Grinding wheel with abrasive segment chips including at...

Details Grinding wheel with abrasive segment chips including at... Grinding wheel with abrasive segment chips including at...

2002-02-25

2003-11-04

Nguyen, Dung Van (Department: 3723)

Abrading

Rigid tool

Rotary cylinder

C451S544000, C125S015000

Reexamination Certificate

active

06641473

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a grindstone or grinding wheel, and more particularly to improvements in such a grinding wheel including a base disk and a multiplicity of abrasive segment chips which are fixed to the base disk.
2. Discussion of the Related Art
There is known a grinding wheel including: a base disk; and a multiplicity of abrasive segment chips which have respective abrasive layers and which are fixed to the base disk so as to be arranged on a circle whose center corresponds to the axis of the base disk. In a grinding operation with this grinding wheel, the base disk is rotated about the axis, so that a workpiece is ground by the abrasive layer of each abrasive segment. One example of such a grinding wheel is disclosed in JP-A-H11-300626 (publication of unexamined Japanese Patent Application laid open in 1999), in which the multiplicity of abrasive segment chips arranged on a circle are spaced apart from each other in a circumferential direction of the base disk. Due to the abrasive segment chips spaced apart from each, the rotating grinding wheel is held in intermittent contact with the workpiece during the grinding operation. In this instance, if the intermittent contact of the grinding wheel with the workpiece is made periodically, i.e., at a constant time interval between the successive contacts, the workpiece tends to suffer from a chattering or self-induced vibration, which would cause deterioration of machining accuracy in the grinding operation. In the disclosed grinding wheel, for the purpose of preventing such a deterioration of the machining accuracy due to the chattering or self-induced vibration, the abrasive segment chips are adapted to be arranged with the spacing interval between the adjacent abrasive segment chips being not constant, or alternatively, the abrasive segment chips are adapted to have respective circumferential lengths which are different from each other.
However, the above-described known grinding wheel is not capable of providing a sufficiently high machining accuracy, particularly, where it is used in an operation such as a centerless grinding operation in which a workpiece is held by a structure having a relatively low degree of rigidity, namely, where the structure to hold the workpiece does not have a rigidity high enough to avoid large amount of displacement of the workpiece which is caused by its intermittent contact with the grinding wheel.
It is considered possible to arrange the multiplicity of abrasive segment chips without a spacing interval or gap between the adjacent abrasive segment chips in the circumferential direction, so that the grinding wheel can be held in continuous or constant contact with the workpiece during the grinding operation. This arrangement, which was not known in the art at the time the present invention was made, was considered to be effective to prevent chattering of the workpiece and accordingly avoid deterioration of machining accuracy, even where the circumferential lengths of the abrasive segment chips are equal to each other.
FIG. 4
shows a grinding wheel
100
, as one example having this arrangement, in which a multiplicity of arcuate abrasive segment chips
104
are disposed on an outer circumferential surface of a base disk
102
having an axis O, with substantially no gap between the abrasive segment chips
104
as viewed in a circumferential direction of the base disk
102
. The arcuate abrasive segment chips
104
are bonded to each other and also to the outer circumferential surface of the base disk
102
by a suitable adhesive. Each of the abrasive segment chips
104
consists of radially inner and outer layers in the form of a support layer
104
a
and an abrasive layer
104
b
which is positioned radially outwardly of the base layer
104
a.
Each abrasive segment
104
is bonded at the support layer
104
a
to the base disk
102
. Each abrasive segment
104
is brought into contact at the abrasive layer
104
b
with the workpiece during the grinding operation. The abrasive layer
104
b
is preferably formed of super abrasive grains such as diamond abrasive grains and CBN (cubic boron nitrides) abrasive grains. It is noted that each abrasive segment
104
may consist of only the abrasive layer
104
b.
However, even such a grinding wheel having the above-described arrangement, in which the multiplicity of abrasive segment chips are arranged with substantially no gap therebetween, is not capable of satisfactorily resolving the problematic deterioration of machining accuracy, for example, when the grinding operation has to be achieved with a relatively high machining accuracy or with a machining condition that inevitably increases the chattering of the workpiece. The chattering can be caused depending upon various factors such as a machining condition and conditions of the used graining machine and grinding wheels. The possibility of chattering can be considerably increased even by a minor change in the machining condition, especially, where the grinding wheel is used in an operation such as a centerless grinding operation in which the workpiece is held by a structure having a relatively low degree of rigidity.
FIG. 5
is a view schematically illustrating a centerless grinding operation in which a cylindrical workpiece
108
is disposed on a work rest blade
106
and is gripped by and between the grinding wheel
100
and a regulating wheel
110
. The workpiece
108
is rotated at a predetermined number of revolutions by the regulating wheel
110
, while the grinding wheel
100
is driven to be rotated at a predetermined number of revolutions, so that an outer circumferential surface of the workpiece
108
is ground. In such a centerless grinding operation, the workpiece tends to suffer from a chattering with a higher possibility than in a grinding operation in which the workpiece is held by and between centers. The grinding operation without a chattering provides the workpiece
108
with a high degree of roundness as shown in FIG.
6
A. If the workpiece
108
chatters during the grinding operation, however, a succession of large lobes
112
, i.e., a succession of large protrusions and recess are formed on the outer circumferential surface in synchronization with successive occurrences of the chattering, whereby the roundness of the workpiece
108
is deteriorated as shown in FIG.
6
B.
FIG. 7
is a table indicating a result of an experiment conducted by the present inventors, in which the workpiece
108
made of a bearing steel and having an outside diameter of about 10 mm was ground at its outer circumferential surface by the grinding wheel
100
having an outside diameter of about 405 mm. The number of the abrasive segment chips
104
disposed on the grinding wheel
100
was thirty three. In this centerless grinding operation as the experiment, the number of revolutions of the regulating wheel
110
, i.e., the number of revolutions of the workpiece
108
was changed in steps while the number of revolutions of the grinding wheel
110
was held constant. After the grinding operation, the roundness of the workpiece
108
was evaluated by counting or measuring the number of the lobes formed on the outer circumferential surface. As is apparent from the table of
FIG. 7
, under conditions (which are indicated in areas defined by thick lines in the table) where the number of revolutions of the regulating wheel
110
was 1.02 times a reference value (=1.00), 1.18 times the reference value, 1.32 times the reference value and 1.49 times the reference value, the number of the lobes was reduced while each of the lobes was enlarged as shown in
FIG. 6B
, whereby the machining accuracy or roundness of the workpiece
108
was deteriorated. It is noted that the variation in the diametrical dimension of the workpiece
108
is somewhat exaggerated in the views of
FIGS. 6A and 6B
which were obtained in a roundness measuring device used in the experiment.
In the prior art, some measures against a chattering are taken as the c

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