Abstract:
A cylindrical internal surface processing method comprises forming a cylinder bore, roughening an upper section of the bore, depositing coating onto the bore, and machining a lower section of the bore and the coating. The forming of the cylinder bore includes forming the upper and lower sections with the lower section being axially spaced from the upper section and having an axial length greater than zero. The roughening creates a roughened surface such that a radially innermost edge of the roughened surface has an internal diameter smaller than an internal diameter of the lower section. The coating is deposited to cover the upper section and at least a portion of the lower section. The machining forms a tapered portion and a cylindrical portion, a radially outermost edge of the cylindrical portion having an internal diameter larger than that of a radially outermost edge of the roughened surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 11/702,060 filed on Feb. 5, 2007. The entire disclosure of U.S. patent application Ser. No. 11/702,060 is hereby incorporated herein by reference. 
         [0002]    This application claims priority to Japanese Patent Application No. 2006-033959 filed on Feb. 10, 2006. The entire disclosure of Japanese Patent Application No. 2006-033959 is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention generally relates to a cylindrical internal surface processing method for applying a finishing machining process to an internal cylindrical surface after a thermally sprayed coating has been formed on the internal cylindrical surface. The invention further relates to a base member having a cylindrical internal surface in which a machining process is performed on the internal cylindrical surface after a thermally sprayed coating has been formed on the internal cylindrical surface. 
         [0005]    2. Background Information 
         [0006]    Typically, aluminum engine blocks of internal combustion engines have cylinder liners provided in their cylinder bores. From the viewpoint of improving the output, fuel economy, and exhaust performance of internal combustion engines having aluminum cylinder blocks and from the viewpoint of reducing the size and weight of such engines, there is a very high demand for an engine design that eliminates the cylinder liners that are used in the cylinder bores of aluminum engine blocks. One alternative to cylinder liners is to use thermal spraying technology to form a thermally sprayed coating on the internal surfaces of the cylinder bores. 
         [0007]    When thermal spraying technology is applied to a cylinder bore, a coating is formed on the internal surface of the cylinder bore using a thermal spray gun configured to spray molten coating material. The coating is deposited by moving the thermal spray gun in the axial direction inside the cylinder bore while rotating the thermal spray gun. After the thermally sprayed coating is formed, the surface of the coating is finished by grinding using a honing process or other machining process. 
         [0008]    Before such a thermally sprayed coating is deposited, the internal surface of the base material of the cylinder bore is roughened using, for example, the surface treatment proposed in Japanese Laid-Open Patent Publication No. 2002-155350 (paragraphs 0002 and 0019). The surface roughening serves to improve the adhesion of the thermally sprayed coating. 
       SUMMARY OF THE INVENTION 
       [0009]    It has been discovered that even though the base material is treated before the thermally sprayed coating is formed on the internal surface of the cylinder bore and finished using honing or another mechanical finishing process, the thermally sprayed coating exfoliates (peels off, flakes) easily at the end portions of the cylinder bore and there is a need for improvement. 
         [0010]    The object of the present invention is to prevent exfoliation of a thermally sprayed coating at an end portion of a cylindrical internal surface in a situation where honing or another mechanical finishing process is applied to the thermally sprayed coating after the coating is formed on the cylindrical internal surface. 
         [0011]    In accordance with one aspect, a cylindrical internal surface processing method is provided that basically comprises forming a cylinder bore in a cylinder block, roughening an upper section of the cylinder bore, depositing a thermally sprayed coating onto an cylindrical internal surface of the cylinder bore, and machining a lower section of the cylinder bore and the thermally sprayed coating along the lower section. The cylinder bore is formed with a cylindrical internal surface including the upper section and the lower section, the lower section being axially spaced from the upper section and having an axial length greater than zero with respect to a central axis of the cylinder bore. The upper section is roughened to create a roughened surface such that a radially innermost edge of the roughened surface with respect to the central axis has an internal diameter smaller than an internal diameter of the lower section. The thermally sprayed coating is deposited onto the cylindrical internal surface to cover the upper section and at least a portion of an axial length of the lower section after the roughening of the upper section. Finally, the lower section and the thermally sprayed coating along the lower section are machined to form a tapered portion and a cylindrical portion. More specifically, they are machined such that the tapered portion extends from the cylindrical portion toward the upper section, and such that a radially outermost edge of the cylindrical portion has an internal diameter that is larger than an internal diameter of a radially outermost edge of the roughened surface with respect to the central axis. 
         [0012]    These and other objects, features, aspects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Referring now to the attached drawings which form a part of this original disclosure: 
           [0014]      FIG. 1  is a transverse cross sectional view of a cylinder block having a cylinder bore with a thermally sprayed coating formed on its cylindrical internal surface in accordance with a first embodiment of the present invention; 
           [0015]      FIG. 2  is an enlarged cross sectional view of an end portion of the cylinder block shown in  FIG. 1  that is closer to a crankcase; 
           [0016]      FIG. 3  is a series of enlarged cross sectional views of a portion of the cylindrical internal surface illustrating the processing applied to the cylinder bore of the cylinder block shown in  FIG. 1 ; 
           [0017]      FIG. 4  is a cross sectional view of the cylinder block in which a roughening process is being applied to the cylindrical internal surface of the base material of the cylinder block shown in  FIG. 1 ; 
           [0018]      FIG. 5A  is an enlarged cross sectional view of a portion of the cylindrical internal surface illustrating how the base material surface roughening process shown in  FIG. 4  is executed using a tool and the discharged cut waste material; 
           [0019]      FIG. 5B  is an enlarged cross sectional view of a portion of the cylindrical internal surface illustrating a typical screw thread cutting process executed using a tool; 
           [0020]      FIG. 6  is a schematic view of an entire thermal spraying apparatus for depositing a thermally sprayed coating onto the internal surface of the cylinder bore of the cylinder block shown in  FIG. 1  after the cylinder bore internal surface has been roughened; 
           [0021]      FIG. 7  is an enlarged cross sectional view of a portion of the cylindrical internal surface illustrating the adhesion between the thermally sprayed coating and the surface onto which the thermally sprayed coating is deposited; 
           [0022]      FIG. 8  is a cross sectional view of the cylinder block shown in  FIG. 1  illustrating the thermally sprayed coating being honed with a honing tool; 
           [0023]      FIG. 9  is a work flow diagram illustrating the flow of processing steps from the base material surface roughening shown in diagram (c) of  FIG. 3  to the finishing (honing) shown in diagram (f) of  FIG. 3 ; 
           [0024]      FIG. 10A  is a schematic illustration of the manner in which a force acts against the thermally sprayed coating when the honing grindstones move upward, showing a case in which a tapered surface is provided on a bottom portion of the coating; 
           [0025]      FIG. 10B  is a schematic illustration of the manner in which a force acts against the thermally sprayed coating when the honing grindstones move upward, showing a case in which a tapered surface is not provided on a bottom portion of the coating; 
           [0026]      FIG. 11  is a transverse cross sectional view of a cylinder block having a cylinder bore with a thermally sprayed coating formed on its cylindrical internal surface in accordance with a second embodiment of the present invention; and 
           [0027]      FIG. 12  is a graph illustrating how the internal diameter of the cylinder bore changes as one moves from the upper end to the lower end thereof after the thermally sprayed coating has been deposited. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0028]    Selected embodiments of the present invention will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments of the present invention are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
         [0029]    Referring initially to  FIG. 1 , a cylinder block is illustrated as a base member in accordance with a first embodiment of the present invention. The cylinder block  1  has a cylinder bore  3  with an internal cylindrical surface  5 . A thermally sprayed coating  7  is formed on the cylinder bore internal surface  5  using a method that is described later. After the thermally sprayed coating  7  is formed, it is finished using a finishing method described later (honing in this embodiment).  FIG. 1  shows the thermally sprayed coating  7  after it has been deposited and before it is finished. 
         [0030]      FIG. 2  is an enlarged cross sectional view showing an axial (crankcase) end portion of the cylinder bore  3  that is closer to a crankcase  9  of the cylinder block  1  as shown in  FIG. 1 . The axial (crankcase) end portion that is closer to the crankcase  9  is larger in diameter than the remaining portion of the cylinder bore  3 , i.e., than the remaining portion of the cylinder bore  3  above the axial (crankcase) end portion. 
         [0031]      FIG. 3  shows the left-hand portion of the view of the cylinder bore  3  shown in  FIG. 2  and illustrates the machining process applied to the cylinder bore internal surface  5 . Diagram (a) of  FIG. 3  shows the state of the cylinder block  1  after casting. The cylinder bore  3  has a tapered section  11  configured to decrease in diameter as one moves downward (i.e., downward from the perspective of  FIG. 3 ) toward the crankcase  9 . 
         [0032]    Diagram (b) of  FIG. 3  shows the cylinder bore  3  after the tapered section  11  shown in diagram (a) of  FIG. 3  has been subjected to a rough boring process with a boring device (not shown). The rough boring is performed to first create an upper section  15  having a uniform internal diameter along its entire length, and then a lower end section  13  whose internal diameter is larger than that of the upper section  15 . The boring device comprises a boring bar with a tool arranged around the outside perimeter of a tip end thereof. The rough boring is accomplished by rotating the boring bar while inserting the boring bar into the cylinder bore  3  from above. 
         [0033]    The larger diameter lower end section  13  is formed by rotating the boring bar eccentrically with respect to the main axis of the boring device. 
         [0034]    After the rough boring shown in diagram (b) of  FIG. 3 , a rough surface  17  is formed in the upper section  15  of the cylinder bore internal surface  5  as shown in diagram (c) of  FIG. 3  by executing a base material surface roughening process. The rough surface  17  serves to increase the adhesion of the thermally sprayed coating  7  that will be formed afterwards. 
         [0035]    The base material surface roughening process is performed as shown in  FIG. 4  using a boring device similar to that used for the rough boring processing shown in diagram (b) of  FIG. 3 . A tool (bit)  21  is mounted to the outer perimeter of the tip end of the boring bar  19  of the boring device. The boring bar  19  is simultaneously rotated and moved axially downward so as to form a screw thread shaped cylinder bore internal surface  5 . More specifically, as shown in diagram (c) of  FIG. 3 , the surface of the base material includes with a plurality of cut portions  23  resembling the recessed portions of a screw thread and a plurality of protruding portions  25  with narrow serrations thereon arranged alternately between the recessed cut portions  23 , similarly to the surface described in Japanese Laid-Open Patent Publication No. 2002-155350 (paragraphs 0002 and 0019). 
         [0036]      FIG. 5A  shows the cut portions  23  and the serrated protruding portions  25  being formed with the tool  21  so as to create the rough surface  17 .  FIG. 5B  shows a reference example illustrating a normal screw thread being cut with a tool  201 . In  FIG. 5B , the tool  201  is rotated and moved downward simultaneously and the cut waste material  203  is discharged in the direction of the arrow A. As a result, a valley portion  205  and a ridge portion  207  are formed with a normal screw thread cutting process. Meanwhile, in  FIG. 5A , while each of the cut portions  23  (which are recessed portions corresponding to the valley portions  205  of  FIG. 5B ) is being cut by the tool  21 , the discharged waste material  27  is used to truncate the peak  29   a  of the ridge portion  29  adjacent to the valley portion (cut portion  23 ) currently being cut, thereby forming the serrated protruding portion  25 . 
         [0037]    The tool  21  shown in  FIG. 5A  is configured such that the angle α 1  of the surface  21   a  (the side facing in the opposite direction as the feed direction of the tool, i.e. upward) with respect to a horizontal plane  30  is approximately 30 degrees, which is larger than the corresponding angle α 2  of the tool  201  shown in  FIG. 5B . Meanwhile, the angle β 1  of the surface  21   b  (the side facing in the same direction as the feed direction of the tool, i.e. downward) with respect to the horizontal plane  30  is approximately 10 degrees, which is smaller than the corresponding angle β 2  of the tool  201  shown in  FIG. 5B . As a result, in the case shown in  FIG. 5A , the waste material  27  discharged when a cut portion  23  is formed is pushed against the adjacent ridge portion  29  by the slanted surface  21   a  facing in the opposite direction of the tool feed direction. The peak  29   a  of the ridge portion  29  is truncated by the waste material  27  in such a manner as to form a finely serrated protruding portion  25 . 
         [0038]    In diagram (c) of  FIG. 3 , the internal diameter at the deepest portion of a cut portion  23  is approximately the same as the internal diameter of the lower end section  13 . After the rough surface  17  shown in diagram (c) of  FIG. 3  is formed, the thermally sprayed coating  7  is deposited onto the cylinder bore internal surface  5  as shown in diagram (d) of  FIG. 3 . The thermally sprayed coating  7  is deposited to as to be substantially uniform with respect to the cylinder bore internal surface  5 . 
         [0039]      FIG. 6  is a schematic view showing the entire thermal spraying apparatus used to form the thermally sprayed coating  7  onto the cylinder bore internal surface  5  of the cylinder block  1  after the cylinder bore internal surface  5  has been roughened as shown in diagram (c) of  FIG. 3 . This thermal spraying apparatus includes a gas-fueled wire-melting type thermal spray gun configured to be inserted into the center of the cylinder bore  3 . A ferrous metal wire material  37  used as the thermal spray coating material is melted and discharged from a thermal spray opening  31   a  in the form of molten droplets  33 . The molten droplets  33  are deposited onto the internal surface  5  of the cylinder bore  3  so as to form a thermally sprayed coating  7 . 
         [0040]    The thermal spray gun  31  is configured to receive the ferrous metal wire material  37  fed from a wire material feeding device  35 , fuel (e.g., acetylene, propane, or ethylene gas) fed from a fuel gas storage tank  39  through a pipe  43 , and oxygen from an oxygen storage tank  41  through a pipe  45 . 
         [0041]    The wire material  37  is fed downward into the thermal spray gun  31  via a wire material feed hole  47  that is formed so as to pass vertically through a center portion of the thermal spray gun  31 . The fuel and oxygen are fed into a gas guide passage  51  that passes vertically through a cylindrical portion  49  disposed around the outside of the wire material feed hole  47 . The mixture of the fuel and oxygen flows out from a lower opening  51   a  (lower from the perspective of  FIG. 6 ) of the gas guide passage  51  and is ignited so as to form a combustion flame  53 . 
         [0042]    An atomizing air passage  55  is provided on an outer portion of the cylindrical portion  49  and an accelerator air passage  61  is formed still farther to the outside between a cylindrical partitioning wall  57  and a cylindrical outer wall  59 . 
         [0043]    The atomizing air passage  55  flowing through the atomizing air passage  55  serves to push the heat of the combustion flame  53  forward (downward in  FIG. 6 ) while cooling the surrounding portions of the gun  31 . It also serves to blow the molten wire material  37  forward. Meanwhile, the accelerator air flowing through the accelerator air passage  61  serves to blow the molten wire material  37  in a direction crosswise to the direction in which the wire material  37  has been blown by the atomizing air. As a result, droplets  33  of the molten wire material  37  are blown toward the cylinder bore internal surface  5  and form a thermally sprayed coating  7  on the cylinder bore internal surface  5 . 
         [0044]    The atomizing air is supplied to the atomizing air passage  55  from an atomizing air supply source  67  through an air supply pipe  71  provided with a pressure reducing valve  69 . The accelerator air is supplied to the accelerator air passage  61  from an accelerator air supply source  73  through an air supply pipe  79  provided with a pressure reducing valve  75  and a micro-mist filter  77 . 
         [0045]    The partitioning wall  57  between the atomizing air passage  55  and the accelerator air passage  61  is provided with a rotary cylinder part  83  configured such that it can rotate with respect to the outer wall  59  on a bearing  81 . The rotary cylinder part  83  is disposed on a lower end portion of the partitioning wall  57  in  FIG. 6 . Rotary vanes  85  are provided on an upper outside portion of the rotary cylinder part  83  so as to be positioned in the accelerator air passage  61 . The accelerator air flowing through the accelerator air passage  61  acts against the rotary vanes  85  and causes the rotary cylinder part  83  to rotate. 
         [0046]    A tip member  87  is fixed to the tip end (bottom end) face  83   a  of the rotary cylinder part  83  such that it rotates integrally with the rotary cylinder part  83 . A protruding portion  91  having a discharge passage  89  passing there-through is provided on a portion of the periphery of the tip member  87 . The discharge passage communicates with the accelerator air passage  61  through the bearing  81 . The aforementioned thermal spray opening  31   a  for discharging the molten droplets  33  is provided at the tip end of the discharge passage  89 . 
         [0047]    The tip member  87  with the thermal spray opening  31   a  is rotated integrally with the rotary cylinder part  83  while the thermal spray gun  31  is moved reciprocally along the axial direction of the cylinder bore  3 . In this way, substantially the entire internal surface  5  of the cylinder bore  3  can be coated with a thermally sprayed coating  7 . 
         [0048]    After the thermally sprayed coating  7  has been deposited onto the cylinder bore internal surface  5  with a thermal spraying apparatus like that shown in  FIG. 6 , the portion of the cylinder bore  3  in the vicinity of the lower end section  13  is machined by grinding as shown in diagram (e) of  FIG. 3 . This grinding is performed using a boring device like that shown in  FIG. 4 , i.e., like boring device that used to perform the roughening of the upper section  15  illustrated in diagram (c) of  FIG. 3 . 
         [0049]    Diagram (e) of  FIG. 3  corresponds to  FIG. 2 . The grinding process applied to the lower end section  13  will now be explained using  FIG. 2 . The double-dot chain line in  FIG. 2  indicates the state shown in diagram (d) of  FIG. 3 , i.e., the state before grinding. The portion indicated with the double-dot chain line, i.e., the un-roughened lower end section  13  and a lower end portion of the rough surface  17  there above are ground such that both the thermally sprayed coating  7  and the roughened and un-roughened portions of the base material indicated by the double-dot chain line are removed. 
         [0050]    The section indicated with the double-dot chain line is ground such that a cylindrical surface  99  is formed at the bottommost portion of the cylinder bore  3 , and a tapered surface  101  configured such that its diameter narrows in the upward direction is formed above the cylindrical surface  99 . The tapered surface  101  is formed so as to span from the base material of the cylinder bore  3  across the thermally sprayed coating  7 . By forming the tapered surface  101  in this manner, the internal diameter of the cylinder bore  3  that exists after the thermally sprayed coating  7  is formed on the cylinder bore internal surface  5  is made to be larger at the end of the cylinder bore  3  that is closer to the crankcase  9  than along the remaining portions of the cylinder bore  3 . 
         [0051]    The grinding just described removes a portion of the lower end (lower end from the perspective of  FIG. 3 ) of the thermally sprayed coating  7 . As a result, the portion of the thermally sprayed coating  7  that is more likely to have poor or low degree of adhesion is removed and the thermally sprayed coating  7  that remains has a high degree of adhesion with respect to the surface of the base material of the cylinder bore  3  (cylinder block  1 ) on which it is formed. For example, even if a gap  103  occurs between the thermally sprayed coating  7  and the surface of the base material at the end of the thermally sprayed coating  7  (where such a gap is most likely to occur) as shown in  FIG. 7 , the portion where the gap  103  exists will be removed and the remainder of the coating  7  will have excellent adhesion. 
         [0052]    Since the portion of the thermally sprayed coating  7  where the adhesion is poor is removed, the thermally sprayed coating  7  can be prevented from exfoliating due to stresses occurring in the poorly adhered portion during the honing process executed after the thermally sprayed coating  7  is formed and the productivity of the cylinder block manufacturing process can be improved. Additionally, exfoliation of the thermally sprayed coating  7  resulting from the sliding resistance of a piston used in an internal combustion engine made with the cylinder block  1  can be prevented and the durability and reliability of the engine product can be improved. 
         [0053]    When the portion of the thermally sprayed coating  7  where the adhesion is poor is removed, an adjacent portion of the thermally sprayed coating  7  where the adhesion is good is also removed. As a result, the thermally sprayed coating  7  that remains after the grinding process can be reliably ensured to have excellent adhesion with respect to the surface of the base material. 
         [0054]    When the portion of the thermally sprayed coating  7  where the adhesion is poor is removed, some of the base material of the cylinder bore  3  is also removed. As a result, the poorly adhered portion of the thermally sprayed coating  7  can be removed reliably even if there is variance in the diameter and/or position of the ground portion from one cylinder bore  3  to the next. 
         [0055]    After the lower end section  13  of the cylinder bore  3  has been ground as shown in diagram (e) of  FIG. 3 , the thermally sprayed coating  7  is honed to finish the surface thereof.  FIG. 8  is a cross sectional view of the cylinder block  1  showing the thermally sprayed coating  7  being honed with a honing tool  105 . The honing tool  105  has a honing head  107  provided with, for example, four grindstones  109  containing grinding particles made of diamond or other material suitable for grinding. The grindstones  109  are arranged around the circumference of the honing head  107  with equal spacing there-between in the circumferential direction. 
         [0056]    An expanding means configured to expand the grindstones  109  radially outward is provided inside the honing head  107 . During the honing process, the expanding means presses the grindstones  109  against the internal surface  5  of the cylinder bore  3  with a prescribed pressure. 
         [0057]    The surface of the thermally sprayed coating  7  is ground, i.e., honed, by rotating the honing tool  105  while simultaneously moving it reciprocally in the axial direction. The honing process completes the processing of the cylinder bore internal surface  5 . The honing process can be contrived to comprise a succession of rough finishing and fine finishing steps executed using grindstones of different particle sizes (grain sizes). 
         [0058]      FIG. 9  shows the flow of processing steps from the base material surface roughening (pretreatment of base material before thermal spraying) shown in diagram (c) of  FIG. 3  to the finishing (bore finishing) shown in diagram (f) of  FIG. 3 . After the base material surface roughening and before deposition of the thermally sprayed coating, a masking member (not shown in figures) is attached to the upper end portion of the cylinder block  1  and inside the crankcase  9  in order to prevent the coating material from adhering to portions where the coating is not required. 
         [0059]    After thermal spraying the coating material, the masking member is removed and the vicinity of the lower end section  13  is ground (lower end coating removal processing) as shown in diagram (e) of  FIG. 3 . Finally, the coating is honed (bore finishing). 
         [0060]    The honing process is conducted by rotating the honing head  107  while moving it in the axial direction. When the bottommost end is reached, the honing head  107  is moved upward while continuing to rotate it. This up and down reciprocal motion is executed repeatedly. When the honing head  107  shown in  FIG. 8  reaches the bottommost end, the lower ends of the grindstones  109  are positioned below the thermally sprayed coating  7 . As a result, the entire surface of the thermally sprayed coating  7  can be honed. 
         [0061]    Since a tapered surface  101  that narrows in the upward direction is formed on the bottom of the thermally sprayed coating  7 , the upward force F that the grindstones  109  exert against the tapered surface  101  of the thermally sprayed coating  7  when the honing head  107  has reached the bottommost position and is being moved upward can be analyzed as shown in  FIG. 10A . The grindstones  109  move upward while being pushed against the surface of the thermally sprayed coating  7  and the resulting upward force F acts on the tapered surface  101  as a component force P that is perpendicular to the tapered surface  101  and a component force Q that is parallel to the tapered surface  101 . 
         [0062]    As a result, particularly due to the perpendicular component P, a force acts against the tapered surface  101  in such a direction as to press the thermally sprayed coating  7  against the surface of the base material and exfoliation of the lower end portion of the thermally sprayed coating  7  can be prevented. In other words, as shown in  FIG. 10A , the tapered surface  101  creates a section that has a larger internal diameter than other parts of the thermally sprayed coating  7  and the larger diameter enables contact with the tool (grindstones  109 ) to be avoided at this section (i.e., at the tapered surface  101 ). As a result, forces acting in such directions as to cause the thermally sprayed coating  7  to peel are suppressed and exfoliation of the thermally sprayed coating  7  can be prevented. 
         [0063]    Conversely, when a tapered surface is not provided at the lower end of the thermally sprayed coating  7  and the lower end of the thermally sprayed coating  7  has a perpendicular surface  7   a  that is substantially perpendicular to the surface of the base material, the grindstones  109  contact the side surface of the bottommost end portion of the thermally sprayed coating  7  as shown in  FIG. 10B . Consequently, when the grindstones  109  are moved upward while being pressed against the surface of the thermally sprayed coating  7 , a large upward force F acts against the perpendicular surface  7   a  and the thermally sprayed coating  7  is more likely to peel. 
         [0064]    In this embodiment, the existence of the tapered surface  101  reduces the amount of honing that must be done at the lower end and enables the processing time to be shortened. 
         [0065]    In this embodiment, a portion of the lower end section  13  where the thermally sprayed coating  7  is not required is also removed when the vicinity of the lower end section  13  is ground in the processing step illustrated in diagram (e) of  FIG. 3 . Consequently, it is not necessary to remove the thermally sprayed coating  7  from the portion where it is not required during the honing process. As a result, the processing time of the honing process can be shortened, the service life of the honing tool can be extended, and the productivity can be increased. 
         [0066]    Although some of a portion  101   a  of the thermally sprayed coating  7  remains on the tapered surface  101  shown in diagram (e) of  FIG. 3  after the honing process, as shown in diagram (f) of  FIG. 3 , most of this portion  101   a  of the tapered surface  101  is removed by the honing process. 
       Second Embodiment 
       [0067]    Referring now to  FIG. 11 , a cylinder block  1 A in accordance with a second embodiment will now be explained. In view of the similarity between the first and second embodiments, the descriptions of the parts of the second embodiment that are similar to the parts of the first embodiment may be omitted for the sake of brevity. The parts of the second embodiment that are similar to the parts of the first embodiment will be indicated with a letter “A”. 
         [0068]      FIG. 11  shows the state of the cylinder bore  3 A after the thermally sprayed coating  7 A has been deposited and before the finishing process (honing) has been executed. In the second embodiment, the rough boring process is different from the rough boring process of the first embodiment (illustrated in diagram (b) of  FIG. 3 ) in that a larger diameter lower end section  13  is not formed. Similarly to the first embodiment, the surface of the base material is roughened (as shown in diagram (c) of  FIG. 3 ) before the thermally sprayed coating  7 A is deposited onto the cylinder bore internal surface  5 A in order to increase the adhesion of the thermally sprayed coating  7 A. The crankcase  9 A is at the lower end of the cylinder bore  3 A. 
         [0069]    The thermally sprayed coating  7 A is formed over the entire vertical length L of the cylinder bore  3 A as shown in  FIG. 11 . A lower end portion of length M is formed so as to have a tapered surface  101   a  that narrows as one moves upward there-along. The portion of the thermally sprayed coating  7  above the tapered surface  101 A has a substantially uniform internal diameter. In other words, a portion of the thermally sprayed coating  7  located at the end of the cylinder bore  3 A that is closer to the crankcase  9 A is made to be thinner than the remaining portions of the thermally sprayed coating  7 . 
         [0070]    In  FIG. 12 , the solid-line curve shows how the internal diameter of the cylinder bore  5 A changes as one moves from the upper end to the lower end after the thermally sprayed coating  7 A is deposited. The curve clearly indicates that the internal diameter increases at the lower end. The broken-line curve indicates the internal diameter after the base material pretreatment; the thermally sprayed coating  7 A is deposited over this diameter. The single-dot chain line indicates the internal diameter after the thermally sprayed coating  7 A has been subjected to a finishing process (honing process). 
         [0071]    The thermally sprayed coating  7 A is deposited using the thermal spraying apparatus shown in  FIG. 6  in a manner similar to the first embodiment. The thermal spraying process is different from first embodiment in that less coating material is sprayed from the thermal spray gun  31  at the end portion that is near the crankcase  9 A than at the remaining portions of the cylinder bore internal surface  5 A. During thermal spraying, the speed of the axial movement of the thermal spray gun  31  shown in  FIG. 6  is held substantially constant. 
         [0072]    Another method of making the portion of the thermally sprayed coating  7 A thinner at the end of the cylinder bore  3 A that is closer to the crankcase  9 A is to increase the axial movement speed of the thermal spray gun  31  at the end portion. Still another method is to move the thermal spray gun  31  up and down reciprocally in such a fashion that the return point where the thermal spray gun  31  stops moving toward the crankcase  9  (i.e., downward in  FIG. 11 ) and starts moving toward the cylinder head (i.e., upward in  FIG. 11 ) is shifted progressively toward the cylinder head mounting end (i.e., upward) as the spray coating processing proceeds. In both of these methods, the discharge rate of the coating material from the thermal spray gun  31  is held substantially constant. 
         [0073]    After the thermally sprayed coating  7 A has been formed, the honing device shown in  FIG. 8  is used to hone, i.e., finish, the thermally sprayed coating  7 A in the same manner as is illustrated in diagram (f) of  FIG. 3  of the first embodiment. 
         [0074]    In the second embodiment, too, a tapered surface  101 A configured to narrow in the upward direction is provided on a lower portion of the thermally sprayed coating  7 A. As a result, when the honing head  107  reaches the bottommost end of the cylinder bore  3 A and starts moving upward, exfoliation of the lower end portion of the thermally sprayed coating  7 A can be prevented from occurring for the same reasons as previously explained in the first embodiment with reference to  FIG. 10 . 
         [0075]    Also, in the second embodiment, since the only processing that is executed after the deposition of the thermally sprayed coating  7 A is a honing process serving simply to finish the cylinder bore internal surface  5 A, it is not necessary to include a process (e.g., the grinding process illustrated in diagram (e) of  FIG. 3 ) for removing the thermally sprayed coating from portions of the cylinder bore internal surface  5 A where the coating is not necessary. As a result, the processing time can be shortened in comparison with the first embodiment. 
       General Interpretation of Terms 
       [0076]    In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. 
         [0077]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such feature(s). Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.