Patent Publication Number: US-6708411-B2

Title: Circular saw with improved corner cutting

Description:
This application claims priority on Japanese Patent Application No. 2001-088391 filed on Mar. 26, 2001, the contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to circular saws. More particularly, the present invention relates to a corner-cutting circular saw including a base plate and a saw blade assembly tiltably mounted on the base wherein when the assembly is tilted rearward, the portion of the blade protruding below the base plate pivots forward to cut a workpiece where the workpiece forms a corner with a wall or similar structure. 
     2. Description of the Related Art 
     A typical circular saw capable of cutting a workpiece such as a wooden board along a corner, for example, where the workpiece board and another board meet to form a right angle therebetween, includes a saw blade assembly which has a saw blade on a front end thereof and a motor disposed on a rear end thereof for driving the saw blade. The circular saw further includes a generally rectangular base plate below which the lower portion of the saw blade extends through a recess cut in the front side edge of the base plate. The saw blade assembly is mounted on the base plate by means of a pair of connecting shafts extending in parallel to the saw blade such that the assembly is tiltable relative to the base plate with the saw blade disposed parallel to the front side edge of the base plate. To perform ordinary bevel cutting, the lower portion of the saw blade is caused to pivot toward the motor by tilting of the assembly outward in the direction opposite to the motor (to the front of the tool). Conversely, in order to perform bevel cutting along or close to a wall or structure extending upward, for example, at a right angle to the workpiece, the lower portion of the saw blade is pivoted in the recess in the direction opposite to the motor by tilting of the assembly toward the motor (bevel cutting along or close to a wall or similar structure extending, for example, at a right angle to the workpiece is hereafter referred to as corner cutting through the specification). 
     The foregoing conventional saw, however, suffers from certain deficiencies that reduce its utility. Referring to FIG. 5, in the conventional electric power tool, the common axis O 1  of the connecting shafts (which couple the saw blade assembly  120  to the base plate  123 ) coincides with or lies in the center plane L of the saw blade  122 . Accordingly, when an operator tilts the saw blade assembly for corner cutting, the upper blade guard  121  covering the upper portion of the saw blade  122  rotates about the connecting shafts. As indicated by the two-dot chain lines, when the assembly is tilted to the maximum angle, whereby the saw blade  122  is located on the front edge of the base plate  123 , the bottom end of the blade guard  121  protrudes beyond the perpendicular plane (P) in which the front side edge of the base plate  123  is located (the portion of the guard  121  protruding beyond the perpendicular plane P is hatched in the figure). Depending on the situation, it may not be possible to bring the base plate  123  into abutment with the wall due to the protruding portion of the upper blade guard  121 , thus making corner cutting difficult or in some cases impossible. If the protruding portion or the bottom end of the blade guard  121  is cut off to avoid possible interference with a wall or other structure, the structural strength of the blade guard is compromised. 
     SUMMARY OF THE INVENTION 
     In view of the above-identified problems, an important object of the present invention is to provide a circular saw capable of corner cutting without interference with a wall or similar structure even in the maximum tilt position of the saw blade while maintaining the tool&#39;s structural strength. 
     Another object of the present invention is to provide a circular saw capable of making cuts along corners without changing the design of the upper blade guard or other components of the saw blade assembly. 
     The above objects and other related objects are realized by the invention, which provides a circular saw capable of cutting a workpiece at a generally right-angled corner formed between the workpiece and a structure. The circular saw comprises a saw blade assembly including a motor on a rear side thereof and a saw blade on a front side thereof. The saw blade has a center plane extending therethrough and is adapted so as to be rotated by the motor. The circular saw further comprises a generally rectangular base plate having a front edge proximal to the saw blade and distal to the motor and a recess provided in the front edge through which the saw blade protrudes below the base plate. Additionally included in the circular saw are a plurality of connecting shafts having a common axis extending in parallel to both the center plane of the saw blade and the front edge of the base plate, with the connecting shafts attaching the saw blade assembly to the base plate such that the saw blade assembly is tiltable toward the rear side about the common axis of the connecting shafts, thereby causing at least the portion of the saw blade protruding below the base plate to pivot to the front side. In this circular saw, when the blade saw assembly is tilted to hold the saw blade at a right angle relative to the base plate, the axis of the connecting shafts is spaced apart from the center plane toward the motor by a predetermined distance. This arrangement ensures excellent corner cutting without interference of the saw blade assembly with walls or other structures which form a right-angle corner with workpieces. Neither is any additional change in the design or shape of the saw blade assembly required to avoid such interference. 
     According to one aspect of the present invention, the predetermined distance of separation between the axis of the connecting shafts and the center plane of the saw blade is approximately 1 centimeter. 
     According to another aspect of the present invention, the location of the common axis of the connecting shafts is selected such that when the assembly is maximally tilted to the rear side, a predetermined amount of clearance is created between the vertical plane located on the front edge of the base plate and the forwardmost part of the saw blade assembly, thus effectively preventing interference between the assembly and a structure forming a corner with the workpiece. Due to this arrangement, no part of the saw blade assembly protrudes forward from the vertical plane lying on the front edge of the base plate when the saw blade assembly is tilted, locating the entire assembly inside (i.e., to the rear of the saw) the front edge of the base plate. 
     According to still another aspect of the present invention, the saw blade assembly further includes an upper blade guard which covers an upper portion of the saw blade and when the assembly is maximally tilted to the rear, the forwardmost part of the saw blade assembly is a bottom end of the upper blade guard. 
     According to yet another aspect of the present invention, the circular saw further comprises: a lower blade guard which normally covers a lower portion of the saw blade and is retractable to expose the lower portion of the saw blade; and a grip plate which is attached to the lower blade guard and extends radially away from the saw blade along the center plane of the saw blade. The grip plate is manually operable with ease so as to retract the lower blade guard even when the front edge of the base plate is set in abutment with a structure forming an approximately 90-degree corner with the workpiece. Due to its configuration, the grip plate can be easily operated by hand even when making a corner cut in limited space without interfering with a wall or similar structure. 
     According to one feature of the present invention, the saw blade, when pivoted to the maximum angle, extends through the recess with a front face of the saw blade substantially intersecting the imaginary front edge of the portion of the base plate removed by the recess, while the bottom end of the upper blade guard is located on the rear side of the vertical plane that is located on the front edge of the base plate. 
     According to another feature of the present invention, the circular saw further includes: an auxiliary base plate including a bottom surface; a pair of guide bars secured to the auxiliary base plate at a right angle; and means for securing the guide bars to the base plate. When the auxiliary base plate is secured to the base plate, the bottom surface of the auxiliary base plate is located on the same plane as a bottom surface of the base plate and the saw blade is interposed between the auxiliary base plate and the base plate. As both the base plate and the auxiliary base plate are set on a workpiece on the front and rear sides of the saw blade, cuts can be made in a more stable manner than without the auxiliary plate. 
     According to still another feature of the present invention, the means for securing includes: a pair of insertion holes provided in the base plate for allowing insertion of the guide bars therethrough from the front side; a pair of threaded holes provided in the base plate in communication with the insertion holes; and a pair of thumbscrews for being tightened into the threaded holes so as to press free ends of the thumbscrews against the guide bars in the insertion holes. This permits the auxiliary base plate to be held against movement relative to the base plate. 
     According to yet another feature of the present invention, the auxiliary base plate further includes an front edge and, when the auxiliary base plate is secured to the base plate, the front edge of the auxiliary base plate extends in parallel to the front edge of the base plate. Due to this feature, the operator can make accurate cuts in workpieces by abutting a ruler or other suitable reference and/or measuring device against the front side edge of the auxiliary base plate. 
    
    
     Other general and more specific objects of the invention will in part be obvious and will in part be evident from the drawings and descriptions which follow. 
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     For a fuller understanding of the nature and objects of the present invention, reference should be made to the following detailed description and the accompanying drawings, in which: 
     FIG. 1 is a front view of a corner-cutting circular saw according to the present invention; 
     FIG. 2 is a plan view of the circular saw shown in FIG. 1 with the rear end of the motor housing omitted from the view; 
     FIG. 3 is a side view of the circular saw shown in FIG. 1 seen from the direction in which cutting is to progress; 
     FIG. 4 shows the positions of the saw blade and the upper blade guard of the circular saw shown in FIG. 1 (hatched) during corner cutting as compared with those of the saw blade and the upper blade guard of a conventional corner-cutting circular saw (two-dot chain lines); and 
     FIG. 5 shows the positions of the saw blade and the upper blade guard of the conventional circular saw shown in FIG. 4 during corner cutting. 
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will be described hereinafter with reference to the attached drawings. 
     FIG. 1 is a front view of a corner-cutting circular saw  1  according to the present invention. The circular saw  1  includes a generally flat rectangular base plate  2  and a saw blade assembly  3  mounted on the base plate  2 . The saw blade assembly  3  includes a circular saw blade  5  disposed generally over a front edge of the base plate  2  and a motor housing  4  encasing a motor (not shown) disposed over a rear edge of the base plate  2 . The saw blade  5  is coupled to and driven by the motor for cutting. The saw blade assembly  3  is disposed on the base plate such that the saw blade  5  is oriented parallel with a longitudinal front edge of the base plate  2  and projects below the base plate through a rectangular recess  6  formed in the longitudinal front edge of the base plate. The saw blade assembly  3  further includes an upper blade guard  7  covering the upper portion of the saw blade  5  and a lower blade guard  9  which is normally biased by a suitable biasing means (not shown) to the position shown in FIG. 1, generally covering the lower portion of the blade  5 . Additionally, a handle  8  for operating the circular saw  1  extends from the motor housing  4  at a right angle to the housing&#39;s axis. Referring to FIGS. 1 and 2, a grip plate  23  is attached to the lower blade guard  9  via a flange  24  provided around the front left corner (as seen in FIG. 1) of the upper blade guard  7 , extending radially away from the saw blade along the plane on which the blade  5  lies. The grip plate  23  is manually operated to retract the lower blade guard  9 , thus exposing the teeth of the blade  5  before making cuts. In order to cut a workpiece along a line, the circular saw  1  is manually moved to the right as seen in FIGS. 1 and 2. 
     The right end (as viewed in FIGS. 1 and 2, i.e., in the direction in which cutting is to be made) of the upper blade guard  7  is coupled to a link  11  (which has a U-shape cross-section in its upper portion) by a bolt  10  in such a manner as to allow the blade guard  7  to pivot about the bolt  10  along the side faces of the blade  5 . The bolt  10  extends through the U-shaped portion along the transverse edge of the base plate  2 . Furthermore, the opposite end of the upper blade guard  7  is coupled to a depth guide  13  by a thumbscrew  14 . The depth guide  13  has an arcuate shape conforming to the longitudinal curvature of the upper blade guard  7  as shown in FIG.  1 . In addition, the depth guide  13  is tiltably connected to the base plate  2  at its bottom end by a first pin  12  that extends in parallel to the longitudinal (front) edge of the base plate  2 . Accordingly, the amount of the saw blade  5  protruding below the base plate  2  (i.e., the depth of cut) can be adjusted by loosening of the thumbscrew  14 , causing the upper blade guard  7  to pivot to a desired vertical position along the depth guide  13 , and re-tightening the thumbscrew  14  to tightly hold the guard  7  against any further movement with respect to the base plate  2 . 
     As best shown in FIG. 3, the bottom surface of the link  11  (to which the upper blade guard  7  is coupled) is tiltably connected to the base plate  2  with a second pin  15  disposed coaxially with the first pin  12 . Furthermore, an arm  16  extends from the bottom surface of the link along the transverse edge of the base plate  2 , reaching a guide plate  17  which is erected from the base plate  2  along the transverse edge of the base plate  2 . The guide plate  17  includes an arcuate guide slot  18  with the center of the circle forming the arcuate slot  18  located at the axis of the second pin  15 . Another thumbscrew  19  is tightened into the free end of the arm  15  through the guide slot  18 , thus securing the arm to the guide plate  17 . 
     Referring to FIGS. 1 and 2, a guide arm  20  extends along the transverse edge of the base plate  2  from the bottom surface of the depth guide  13 . The guide arm  20  includes an arcuate guide slot (not shown) with the center of the circle forming the arcuate slot located at the axis of the first pin  12 . The guide arm  20  is secured to the upright plate  21  provided on the base plate  2  by a thumbscrew  22  tightened into the arm  20  through the arcuate slot. Accordingly, loosening of both thumbscrews  19  and  22  permits tilting of the saw blade assembly  3  either to the front or rear direction about the first and second pins. By re-tightening the thumbscrews  19  and  22  after tilting the saw blade assembly  3  and thus the saw blade  5  to a desired angular position, the blade remains securely tilted at that angle with respect to the base plate  2 . 
     As shown in FIG. 3, when the saw blade  5  is disposed at 90 degrees relative to the top surface of the base plate  2 , the axis O 2  going through the first and second pins  12  and  15  is spaced apart from the center plane L of the saw blade  5  toward the motor by a predetermined distance S (approximately 10 mm in this embodiment) in the orthogonal direction of the center plane of the saw blade or as measured along a path orthogonally intersecting both the center plane and the axis. 
     Referring to FIGS. 1 and 2, in one embodiment, the circular saw  1  can include an auxiliary base plate  30  that has a length similar to the base plate  2  and a smaller width than that of the plate  2 . A pair of guide bars  32  are permanently secured to the top surface of the auxiliary base plate  30  at a right angle. When the use of the auxiliary base plate  30  is desired, the guide bars  32  are manually passed through insertion holes  34  provided in the base plate  2  close to the right and left (as seen in FIG. 1) transverse edges of the base plate  2 . The auxiliary base plate  30  can be securely attached to the base plate  2  by tightening of thumbscrews (not shown) into threaded holes  36  which are in communication with the guide holes  34  so as to press the free ends of the thumbscrews firmly against the guide bars  32 . Those with ordinary skill in the art to which the invention pertains will readily understand that the guide bars  32  can be secured to the base plate  2  by many other suitable means than the foregoing. When attached to the circular saw  1  in this manner, the auxiliary base plate  30  extends in parallel to the base plate  2  with its bottom surface located on the same plane as that of the (main) base plate  2  with the two base plates  2  and  30  interposing the saw blade  5  therebetween. When unnecessary, the auxiliary base plate  30  can be removed from the circular saw  1  for storage by following the attachment steps in reverse. 
     The provision of the auxiliary base plate  30  augments the utility of the circular saw  1  when the tool  1  is used as an ordinary circular saw (i.e., as opposed to an corner-cutting saw). Specifically, as both the base plate  2  and the auxiliary base plate  30  are set on a workpiece on the front and rear sides of the saw blade  5 , cuts can be made in a more stable manner than without the auxiliary plate  30 . This effect is particularly enhanced during normal bevel cutting as the center of gravity of the saw blade assembly  3  is shifted toward the auxiliary base plate  30  in this type of cutting. 
     In addition to the foregoing advantage, the auxiliary base plate  30  allows the operator to make accurate cuts by abutment of a ruler against the right (as seen in FIG. 2) side edge of the auxiliary base plate. Without the plate  30 , the operator will experience difficulty in using a ruler as a guide or reference on the right or front edge of the saw blade during cutting since the circular saw  1  has no other structure on the front side of the saw blade  5  that a ruler or similar device can be held against. 
     In a corner cutting circular saw  1  thus constructed, when the thumbscrews  19  and  22  are loosened and the saw blade assembly  3  is tilted about the pins  12  and  15  toward the front, the lower portion of the saw blade  5  protruding below the base plate moves toward the rear. When thumbscrews  19  and  22  are tightened again, bevel cuts can be made. Conversely, as shown in FIGS. 3 and 4, when the saw blade assembly  3  is tilted about the pins  12  and  15  toward the rear, the upper blade guard  7  pivots to the rear on the axis O.sub.2 of the two pins, moving the lower portion of the saw blade  5  below the base plate  2  toward the front. When the saw blade  5  is rotated to the maximum angle, at which the thumbscrew  19  is slid to the lower end of the guide slot  18  of the guide plate  17  (FIG.  3 ), the saw blade extends through the recess, with a front face of the saw blade substantially intersecting the imaginary front edge of the portion of the base plate removed by the recess. At the same time, the bottom end of the upper blade guard  7  is located inside of(i.e., on the rear side of) the vertical plane P that lies on the front edge of the base plate  2  (FIG.  4 ). This permits unobstructed corner cutting as the plate&#39;s front edge can abut a wall or other structure without interference of the blade guard  7  against the wall. 
     FIG. 4 shows the positions of the saw blade  5  and the upper blade guard  7  of the corner-cutting circular saw  1  (hatched) during corner cutting as compared with those of the saw blade  122  and the upper blade guard  121  of the conventional corner-cutting circular saw shown in FIG. 5 (two-dot chain lines). In the circular saw  1  of the present invention, when the assembly  3  is maximally tilted for corner cutting, the saw blade  5  is positioned further outward (to the front) than the saw blade  122  of the conventional circular saw at the same angle of tilt, with the lower edge of the upper blade guard  7  located inside the vertical plane P as described above. Accordingly, when the longitudinal front edge of the base plate  2  is aligned with the line of cut by the saw blade  5 , there is still clearance A between the vertical plane P and the outermost edge (bottom end) of the upper blade guard  7 , thus advantageously preventing interference of the blade guard against the wall. 
     To perform corner cutting, after the saw blade assembly  3  is tilted as shown in FIGS. 3 and 4, the lower blade guard  9  is retracted by manually operating the grip plate  23  (see FIGS. 1 and 2) to expose the teeth of the blade  5 . The base plate  2  is then positioned on or in close proximity to the wall while allowing the blade  5  cut into the desired location of the workpiece. A typical conventional circular saws for corner cutting includes a small, narrow grip plate adjacent to the front side of the upper blade guard. Such a narrow grip plate, when placed in tight space between the upper blade case and a wall, cannot be easily operated so as to expose the saw teeth for corner cutting. According to the present embodiment, however, as the grip plate  23  extends to the side of the upper blade guard  7 , the operator can quite easily manipulate the plate  23  even when making a corner cut in limited space. It should be noted that the grip plate  23  is configured and dimensioned to avoid interference with a wall or similar structure when the saw  1  is set on the wall prior to corner cutting. 
     As described above, according to the foregoing embodiment, when the blade saw assembly  3  is tilted to maintain the saw blade  5  at 90 degrees relative to the top surface of the base plate  2 , the common axis O 2  of the first and second pins  12  and  15  is spaced apart from the center plane L of the saw blade  5  toward the motor by the distance S as measured along a path orthogonally intersecting the center plane and the axis. This arrangement prevents the upper blade guard  7  from protruding from the vertical plane located or lying on the front edge of the base plate  2  under any operating conditions of the saw  1 . In addition, when the saw blade assembly  3  is tilted as shown in FIG. 4, the blade guard  7  is located inside (i.e., to the rear of the saw  1 ) the front edge of the base plate  2  with a predetermined amount of clearance. This feature provides for excellent corner cutting without interference of the blade guard  7  with walls or other structures. It should be noted that such interference can be avoided without compromising the structural strength of the blade guard by changing the design or shape of the guard. 
     According to the embodiment, the common axis of the first and second pins  12  and  15  is horizontally shifted to the saw&#39;s rear in contrast to the corresponding axis of the conventional corner cutting saw shown in FIG.  5 . Those of ordinary skill in the art will readily understand that the axis of the first and second pins may lie lower toward the base plate  2  or higher than in the embodiment as long as it is located inside or behind the center plane of the saw blade  5  when the blade  5  is held at 90 degrees relative to the base plate  2 . 
     Equivalents 
     It will thus be seen that the present invention efficiently attains the objects set forth above, among those made apparent from the preceding description. As other elements may be modified, altered, and changed without departing from the scope or spirit of the essential characteristics of the present invention, it is to be understood that the above embodiments are only an illustration and not restrictive in any sense. The scope or spirit of the present invention is limited only by the terms of the appended claims.