in order to clarify the present invention in further detail , description will be made referring to the accompanying drawings in the following . fig1 to 3 exemplify the basic structure of the pen tip according to the present invention . the pen tip is made up of a ball retaining tube ( 1 ), a ball receiving stem ( 2 ) having ink flow paths ( b ) on its end surface portion ( 2a ), and a rotatable member in the form of a ball ( 3 ). the ball retaining tube ( 1 ) has a bore a little larger in diameter than the ball ( 3 ) and contains therein the ball receiving , elongated stem ( 2 ), which is of smaller diameter so that a gap is formed between the same and the inner surface of the tube , and the tube is caulked onto the stem from the outside at proper positions such that the tube is made integral with the stem and there is formed an ink flow path ( a ) axially running between the inner surface of the tube and the peripheral surface of the ball receiving stem ( 2 ), and within the tube at its end there is formed a ball retaining portion ( 4 ) having the end surface portion ( 2a ) of the ball receiving stem ( 2 ) as its bottom and operably holding the ball ( 3 ) for rotation . the caulked portions ( 5 ) of the ball retaining tube ( 1 ) are made into protruded stripes uniformly projecting toward the center axis at equal intervals on the periphery of the ball receiving stem ( 2 ) starting at the vicinity of the end surface portion ( 2a ) of the ball receiving stem ( 2 ) and ending at the position suitably separated therefrom in the axial direction , whereby the ball receiving stem ( 2 ) is uniformly pressed from the surroundings and good strength against bending , or rigidity , of the integrated body of the ball retaining tube ( 1 ) and ball receiving stem ( 2 ) is provided and , at the same time , the ink flow path ( a ) is formed in uniform thickness in axial and circumferential directions . the gap width in the ink flow path ( a ) will produce no problem if it is larger than 3 - 6μ when the thickness of the ink adhering to the surface being 3 - 4μ is taken into consideration , but 25 - 100 thick gap is given , in practice , subject to adjustment according to the properties of the materials used such as viscosity of the ink . when an oily ink is used , a somewhat larger gap is considered suitable . the ball receiving stem ( 2 ) is of a solid stem structure and has a diameter such that the ink flow path ( a ) is formed between the same and the inner surface of the ball retaining tube ( 1 ), and the same is provided disposed on its end surface portion ( 2a ) with groove - like ink flow paths ( b ) communicating with the ink flow space ( a ). the ink flow paths ( b ) are adapted to conduct the ink from the ink flow path ( a ) to the bottom of the ball retaining portion ( 4 ), i . e ., the portion with which the ball rubs , or its vicinity , by capillary action . the ink flow paths ( b ), extended as far as the position of a ball receiving surface portion ( 6 ) in the center of the end surface portion ( 2a ), or the vicinity of the ball receiving surface portion ( 6 ), is approximately 25 - 110μ in width of the groove and approximately more than 20μ , or preferably 40 - 100μ , in depth . generally speaking , the width and the depth are inversely proportional to each other . the cross section of the ink flow path ( b ) takes various shapes , such as v - shape , u - shape , ␣ shape , shape , and so on . the ball ( 3 ), prevented from getting out of place by the drawn portion ( 7 ) and provided at the end of the ball retaining tube ( 1 ), is received within the ball retaining portion ( 4 ) and adapted to draw the ink from the ink flow paths ( b ) as it rotates , being supported on the ball receiving surface portion ( 6 ). the diameter of the ball ( 3 ) is approximately 0 . 3 to 1 . 0 mm . fig4 to 11 exemplify pen tip structures according to the invention in which the cross - sectional shape of the inner surface of the ball retaining tube ( 1 ) and the cross - sectional profile of the ball receiving stem ( 2 ) are all alike similar to each other and the ink flow paths ( b ) are similarly communicating with each other on the ball receiving surface portion ( 6 ) but are different in their configurations . these pen tips are all made up of three parts , that is , the ball retaining tube ( 1 ), the ball receiving stem ( 2 ) having , on its end surface portion ( 2a ), a plurality of ink flow paths ( b ) communicating with each other reaching as far as the center of the ball receiving surface portion ( 6 ), and the ball ( 3 ), and the ball retaining tube ( 1 ) and the ball receiving stem ( 2 ) are arranged in the combination of a circular tube and a circular stem or of a square tube and a square stem . the ball retaining tube ( 1 ) has a bore somewhat larger in diameter than the ball ( 3 ), contains therein the ball receiving stem ( 2 ) of a smaller diameter such that there may be provided a space between the same and the inner surface of the tube , and is inwardly caulked from outside the tube at proper positions such that the tube is pressed against the stem and made integral therewith and , at the same time , there is provided the ink flow space ( a ) axially running between the peripheral surface of the ball receiving stem ( 2 ) and the inner surface of the tube . the tube is provided with the ball retaining portion ( 4 ) within the same at its end having the end surface portion ( 2a ) of the ball receiving stem ( 2 ) serving as the bottom and the ball retaining portion ( 4 ) operably retains the ball ( 3 ) for rotation . the caulked portions ( 5 ) of the ball retaining tube ( 1 ) are made into protruded stripes uniformly projecting toward the center axis at equal intervals on the periphery of the ball receiving stem ( 2 ) starting in the vicinity of the end surface portion ( 2a ) of the ball receiving stem ( 2 ) and ending at the position suitably separated therefrom in the axial direction , whereby the ball receiving stem ( 2 ) is uniformly pressed from the surroundings and good strength against bending , or rigidity , of the integrated body of the ball retaining tube ( 1 ) and ball receiving stem ( 2 ) is provided and , at the same time , the ink flow space ( a ) is formed in uniform thickness axially and circumferentially . the gap width in the ink flow space ( a ) will produce no problem if it is larger than 3 - 6μ when the thickness of the ink adhering to the surface being 3 - 4μ is taken into consideration , but 25 - 100μ thick gap is given , in practice , subject to adjustment according to the properties of the materials used such as viscosity of the ink . when an oily ink is used , somewhat larger gap is considered suitable . the ball receiving stem ( 2 ) is of a solid stem structure and has a diameter such that the ink flow space ( a ) is formed between the same and the inner surface of the ball retaining tube ( 1 ), and the end surface portion ( 2a ) is provided thereon with the ink flow paths ( b ) in groove forms extending from the end portion of the ink flow space ( a ) centripetally to the center of the ball receiving surface portion ( 6 ) where they communicate with each other , whereby it is arranged that the ink is conducted by capillarity from the ink flow space ( a ) to the ball receiving surface portion ( 6 ), i . e ., the portion where the ball rubs therewith , in the center of the end surface portion ( 2a ). the ink flow paths ( b ) is provided in such a configuration that is easy to process , i . e ., radially extended at equal angular intervals in four directions ( fig5 ), in two directions ( fig9 ), or in six directions ( not shown ). the groove width , preferably , is approximately 25 - 110μ and the depth is approximately over 20μ or , preferably , 40 - 100μ , where the groove width and the depth are generally inversely proportional to each other . and , the corner portion ( 6a ) of the center portion , with which the ball ( 3 ) rubs , of the ink flow paths ( b ) may be made round so that the frictional resistance to the rotation of the ball ( 3 ) may be further decreased ( fig6 ). the ball ( 3 ), prevented from getting out of place by the drawn portion ( 7 ) provided at the end of the ball retaining tube ( 1 ), is received within the ball retaining portion ( 4 ) and adapted to draw the ink from the center and its vicinity of the ink flow paths ( b ) as it rotates being supported by the ball receiving surface portion ( 6 ). diameter of the ball 3 is approximately 0 . 3 to 1 . 0 mm . fig1 and 11 show an embodiment in which the ball ( 3 ) is formed into a roller and both the ball retaining tube ( 1 ) and ball receiving stem ( 2 ) are made square in their cross sections . in this embodiment , the width of the roller becomes the width of the written line . fig1 to 19 exemplify pen tip structures according to the invention in which the cross - sectional shape of the inner surface of the ball retaining tube ( 1 ) and the cross - sectional profile of the ball receiving stem ( 2 ) are all alike similar to each other and the ink flow paths ( b ) in communication with each other are similarly arranged to surround the ball receiving surface portion ( 6 ) but are different in their configurations . these pen tips are all made up of three parts , that is , the ball retaining tube ( 1 ), the ball receiving stem ( 2 ) having , on its end surface portion ( 2a ), the ink flow paths ( b ) reaching as far as the vicinity of the ball receiving surface portion ( 6 ) and arranged to be communicating with each other and surrounding the ball receiving surface portion ( 6 ), and the ball ( 3 ), and the ball retaining tube ( 1 ) and the ball receiving stem ( 2 ) are arranged in the combination of a circular tube and a circular stem or of a square tube and a square stem . the ball retaining tube ( 1 ) has a bore somewhat larger in diameter than the ball ( 3 ), contains therein the ball receiving stem ( 2 ) of a smaller diameter such that there may be provided a space between the same and the inner surface of the tube , and is inwardly caulked from outside the tube at proper positions such that the tube is pressed against the stem to be made integral therewith and , at the same time , there is provided the ink flow space ( a ) axially running between the peripheral surface of the ball receiving stem ( 2 ) and the inner surface of the tube . the tube is provided with the ball retaining portion ( 4 ) within the same at its end having the end surface portion ( 2a ) of the ball receiving stem ( 2 ) serving as the bottom . the ball retaining portion ( 4 ) operably retains the ball ( 3 ) for rotation . the caulked portions ( 5 ) of the ball retaining tube ( 1 ) are made into protruded stripes uniformly projecting toward the center axis at equal intervals on the periphery of the ball receiving stem ( 2 ) starting at the vicinity of the end surface portion ( 2a ) of the ball receiving stem ( 2 ) and ending at the position suitably separated therefrom in the axial direction , whereby the ball receiving stem ( 2 ) is uniformly pressed from the surroundings and good strength against bending , or rigidity , of the integrated body of the ball retaining tube ( 1 ) and ball receiving stem ( 2 ) is provided and , at the same time , the ink flow space ( a ) is formed in uniform thickness axially and circumferentially . the gap width in the ink flow space ( a ) will produce no problem if it is larger than 3 - 6μ when the thickness of the ink adhering to the surface being 3 - 4μ is taken into consideration , but 25 - 100μ thick gap is given , in practice , subject to adjustment according to the properties of the materials used such as viscosity of the ink . when an oily ink is used , somewhat larger gap is considered suitable . the ball receiving stem ( 2 ) is of a solid stem structure and has a diameter such that the ink flow space ( a ) is formed between the same and the inner surface of the ball retaining tube ( 1 ), and the same is provided with ball receiving surface portion ( 6 ) on its end surface portion ( 2a ) and with the groove - like ink flow paths ( b ) communicating with the ink flow space ( a ) disposed on the surroundings of the ball receiving surface portion ( 6 ), whereby the ink is adapted to be conducted by capillarity from the ink flow space ( a ) to the surroundings of the ball receiving surface portion ( 6 ), i . e ., the vicinity of the portion with which the ball rubs . the ink flow paths ( b ) consist of the flow path portion to surround the ball receiving surface portion ( 6 ) in such a configuration as parallel crosses ( fig1 ) or others as shown in fig1 , 17 , and 19 , and the flow path portion communicating thence with the ink flow space ( a ). the groove width is approximately 25 - 110μ , and the depth is approximately 20μ or more , or , preferably , 40 - 100μ , and the groove width and depth are generally inversely proportional to each other . the ball receiving surface portion ( 6 ) being flat and smooth supports the ball ( 3 ) on a point . by the way , the ball receiving surface portion ( 6 ) may optionally be arranged in a slightly concaved form ( fig1 ). the ball ( 3 ), prevented from getting out of place by the drawn portion ( 7 ) provided at the end of the ball retaining tube ( 1 ), is received within the ball retaining portion ( 4 ) and adapted to draw the ink from the ink flow paths ( b ) in the vicinity of the ball receiving surface portion ( 6 ) as it rotates being supported on a point on the ball receiving surface portion ( 6 ). diameter of the ball ( 3 ) is approximately 0 . 3 to 1 . 0 mm . fig1 and 19 show an embodiment in which the ball ( 3 ) is formed into a roller and both the ball retaining tube ( 1 ) and ball receiving stem ( 2 ) are shaped square in their cross sections . this embodiment has a feature that the roller advances straight in writing and the width of the roller becomes the width of the written line . the caulked portions ( 5 ) are provided in a dotted arrangement . fig2 to 30 exemplify pen tip structures according to the invention in which the cross - sectional shapes of the inner surfaces of the ball retaining tubes ( 1 ) are all alike circular and the cross - sectional profiles of the ball receiving stems ( 2 ) are likewise square but are different from each other in the configurations of the ink flow paths ( b ). these pen tips are all made up of three parts , i . e ., the ball retaining tube ( 1 ) whose inner surface is circular in cross section , the ball receiving stem ( 2 ) whose cross - sectional profile is square and whose end surface portion ( 2a ) is provided with the ink flow paths ( b ) reaching as far as the center or its vicinity of the ball receiving surface portion ( 6 ), and the ball ( 3 ). the ball retaining tube ( 1 ) is circular in cross section and has a bore somewhat larger in diameter than the ball ( 3 ), containing therein the ball receiving stem ( 2 ) being square in cross section and having a diagonal width slightly larger than , equal to , or smaller than the bore of the tube , and is inwardly caulked from outside the tube at proper positions , the caulked portions ( 5 ), such that the tube is pressed against the stem to be made integral therewith and , at the same time , there is provided the ink flow space ( a ) axially running between the peripheral surface of the ball receiving stem ( 2 ) and the inner surface of the tube . the tube is provided with the ball retaining portion ( 4 ) within the same at its end having the end surface portion ( 2a ) of the ball receiving stem ( 2 ) serving as the bottom . the ball retaining portion ( 4 ) operably retains the ball ( 3 ) for rotation . the ball receiving stem ( 2 ) is of a stem structure being square in cross section and the ball receiving stem ( 2 ) is provided with the ball receiving surface portion ( 6 ) at its end surface portion ( 2a ) and , on the ball receiving surface portion ( 6 ), provided with the groove - shaped ink flow paths ( b ) communicating with the ink flow space ( a ) and extended as far as the center or its vicinity of the ball receiving surface portion ( 6 ), whereby the ink is adapted to be guided from the ink flow space ( a ) to the ball receiving surface portion ( 6 ), the portion with which the ball rubs , or its vicinity by capillary action . the ball ( 3 ), prevented from getting out of place by the drawn portion ( 7 ) provided at the end of the ball retaining tube ( 1 ), is received within the ball retaining portion ( 4 ) and adapted to draw the ink from the ball receiving surface portion ( 6 ) or the ink flow paths ( b ) at its vicinity as it rotates supported on the ball receiving surface portion ( 6 ). the ball is made into a sphere or a roller of approximately 0 . 3 - 1 . 0 mm in diameter . the ink flow space ( a ) is formed of the space left between the inner surface of the ball retaining tube ( 1 ) in circular cross section and the peripheral surface of the ball receiving stem ( 2 ) in square cross section and running in the axial direction . the ink flow space ( a ), when the maximum thickness of the cross section of the ball receiving stem ( 2 ) is equal to or larger than the bore of the ball retaining tube ( 1 ) ( fig2 and fig2 to 29 ), is separated into divisions in number corresponding to the number of the faces of the ball receiving stem ( 2 ), and when the maximum thickness is smaller than the bore of the ball retaining tube ( 1 ) ( fig3 ), the divisions communicates with each other in the circumferential direction across the corners of the ball receiving stem ( 2 ). in either case , the ink flow space ( a ) is kept in communication with the ink flow paths ( b ) to supply the same with the ink and also to prevent the ink from being drained back . the gap width in the ink flow space ( a ) will produce no problem if it is larger than 3 - 6μ when the thickness of the ink adhering to the surface being 3 - 4μ is taken into consideration , but 25 - 100μ thick gap is given , in practice , subject to adjustment according to the properties of the materials used such as viscosity of the ink . when an oily ink is used , somewhat larger gap is considered suitable . the ink flow paths ( b ) are extended as far as the center or the vicinity of the flat ball receiving surface portion ( 6 ) on the ball receiving stem ( 2 ) so as to communicate with each other , arranged in practice in various configurations such as a flat cross ( fig2 and 30 ), parallel crosses ( fig2 and 29 ), or something like that ( fig2 , fig2 to 28 ), and are in communication with the ink flow space ( a ). the groove width is approximately 25 - 110μ and the depth is over 20μ or , preferably , 40 - 100μ , and the groove width and depth are generally inversely proportional with each other . fig3 exemplifies a structure of the pen tip according to the present invention in which the ball retaining tube ( 1 ) whose inner surface is square in cross section is combined with the ball receiving stem ( 2 ) whose cross - sectional profile is circular . in this case , the ball ( 3 ) is preferred to be of a roller type and then the ink flow paths ( b ) on the end surface portion ( 2a ) are adapted to form the flow paths along the both sides of the ball retaining portion ( 4 ) in a line shape and thereby to allow the ink to be smoothly drawn by the ball from the flow paths . fig3 exemplifies a structure of the pen tip according to the invention in which a spiral groove portion ( 8 ) is formed on the peripheral surface of the ball receiving stem ( 2 ), whereby the straight flow of the ink in the axial direction is restrained . fig3 and 34 exemplify a structure of the pen tip according to the invention in which concaved portions ( 9 ) are formed on the peripheral surface of the ball receiving stem ( 2 ) in the axial direction , and the concaved portions ( 9 ) are suited for application to the arrangement in which the cross section of the inner surface of the ball retaining tube ( 1 ) is circular and the cross - sectional profile of the ball receiving stem ( 2 ) is square when an ink of high viscosity is used since the ink flow space is magnified by the concave . fig3 exemplifies a structure of the pen tip according to the invention in which the outer diameter of the ball receiving stem ( 2 ) is larger than that of the ball ( 3 ). the ball receiving stem ( 2 ) of this structure has a feature to enlarge the annular ink flow space ( a ) and thereby to enhance the ink conducting performance in proportion to the diameter by providing increased flow space without increasing the size of the gap . in the case of the pen tip according to the invention where the three parts are formed of special materials such that the ball retaining tube ( 1 ) and ball receiving stem ( 2 ) are formed of stainless steel and the ball ( 3 ) is formed of stainless steel or ceramic , there is not occurring any electrostatic action among these three members and therefore there is not produced any corrosion or rusting due to such action . the same may be said with the arrangement in which the ball receiving stem ( 2 ) is formed of a stainless steel stem portion ( 10 ) with a synthetic resin film ( 11 ) coated thereover . in either case , durability characteristic of these hard materials is provided . fig3 to 38 indicate the pen tip structure according to the invention when concretely embodied in a ballpoint pen . the pen tip is structured such that a ball receiving stem ( 2 ) whose cross - sectional profile is square is inserted in a ball retaining tube ( 1 ) whose cross - sectional shape is circular and the lower half of the ball retaining tube ( 1 ) is caulked so as to be reduced in diameter and pressed against the ball receiving stem ( 2 ) at chamfered corner portions ( 2b ) thereby to be made integral therewith , and the space between the inner surface of the ball retaining tube ( 1 ) and the peripheral surface of the ball receiving stem ( 2 ) running in the axial direction is made into the ink flow space ( a ). and , ink flow paths ( b ) are disposed substantially in a parallel - cross configuration on a flat ball receiving surface portion ( 6 ) serving as the bottom of the ball retaining portion ( 4 ) of the pen tip . the spherical ball ( 3 ) is supported on the flat surface on a point so that the same may sufficiently endure impulsive writing - pressure and high writing - pressure and the ink may be forcedly supplied very close to the point of support by capillary action . the caulked portion ( 5 ) is tightly fitted into a pen tip holder ( 12 ) as far as the jaw portion of the caulked portions ( 5 ) serving as the stopper ( 5a ) is caught by the end of the pen tip holder , while the ball receiving stem ( 2 ) is inserted into an ink reservoir ( 13 ), whereby the ink is fed through the ink flow space ( a ) and the ink flow paths ( b ) to the ball ( 3 ). fig3 to 41 indicates a ballpoint pen for drawing purposes as another concrete embodiment of the invention . the pen tip is structured such that a ball receiving stem ( 2 ) whose cross - sectional profile is square is inserted in a ball retaining tube ( 1 ) whose cross - sectional shape is circular and the same is caulked so as to be constricted in the middle and at the lower portion and pressed against the ball receiving stem ( 2 ) at chamfered corner portions ( 2b ) thereby to be integrated therewith , and the space between the inner surface of the ball retaining tube ( 1 ) and the surface of the ball receiving stem ( 2 ) running in the axial direction is made into the ink flow space ( a ). and the ink flow paths ( b ) are disposed in a substantially - like configuration on a flat ball receiving surface portion ( 6 ) serving as the bottom of the ball retaining portion ( 4 ) of the pen tip and a roller type ball ( 3 ) is supported on the flat surface on a line so that the ball may fully endure impulsive writing - pressure and high writing - pressure and the ink may be forcedly supplied very close to the support line by capillarity . the caulked portions ( 5 ) are tightly fitted into a pen tip holder ( 12 ) as far as the constricted portions thereof serving as engagement recess ( 5a ) are put in engagement with annular protrusions ( 12a ) provided in the pen tip holder , while the ball receiving stem ( 2 ) is inserted into an ink reservoir ( 13 ), whereby the ink is fed through the ink flow space ( a ) and the ink flow paths ( b ) to the ball ( 3 ). in both the above mentioned embodiments , the caulked portions ( 5 ) perform two functions , i . e ., to fixedly integrate the ball retaining tube ( 1 ) with the ball receiving stem ( 2 ) and to fixedly connect itself with the pen tip holder ( 12 ), and since the caulked portions ( 5 ) is pressed against the ball receiving stem ( 2 ) at its chamfered corner portions ( 2b ) , these members can be integrated in a condition tightly fixed together between faces . and , since the roller width of the ball ( 3 ) becomes the width of a drawn line and various widths of lines can be drawn with various widths of rollers , the arrangement provides convenience in drawing . a roller type ball ( 3 ) made of flexible synthetic resin will be suited for toilet purposes . fig4 shows another embodiment , in which an intermediate conducting core ( 14 ) formed of a bundle of fibers is interposed between the ink flow space ( a ) in the pen tip and the ink reservoir ( 13 ). by means of the intermediate conducting core ( 14 ) as the junction path between the path of the ink flow space ( a ) and the ink reservoir ( 13 ), the ink &# 34 ; sucking up &# 34 ; action by the ink reservoir ( 13 ) is minimized , and it is adapted , even when the ink quantity in the ink reservoir ( 13 ) becomes very small , such that the ink is smoothly conducted by capillarity through the intermediate core ( 14 ) and the ink flow space ( a ) to the ink flow paths ( b ) to be extracted therefrom by the ball ( 3 ). fig4 to 45 show data on the ink extracting performance of the pen tips , i . e ., the data of the pen tips which are not provided with the ink flow paths ( b ) of the invention ( fig4 ) and the data of the pen tips provided with the ink flow paths ( b ) of the invention ( fig4 and 45 ). these data were all obtained through the tests made on five random samples of pen tips . as apparent from these data , the pen tip structures without the ink flow paths ( b ) of the invention exhibited very wide dispersion and instability in the ink extracting performance , while the pen tip structures with the ink flow paths ( b ) of the invention showed almost unnoticeable dispersion and proved their stability in the ink extracting performance . thus it is understood that they are quite reliable when applied to the products for practical uses . as described in the foregoing , the pen tip structure according to the present invention is useful as the pen tips for ballpoint pens for writing , drawing , and toilet purposes and the same is especially suited for the applications requiring smooth and reliable ball rotation , stable ink extracting performance , and durability against high writing - pressure .