Abstract:
[Problem] Because the knife edge of medical knives such as straight knives manufactured with austenite stainless steel is thin and sharp, there is the problem that the knife edge bends too easily when making incisions in the cornea, sclera, etc. during ophthalmologic surgery. The invention provides a medical knife capable of increasing the strength of the thin, sharp knife edge and preventing the reduction of sharpness. [Solution] This medical knife has a flat cutting part and an inclined surface formed along the border of said cutting part. By electrolytic polishing or chemical polishing of the inclined surface, a convexly curved cutting edge is formed on at least the edge of the inclined surface of the knife. The knife edge has a rounded configuration. The knife edge being rounded increases the thickness of the knife edge, increases the strength thereof and hinders bending.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a medical knife used in surgery, such as a straight ophthalmic knife, an LRI knife for astigmatism correction, and the like. 
       BACKGROUND ART 
       [0002]    The straight knife and the LRI knife used for ophthalmic surgery have a thin, sharp front end and a straight cutting edge several millimeters long extending from the end. When using them, mainly the front end and a cutting edge portion approximately 1 mm from the front end are shifted in a direction orthogonal to the axis of the knife and direction parallel to the cutting portion, so as to cut into a cornea or sclera. 
         [0003]    The medical knife is conventionally made of martensitic stainless steel. Martensitic stainless steel can be quenched, and can thereby attain a desired hardness and favorable sharpness. 
         [0004]    However, since martensitic stainless steel rusts easier than austenitic stainless steel, use of austenitic stainless steel that does not rust as easily for medical applications is desired. However, austenitic stainless steel wire cannot be quenched nor attain a predetermined hardness. 
         [0005]    In response, an austenitic stainless steel wire rod is stretched and work hardened, so as to attain a predetermined hardness. The stainless steel wire that is stretched and work hardened as such has a crystalline structure spreading in strips along the length of the wire. This is referred to as a fibrous structure. 
         [0006]    The method of manufacturing a medical knife from austenitic stainless steel having such a fibrous structure is almost the same as the case of manufacturing from martensitic stainless steel. That is, the medical knife is manufactured by cutting an austenitic stainless steel wire rod to a predetermined length to make a round bar, pressing an end of the round bar flat, molding the pressed portion into a knife shape through the following pressing step, and forming a cutting blade through grinding using a grinding stone or the like. In the martensitic case, it then undergoes heat treatment and final polishing, but in the austenitic case, it does not proceed past the heat treatment step. 
         [0007]    The medical knife has a problem with sharpness, and therefore various measures have been taken to improve sharpness. Application of silicone is often performed as one of these measures. This is because a silicone film reduces frictional resistance (e.g., Patent Document 1). 
         [0008]    A suture needle having grooves running almost orthogonal to the central axis thereof and grooves running along the length of the central axis is proposed in Patent Document 2. This is attained by polishing the raw material having a crystalline fibrous structure in a direction orthogonal to the length of the fibrous structure, making grooves running orthogonal to the length of the fibrous structure by abrasive grains, and then carrying out processing such as electrolytic polishing and chemical polishing, thereby etching the grooves by the abrasive grains so as to expose a part of the fibrous structure. The grooves running along the central axis result from performing electrolytic polishing on the pattern of the fibrous structure, the grooves running orthogonal to the central axis are made by abrasive grains during polishing. Silicone is applied to the suture needle that has vertical and horizontal grooves and then used. Such a structure allows improvement in adherence of silicone. 
         [0009]    In addition, technology of electrolytically polishing a blade tip portion so as to remove burrs resulting from abrasive polishing, making the blade tip portion into a mirror-finished surface, finishing a sharp cutting blade, and improving sharpness is proposed in Patent Document 3. 
       PRIOR ART DOCUMENTS 
     Patent Documents 
       [0000]    
       
         [Patent Document 1] JP 2003-116866 A 
         [Patent Document 2] JP 3140508 
         [Patent Document 3] JP H08-238245 A 
       
     
       DISCLOSURE OF THE INVENTION 
     Problem to be Solved by the Invention 
       [0013]      FIG. 5  are diagrams illustrating a conventional straight knife, where  FIG. 5(   a ) is a top view,  FIG. 5(   b ) is a cross section of a cutting blade cut along a line E-E of  FIG. 5(   a ),  FIG. 5(   c ) is an enlarged view of a blade tip portion of the knife viewed from F of  FIG. 5(   a ), and  FIG. 5(   d ) is a diagram illustrating a bent blade tip. 
         [0014]    A straight knife  1  shown in the diagrams is made of austenitic stainless steel. As described before, a stainless steel round bar  1   a  is cut to a predetermined length, a front end thereof is pressed flat into a planar cutting portion  1   b , a side of the cutting portion  1   b  is ground from either side to form a slanted surface  1   c , and a linear cutting blade  1   d  and an edge  1   e  are formed at the end of the slanted surface  1   c.    
         [0015]    The straight knife  1  generally has a small angle α of a blade tip  1   f  and thin thickness t as shown in  FIG. 5(   a ) so as to improve sharpness. As a result, there is a problem that when forming an incision in a cornea, a sclera, or the like during ophthalmic surgery, the blade tip if of the austenitic stainless steel knife is easily bent.  FIG. 5(   d ) is a diagram illustrating peripheral parts of the same blade tip if (referred to as ‘blade tip portion’ hereafter) as in  FIG. 5(   c ), illustrating a bent state due to elasticity when cutting a cornea, a sclera, or the like. If the blade tip portion easily bends, it is difficult to use as a knife. Moreover, there is a problem that if deformation is great, it does not return to its original shape and cannot be used as a knife thereafter. 
         [0016]    The present invention aims to solve the above problems, and to provide a medical knife made of austenitic stainless steel that can increase strength of a thin and sharp blade tip portion, is strong against deformation, and can prevent decrease in sharpness. 
       Solution to the Problem 
       [0017]    A first medical knife of the present invention for achieving the above object is characterized by including a planar cutting portion and a slanted surface formed along the periphery of at least a part of the cutting portion. The slanted surface is electrolytically polished or chemically polished so as to form a cutting blade at a front end portion of the slanted surface, and a blade tip of the knife is rounded. 
         [0018]    Note that ‘a blade tip of the knife is rounded’ here includes a convex rounded surface on the whole not only having a circular arc with a fixed curvature radius, but also an elliptic arc, a parabola, and some irregularities. 
         [0019]    A second medical knife of the present invention for achieving the above object is characterized by including a planar cutting portion and a slanted surface formed along the periphery of at least a part of the cutting portion. The slanted surface is electrolytically polished or chemically polished so as to form a cutting blade at a front end portion of the slanted surface, and the cutting blade is gradually curved in a centrally protruding form. 
         [0020]    A third medical knife of the present invention for achieving the above object is characterized by including a planar cutting portion and a slanted surface formed along the periphery of at least a part of the cutting portion. The slanted surface is electrolytically polished or chemically polished so as to form a cutting blade constituted by a convex rounded surface at a front end portion of the slanted surface. 
         [0021]    A combined configuration of any two of the first, the second and the third structure, or a combined configuration of all of the first, the second, and the third structures may be provided. Alternatively, a configuration where the electrolytic polishing or chemical polishing is applied to the entire length of the cutting blade, a configuration where the cutting blade is formed on two sides of the cutting portion adjacent to each other, or a configuration formed by a wrapping film may be provided. 
       Advantageous Effect of the Invention 
       [0022]    According to the medical knife of the present invention, since the blade tip is rounded through electrolytic polishing or chemical polishing, the blade tip portions is thick and difficult to bend, thereby allowing prevention of bending during use even if austenitic stainless steel is used. This brings about a beneficial effect of easy usability. Moreover, since austenitic stainless steel is used, a medical knife that does not rust as easily can be provided. 
         [0023]    It seems that sharpness of the knife decreases when the blade tip is rounded. However, since it is rounded through electrolytic polishing or chemical polishing, the blade tip is the very small portion that is rounded. Furthermore, the burrs generated through grinding when forming the cutting blade are removed through electrolytic polishing or chemical polishing, thereby improving sharpness. Therefore, the same level of sharpness on the whole as the case of only grinding can be secured. 
         [0024]    Furthermore, the knife according to the present invention is mainly used for cutting by thrusting the blade tip into an object, such as sclera, to a predetermined depth (axial direction), and moving orthogonally to the axis while maintaining it around that depth. Therefore, since the sharpness of the blade tip only when the knife is first thrust into tissue affects the technician&#39;s perception of the sharpness during use, and it is then cut by the edge lower (handle side) than the rounded portion of the blade tip being pulled back, the technician&#39;s perception of sharpness during use is not affected even if the blade tip is rounded. 
         [0025]    Yet further, since the knife according to the present invention is often used for cutting by inserting approximately 1 to 2 mm from the front end and pulling back, unlike knife having the precisely formed maximal blade width for forming an incision into which an intraocular lens is to be inserted where the latter knife is not according to the present invention and is used to be moved along the axis and in the direction parallel to the cutting portion and to be inserted into a tissue, the angle of the cutting blade does not affect sharpness as is, and the sharpness is clearly perceived even if the angle of the cutting blade and the knife for forming an incision in order to insert the intraocular lens are the same. 
         [0026]    The knife for forming an incision in order to insert the intraocular lens is not according to the present invention because formation of the knife, through only grinding, used for inserting the intraocular lens allows easy control of dimensions of the maximum blade width, and precise formation. 
         [0027]    Yet even further, the configuration having the cutting blade gradually bending in a centrally protruding form makes a smaller angle of gradient of the edge to the tissue than the conventional knife which doesn&#39;t have a bent edge but have a straight edge while the angle of gradient of the knife handle to the tissue is large. As a result, sharpness is improved, thereby making a user-friendly knife. Yet even further, the configuration having the slanted surface formed by a wrapping film makes the slanted surface gradually bending in a centrally protruding form, and both end portions (ends on the front end side and the shank side) of the slanted surface are convex rounded surfaces, thereby increasing the angle of the slanted surface. Through administration of electrolytic polishing and chemical polishing, a cutting blade is molded exactly on the central protruding form, and both end portions with large angles are polished a little while the central portion with a small angle is polished a lot, thereby allowing formation of overall uniform cutting blade angles, and improving usability. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIGS. 1(   a ) to  1 ( f ) are diagrams describing a manufacturing method for a straight knife according to the present invention, and  FIGS. 1(   g ) and  1 ( h ) are enlarged views of a blade tip portion viewed from A of  1 ( e ), where  FIG. 1(   g ) illustrates a view before electrolytic polishing and  FIG. 1(   h ) illustrates a view after electrolytic polishing; 
           [0029]      FIG. 2(   a ) is a top view of the straight knife, and  FIG. 2(   b ) is an enlarged view of the blade tip portion; 
           [0030]      FIG. 3  are cross sections cut along a line B-B of  FIG. 2(   b ), where  FIG. 3(   a ) illustrates a view before electrolytic polishing and  FIG. 3(   b ) illustrates a view after electrolytic polishing; 
           [0031]      FIG. 4  are diagrams of an LRI knife for astigmatism correction, where  FIG. 4(   a ) is a top view,  FIG. 4(   b ) is a cross section cut along a line C-C of  FIG. 4(   a ) illustrating the state before electrolytic polishing, and  FIG. 4(   c ) is a view illustrating the state after electrolytic polishing;  FIG. 4(   d ) is a cross section cut along a line D-D of  FIG. 4(   a ) illustrating the state before electrolytic polishing, and  FIG. 4(   e ) is a view illustrating the state after electrolytic polishing; and 
           [0032]      FIG. 5  are diagrams illustrating a conventional straight knife, where  FIG. 5(   a ) is a top view,  FIG. 5(   b ) is a cross section of a cutting blade cut along a line E-E of  FIG. 5(   a ),  FIG. 5(   c ) is an enlarged view of a blade tip portion of the knife viewed from F of  FIG. 5(   a ), and  FIG. 5(   d ) is a diagram illustrating the blade tip portion in a bent state. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0033]    An embodiment of the present invention is described while referencing the attached drawings. 
         [0034]      FIGS. 1(   a ) to  1 ( f ) are diagrams describing a manufacturing method for a straight knife  10  according to the present invention. To begin with, a raw, round bar  11  is cut at a predetermined length shown in  FIG. 1(   a ). This is made from an austenitic stainless steel round bar having a fibrous crystalline structure. Then, as shown in  FIG. 1(   b ), the front end side is flattened by a press, and excessive portions indicated by dotted lines are cut in a subsequent pressing step so as to have a planar pointed end, thereby forming an approximately rhombus cutting portion  12  viewed in the top view. As shown in  FIGS. 1(   c ) and  1 ( d ), both flattened surfaces  12 ′ are polished and made into smooth surfaces and made into a predetermined thickness t. The flattened surfaces  12 ′ of the cutting portion  12  are then made into irregular reflection surfaces through blasting. Making irregular reflection surfaces keeps the knife from shining and making it difficult to see an affected area when used under a microscope. Next, as illustrated in  FIG. 1(   e ), a slanted surface  13  is formed on the respective flattened surfaces  12 ′ through grinding, a front end portion of the slanted surface  13  is made into a cutting blade, and an edge  15  of the cutting blade is sharpened to a point. The slanted surfaces  13  are formed extending to the vicinity of the largest width of the cutting portion  12 . 
         [0035]    The above description is nearly the same as the manufacturing method of the conventional straight knife. The present invention is characterized by forming a cutting blade  14  through electrolytic polishing instead of grinding. 
         [0036]      FIG. 1(   g ) is an enlarged view of a blade tip portion  16  viewed from A of  FIG. 1(   e ), where  FIG. 1(   g ) illustrates a view before electrolytic polishing and  FIG. 1(   h ) illustrates a view after electrolytic polishing. The slanted surface  13  is formed through grinding using a grinding stone, has grooves orthogonal to the edge  15  formed across the entirety, and multiple burrs are generated on the slanted surface  13  and the edge  15 . 
         [0037]    With the present invention, once the slanted surfaces  13  are formed through grinding using a grinding stone, electrolytic polishing is carried out. Electrolytic polishing can be carried out using a typical method, such as a method of using phosphoric acid fluid as an electrolyte, soaking an entire cutting portion of a straight knife in an electrolytic solution, connecting the cutting portion to a positive electrode, connecting an electrolytic bath to a negative electrode, and electrify. The metal of the straight knife is eluted into the electrolytic solution, and is thereby polished. At this time, the metal at the pointed portion of the work is eluted first. 
         [0038]    With electrolytic polishing, the burrs on the slanted surface  13  and the edge  15  are eluted into the electrolytic solution, thereby burr removal is carried out by priority. Typical electrolytic polishing is completed at this stage; however, with the present invention, electrolytic polishing is further continued. 
         [0039]    Once the burrs are removed and electrolytic polishing is continued, grooves resulting from polishing are reduced, and the slanted surface  13  is melted so as to form the cutting blade  14  having a convex curved surface on an end of the slanted surface  13 . The tip end of the cutting blade  14  is the edge  15 . 
         [0040]      FIG. 2(   a ) is an enlarged top view of the cutting portion  12  of the straight knife  10 , and  FIG. 2(   b ) is an enlarged view of the blade tip portion, where both  FIGS. 2(   a ) and  2 ( b ) illustrate the state after electrolytic polishing. A blade tip  16   a  has a dotted-line portion melted into a circular arc surface having radius R 1 . Carrying out electrolytic polishing up to this state after burr removal thickens the blade tip  16   a , increasing its strength. Increase in strength allows prevention of bending of the blade tip portion  16  as shown in  FIG. 5(   d ). Moreover, while the edge  15  of the cutting blade  14  is almost straight, it has a nearly circular form, gradually curving like the curvature of a Japanese sword. A straight line n shown in  FIG. 2(   a ) is tangent to the edge  15 . 
         [0041]    Even the blade tip  16   a  of the straight knife  10  using the conventional martensitic stainless steel is not pointed but shows a certain amount of roundness when viewed from a microscope. Conventionally, this roundness has less than an R of 0.1 mm at the blade tip  16   a ; however, R 1  of the blade tip according to the present invention is 0.3 to 0.7 mm, which is larger than the conventional radius. This is because R 1  of less than 0.3 mm makes it thinner, thereby making it easier to be bent. If R 1  is greater than 0.7 mm, a desired sharpness cannot be attained. 
         [0042]    Furthermore, while radius R 2  of the front end portion of the edge  15  is conventionally straight, with the present invention, polishing is continued after the burrs are removed, thereby becoming a curved line in nearly a circular arc form with R 2  of 100 to 500 mm as illustrated in the drawing having the centrally protruding area. The centrally protruding curvature at R 2  is also generated through electrolytic polishing; however, such a curvature allows improvement in sharpness of the knife. Particularly, since a portion approximately 1 mm from the blade tip  16   a  is used with the straight knife  10 , providing curvature to that portion can improve sharpness of the blade tip portion  16 . Note that, as illustrated in  FIG. 2(   a ), curvature may be provided near the widest portion of the cutting portion  12 . If electrolytic polishing or chemical polishing is given to the whole cutting blade  14 , curvature is formed near the widest portion. The reason for keeping R 2  from becoming less than 100 mm is that when it is less than 100 mm, too much is ground, and sharpness decreases. Meanwhile, since cross-sectional angle of the blade (blade angle) when cutting by pulling looks smaller, the upper limit of R 2  should be set to 500 mm or less. If it exceeds 500 mm, there is hardly any influence of reducing the cross-sectional angle of the blade. 
         [0043]    By carrying out grinding for forming the slanted surface  13  using a wrapping film, the cutting portion  12  may be curved due to processing pressure during the grinding, making it easy to form a protruding curve indicated by R 2  in  FIG. 2 . Moreover, the front end side and the shank side of the slanted surface  13  are curved convexly, and angle of gradient of the slanted surface  13  increases. Through administration of the same electrolytic polishing and chemical polishing as described above in this state, the cutting blade  14  is molded exactly on the centrally protruding form, and both end portions with large angles are polished a little while the central portion with a small angle is polished a lot, thereby allowing formation of overall uniform cutting blade angles. 
         [0044]    While the blade tip  16   a  in  FIG. 2(   b ) is a circular arc, it may actually have some irregularities, not being such a clean circular arc. However, ‘become round’ according to the present invention includes various rounded surfaces aside from the circular arc surface such as an elliptic surface, a paraboloidal surface, and a convex rounded surface with some irregularities. 
         [0045]    The blade tip that is conventionally pointed as indicated by a dotted line in  FIG. 2(   b ) is rounded as the blade tip  16   a  indicated by a solid line, however, sharpness is improved by making the convexly curved surface small as described above and eliminating the burrs, where sharpness on the whole is on par with that before electrolytic polishing. On the other hand, with the conventional pointed blade tip  16   a , while the blade tip portion  16  is easily bent when cutting open a cornea or a sclera as described in  FIG. 5(   d ), if the blade tip portion  16  at the front end of the cutting blades  14  constituted by convex rounded surfaces is used, cutting open without bending can be performed reliably and easily. 
         [0046]      FIG. 3(   a ) is a diagram corresponding to  FIG. 5(   b ) and is a cross section cut along a line B-B of  FIG. 2(   b ). This illustrates the slanted surface  13  formed by grinding and the edge  15  of the cutting blades  14 . The cutting blades  14  constituted by convex rounded surfaces are formed between the slanted surface  13  and the edge  15  through electrolytic polishing or chemical polishing. The cutting blades  14  have approximately circular arc surfaces and a central axis along the length of the slanted surface  13 . Use of such a configuration allows easier penetration of the cutting blades  14  into bodily tissue and improvement in sharpness. 
         [0047]    A knife having the slanted surface  13  and the cutting blades  14  formed symmetrically on either side of the cutting portion  12  has been described above; however, this also applies for a knife having the slanted surface  13  and the cutting blade  14  formed only one side of the cutting portion  12 . Moreover, while a straight knife has been described as an example, this is applicable to other medical knives such as an LRI knife for astigmatism correction, for example. 
         [0048]      FIG. 4  are diagrams of an LRI knife for astigmatism correction, where  FIG. 4(   a ) is a top view,  FIG. 4(   b ) is a cross section cut along a line C-C of  FIG. 4(   a ) illustrating the state before electrolytic polishing, and  FIG. 4(   c ) is a view illustrating the state after electrolytic polishing.  FIG. 4(   d ) is a cross section cut along a line D-D of  FIG. 4(   a ) illustrating the state before electrolytic polishing, and  FIG. 4(   e ) is a view illustrating the state after electrolytic polishing. 
         [0049]    An LRI knife  40  has a cutting portion  41  having a pointed, almost 90 degree V-shaped front end, and slanted surfaces  42  formed on two sides constituting the front end V shape. Edges of the slanted surfaces  42  are edges  43  and  43 . A blade tip  45  is at the intersecting point of the edges  43  and  43 . 
         [0050]    If the slanted surfaces  42  and the edges  43  are formed at the cutting portion  41  through grinding, the LRI knife  40  has cross-sectional shapes as illustrated in FIGS.  4 ( b ) and  4 ( d ), and if electrolytic polishing is carried out after burr removal according to the present invention, it has cross-sectional shapes as in  FIGS. 4(   c ) and  4 ( e ). 
         [0051]    That is, cutting blades  44  constituted by convex rounded surfaces are formed on the front end side of the slanted surfaces  42 , and the edges  43  are formed on front ends thereof. However, a blade tip  45  in the top view of  FIG. 4(   a ) is not rounded as the straight knife  10  illustrated in  FIG. 2(   b ). This is because the LRI knife  40  is thick from the start, the V-shaped angle is large, and if the cutting blades  44  constituted by convex rounded surfaces are formed, the problem of bending does not occur without rounding the blade tip  45 . Moreover, the blade tip  45  may be rounded in the top view of  FIG. 2(   b ). 
         [0052]    Tables 1 and 2 are examples comparing sharpness of the straight knife  10  according to the present invention and a conventional straight knife. Table 1 shows the results from examining sharpness on ten specimens  1  to  10  using the straight knife of the present invention. Table 2 shows the results from examining sharpness on two specimens  11  and  12  using a conventional straight knife made of martensitic stainless steel. Both the straight knife according to the present invention and the conventional straight knife have a knife angle of 15 degrees, board thickness (value of t) of 0.11 mm, and the same shape. Testing has been conducted by measuring pierce force (unit of millinewtons ‘in N’) when the straight knife is pierced through a 0.45 mm-thick membrane of imitation leather (Porvair) having a hardness close to that of the cornea or sclera. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Working Example 
               
             
          
           
               
                 Specimen 
                 1st 
                 2nd 
                 3rd 
                 Average 
               
               
                 Number 
                 mN 
                 mN 
                 mN 
                 mN 
               
               
                   
               
               
                 1 
                 237.0 
                 297.0 
                 383.0 
                 305.7 
               
               
                 2 
                 269.0 
                 338.0 
                 396.0 
                 334.3 
               
               
                 3 
                 248.0 
                 319.0 
                 392.0 
                 319.7 
               
               
                 4 
                 246.0 
                 293.0 
                 334.0 
                 291.0 
               
               
                 5 
                 192.0 
                 280.0 
                 313.0 
                 261.7 
               
               
                 6 
                 241.0 
                 349.0 
                 383.0 
                 324.3 
               
               
                 7 
                 227.0 
                 315.0 
                 414.0 
                 318.7 
               
               
                 8 
                 246.0 
                 315.0 
                 379.0 
                 313.3 
               
               
                 9 
                 255.0 
                 310.0 
                 351.0 
                 305.3 
               
               
                 10  
                 295.0 
                 349.0 
                 383.0 
                 342.3 
               
               
                 min. 
                 192.0 
                 280.0 
                 313.0 
                 261.7 
               
               
                 max. 
                 295.0 
                 349.0 
                 414.0 
                 342.3 
               
               
                 Average 
                 245.6 
                 316.5 
                 372.8 
                 311.6 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Conventional Example 
               
             
          
           
               
                 Specimen 
                 1st 
                 2nd 
                 3rd 
                 Average 
               
               
                 Number 
                 mN 
                 mN 
                 mN 
                 mN 
               
               
                   
               
               
                 11 
                 333.0 
                 393.0 
                 428.0 
                 384.7 
               
               
                 12 
                 293.0 
                 345.0 
                 393.0 
                 343.7 
               
               
                 min. 
                 293.0 
                 345.0 
                 393.0 
                 343.7 
               
               
                 max. 
                 333.0 
                 393.0 
                 428.0 
                 384.7 
               
               
                 Average 
                 313.0 
                 369.0 
                 410.5 
                 364.2 
               
               
                   
               
             
          
         
       
     
         [0053]    Table 1 shows results of a pierce test conducted three times for each of ten specimens using the straight knife according to the present invention. As a result, the blade tip portion has pierced through all of the specimens without even a single bent one. In addition, pierce resistance, although slightly, has improved more than the conventional straight knife made of martensitic stainless steel. 
         [0054]    This shows that according to the present invention, a straight knife can be manufactured from austenitic stainless steel, with sharpness that bears comparison with the conventional knife. 
         [0055]    Note that electrolytic polishing has been carried out in the above working examples, but chemical polishing is also possible. However, since roundness of the blade tip  16   a  of the cutting blades  14  shown in  FIG. 1(   g ) is small, the blade tip  16   a  with such a small R cannot be formed by grinding. This roundness can be formed only by electrolytic polishing or chemical polishing. 
       EXPLANATION OF REFERENCES 
       [0000]    
       
           10 : straight knife 
           11 : round bar 
           12 : cutting portion 
           13 : slanted surface 
           14 : cutting blade 
           15 : cutting blade edge 
           16 : blade tip portion 
           16   a : blade tip 
           17 : curved surface