatrix of the ground cylindrical surface, the front 
edge E of the grinding surface 1 as viewed in the direction of movement as 
shown in FIG. 1 is worn too quickly. 
In the conventional traverse grinding, it is difficult to obtain a desired 
surface finish and quite accurate dimensional tolerances if the grinding 
wheel traverses the cylindrical surface of the workpiece only once. 
Therefore, three machining steps, i.e., rough grinding, accurate grinding, 
and finishing grinding, are normally needed. Consequently, it is 
impossible to machine the cylindrical surface in a short time. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method and a machine 
which prevent the grinding surface of the used grinding wheel from being 
worn locally. 
It is another object of the invention to provide a method and a machine 
which can complete the grinding of the cylindrical surface of a workpiece 
by moving a grinding wheel along the cylindrical surface of the workpiece 
only once. 
In brief, in accordance with the present invention, a circular grinding 
wheel is used which has a cylinder-grinding surface comprising a parallel 
grinding surface and a tilted grinding surface. The parallel grinding 
surface has a generatrix parallel to the generatrix of the cylindrical 
surface of a workpiece to be ground. The tilted grinding surface is 
continuous with the parallel grinding surface and has a generatrix tilted 
away from the generatrix of the cylindrical surface. This circular 
grinding wheel is moved relative to the workpiece toward the central axis 
of the workpiece in a direction intersecting the cylindrical surface over 
a distance corresponding to the grinding allowance. Then, the cylindrical 
surface is machined by the tilted grinding surface. Subsequently, the 
grinding wheel is moved relative to the workpiece along the generatrix of 
the cylindrical surface in such a direction that the cylindrical surface 
is ground by the parallel grinding surface. Thus, the cylindrical surface 
is machined accurately. As a result, desired dimensions are obtained. 
In the above-described method according to the invention, the tilted 
grinding surface makes no local contact with the cylindrical surface of 
the workpiece. Hence, the grinding wheel is prevented from wearing down 
locally too quickly. After the tilted grinding surface grinds the 
workpiece, the parallel grinding surface continuous with the tilted 
grinding surface performs a finishing grinding operation on the 
cylindrical surface. In consequence, no separate finishing grinding 
operation is needed. Also, the machining efficiency can be enhanced. 
A grinding machine according to the invention comprises a circular grinding 
wheel having a parallel grinding surface and a tilted grinding surface 
continuous with the parallel grinding surface. This parallel grinding 
surface has a generatrix parallel to the generatrix of a cylindrical 
surface to be ground. The tilted grinding surface has a generatrix tilted 
away from the generatrix of the cylindrical surface. A control means 
causes the grinding wheel to move relative to the workpiece into the 
cylindrical surface. The wheel is fed into the cylindrical surface to a 
depth corresponding to the grinding allowance. Then, the cylindrical 
surface is ground by the tilted grinding surface. Subsequently, the 
grinding wheel is moved relative to the workpiece along the generatrix of 
the cylindrical surface in such a direction that the cylindrical surface 
is ground by the parallel grinding surface. In this way, the 
above-described objects of the invention are achieved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 2, there is shown a CNC grinding machine according to the 
invention. This machine has a bed 10 on which a wheel spindle stock 12 and 
a work table 11 are guided so a to be movable in the directions of X- and 
Y-axes, respectively, which are perpendicular to each other. A wheel 
spindle is held to the spindle stock 12 so as to be rotatable about an 
axis which is inclined at a given angle .gamma. to the axis of rotation of 
a cylindrical workpiece W (described later) within a horizontal plane. An 
angular grinding wheel G is mounted to one end of the wheel spindle and 
driven by an electric motor (not shown). This grinding wheel G comprises a 
metallic disk and a layer of abrasive grains of CBN (cubic system of boron 
nitride) formed on the outer periphery of the disk. The abrasive grains 
are bonded together with a metal bond. This wheel G is narrower than the 
conventional grinding wheel. 
A headstock 17 and a tailstock 18 are disposed opposite to each other on 
the table 11. The workpiece W is held by the headstock 17 and the 
tailstock 18 in such a way that the workpiece can rotate about an axis 
parallel to the direction of the Z-axis in which the table 11 is moved. 
The workpiece W is rotated by a spindle motor (not shown). Feed screws 14 
and 13 are screwed to the spindle stock 12 and the table 11, respectively. 
These screws 13 and 14 are rotated by servomotors 15 and 16, respectively. 
The servomotors 15 and 16 are connected with drive circuits 28 and 27, 
respectively, and are controlled by instruction pulses supplied from a 
control unit 20 that is connected with the drive circuits 27, 28 to 
provide a numerical control of the servomotors. 
FIG. 3 is an enlarged view of the angular grinding wheel G, for showing its 
shape. The workpiece W has a cylindrical surface Wc. A cylinder-grinding 
surface Ga for grinding the cylindrical surface Wc and a shoulder-grinding 
surface Gb are formed on the grinding wheel G. The shoulder-grinding 
surface Gb acts to grind the end surface of the shoulder portion adjacent 
to the cylindrical surface Wc. An arc-shaped apical portion Gc having a 
given radius is formed between the cylinder-grinding surface Ga and the 
shoulder-grinding surface Gb. The cylinder-grinding surface Ga is composed 
of a cylinder-grinding tilted surface 31 and a cylinder-grinding parallel 
surface 33 formed between the tilted surface 31 and the apical portion Gc. 
This tilted surface 31 is a truncated conical surface which continues to 
the cylinder-grinding parallel surface 33 at the end on the side of the 
apical portion Gc. The distance between the truncated conical surface and 
the generatrix of the cylindrical surface Wc increases in going away from 
the cylinder-grinding parallel surface 33. The conical surface is tilted 
at angle .alpha. to the cylindrical surface Wc. The shoulder-grinding 
surface Gb comprises a shoulder-grinding tilted surface 32 and a 
shoulder-grinding parallel surface 34 formed between the tilted surface 32 
and the apical portion Gc. This tilted surface 32 is a truncated conical 
surface which continues to the shoulder-grinding parallel surface 34 at 
the end on the side of the apical portion Gc. The distance between this 
conical surface and the end surface Ws of the shoulder portion of the 
workpiece increases in going away from the shoulder-grinding parallel 
surface 34. This conical surface is inclined at angle .beta. to the end 
surface Ws of the shoulder portion. 
As described above, the generatrix of the cylinder-grinding tilted surface 
31 is inclined at the preset angle .alpha. in the direction to move away 
from the generatrix of the cylindrical surface Wc of the workpiece W. The 
generatrix of the shoulder-grinding tilted surface 32 is inclined at the 
preset angle .beta. in the direction to move away from the end surface Ws 
of the shoulder portion of the workpiece W. Let L.sub.1 and L.sub.2 be the 
cross-sectional lengths of the cylinder-grinding tilted surface 31 and the 
shoulder-grinding tilted surface 32, respectively. The angles .alpha. and 
.beta. are so set that L.sub.1 sin.alpha. and L.sub.2 sin.beta. correspond 
to the finishing grinding allowances for the cylindrical surface Wc and 
the end surface Ws of the shoulder portion, respectively. 
The cylinder-grinding parallel surface 33 and the shoulder-grinding 
parallel surface 34 are parallel to the cylindrical surface Wc and the end 
surface Ws of the shoulder portion, respectively, at the grinding point. 
Since the diameter of the grinding wheel is large, the cylinder-grinding 
parallel surface 33 has a larger peripheral speed and experiences less 
resistance compared with the cylinder-grinding tilted surface 31. 
Therefore, during grinding operation, the workpiece W flexes only a 
little. The cylinder-grinding parallel surface 33 functions well as a 
finishing grinding portion for the cylindrical surface Wc of the workpiece 
W. For the same reason, the shoulder-grinding parallel surface 34 
functions well as a finishing grinding portion for the end surface Ws of 
the shoulder portion of the workpiece W. 
The manner in which the grinding machine constructed as described above 
grinds the workpiece is next described by referring to FIGS. 4 and 5. FIG. 
4 is a flowchart illustrating the operation of the control unit 20. First, 
the table 11 is moved in the direction of the Z-axis (step 50). The first 
cylindrical surface Wc1 is placed at the machining position. Then, the 
table 11 is moved to the right and, at the same time, the spindle stock 12 
is advanced to quickly place the grinding wheel G at the position 
corresponding to the end of the first cylindrical surface Wc1 close to the 
end surface Ws of the shoulder portion. (step 52). A decision is made to 
determine whether there exists a shoulder portion end surface which is 
adjacent to the cylindrical surface Wc1 and should be machined (step 54). 
If such a shoulder portion does not exist, then step 56 is skipped, and 
control goes to step 58. In this case, there exists the end surface Ws of 
the shoulder portion to be machined and so control goes from step 52 to 
step 56, where the end surface Ws of the shoulder portion is ground. In 
this step 56, the table 11 is first moved to the right over a given 
distance at a given infeed speed. The shoulder-grinding surface Gb of the 
grinding wheel G is fed into the end surface W2 of the shoulder portion by 
a given grinding allowance. Thereafter, the spindle stock 12 is moved 
backward at a given grinding speed. Thus, the end surface Ws of the 
shoulder portion of the workpiece W is first ground by the 
shoulder-grinding tilted surface 32. Subsequently, a finishing grinding 
operation is performed by the shoulder-grinding parallel surface 34. When 
the machining of the end surface Ws of the shoulder portion is completed, 
the table 11 is moved to the left over a given distance to form a certain 
clearance between the grinding wheel G and the end surface Ws of the 
shoulder portion. Thereafter, the spindle stock 12 is advanced again at a 
high speed back into its original radial position. Then, the spindle stock 
12 is fed into the workpiece W toward the axis of rotation of the 
workpiece to feed the wheel into the first cylindrical surface Wc1 to a 
given depth corresponding to the grinding allowance (step 58). The table 
11 is moved to the left. In this process, the first cylindrical surface 
Wc1 of the workpiece W is first roughly ground by the cylinder-grinding 
tilted surface 31. Then, the cylinder-grinding parallel surface 33 
performs a finishing grinding operation (step 60). At this time, as shown 
in FIG. 3, the cylindrical surface Wc is ground by the whole of the 
cylinder-grinding tilted surface 31 and, therefore, excessive local wear 
of the angular grinding wheel G is prevented. Also, the machining of the 
cylindrical surface Wc1 is completed by a single traverse grinding 
operation, because a finishing grinding operation is carried out by the 
cylinder-grinding parallel surface 33 after the cylindrical surface Wc1 is 
roughly ground by the cylinder-grinding tilted surface 31. Consequently, 
the grinding time can be shortened. Similarly, the prevention of the 
excessive wear and the shortening of the grinding time can be attained by 
the shoulder-grinding tilted portion 32 and the shoulder-grinding parallel 
surface 34. After the completion of the machining of the first cylindrical 
surface Wc1, control proceeds to step 62, where a decision is made to 
determine whether there exists any other portion to be ground. If not so, 
the grinding process is ended. On the other hand, if the result of the 
decision is that there exists any portion to be ground other than the 
first cylindrical surface as in the present example, then control goes to 
step 64, where the next second cylindrical surface Wc2 is brought into the 
machining position. The grinding wheel G is placed at the left end of the 
second cylindrical surface Wc2. Subsequently, the processing beginning 
with step 54 is performed again to machine the second cylindrical surface 
Wc2. The third machined surface Wc3 is machined in the same way. 
It is to be understood that the present invention is also applicable to the 
case in which a taper is ground on a workpiece. In this case, the table is 
inclined in such a way that the generatrix of the tapering cylindrical 
surface is parallel to the direction of movement of the table at the 
machining position. Under this condition, the taper is ground. 
In the above example, an angular grinding wheel is used. As shown in FIG. 
6, a grinding wheel having only an outer surface parallel to the axis of 
rotation of the workpiece may also be employed. In this case, this outer 
surface has a grinding parallel surface 33 and a grinding tilted surface 
31. The parallel surface 33 has a generatrix parallel to the generatrix of 
the cylindrical surface to be ground. The tilted surface 31 is continuous 
with the parallel surface 33 and has a generatrix inclined away from the 
generatrix of the cylindrical surface. 
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.