Master cylinder, master cylinder body, and method for producing master cylinder

A master cylinder that can prevent impairment of a piston seal, and a method of producing the master cylinder are provided. A master cylinder has a circumferential groove (47) in which a piston seal is provided, the piston seal having an outer circumference which enables operating fluid to be supplied, and a communication groove (96) which opens into the circumferential groove (47) and extends from the circumferential groove (47) toward a bottom portion of a cylinder body (15) to connect a pressure chamber and the circumferential groove (47). The communication groove (96) is formed with its center made eccentric relative to the center of the circumferential groove (47). A chamfered portion (100) is formed over an entire extent of a corner portion formed by the circumferential groove (47) and the communication groove (96), along a circular path concentric with the communication groove (96).

BACKGROUND OF THE INVENTION

The present invention relates to a master cylinder for supplying operating fluid to a brake system of a vehicle, a master cylinder body, and a method of producing the master cylinder.

As a master cylinder for supplying operating fluid to a brake system of a vehicle, there is one of a structure in which a piston is directly fittingly inserted in a cylinder body of bottomed cylindrical shape, putting the sleeve for a cylinder in disuse. This type of a master cylinder comprises: a cylinder body of bottomed cylindrical shape, a supply passage provided in this cylinder body for supplying operating fluid therethrough from a reservoir; a pressure chamber defined inside the cylinder body by a piston which slides therein; a piston seal provided in a circumferential groove formed in the cylinder body, and having an inner circumference in sliding contact with the piston and an outer circumference which enables the operating fluid to be supplied from the supply passage into the pressure chamber; and a communication groove which opens into the circumferential groove and extends from this circumferential groove toward a bottom portion of the cylinder body to connect the pressure chamber side and the circumferential groove. This communication groove is formed with its center made eccentric relative to the center of the circumferential groove, and is provided to send the operating fluid, which is supplied through a portion between a bottom portion of the circumferential groove and the outer circumference of the piston seal, into the pressure chamber when the piston returns to its initial position after generating a fluid pressure in the pressure chamber. (See, for example, Japanese Patent Application Publication No. 2004-299568.)

SUMMARY OF THE INVENTION

In the above master cylinder, as a result of the structure in which the cylinder sleeve is disused and the piston is directly fittingly inserted in the cylinder body, in order to dispose the piston seal around an inner circumferential portion of the cylinder body, it is necessary, as shown inFIG. 7, that a circumferential groove200and a communication groove201, which is eccentric relative to the circumferential groove200so as to open into the circumferential groove200, be formed by cutting work. However, such a circumferential groove200and a communication groove201, when formed, causes a corner portion202between them, resulting in the possibility of the piston seal impaired by the corner portion202between the circumferential groove200and the communication groove201during, for example, deformations or minute movements of the piston seal.

Therefore, it is an object of the present invention to provide a master cylinder that can prevent impairment of the piston seal, as well as a method of producing the master cylinder.

In order to attain the above object, the invention provides a master cylinder comprising: a cylinder body of a bottomed cylindrical shape; a supply passage provided on the cylinder body to which operating fluid is supplied from a reservoir; a pressure chamber defined inside said cylinder body by a piston which slides inside the cylinder body; a circumferential groove formed in said cylinder body; a piston seal provided in said circumferential groove, said piston seal having an inner circumference thereof in sliding contact with said piston and an outer circumference which enables the operating fluid to be supplied from said supply passage to said pressure chamber; and a communication groove which opens into said circumferential groove and extends from the circumferential groove toward a bottom portion of said cylinder body to connect said pressure chamber and said circumferential groove, said communication groove being formed with a center thereof eccentric relative to a center of said circumferential groove, wherein a chamfered portion is formed over an entire extent of a corner portion formed by said circumferential groove and said communication groove, along a circular path concentric with said communication groove.

The communication groove may be partially formed outwardly beyond a bottom portion of said circumferential groove relative to a radial direction of the cylinder body.

The chamfered portion may be formed to be enlarged in diameter from a bottom portion of said communication groove toward said circumferential groove.

The chamfered portion may be formed simultaneously when forming said communication groove.

Further, the present invention provides a master cylinder body which is formed in a bottomed cylindrical shape, and inside which a piston is slidable, said master cylinder body comprising a supply passage to which operating fluid is supplied from a reservoir; a cylinder bore in which said piston is slidably inserted and a pressure chamber is formed, the pressure chamber being defined by said piston; a circumferential groove in which a piston seal is fitted, said piston seal having an inner circumference thereof in sliding contact with said piston and an outer circumference which enables the operating fluid to be supplied only in one direction from said supply passage to said pressure chamber; and a communication groove which opens into said circumferential groove and extends from the circumferential groove toward a bottom portion of said cylinder body to connect said pressure chamber and said circumferential groove, wherein said communication groove being formed with a center thereof eccentric relative to a center of said circumferential groove, and wherein a chamfered portion is formed over an entire extent of a corner portion formed by said circumferential groove and said communication groove, along a circular path concentric with said communication groove.

The communication groove may be partially formed outwardly beyond a bottom portion of said circumferential groove relative to a radial direction of the cylinder body.

The chamfered portion may be formed to be enlarged in diameter from a bottom portion of said communication groove toward said circumferential groove.

The chamfered portion is formed simultaneously when forming said communication groove.

Further, the present invention provides a method for producing a master cylinder having a cylinder bore formed inside a cylinder body, a part of said cylinder bore constituting a pressure chamber, a circumferential groove formed in the cylinder bore to receive a piston seal therein, and a communication groove which opens into the circumferential groove and extends from the circumferential groove toward a bottom portion of said cylinder bore to connect said pressure chamber and said circumferential groove, said communication groove being formed with a center thereof eccentric relative to a center of said circumferential groove, the method comprising: after completion of cutting said circumferential groove, cutting said communication groove; and cutting a chamfered portion on a corner portion formed by said circumferential groove and said communication groove, said chamfered portion being enlarged in diameter from a bottom portion of said communication groove toward said circumferential groove.

The communication groove may be partially cut outwardly beyond a bottom portion of said circumferential groove relative to a radial direction of the cylinder body.

The chamfered portion may be formed by an entire extent of said corner portion being cut along a circular path concentric with said communication groove.

The communication groove and said chamfered portion are simultaneously cut with the same cutting tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Reference numeral11inFIG. 1denotes a master cylinder of the present embodiment which generates a brake fluid pressure by the force corresponding to the operation amount of a brake pedal introduced through a brake booster (not shown). A reservoir12which discharges and supplies brake fluid (operating fluid) is mounted on an upper side of the master cylinder11.

The master cylinder11is of a tandem type having a cylinder body15machined and formed from a monolithic material into a bottomed cylindrical shape having a bottom portion13and a cylindrical portion14with a cylinder bore17defined therein, which cylinder body is disposed in a horizontal posture on a vehicle; a primary piston (piston)18inserted into the cylinder body15and positioned at an opening16side in the cylinder body (right side inFIG. 1) so as to be slidable along a longitudinal axis of the cylindrical portion14of the cylinder body15(hereinafter referred to as “cylinder axis”); and a secondary piston (piston)19inserted into the cylinder body15and positioned at a portion more closed to the bottom portion13than the primary piston18(left side inFIG. 1) so as to be slidable along the direction of the cylinder axis.

The cylinder body15has two mount portions22and23integrally formed on the cylindrical portion14at a predetermined position in a circumferential direction thereof (hereinafter referred to as “cylinder circumferential direction”), which extend outwardly in a radial direction of the cylindrical portion14(hereinafter referred to as “cylinder radial direction”), and are juxtaposed in the direction of the cylinder axis. In the mount portions22and23, mount holes24and25for mounting the reservoir12are formed. The position of the mount holes are coincided with each other in the cylinder circumferential direction.

A secondary discharge passage26and a primary discharge passage27which lead to the cylinder bore17are formed on a side of the mount portions22and23of the cylindrical portion of the cylinder body15. The secondary discharge passage26and primary discharge passage27are secured brake lines (not shown) for supplying a brake fluid to a brake system (not shown). These secondary discharge passage26and primary discharge passage27are spaced apart from each other in the direction of the cylinder axis, while positions of the secondary discharge passage26and primary discharge passage27are coincided in the cylinder circumferential direction.

On the bottom portion13side of the cylindrical portion14forming the cylinder bore17of the cylinder body15, inner diameter slide portions30and31, in which the secondary piston19is slidably fitted, are formed in order from the bottom portion13side. Further, on an opening16side of the cylindrical portion of the cylinder body15, inner diameter slide portions35and36, in which the primary piston18is slidably fitted, are formed in order from the bottom portion13side. These inner diameter slide portions30,31, and36are coaxial and have the same diameter.

In the cylindrical portion14of the cylinder body15, a large-diameter hole portion40which is coaxial with, but of larger diameter than, the inner diameter slide portions30and31are formed between the inner diameter slide portion30and the bottom portion13. Further, between the inner diameter slide portion31and the inner diameter slide portion35, a large-diameter hole portion41which is coaxial with, but of larger diameter than, the inner diameter slide portions35and36are formed. These large-diameter hole portions40and41have the same diameter.

On the bottom portion13side in the cylindrical portion14of the cylinder body15, at an intermediate position of the inner diameter slide portion30in the direction of the cylinder axis, a circumferential seal groove (circumferential groove)43is formed. The circumferential seal groove is annular in the cylinder circumferential direction, and has a shape recessed outwardly in the cylinder radial direction. In the circumferential seal groove, a piston seal51, which will be described later, is fitted. In addition, between the inner diameter slide portions30and31, an open groove44, which is annular in the cylinder circumferential direction and has a shape recessed outwardly in the cylinder radial direction, is formed. Furthermore, at an intermediate position of the inner diameter slide portion31in the direction of the cylinder axis, a circumferential seal groove45is formed which is annular in the cylinder circumferential direction and has a shape recessed outwardly in the cylinder radial direction.

Also on the opening16side in the cylindrical portion14of the cylinder body15, at an intermediate position of the inner diameter slide portion35in the direction of the cylinder axis, a circumferential seal groove (circumferential groove)47is formed. The circumferential seal groove is annular in the cylinder circumferential direction, and has a shape recessed outwardly in the cylinder radial direction. In the circumferential seal groove, a piston seal56, which will be described later, is fitted. In addition, between the side-permitting inner diameter portions35and36, an open groove48is formed which is annular in the cylinder circumferential direction and has a shape recessed outwardly in the cylinder radial direction. Furthermore, at an intermediate position of the inner diameter slide portion36in the direction of the cylinder axis, a circumferential seal groove49is formed which is annular in the cylinder circumferential direction and has a shape recessed outwardly in the cylinder radial direction.

The circumferential seal groove43of the inner diameter slide portion30of the cylinder body15is located near the mount hole24on bottom portion13side. The piston seal51is fitted in the circumferential seal groove43. The piston seal51is a cup seal of C-shaped cross section having lip portions at inner and outer circumferences thereof, and is mounted in the circumferential seal groove43, with an open side thereof disposed on the bottom portion13side.

A communication hole52, which is boredly formed from the mount hole24on the bottom portion13side, opens into the open groove44between the inner diameter slide portions30and31of the cylinder body15. By connecting the cylinder body15and the reservoir12in a communicable manner and being always kept in communication with the reservoir12, the open groove44and the communication hole52mainly constitute a secondary supply passage (supply passage)53to which the brake fluid (operating fluid) is supplied from the reservoir12.

A partition seal54is fitted in the circumferential seal groove45on the inner diameter slide portion31of the cylinder body15. This partition seal54is also a cup seal of C-shaped cross section having lip portions at inner and outer circumferences thereof, and is mounted in the circumferential seal groove45, with an open side thereof disposed on the opening16side.

The circumferential seal groove47of the inner diameter slide portion35of the cylinder body15is located near the mount hole25on the opening16side. The piston seal56is fitted in the circumferential seal groove47. The piston seal56is a cup seal of C-shaped cross section having lip portions at inner and outer circumferences thereof, and is mounted in the circumferential seal groove47, with an open side thereof disposed on the bottom portion13side.

A communication hole57, which is boredly formed from the mount hole25on the opening16side, opens into the open groove48between the inner diameter slide portions35and36of the cylinder body15. By connecting the cylinder body15and the reservoir12in a communicable manner and being always kept in communication with the reservoir12, the open groove48and the communication hole57mainly constitute a primary supply passage (supply passage)58to which the brake fluid is supplied from the reservoir12.

A partition seal59is fitted in the circumferential seal groove49on the inner diameter slide portion36of the cylinder body15. This partition seal59is also a cup seal of C-shaped cross section having lip portions at inner and outer circumferences thereof, and is mounted in the circumferential seal groove49, with an open side thereof disposed on the bottom portion13side.

The secondary piston19, which is fitted in the cylinder body15on the bottom portion13side, is of bottomed cylindrical shape having a cylindrical portion61, and a bottom portion62formed inside the cylindrical portion61, at a position close to one side in the longitudinal axis direction. The secondary piston19is slidably fitted in the inner diameter slide portions30and31of the cylinder body15, with its bottom portion62disposed on the opening16side of the cylinder body15. In addition, an annular stepped portion64is formed at the outer circumference of an end portion on the opposite side of the cylindrical portion61from the bottom portion62. The stepped portion is of slightly smaller diameter than the remaining portion. Furthermore, on its bottom portion62side of the stepped portion64of the cylindrical portion61, a plurality of radial ports65are formed. The ports extend through the stepped portion64in the cylinder radial direction.

A portion which is surrounded by the bottom portion13and the cylindrical portion14, mainly the large-diameter hole portion40, which define the cylinder bore17of the cylinder body15, and the secondary piston19, forms the secondary pressure chamber (pressure chamber)68which supplies the fluid pressure to the secondary discharge passage26. This secondary pressure chamber68communicates to the secondary supply passage53when the secondary piston19is in position to allow the ports65to open into the open groove44. The inner circumference of the piston seal51provided in the circumferential seal groove43on the bottom portion13side of the cylinder body15slidingly contacts the outer circumference of the secondary piston19and, in a state where the secondary piston19has the ports65located on a position more close to the bottom13than the piston seal51, the piston seal51can provide a seal between the secondary supply passage53and the secondary pressure chamber68, i.e., the piston seal51can cut off the communication between the secondary pressure chamber68and the secondary supply passage53and reservoir12.

The piston seal51, provided in the circumferential seal groove43formed in the cylinder body15and having the inner circumference which slidingly contact with the secondary piston19, cuts off the communication between the secondary pressure chamber68and the secondary supply passage53and reservoir12when the fluid pressure in the secondary pressure chamber68becomes greater than that in the secondary supply passage53(i.e., atmospheric pressure); and provides communication between the secondary pressure chamber68and the secondary supply passage53and reservoir12when the fluid pressure in the secondary pressure chamber68becomes smaller (negative pressure) than that in the secondary supply passage53, via a gap between the outer circumference of the outer lip portion and the circumferential seal groove43which is produced by deformation of the outer lip portion, thereby enabling supply of the brake fluid into the secondary pressure chamber68.

Between the secondary piston19and the bottom portion13of the cylinder body15, there is provided a spacing adjustment unit71including a secondary piston spring70which defines the spacing between the secondary piston19and the bottom portion13of the cylinder body15in the initial state where no input is applied from the not-shown brake pedal (right side inFIG. 1).

This spacing adjustment unit71has a spring retainer72which abuts with the bottom portion13of the cylinder body15, a spring retainer73which abuts with the bottom portion62of the secondary piston19and coupled to the spring retainer72so that the spring retainer73is slidable only within predetermined limits. The secondary piston spring70is interposed between the spring retainers72and73on both sides.

The primary piston18, which is fitted in the cylinder body15on the opening16side, is shaped to have a first cylindrical portion77, a bottom portion78formed on one side in the longitudinal axis direction of the first cylindrical portion77, and a second cylindrical portion79formed on the opposite side of the bottom portion78from the first cylindrical portion77; and is slidably fitted in the inner diameter slide portions35and36of the cylinder body15, with its first cylindrical portion77disposed on the secondary piston19side in the cylinder body15. A not-shown brake booster output shaft is inserted into the secondary cylindrical portion79, and this output shaft pushes the bottom portion78.

An annular recessed portion81is formed at the outer circumference of an end portion on the opposite side of the first cylindrical portion77from the bottom portion78. The recessed portion is of slightly smaller diameter than the remaining portion. Furthermore, on its bottom portion78side, the recessed portion81of the first cylindrical portion77is provided with a plurality of radial ports82which extend through the first cylindrical portion in the radial direction.

A portion which is surrounded by the cylindrical portion14, mainly the large-diameter hole portion41, which defines the cylinder bore17of the cylinder body15, the primary piston18, and the secondary piston19, forms a primary pressure chamber (pressure chamber)84which supplies the fluid pressure to the primary discharge passage27. This primary pressure chamber84communicates to the primary supply passage58when the primary piston18is in position to allow the ports82to open into the open groove48. The inner circumference of the piston seal56provided in the circumferential seal groove47of the cylinder body15slidingly contacts the outer circumference of the primary piston18and, in a state where the primary piston18has the ports82located on position more close to the bottom13than the piston seal56, the piston seal56can provide a seal between the primary supply passage58and the primary pressure, chamber84, i.e., can cut off the communication between the primary pressure chamber84and the primary supply passage58and reservoir12.

The piston seal56, provided in the circumferential seal groove47formed in the cylinder body15and having the inner circumference which slidingly contact with the primary piston18, cuts off the communication between the primary pressure chamber84and the primary supply passage58and reservoir12when the fluid pressure in the primary pressure chamber84becomes greater than that in the primary supply passage58(i.e., atmospheric pressure); and provides communication between the primary pressure chamber84and the primary supply passage58and reservoir12when the fluid pressure in the primary pressure chamber84becomes smaller (negative pressure) than that in the primary supply passage58, via a gap between the outer circumference of the outer lip portion and the circumferential seal groove47which is produced by deformation of the outer lip portion, thereby enabling supply of the brake fluid into the primary pressure chamber84.

The partition seal54provided in the circumferential seal groove45slidingly contacts the secondary piston19so as to seal between the secondary pressure chamber68and secondary supply passage53and the primary pressure chamber84, and the partition seal59provided in the circumferential seal groove49slidingly contacts the primary piston18so as to seal the primary supply passage58and the primary pressure chamber84against the external air.

Between the secondary piston19and the primary piston18, there is provided a spacing adjustment unit88including a primary piston spring87which defines the spacing between the secondary piston19and the primary piston18in the initial state where no input is applied from the not-shown brake pedal (right side inFIG. 1).

This spacing adjustment unit88has a spring retainer89which abuts with the bottom portion62of the secondary piston19, a spring retainer90which abuts with the bottom portion78of the primary piston18, and a shaft member91fixed at one end portion to the spring retainer89and supporting the spring retainer90in a slidable manner within predetermined limits. The primary piston spring87is interposed between the spring retainers89and90on both sides.

At a portion of the inner diameter slide portion30of the cylinder body15on a side more close to a large-diameter hole portion40than the circumferential seal groove43, a communication groove94is formed which is recessed outwardly in the cylinder radial direction. The communication groove94opens, at one end thereof in the direction of the cylinder axis, into the circumferential seal groove43, and extends toward the bottom portion13of the cylinder bore17, and opens, at the other end, into the large-diameter hole portion40. This communication groove94provides communication between the secondary discharge passage26formed in the large-diameter hole portion40and the circumferential seal groove43via the secondary pressure chamber68. This communication groove94is formed in an arc shape of smaller diameter than the inner diameter slide portion30and having a center eccentric relative to the center of the circumferential seal groove43. Owing to the presence of this communication groove94, the brake fluid can efficiently be supplied from the secondary supply passage53to the secondary pressure chamber68when a negative pressure is produced in the above secondary pressure chamber68.

Likewise, at a portion of the inner diameter slide portion35of the cylinder body15on a side more close to a large-diameter hole portion41than the circumferential seal groove47, a communication groove96is formed which is recessed outwardly in the cylinder radial direction. The communication groove96opens, at one end thereof in the direction of the cylinder axis, into the circumferential seal groove47, and extends toward the bottom portion13of the cylinder bore17, and opens, at the other end, into the large-diameter hole portion41. This communication groove96provides communication between the primary discharge passage27formed in the large-diameter hole portion41and the circumferential seal groove47via the primary pressure chamber84. This communication groove96is also formed in an arc shape of smaller diameter than the inner diameter slide portion35and having a center eccentric relative to the center of the circumferential seal groove47. Owing to the presence of this communication groove96, the brake fluid can efficiently be supplied from the primary supply passage58to the primary pressure chamber84when a negative pressure is produced in the above primary pressure chamber84.

Moreover, in the present embodiment, as exemplified inFIGS. 2 to 5in connection with the communication groove96on the primary pressure chamber84side of the cylinder body15, chamfered portions100are formed over an entire extent of a corner portion formed by the circumferential seal groove47and the communication groove96, and over an entire extent of a corner portion formed by the circumferential seal groove43and the communication groove94. As shown, this chamfered portion100is formed along a circular path that is concentric with the communication groove96, and machined simultaneously with the communication groove96. The primary side is described by way of example in the following description for the reason of illustration, but the same also applies to the secondary side. InFIGS. 2,3,5and7, the piston seal56provided in the circumferential seal groove47is omitted, for the reason of illustration.

A T-slot cutter (tool)101for simultaneously machining the communication groove96and chamfered portion100, as shown inFIG. 6, has: a first end face102located at one end in the direction of the axis of the tool and extending along a direction perpendicular to the axis; a first cutting portion103having a tapered cutting face which extends so as to increase in diameter as it proceeds in the longitudinal axis direction away from an outer edge portion of the first end face102; a second cutting portion104which extends, with a cutting face having a constant outer diameter, in the longitudinal axis direction from the outer edge portion of the first cutting portion103on the side opposite from the first end face102; a third cutting portion105provided on the opposite side of the second cutting portion104from the first cutting portion103and having a tapered cutting face which increases in diameter as it proceeds in the longitudinal axis direction away from the second cutting portion104; a fourth cutting portion106with a cutting face having a constant outer diameter which extends from the outer edge portion of the third cutting portion105in an opposite direction to the direction in which the second cutting portion104extends; a fifth cutting portion107provided on the opposite side of the fourth cutting portion106from the third cutting portion105and having a tapered cutting face which decreases in diameter as it proceeds in the longitudinal axis direction away from the fourth cutting portion106; and a second end face108located on the opposite side of the fifth cutting portion107from the fourth cutting portion106and extending along a direction perpendicular to the axis. The minimum diameter of the cutting face of the fifth cutting portion107is greater than the minimum diameter of the cutting face of the third cutting portion105, i.e., than the outer diameter of the cutting face of the second cutting portion104.

FIGS. 4A,4B show a circumferential seal groove and a communication groove in the master cylinder, of whichFIG. 4Ais a cross-sectional view taken along A-A inFIG. 3, andFIG. 4Ba cross-sectional view taken along B-B inFIG. 3.

After completion of cutting a circumferential seal groove47of substantially constant circular cross section with another not-shown cutting tool which is inserted through the opening16into the cylinder body15, the above T-slot cutter101, which is also inserted through the opening16, is used, with its center being off-centered in the direction A (FIG. 6) towards the outer circumference of the cylinder body15relative to the center of the circumferential seal groove47, to cut the communication groove96on the bottom portion13side of the circumferential seal groove47. In this instance, the second cutting portion104is fed in the direction A outwardly beyond the bottom portion of the circumferential groove in the cylinder radial direction, with an intermediate position in the longitudinal axis direction of the third cutting portion105aligned, in the cylinder longitudinal axis direction, with the position of a wall portion110of the circumferential seal groove47on the cylinder bottom portion13side. Thereupon, a portion more close to the cylinder bottom portion13of the inner diameter slide portion35than the circumferential seal groove47is cut out in the cylinder longitudinal axis direction by the T-slot cutter101.

In this instance, a wall portion112shown inFIGS. 4and5, which is at the boundary between the large-diameter hole portion41and the communication groove96and extends along a direction perpendicular to the cylinder axis, is formed by the first cutting portion103, along with a tapered bottom chamfered portion113which increases in diameter as it proceeds in the longitudinal axis direction away from the wall portion112; and a bottom portion114of the communication groove96which lies along the cylinder longitudinal axis is formed by the second cutting portion104. In addition, a chamfered portion100is formed, by the third cutting portion105, over a portion from a corner portion between the bottom portion114of the communication groove96and the wall portion110of the circumferential seal groove47to a corner portion between the bottom portion114of the communication groove96and the bottom portion111of the circumferential seal groove47as shown inFIG. 5(i.e., over an entire extent of the corner portion formed by the circumferential seal groove47and the communication groove96), along a circular path concentric with the communication groove96. This chamfered portion100has a shape enlarged in diameter as it proceeds from the bottom portion114of the communication groove96toward the circumferential seal groove47. Furthermore, a bottom portion115which is at a greater depth than the bottom portion111of the circumferential seal groove47and along the direction of the cylinder axis is formed in the circumferential seal groove bottom portion111by the fourth cutting portion106, and a tapered portion116as shown inFIG. 4Awhich decreases in diameter as it proceeds away from the communication groove96is formed in the bottom portion111of the circumferential seal groove47by the fifth cutting portion107, along with a wall portion117which is at the boundary between the tapered portion116and the bottom portion111of the circumferential seal groove47and along a direction perpendicular to the cylinder axis.

As mentioned above, by feeding the third cutting portion104of the T-slot cutter101outwardly beyond the bottom portion111of the circumferential seal groove47relative to the cylinder radial direction, the communication groove96is partially formed deeper outwardly beyond the bottom portion111of the circumferential seal groove47relative to the cylinder radial direction.

According to the present embodiment as described above, after completion of cutting the circumferential seal groove47, the communication groove96is cut, and the chamfered portion100is cut on the corner portion formed by the circumferential seal groove47and the communication groove96. Since this chamfered portion100is formed over an entire extent of the corner portion formed by the circumferential seal groove47and the communication groove96, the piston seal56is not impaired on the corner portion between the circumferential seal groove47and the communication groove96if it experiences deformations, minute displacements, or the like. Impairment of the piston seal56is thus prevented.

In addition, since the chamfered portion100is formed over the entire extent of the corner portion formed by the circumferential seal groove47and the communication groove96, along a circular path concentric with the communication groove96, the communication groove96and the chamfered portion100can be cut at a time with the same T-slot cutter100. Therefore, an easy machining can be obtained as compared with the case where the corner portion, which is eccentric relative to the circumferential seal groove47and thus intricate in shape and difficult to machine, is cut anew with another tool, thereby improving the productivity.

Furthermore, since the communication groove96is partially formed by cutting work outwardly beyond the bottom portion111of the circumferential seal groove47relative to the cylinder radial direction, the fluid can be supplied satisfactorily through the gap between the outer circumference of the piston seal56and the bottom portion111of the circumferential seal groove47, and through the communication groove96.

Additionally, since the chamfered portion100is formed to be enlarged in diameter as it proceeds from the bottom portion114of the communication groove96toward the circumferential seal groove47, the fluid can be supplied further satisfactorily through the gap between the outer circumference of the piston seal56and the bottom portion111of the circumferential seal groove47, and through the communication groove96.

The above effects are also applicable to the secondary side where the same construction as described above is employed.

According to the master cylinder of the invention, since a chamfered portion is formed over an entire extent of the corner portion formed by the circumferential groove which receives the piston seal therein and the communication groove which opens into the circumferential groove, the piston seal is not impaired by the corner portion between the circumferential groove and the communication groove if it undergoes deformations, minute movements, or the like. Therefore, impairment of the piston seal can be prevented.

Further, since the communication groove is partially formed outwardly beyond the bottom portion of the circumferential groove relative to the cylinder radial direction, the fluid can be supplied satisfactorily through a gap between the outer circumference of the piston seal and the bottom portion of the circumferential groove, and through the communication groove.

Further, since the chamfered portion is formed to be enlarged in diameter from the bottom portion of the communication groove toward the circumferential groove, the fluid can be supplied further satisfactorily through the gap between the outer circumference of the piston seal and the bottom portion of the circumferential groove, and through the communication groove.

Further, since the chamfered portion is formed simultaneously when forming the communication groove, its machining can be done easily as compared with the case where the corner portion, which is eccentric relative to the circumferential groove and thus difficult to machine, is cut anew with another tool to form the chamfered portion, resulting in an improved productivity.

According to the master cylinder body of the invention, since a chamfered portion is formed over an entire extent of the corner portion formed by the circumferential groove which receives the piston seal therein and the communication groove which opens into the circumferential groove, the piston seal is not impaired by the corner portion between the circumferential groove and the communication groove if it undergoes deformations, minute movements, or the like. Therefore, impairment of the piston seal can be prevented.

Further, since the communication groove is partially formed outwardly beyond the bottom portion of the circumferential groove relative to the cylinder radial direction, the fluid can be supplied satisfactorily through a gap between the outer circumference of the piston seal and the bottom portion of the circumferential groove, and through the communication groove.

Further, since the chamfered portion is formed to be enlarged in diameter from the bottom portion of the communication groove toward the circumferential groove, the fluid can be supplied further satisfactorily through the gap between the outer circumference of the piston seal and the bottom portion of the circumferential groove, and through the communication groove.

Further, since the chamfered portion is formed simultaneously when forming the communication groove, its machining can be done easily as compared with the case where the corner portion, which is eccentric relative to the circumferential groove and thus difficult to machine, is cut anew with another tool to form the chamfered portion, resulting in an improved productivity.

According to the method for producing the master cylinder, since, after cutting the circumferential groove, the communication groove is cut, and a chamfered portion is cut on the corner portion formed by the circumferential groove and the communication groove, which chamfered portion is enlarged in diameter from the bottom portion of the communication groove toward the circumferential groove, the piston seal is not impaired by the corner portion between the circumferential groove and the communication groove if it undergoes deformations, minute movements or the like. Therefore, impairment of the piston seal can be prevented. In addition, since the chamfered portion is formed to be enlarged in diameter as it proceeds from the bottom portion of the communication groove to the circumferential groove side, the fluid can be supplied satisfactorily through a gap between the outer circumference of the piston seal and a bottom portion of the circumferential groove, and through the communication groove.

Further, since the communication groove is partially cut outwardly beyond the bottom portion of the circumferential groove relative to the cylinder radial direction, the fluid can be supplied further satisfactorily through the gap between the outer circumference of the piston seal and the bottom portion of the circumferential groove, and through the communication groove.

Further, since an entire extent of the corner portion formed by the circumferential groove and the communication groove is cut along a circular path which is concentric with the communication groove so as to form the chamfered portion, it becomes possible to cut the communication groove and the chamfered portion at a time with the same tool. Therefore, an easy machining becomes possible as compared with the case where the corner portion, which is eccentric relative to the circumferential groove and thus intricate in shape and difficult to machine, is cut anew with another tool, resulting in an improved productivity.

Further, since the communication groove and the chamfered portion are simultaneously cut with the same cutting tool, an easy machining can be done as compared with the case where the corner portion, which is eccentric relative to the circumferential groove and thus intricate in shape and difficult to machine, is cut anew with another tool, resulting in an improved productivity.

The present application claims priority under 35 U.S.C. section 119 to Japanese Patent Application No. 2006-23693, filed on Aug. 31, 2006. The entire disclosure of Japanese Patent Application No. 2006-236932 filed on Aug. 31, 2006 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.

The Japanese Patent Public Disclosure No. 2004-299568 is incorporated herein by reference in its entirety.