Preformed pieces for outer and inner races of bearings

A bearing outer-race performed piece has a turning stock on an outer periphery thereof and has a circular bearing groove on an inner surface thereof. The bearing groove has a cross-section which includes circular portions defined by a circle of a greater diameter than that of a bearing groove of a bearing outer-race product, and a flat portion being arranged between the circular portions at a location shifted toward the center of the circle of the circular portions from a bottom locus of the circular portions.

BACKGROUND OF THE INVENTION 
1. Field of The Invention 
The present invention relates generally to a preformed piece for an outer 
or inner race of a bearing, method for manufacturing same, and a mandrel 
and a roll used therein. More specifically, the invention relates to the 
shape of a forming die comprising a mandrel and a roll, which is used for 
forming a preformed piece for an outer or inner race of a bearing having a 
bearing ring, using the cold rolling working. 
2. Description of The Prior Art 
In a conventional method for manufacturing an outer or inner race of a 
bearing, a steel blank is hot-forged to obtain an annular blank. Then, 
after the annular blank is annealed and shot-blasted, it is cold-rolled to 
form a preformed piece 1 for a bearing outer race (which will be 
hereinafter referred to as "a bearing outer-race preformed piece") or a 
preformed piece 14 for a bearing inner race (which will be hereinafter 
referred to as a "bearing inner-race preformed piece"), as illustrated in 
FIGS. 6 and 7. The term "preformed piece" as used herein means an object 
that has been subjected to preliminary incomplete shaping before 
undergoing complete or final processing. The preformed piece thus obtained 
is turned by means of a lathe to produce a bearing outer-race product or a 
bearing inner-race product of a product shape 6 or 19 illustrated by a 
two-dot chain line in FIGS. 6 and 7. In the cold rolling working, an inner 
raceway groove 7 or an outer raceway groove 20, which has a circular 
curved surface, is provided on an inner surface 2a of the bearing outer 
race or an outer surface 15b of the bearing inner race to form the bearing 
outer-race preformed piece 1 or the bearing inner-race preformed piece 14. 
In the case of the bearing inner-race preformed piece 14 having the raceway 
groove on the outer surface 15b thereof, the core run out is easy to occur 
since the bearing inner-race preformed piece 14 is held by at tightening 
chuck when it is turned to form a final product. For that reason, an 
unbalanced load is easy to be applied thereto, so that the product yield 
is poor. Therefore, the number of the bearing inner-race preformed piece 
14 having the raceway groove is less than the number of the bearing 
outer-race preformed piece 1 having the raceway groove. In addition, the 
forming-die surface of a forming die of a mandrel or a roll used for 
forming these bearing preformed pieces using the cold rolling working, has 
a bearing ring shape, which is obtained by adding a turning stock y of 1.5 
mm shown in the drawing to the turned surface of the bearing ring of the 
bearing outer-race product or the bearing inner-race product, in order to 
facilitate the turning working. 
In the turning of the bearing preformed piece formed by the mandrel or 
roll, when a turning tool for finishing having a turned curved-surface 
cuts into the bearing outer-race product or the bearing inner-race product 
to the turned curved-surface of the bearing ring, elongated thin chips are 
produced particularly at the beginning of turning. In addition, when the 
center of the bearing ring is offset, the direction for discharging the 
chips can not be determined, so that the chips enter a gap between the 
working face and knock-out face of a collet chuck which holds the 
preformed piece. Thus, the nipped chips may get into trouble such as the 
production of defective products. In addition, the chipping of the turning 
tool for finishing may occur due to the unbalanced load, so that the 
turning tool for finishing must be used while being rotated by three or 
four surfaces, thereby decreasing the life time of the tool. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to eliminate the 
aforementioned problems and to prevent elongated fine chips from occurring 
at the beginning of turning when a bearing outer-race preformed piece or a 
bearing inner-race preformed piece is turned to form a bearing ring of a 
bearing outer-race product or a bearing inner-race product, so as to 
prevent the chips from being nipped and to prevent the chipping of a tool 
due to unbalanced load. 
In order to accomplish the aforementioned and other objects, according to 
one aspect of the present invention, there is provided a bearing 
outer-race preformed pieces, which has a turning stock on an outer 
periphery thereof and which has a circular bearing ring on an inner 
surface thereof, the bearing ring having a cross-section which comprises 
circular portions defined by a circle of a greater diameter than that of a 
bearing ring of a bearing outer-race product, and a flat portion being 
arranged between the circular portions at a location shifted toward the 
center of the circle of the circular portions from a bottom locus of the 
circular portions. 
According to another aspect of the present invention, there is provided a 
bearing inner-race preformed pieces, which has a turning stock on an inner 
periphery thereof and which has a circular bearing ring on an outer 
surface thereof, the bearing ring having a cross-section which comprises 
circular portions defined by a circle of a greater diameter than that of a 
bearing ring of a bearing inner-race product, and a flat portion being 
arranged between the circular portions at a location shifted toward the 
center of the circle of the circular portions from a bottom locus of the 
circular portions. 
According to another aspect of the present invention, there is provided a 
method for manufacturing a bearing outer-race preformed piece, comprising 
the steps of: hot-forging a steel blank to form an annular steel blank; 
and cold-rolling the annular steel blank to form a bearing outer-race 
preformed piece, the cold-rolling is carried out while a forming-die 
surface of a mandrel, which has a circular projecting bearing-ring surface 
and a flat portion arranged at the top portion of the circular projecting 
bearing-ring surface, is brought into contact with an inner surface of the 
annular steel blank. 
According to another aspect of the present invention, there is provided a 
method for manufacturing a bearing inner-race preformed piece, comprising 
the steps of: hot-forging a steel blank to form an annular steel blank; 
and cold-rolling the annular steel blank to form a bearing inner-race 
preformed piece, the cold-rolling is carried out while a forming-die 
surface of a roll, which has a circular projecting bearing-ring surface 
and a flat portion arranged at the top portion of the circular projecting 
bearing-ring surface, is brought into contact with an outer surface of the 
annular steel blank. 
According to further aspect of the present invention, there is provided a 
mandrel used for manufacturing a bearing outer-race preformed piece by a 
cold rolling working, the mandrel having a profile of a forming-die 
surface which comprises: circular portions corresponding to a profile of a 
cross-section of an internal bearing ring of the bearing outer-race 
preformed piece which has a greater diameter of an internal bearing ring 
of a bearing outer-race product formed by turning the bearing outer-race 
preformed piece; and a flat portion between the circular portions, the 
flat portion being parallel to an inner surface of the internal bearing 
ring of the bearing outer-race preformed piece at a lower location than a 
top of locus of the internal bearing ring of the bearing outer-race 
preformed piece. 
According to still further aspect of the present invention, there is 
provided a roll used for manufacturing a bearing inner-race preformed 
piece by a cold rolling working, the roll having a profile of a 
forming-die surface which comprises: a profile of a cross-section of an 
external bearing ring of the bearing inner-race preformed piece which has 
a greater diameter of an external bearing ring of a bearing inner-race 
product formed by turning the bearing inner-race preformed piece; and a 
flat portion which is parallel to an outer surface of the external bearing 
ring of the bearing inner-race preformed piece at a lower location than a 
top of locus of the external bearing ring of the bearing inner-race 
preformed piece. 
According to the present invention, since the bearing outer-race preformed 
piece or the bearing inner-race preformed piece which have a flat portion 
on the bottom of the circular bearing ring, the forming-die surface of the 
mandrel or roll for the cold rolling working has a shape obtained by 
turning over the cross-section of the bearing ring which has the flat 
portion the bottom of the internal or external bearing ring of the bearing 
outer-race preformed formed piece or the bearing inner-race preformed 
piece. 
Thus, it is possible to easily form the bearing outer-race preformed piece 
or the bearing inner-race preformed piece, which have the flat portion on 
the bottom of the circular bearing ring, by cold-rolling a hot-forged 
steel blank by means of the mandrel or roll. In addition, since the formed 
bearing outer-race preformed piece or bearing inner-race preformed piece 
has a flat surface on the bottom of the circular raceway groove formed on 
the inner or outer surface thereof, it is possible to easily cut into the 
bearing ring at the beginning of turning when the bearing outer-race 
product or bearing inner-race product is finished by a turning tool for 
finishing, so that chips are discharged in a constant direction without 
being nipped. Therefore, it is possible to prevent the core run out and to 
prevent defective products from occurring.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the accompanying drawings, particularly to FIGS. 1 through 
5, the preferred embodiments of the present invention will be described 
below. 
First, after a steel blank is hot-forged, annealed and shot-blasted, the 
resulting blank is cold-rolled to form a bearing outer-race preformed 
piece 1 or a bearing inner-race preformed piece 14 by the cold rolling 
working. In this cold rolling working, an internal raceway groove serving 
as an internal bearing ring 7 is formed in an inner surface 2a of the 
bearing outer-race preformed piece 1, or an external raceway groove 
serving as an external bearing ring 20 is formed in an outer surface 15b 
of the bearing inner-race preformed piece 14. The bearing outer-race 
preformed piece 1 or bearing inner-race preformed piece 14 is turned to 
form a bearing outer-race product or a bearing inner-race product. 
Therefore, the bearing outer-race preformed piece 1 or bearing inner-race 
preformed piece 14 is greater than the bearing outer-race product and 
bearing inner-race product by a finishing stock for turning the outer 
peripheral portion thereof. 
Referring to FIGS. 1A and 1B, the bearing outer-race preformed piece 1 will 
be described below. 
The profile of a cross-section of the bearing outer-race preformed piece 1 
is expressed by a full line in FIG. 1A, and this corresponds to the left 
profile of the cross-section of the bearing outer-race preformed piece 
shown in FIG. 1B. The region surrounded by a two-dot chain line in FIG. 1A 
corresponds to a finishing stock to be turned. The internal raceway groove 
serving as the internal bearing ring 7 is formed in the inner surface of 
the bearing outer-race preformed piece 1. The profile of the internal 
bearing ring 7 comprises circular profile portions 11 on both sides 
thereof, and a flat profile portion 12 at the center thereof. The 
straight-line portion of the flat profile portion 12 is formed at a height 
of about 0.2 mm toward the central axis C of the outer race from a bearing 
ring bottom 13 expressed by a dotted line in FIG. 1A. The height x of the 
flat profile portion 12 from the bearing ring bottom 13 is optimally about 
0.2 mm to prevent the offset of center of a tool when turning and to 
easily carry out the working. When the height x is 0.1 mm, it is not 
possible to prevent the offset of center. When the height x is greater 
than 0.2 mm, although it is possible to prevent the offset of center, the 
excessive turning must be carried out, so that the efficiency is low. 
In addition, the radius of curvature of the circular profile portion 11 is 
greater than the radius of curvature of a product internal bearing ring 9 
expressed by the two-dot chain line in FIG. 1A. Therefore, the thickness 
of the finishing stock from the bottom of the product internal bearing 
ring 9 to the flat profile portion 12 is about 1.7 mm. In the case of this 
thickness of the finishing stock, the most effective turning can be 
carried out. 
The turning of the bearing ring is carried out while a turning tool for 
finishing 10, which has a shape corresponding to that of the product 
internal bearing ring 9, is moved in direction D. Therefore, according to 
the present invention, the tip of the turning tool for finishing 10 
point-contacts the center of the flat profile portion 12 to start the 
turning. Thus, at the beginning of the cutting into the bearing ring, a 
sufficiently large gap occurs between the edge of the turning tool for 
finishing 10 and the preformed piece to be worked, so that the chips can 
easily escape without being nipped therebetween. In addition, it is 
difficult to cause the offset of center of the bearing ring, so that the 
discharging direction of the chips is constant. Therefore, according to 
the present invention, it is possible to prevent fine chips from entering 
a gap between the working face and knock-out face of a collet chuck to 
prevent the offset of center and to prevent defective products from 
occurring, so that it is possible to achieve a good turning without 
getting into trouble. 
The profile of the bearing outer-race preformed piece 1 expressed by the 
full line in FIG. 1A, also shows a profile 3 of a forming-die surface of a 
mandrel (which will be hereinafter referred to as a "mandrel forming-die 
surface profile") and a profile 4 of a forming-die surface of a roll 
(which will be hereinafter referred to as a "roll forming-die surface 
profile"), which are used for the cold rolling working of the bearing 
outer-race preformed product 1. The right profile shows the mandrel 
forming-die surface profile 3, and the left profile shows the roll 
forming-die surface profile 4. Therefore, the mandrel forming-die surface 
profile 3 comprises the circular profile portions 11 on both sides, which 
correspond to the sectional profile of the preformed-piece internal 
bearing ring having a greater diameter than that of the product internal 
bearing ring obtained by turning the bearing outer-race preformed piece, 
and the flat profile portion 12, which is parallel to the inner surface of 
the bearing arranged between the circular profile portions 11 at a 
location lower than the top of locus expressed by the dotted line in FIG. 
1A by about 0.2 mm to the right. Since the outer surface 15a of the 
bearing outer-race profiled piece 1 is a flat surface, the roll 
forming-die surface profile 4 is also a simple flat surface. 
Referring to FIGS. 2A and 2B, the bearing inner-race profiled piece 14 will 
be described below. 
The profile of a cross-section of the bearing inner-race preformed piece 14 
is expressed by a full line in FIG. 2A, and this corresponds to the left 
profile of the cross-section of the bearing inner-race preformed piece 14 
expressed in FIG. 2B. In the case of this bearing inner-race preformed 
piece 14, a preformed-piece external bearing ring 20 is formed in the 
outer surface 15b thereof, unlike the bearing outer-race preformed piece 1 
of FIG. 1 having the preformed-piece internal bearing ring 7 in the inner 
surface thereof. Therefore, the bearing ring itself of the bearing 
inner-race preformed piece 14 is the same as that of the bearing 
outer-race preformed piece 1, except that the bearing ring is formed in 
the inner surface, not in the outer surface. 
The region surrounded by a two-dot chain line in FIG. 2A corresponds to a 
finishing stock to be turned. The external raceway groove serving as the 
external bearing ring 20 is formed in the outer surface of the bearing 
inner-race preformed piece 14. The profile of the external bearing ring 20 
comprises circular profile portions 23 on both sides of the bearing ring 
and a flat profile portion 24 at the center of the bearing ring. The 
straight-line portion of the flat profile portion 24 is formed at a height 
of about 0.2 mm toward the central axis C of the inner race from a 
bearing-ring bottom 25 expressed by a dotted line in FIG. 2A. The height x 
of the flat profile portion 24 from the bearing-ring bottom 25 is 
optimally about 0.2 mm to prevent the offset of center of the tool when 
turning and to easily carry out the working. When the height x is 0.1 mm, 
it is not possible to prevent the offset of center. When the height x is 
greater than 0.2 mm, although it is possible to prevent the offset of 
center, the excessive turning must be carried out, so that the efficiency 
is low. In addition, the radius of curvature of the circular profile 
portion 23 is greater than the radius of curvature of a product external 
bearing ring 22 expressed by the two-dot chain line in FIG. 2A. Therefore, 
the thickness of the finishing stock from the bottom 25 of the product 
external bearing ring 22 to the flat profile portion 24 is about 1.7 mm. 
In the case of this thickness of the finishing stock, the most effective 
turning can be carried out. 
The turning of the bearing ring is carried out while a turning tool for 
finishing 10, which has a shape corresponding to that of the product 
external bearing ring 22, is moved in direction D. Therefore, according to 
the present invention, in the bearing inner race similar to the bearing 
outer race shown in FIGS. 1A and 1B, the tip of the turning tool for 
finishing 10 point-contacts the center of the flat profile portion 12 to 
start the turning. Thus, at the beginning of the cutting into the bearing 
ring, a sufficiently large gap occurs between the edge of the turning tool 
for finishing 10 and the preformed piece to be worked, so that the chips 
can easily escape without being nipped therebetween. In addition, it is 
difficult to cause the offset of center of the bearing ring, so that the 
discharging direction of the chips is constant. Therefore, according to 
the present invention, it is possible to prevent the offset of center due 
to fine chips, so that it is possible to achieve a good turning without 
producing defective products. 
The profile of the bearing inner-race preformed piece 14 expressed by the 
full line in FIG. 2A, also shows a profile 16 of a forming-die surface of 
a roll (which will be hereinafter referred to as a "roll forming-die 
surface profile") and a profile 17 of a forming-die surface of a mandrel 
(which will be hereinafter referred to as a "mandrel forming-die surface 
profile"), which are used for the cold rolling working of the bearing 
inner-race preformed product 14. The left profile shows the roll 
forming-die surface profile 16, and the right profile shows the mandrel 
forming-die surface profile 17. Therefore, the roll forming-die surface 
profile 16 comprises the circular profile portions 23 on both sides, which 
correspond to the sectional profile of the preformed-piece external 
bearing ring having a greater diameter than that of the product external 
bearing ring obtained by turning the bearing inner-race preformed piece, 
and the flat profile portion 24, which is parallel to the outer surface of 
the bearing arranged between the circular profile portions 23 at a 
location lower than the top of locus expressed by the dotted line in FIG. 
2A by about 0.2 mm to the left. Since the inner surface 2a of the bearing 
inner-race profiled piece 14 is a flat surface, the mandrel forming-die 
surface profile 17 is also a simple flat surface. 
FIGS. 3A through 3D show cross-sections of the bearing rings of the bearing 
outer-race preformed piece 1 and the bearing inner-race preformed piece 
14, and partially enlarged portions thereof, according to the present 
invention. In FIGS. 3A through 3D, the turning stocks of the bearing 
outer-race preformed piece 1 and the bearing inner-race preformed piece 
14, which are formed by the cold rolling working, for manufacturing the 
products shown in FIGS. 1 and 2 are previously turned except for the 
portions of the bearing rings. FIG. 3A shows the bearing outer-race 
preformed piece 1. As shown in FIG. 3A, the internal bearing ring 7 is 
formed on the inner surface 2a of the bearing outer-race preformed piece 1 
by the cold rolling working. FIG. 3B shows an enlarged portion in the 
circle of FIG. 3A. As can be clearly seen from FIG. 3B, the internal 
bearing ring 7 formed in the inner surface 2a comprises the right and left 
circular profile portions 11 and the flat profile portion 12 therebetween. 
FIG. 3C shows the bearing inner-race preformed piece 14. As shown in FIG. 
3C, the external bearing ring 20 is formed in the outer surface 15b of the 
bearing inner-race preformed piece 14 by the cold rolling working. FIG. 3D 
shows an enlarged portion in the circle of FIG. 3C. As can be clearly seen 
from FIG. 3D, the external bearing ring 20 formed in the outer surface 15b 
comprises the right and left circular profile portions 23 and the flat 
profile portions 24 therebetween. 
FIG. 4 shows a method for holding the bearing outer-race preformed pieces 
1, which was manufactured by the cold rolling working and wherein the 
turning stock of a portion other then the bearing ring was turned, on a 
single purpose lathe by means of an outer-diameter chuck 26 of a collet 
chuck, to turn and finish the inner surface 2a of the bearing outer-race 
preformed piece 1 by means of a turning tool for finishing 28 to 
manufacture a bearing outer-diameter product. The turning tool for 
finishing 28 moves in a direction of arrow toward the inner surface 2a of 
the bearing outer-race preformed piece 1 to turn the previously rolled 
internal bearing ring 7 of the bearing outer-race preformed piece 1 to 
finish the product internal bearing ring. 
FIG. 5 shows a method for holding the bearing inner-race preformed pieces 
14, which was manufactured by the cold rolling working and wherein the 
turning stock of a portion other then the bearing ring was turned, on a 
single purpose lathe by means of an inner-diameter chuck 27 of a collet 
chuck, i.e., a tightening chuck, to turn and finish the outer surface 15b 
of the bearing inner-race preformed piece 14 by means of the turning tool 
for finishing 28 to manufacture a bearing inner-diameter product. The 
turning tool for finishing 28 moves in a direction of arrow toward the 
outer surface 15b of the bearing outer-race preformed piece 1 to turn the 
previously rolled external bearing ring 20 of the bearing outer-race 
preformed piece 14 to finish the product internal bearing ring. 
While the present invention has been disclosed in terms of the preferred 
embodiments in order to facilitate better understanding thereof, it should 
be appreciated that the invention can be embodied in various ways without 
departing from the principle of the invention. Therefore, the invention 
should be understood to include all possible embodiments and modification 
to the shown embodiments which can be embodied without departing from the 
principle of the invention as set forth in the appended claims.