Internal grinding method

In a method of internal grinding with a grinding wheel composed of cubic boron nitride, in order to maintain the processing accuracy of workpieces in each grinding cycle after the completion of dressing, the grinding wheel undergoes a corrective retracting movement from the roughly ground surface of a workpiece at the larger amount than the normal retraction amount in the predetermined time or in a time for grinding the predetermined pieces of the workpiece just after completion of dressing.

BACKGROUND OF THE INVENTION 
This invention relates to a method which executes a internal grinding of a 
workpiece by use of a grinding wheel, which is composed of abrasive grains 
of ultra hard material such as a cubic boron nitride (CBN). 
Conventionally, in a case of executing the internal grinding, by use of the 
grinding wheel such as CBN grinding wheel (hereinafter called so) which is 
composed of the abrasive grains of the ultra hard material such as a cubic 
boron nitride and etc., there is a problem that just after the completion 
of dressing, the sharpness of the grinding wheel becomes dull and thereby 
several and/or several tens of workpieces are ground through the internal 
grinding in a manner that the ground hole is shaped in progressively 
narrowed toward the end, namely the cylindricity becomes poor. Thus, 
conventionally, the workpeice just after the dressing is inspected and 
selected by the man power. Otherwise, several and/or several tens of the 
workpieces are abolished just after the dressing, which is called a dummy 
work. 
SUMMARY OF THE INVENTION 
As set forth above, there are problems of the difficulty of no-labor 
driving in the selection process and of occurrence of the loss of raw 
material and the loss of time in case of using a dummy workpiece. 
Accordingly, this invention is intended to settle the above mentioned 
drawbacks and to provide a method for accurately processing the workpiece 
even if the CBN grinding wheel becomes dull just after the dressing. 
This invention is intended to settle the problems and is characterized in 
that it executes the dressing of CBN grinding wheel by use of a diamond 
dresser and after completion of dressing the grinding wheel, it executes 
the grinding according to the infeeding pattern of the grinding cycle in 
which a retraction amount of the grinding wheel is larger than the normal 
retraction amount in the predetermined time and/or in a time for grinding 
the predetermined number of the workpieces and thereafter, it executes the 
grinding according to the infeeding pattern of the above mentioned normal 
retraction amount so as to accurately process the workpiece. 
According to this invention, just after the completion of the dressing, the 
sharpness of the grinding wheel becomes dull and the degree that the 
ground hole is narrowed toward the end becomes larger than the normal, but 
by grinding at the retraction amount larger than the normal amount, the 
hole is repeatedly ground from its end residual stock-removal such that 
the workpiece of good cylindricity can be obtained just after the dressing 
and in addition, as the sharpness of the grinding wheel becomes improved 
after several and/or several tens of the workpieces are ground, it is 
possible that the workpiece can be processed in more shortened time by 
reverting to the normal retraction amount.

DETAILED DESCRIPTION OF THE INVENTION 
Hereinafter, the embodiments of this invention will be explained in 
conjunction with the drawings. FIG. 2 illustrates an embodiment of 
conventional infeeding pattern in which the grinding wheel executes a 
rapid advance, a rough or coarse grinding, a retraction, a second rough 
grinding, a fine grinding, a spark-out and a rapid return successively and 
repeats in turn at every workpiece. FIG. 1 illustrates an embodiment of 
grinding in accordance with the method of this invention wherein, the 
grinding wheel operates to grind two workpieces just after the completion 
of the repeated dressing at the infeeding pattern of a grinding cycle 
during which the retraction amount of the grinding wheel is larger than 
the normal retraction amount and moreover, the grinding wheel executes to 
grind the third and further workpieces at the infeeding pattern of the 
normal retraction amount. Namely, just after completion of the dressing, 
it executes a rapid advance, a rough grinding, an increased retraction, a 
second rough grinding, a fine grinding, a spark-out and a rapid return 
successively and the second workpiece is ground at the same infeeding 
pattern of the grinding cycle and the third workpeice is ground at the 
infeeding pattern of the normal retraction amount. 
More detailedly speaking, FIGS. 3A-3C illustrate a relation between the 
workpiece 1 and the grinding wheel or tool 2 in a case that the sharpness 
or the abrading capacity of the grinding wheel is normal. FIG. 3A 
illustrates a state that a rough grinding is over and FIG. 3B illustrates 
a state that the grinding wheel executes a retraction and FIG. 3C 
illustrates a state that it executes a spark-out. 
When the grinder shaft 3 is comparatively poor in rigidity as in the 
internal grinding machine, as illustrated in FIG. 3A, the bending of the 
grinder shaft 3 will occur under the grinding process due to the grinding 
resistance, therefore, if the grinding is completed at this stage, the 
cylindricity of the ground surface of the work piece is deteriorated. 
Accordingly, the retraction is needed in order to eliminate this drawback. 
In FIG. 3B, the grinding wheel executes a retraction movement from the 
roughly ground surface through the relatively small correction distance of 
the amount R so as to correct the bending of the grinder shaft and 
further, it executes a second rough grinding such that the hole is 
progressively ground from the residual stock removable in the end of the 
hole of the workpiece and eventually, it executes a fine grinding and a 
spark-out and executes a finishing as shown in FIG. 3C. Such finishing 
method is a popular method that is used in order to shorten the processing 
time and to attain high accurate cylindricity. However the sharpness of 
the CBN grinding wheel becomes dull, just after the completion of the 
dressing, because the dresser smoothes the surface of the abrasive grain 
so as to round the edge of the abrasive grains. FIGS. 4D-4F illustrate a 
relation between the workpiece and the grinding wheel when the grinding 
wheel executes a processing just after the completion of the dressing. 
FIG. 4D illustrates a state that a rough grinding is completed and it 
shows that the bending of the grinder shaft 3 is more increased due to the 
dull sharpness of the grinding wheel in comparison with the state shown in 
FIG. 3A and the cylindricity of the workpiece is deteriorated. Next, if 
the grinding wheel executes a retraction by the same small amount R, the 
workpiece 1 do not leave the grinding wheel 2, as shown in FIG. 4E, 
because the taper degree is larger than in the normal processing. Thus, 
the grinder shaft 3 maintains its bent state. Further, if the grinding 
wheel executes a second rough grinding and a spark-out as shown in FIG. 
4F, the grinder shaft 3 maintains its bent state such that the hole 
maintains its tapered state. FIG. 5 illustrates a result of grinding owing 
to the conventional method. In the drawing, the mark "-" means that the 
diameter of the hole is smaller in the back portion thereof. It is clear 
that the n pieces of the workpieces (being three in the drawing) are 
smaller in its diameter of the back portion of the hole due to the 
dullness of the grinding wheel, but the fourth and further ones are 
normalized. 
As a means for solving this fault, the grinding wheel executes to grind 
workpieces at the larger retraction amount than the normal amount, namely, 
the grinding wheel undergoes the retracting movement through a relatively 
large correction distance just after the dressing such that the grinding 
wheel is caused to perfectly leave the workpiece so as not to exert the 
bending of the grinder shaft 3 and further, the grinding wheel executes a 
second rough grinding, a fine grinding and a spark-out successively, 
thereby the cylindricity thereof is extremely improved. Furthermore, if it 
executes to repeatedly grind several and/or several tens of the workpieces 
which is differ in response to the workpeice material and/or the kind of 
the grinding wheel after the dressing thereof, the sharpness or the 
abrading capacity of the grinding wheel is restored or recovered again, 
therefore even though the retraction amount is changed to the normal small 
amount the accuracy of the cylindricity can be maintained. Abovementioned 
explanation of the drawing describes an exemplified embodiment that the 
length of the hole is shorter than the length of the grinding wheel. 
Moreover, it is very effective to the socalled reciprocation grinding that 
carry out the reciprocation of the grinding wheel in a case that the 
length of the hole is comparatively longer than the diameter of the hole 
and the length of the grinding wheel is comparatively shorter than the 
length of the hole. 
As set forth above, this invention has superior advantages that it is 
possible to carry out the accurate and stable grinding just after the 
dressing and of omitting the accuracy inspection and/or the selection of 
the workpiece just after the dressing which is carried out by the man 
power in the past time and to utilize the workpeice which is abolished in 
the past time as a dummy work, by employing a method of grinding a 
workpiece at the more retraction amount than the normal amount just after 
the completion of the dressing and thereafter grinding workpieces at the 
normally reverted retraction amount after the predetermined pieces of the 
workpiece are ground.