Patent Publication Number: US-6983541-B2

Title: Utility knife with retracting shield

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to replaceable-blade utility knives, such as used in the construction trades for cutting sheet materials by hand. 
   2. Description of Related Art 
   Hand-held utility knives that have a thin cutting blade with at least one razor-like edge are ubiquitous tools found in the toolbox of almost every tradesman and do-it-yourselfer. These utility knives come in different styles and types, including disposable and reusable types. Reusable types typically include a metallic handle and are generally regarded as stronger and more reliable than disposable, plastic-handled types. Therefore, reusable utility knives are preferred by professionals and serious craftspeople. With disposable knives, the entire knife is discarded when the blade is spent. With reusable knives, only the blade is discarded and replaced with a new blade; hence, such knives may also be referred to as replaceable-blade utility knives. 
   Replaceable-blade utility knives are generally available in two basic types: retractable-blade knives and fixed-blade knives. Retractable-blade knives, such as knife  90  shown in  FIGS. 1A-B , typically use a single-edged blade  92  that is mounted to a sliding blade holder inside of handle  94 . The sliding blade holder is connected to a button  96  on the outside of handle  94 . By sliding the button  96  along handle  94  to the position shown in  FIG. 1B , blade  92  may be retracted into handle  94 . 
   Retractable-blade utility knives are convenient light-duty cutting tools, and their retractable blade enhances their convenience and safety. At the same time, however, the use of a sliding blade holder adds a certain amount of imprecision to the blade mount and weakens the connection between the blade and the handle. For this reason, many professionals and serious craftspeople prefer to use a fixed-blade utility knife for more demanding applications, such as the installation of wall-to-wall carpeting or precision detailed cutting. 
   As the name implies, in a fixed-blade utility knife, the cutting blade is securely fixed to the handle of the knife. To secure the blade, a thumbscrew or other removable fastener may be used, thereby facilitating convenient replacement of spent blades. But when in use, the blade is securely clamped in place relative to the handle, providing a more trustworthy and precise cutting edge. Consequently, the cutting edge of the blade remains exposed when the knife is not in use, such as when lying in the tool box or carried in a tool pouch. The exposed blade may be covered using a removable safety cover, but such covers are prone to become lost, or may be perceived as inconvenient by some users. 
   Accordingly, there is a need for a replaceable-blade utility knife that combines the safety and convenience of a retractable-blade knife with the precision and strength of a fixed-blade knife, at an affordable cost. 
   SUMMARY OF THE INVENTION 
   The present invention provides a replaceable-blade utility knife that combines the safety and convenience of a retractable-blade knife with the precision and strength of a fixed-blade knife, at an affordable cost. In essence, the knife blade is fixed to the handle, as in a conventional fixed-blade utility knife. Unconventionally, however, a retractable shield (blade guard) can be extended from the handle when the knife is not in use, covering the cutting edges of the blade. The shield can be conveniently retracted out of the user&#39;s way and into the handle using an internal mechanism. The retractable shield does not interfere with the removal and replacement of spent cutting blades, which can be accomplished in a manner similar to that used for a conventional fixed-blade utility knife. 
   In an embodiment of the invention, the retractable shield is operated using a toggle-type spring-loaded reversing mechanism. The reversing mechanism uses a “push and click” operation like the plunger of a ball-point pen. To extend the shield, the plunger is depressed until the shield “clicks” into position. To retract the shield, another push on the plunger disengages the shield and activates a spring-loaded slider that pulls the shield into the handle. The convenient operation of the shield should be particularly appealing to busy professionals and others who desire the safety of a shielded blade without paying a penalty in convenience, strength, or precision. 
   A more complete understanding of the utility knife will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are simplified conceptual views of a prior-art retractable-blade utility knife, respectively showing the cutting blade in extended and retracted positions. 
       FIGS. 2A and 2B  are simplified conceptual views of a retractable-shield knife using an internal sliding mechanism attached to a sliding button, respectively showing the shield in retracted and extended positions. 
       FIG. 2C  is an end view of the knife shown in FIG.  2 B. 
       FIGS. 3A and 3B  are simplified conceptual views of a retractable-shield knife using an internal sliding mechanism actuated by a rear-mounted plunger, respectively showing the shield in retracted and extended positions. 
       FIGS. 4A and 4B  are simplified conceptual views of a retractable-shield knife using an internal toggle-type spring-loaded reversing mechanism actuated by a rear-mounted plunger, respectively showing the shield in retracted and extended positions. 
       FIG. 5  is a perspective view of an exemplary utility knife according to the invention, with the retractable shield in an extended position. 
       FIG. 6  is a perspective view of the knife shown in  FIG. 5 , with the retractable shield in a retracted position. 
       FIG. 7  is a left plan view of the knife shown in  FIG. 5 , with the retractable shield in an extended position. 
       FIG. 8  is a right plan view of the knife shown in  FIG. 5 , with the retractable shield in a retracted position. 
       FIG. 9  is a perspective view of the knife shown in  FIG. 5 , with the retractable shield in an extended position and the blade access door swung open. 
       FIG. 10  is a right plan view of the knife shown in  FIG. 5 , with the right-side knife handle removed to reveal the internal shield-retracting mechanism. 
       FIG. 11  is an exploded plan view of the knife shown in FIG.  5 . 
       FIG. 12  is a perspective view of an exemplary utility knife according to an alternative embodiment of the invention, with the retractable shield in an extended position. 
       FIG. 13  is a perspective view of the knife shown in  FIG. 12 , with the retractable shield in a retracted position. 
       FIG. 14  is an exploded plan view of the knife shown in FIG.  12 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention provides a replaceable fixed-blade utility knife, that includes a retractable blade guard, also called a shield, for greater safety and convenience. In the detailed description that follows, like element numerals are used to designate like elements appearing in one or more of the figures. 
   In an embodiment of the invention, a retractable-shield knife  100  as shown in  FIGS. 2A-C  has a retractable shield  102  attached to a movement mechanism (not shown) internal to handle  104 . The movement mechanism provides sliding motion, and is therefore referred to as a sliding mechanism. It should be appreciated, however, that the movement mechanism may, in the alternative or in addition, provide other types of motion, such as rotational movement. Blade  108  is fixed relative to handle  104 , and has a cutting edge  109  that extends from the handle for a fixed distance. The sliding mechanism, in turn, is attached to a sliding button  106 , by which the sliding action of shield  102  is actuated. Pushing the sliding button  106  to the left, to the position shown in  FIG. 2A , actuates the sliding mechanism so as to retract the shield into the handle. Pushing the sliding button to the right, to the position shown in  FIG. 2B , actuates the sliding mechanism to extend the retractable shield  102  to cover the cutting edge  109  of blade  108 . An end view of knife  100  is shown in FIG.  2 C. Shield  102  is free to move in and out of handle  104  through opening  105 . Blade  108  may be fixed to handle  104  in any suitable manner. 
   In an alternative embodiment, as shown in  FIGS. 3A-3B , a retractable-shield knife  110  may be provided with a shield  112  operated by an internal sliding mechanism, which is, in turn, actuated by a rear-mounted plunger  113 . Motion of the plunger  113  causes a corresponding motion of the shield  112  in the same direction.  FIG. 3A  shows the shield in a retracted position; in this position the plunger is extended from handle  114  and the cutting edge of blade  118  is exposed.  FIG. 3B  shows the shield in an extended position, covering the cutting edge of the blade; in this position, the plunger has been pushed deeper inside of the handle  114 . 
   Plunger  113  may be provided with a spring-loaded toggle mechanism, such as the examples discussed in connection with  FIGS. 11 and 14  below. With a toggle mechanism, depressing the plunger when it is in the extended position moves it against the force of a spring to the position shown in  FIG. 3B , where the plunger is latched in position. Depressing the plunger a second time releases the latch, and the plunger is returned by the stored spring force to the extended position shown in FIG.  3 A. Knife  110  may additionally be provided with a safety latch (not shown), to prevent inadvertent toggling of the plunger that might otherwise expose blade  118  at an inopportune time. 
   Advantageously, a plunger-actuated mechanism provides a more direct coupling of sliding force to the retractable shield, as compared to the offset sliding button shown in  FIGS. 2A-C . Disadvantageously, however, the plunger may be somewhat bulkier, and may interfere with handling of knife  110  when cutting; i.e., when shield  112  is in its retracted position. A further disadvantage is that the plunger, when equipped with a spring-loaded toggle mechanism, may be prone to inadvertently exposing the blade unless equipped with a separate safety lock. 
   Knife  120 , shown in  FIGS. 4A and 4B , offers all of the advantages of a plunger-actuated mechanism, while minimizing its disadvantages. In brief, knife  120  includes a reversing mechanism inside of handle  124 , that generally reverses the direction of motion of shield  122 , relative to actuator  123 . Hence, when shield  122  is in a retracted position as shown in  FIG. 4A , exposing the cutting edge of blade  128 , plunger  123  is also retracted into handle  124 . In this retracted position, plunger  123  is held in place by a toggle latch against a spring. Depressing plunger  123  releases the latch, causing the plunger  123  to extend under the force of a spring installed inside of handle  124  to the position shown in FIG.  4 B. In this position, shield  122  is also extended, covering the cutting edge of blade  128 . Thus, knife  120  provides at least two advantages: (1) the shield is maintained in position by the force of a spring when the cutting blade is not in use, and (2) the plunger is retracted into the handle and out of the user&#39;s way when knife is used for cutting; i.e., when the blade is exposed. 
   One of ordinary skill will understand knives  100 ,  110 , and  120  to exemplify various basic types of retractable shield knives according to the invention. Further mechanical details that may be applied in the construction of these and other knives according to the invention should be apparent from the more particularized descriptions of the embodiments that follow. The first of these embodiments utilizes a double-edged, thin replaceable blade such as conventionally used in fixed-blade utility knives intended for cutting of carpeting, and the like. The second of these embodiments, also a fixed-blade design, utilizes a single-edged blade such as conventionally used in retractable-blade, general-purpose utility knives. While the present invention may be adapted for use with a blade of any type, use of a commonly-available utility blade such as those shown may enhance convenience for the user. 
   Referring generally to  FIGS. 5-11 ,  FIGS. 5-9  show various views of an exemplary utility knife  200  according to the invention.  FIG. 10  shows a portion of knife  200 , with a portion of the handle removed to reveal the movement mechanism and other parts positioned in the interior of the knife handle.  FIG. 11  is an exploded assembly drawing showing individual parts of knife  200 . Like knife  120  shown in  FIGS. 4A-B , knife  200  comprises a cutting blade  208  fixed to a handle  204 , and a plunger-type actuator  203  connected by a reversing mechanism to a retractable shield  202 .  FIGS. 5 ,  7 , and  9  show knife  200  with shield  202  in an extended position, covering the edges of blade  208 .  FIGS. 6 and 8  show knife  200  with shield  202  retracted into handle  204 . 
   Handle  204  comprises a right casing  210  and left casing  212 . Casings  210 ,  212  may be fastened together using fasteners  220 , and may include miscellaneous other internal and external small parts, including a cover piece  214  and clamp  216 . Casings  210 ,  212  may be constructed of any suitable material, such as plastic or metal. In an embodiment of the invention, casings  210 ,  212  are cast aluminum alloy pieces. When assembled, handle  204  preferably presents a smooth, comfortable surface for gripping. 
   In addition, a finger grip  206  may be provided along an undersurface of the handle  204 , to provide a more ergonomic handle. Finger grip  206  may comprise a generally smooth protrusion from handle  204 , with contoured sides  207  shaped to fit between fingers of one hand, and a rounded tip  211 . The finger grip may be positioned on handle  204  to fit between the index and middle fingers, and be symmetrical in shape so as to fit both right-handed and left-handed users. In addition, or in the alternative, finger grip  206  may be placed in a different position along handle  204 , or additional finger grips may be placed on the handle, or may be omitted altogether. Use of a single finger grip  206  is believed to provide a more ergonomic handle than prior art utility knives, as well as contributing to an elegant but utilitarian appearance of the knife. Optionally, a gripping portion of the handle (e.g., to the left of cover piece  214  and grip  206  in  FIG. 5 ) may be covered by a soft material suited for gripping, such as a synthetic rubber material (not shown). 
   For more comfortable gripping, the gripping portion of handle  204  may be inclined at an angle relative to axis ‘a’, as shown in  FIGS. 5-8 . Axis ‘a’ is an axis parallel to at least one cutting edge of blade  208 . In the embodiment shown in  FIGS. 5-9 , blade  208  has two parallel cutting edges  209 , as indicated in FIG.  6 . Blade  208  is fixed to handle  204 , and has cutting edges  209  extending for a fixed distance from handle  204 . That is, blade  208  is essentially not movable relative to the handle, except when being removed and replaced with a new blade. Although the invention does not preclude the use of a moveable blade, a fixed-blade configuration is preferred. Blade  208  may comprise any suitable knife or razor blade. One of ordinary skill will recognize the exemplary blade  208  as one of a great variety of commonly-available utility blades. 
   Knife  200  further includes a retractable shield  202 . Shield  202  comprises a piece of hard, durable material shaped to cover one or both cutting edges of blade  208 . In the embodiment shown in  FIGS. 5-9 , shield  202  is a primarily flat metal piece with opposing lips  278  that cover the opposing cutting edges of blade  208 . Referring to  FIG. 11 , shield  202  includes two guide slots  274  that cooperate with other features of the knife to guide the shield back and forth over its intended range of motion. Slots  274  may be formed to accommodate legs  270  of blade retainer  224 . Shield  202  may also be guided by channel  228 , and any other suitable features may be used to guide the shield. Shield  202  may additionally include an attachment feature  276  for attaching the shield to a movement mechanism. 
   Plunger  203  is connected to shield  202  by a movement mechanism, comprising pulleys  240   a-d  and cable  238 . In an embodiment of the invention, the movement mechanism provides linear motion over a desired linear range for shield  202 , in the direction of axis ‘a’. The movement mechanism may, in the alternative, provide motion over an arcuate path. The movement mechanism may be located between casings  210 ,  212 , such as in recess  236  in left casing  212 . Cable  238  may comprise a loop of any suitable tensile material. A first attachment feature  244  on cable  238  attaches to a corresponding attachment  290  in plunger  203 . A second attachment feature  248  attaches to shield  202 . Pulleys  240   a-d  may be placed in suitable portions of recess  236  and fastened in place using any suitable fasteners. Loop  238  is looped around the pulleys, and may be retained by grooves in the pulley perimeters. Pulleys  240   b ,  240   d  may be stacked to guide loop  238  in opposing directions. 
   It should be apparent that placing the attachment features  244 ,  248  on opposite sides of loop  238  (as shown in  FIG. 11 ) provides a reversing mechanism. That is, movement of the plunger  203  (and hence, the first attachment feature  244 ) from left to right causes a generally opposite movement from right to left of attachment feature  248  and its attached shield  202 . In the alternative, the attachment features may be placed on the same side of loop  238 , to provide a non-reversing mechanism. 
   The invention is not limited to knives that incorporate a loop-and-pulley type movement mechanism. Other types of movement mechanisms may be employed, and may also be mounted inside or adjacent to the knife handle. For example, other suitable linear movement mechanisms may include rack-and-pinion mechanisms, tongue-and-groove sliders, rails, rollers, worm gears, or other geared mechanisms. Rotating mechanisms may be used to provide motion over an arc. Cam/cam follower mechanisms may also be suitable. Instead of using a plunging or sliding actuator, the movement mechanism may be actuated by turning a knob, or the like. The depicted loop-and-pulley type mechanism, however, is believed advantageous for providing rapid shield movement with a toggled reversing action, using a relatively simple assembly. 
   A suitable spring, such as coil spring  242 , may be attached to the movement mechanism and biased against handle  204 . Pushing plunger  203  into handle  204  stretches spring  242 , opposite ends of which are attached to handle  204  and loop  238 , respectively. A pin  250  may be used to attached the spring to the left casing  212 . To prevent the plunger and its attached loop from immediately springing back, the movement mechanism may additionally include a toggle latch  234 , shown in FIG.  11 . Latch  234  may be pinned to handle  204  using a suitable pin  282 , and biased against a suitable spring  280 . When shield  202  is fully retracted, toggle latch  234  drops into a latched position, preventing loop  238  from springing back under the tension of spring  242 . Pushing plunger  203  a second time releases the latch, causing the plunger and shield  202  to extend from the handle, releasing energy stored in spring  242 . Various toggle latches are known in the art, and any suitable latch may be used. In the alternative, or in addition, a manually-operated latch may be used to hold the shield  202  in an extended and/or retracted position. Disadvantageously, a manual latch may make the movement mechanism somewhat less convenient for a user to operate. 
   Referring to  FIGS. 10 and 11 , plunger  203  may also rest in recess  236 . The plunger includes an attachment feature  290  for attaching to loop  238 , as previously described. Plunger  203  may include a guide slot  292  to accommodate a corresponding guide post in recess  236 , for guiding the back-and-forth motion of the plunger. The plunger may be formed from any durable material, for example, metal or plastic. Portions of the plunger that protrude from the handle should be free of sharp edges. 
   Blade  208  is fixed in position relative to handle  204  by cover piece  214 , yet may readily be removed and replaced by loosening clamp  216  and pivoting the cover piece to an open position, such as shown in FIG.  9 . When in this position, the blade is free to be lifted off of the retainer  224 . A replacement blade may then be placed on the retainer, as shown. Or, the original blade may be replaced in a different position, for example, turned 180°. 
   A user of the knife may reposition the blade several times before it is spent and discarded. For example, the double-edged blade  208  has four useable cutting edge portions, only one of which is in use at any given time. Two of the four edge portions are encased between casings  210 ,  212 , and two are exposed. The two exposed edge portions may be used successively by turning the entire knife over, or by opening the cover  214  and turning the blade over. In any case, it is desirable to provide a blade holding structure that permits convenient replacement or repositioning of the blade, as both actions may be frequently needed in the ordinary course of work. 
   Knife  200  embodies one such convenient blade holder. Referring generally to  FIGS. 5 ,  9 , and  11 , cover piece  214  is connected to right casing  210  by pin  260 , about which it can be pivoted. As shown in  FIG. 9 , a bearing surface  230  is provided on the underside of piece  214 . The bearing surface bears against blade  208 , holding it against the mounting surface  268  of retainer  224  when the cover  214  is closed and compressed by clamp  216 . A relief well  232  is also provided in the underside of cover piece  214 , to provide clearance for the central bar  272  of the retainer  224 , thereby assuring contact between bearing surface  230  and blade  208 . 
   Clamp  216  is rotatably fastened to the upper surface of the cover piece by a fastener  258 , and is disposed in a central recess  254 . The clamp includes a gripping feature  218  for permitting a user to rotate the clamp in the recess  254  around fastener  258 , without loosening the fastener. Preferably, the gripping feature comprises a handle, such as bar  218 , that can readily be grasped by hand, and turned. In the alternative, or in addition, the gripping feature may comprise a stud, indentation, ring, or other feature for gripping by hand or with a separate tool. As shown in  FIG. 11 , clamp  216  further comprises annular tapered edges  252  around portions of its outer periphery. Each edge  252  is relatively thin at one end, and is progressively tapered to a greater thickness around the circumference of the clamp. Hence, each edge  252  provides a ramp that may be wedged into an opposing recess  226  (one of two shown in  FIG. 9 ) in the left casing  212  when the cover piece  214  is closed. 
   Clamp  216  is operated by closing the cover piece  214  with the blade  208  in the position shown in FIG.  9  and the clamp turned to its counter-clockwise limit. To define this limit, a suitable stop (not shown) may be provided between the cover and the clamp. When the cover is closed, the clamp is turned in a clockwise direction to engage edges  252  in their respective recesses  226 . Further turning in a clockwise direction compresses the cover against the blade, as the edges  252  advance in a screw-like fashion through recesses  226  of casing  212 . Maximum compression may be reached at a clockwise limit of clamp  216 . 
   The combination of a pivoting cover  214  and clamp  216  advantageously provides for quick and convenient clamping and un-clamping of blade  208 , without requiring a threaded fastener. In the alternative, a conventional threaded fastener may be used for clamping the blade, as in prior-art utility knives, by passing through the blade and engaging the left casing  212 . Of course, the movement mechanism and shield  202  should remain free to move through or around any clamped region. Other types of clamps may also be used, and the invention should not be limited to a blade clamp of a particular type. 
   The method of blade support used by knife  200  may also be of interest to one of ordinary skill, although the invention should not be limited to the particular embodiment described. Blade  208  rests on the mounting surface  268  of retainer  224 . Bar  272  protrudes through a central opening  262  in blade  208 . Bar  272  may be shaped so that it is closely accommodated by the opening, thereby ensuring precise locating of the blade relative to the retainer  224 , in the plane of mounting surface  268 . To avoid dulling of its cutting edges, retainer  224  should hold blade  208  so that its edges do not contact surfaces inside of channel  228 . Accordingly, these edges are preserved, permitting renewal of the blade edges by opening the cover piece  214  and rotating the blade 180°, thereby doubling the blade life. 
   Retainer  224  further includes stand-offs  270  (shown in dashed line in  FIG. 11 ) on the underside of the retainer. Stand-offs  270  may comprise cylindrical support legs, and are configured to engage in recesses  266  in the left casing  212 . The stand-offs are positioned to pass through the guide slots  274  of shield  202 . Stand-offs  270  may be configured to perform at least two separate functions: to provide support for the blade retainer  224  while leaving the shield  202  free to slide in channel  228  between the blade and the casing, and guiding shield  202  in cooperation with guide slots  274 . The blade  208  may thus be securely clamped to the retainer  224 , while leaving the shield  202  free to slide in channel  228  under the blade. 
   Knife  200  optionally includes an internal tray  220 , a small portion of which is shown in  FIGS. 5-6  at the rear of handle  204  near plunger  203 . A plan view of tray  220  is shown in FIG.  11 . Tray  220  may be pulled out of handle  204  through an opening near the rear of the handle. It may include a recess for holding spare utility blades, such as blade  286 , or other useful implements. Tray  220  may be separated from plunger  203  by a partition  288 . 
   According to an alternative embodiment of the invention, a knife  300  is provided with a single-edged blade  308 , as shown in  FIGS. 12-14 . Many of the other features of knife  200  have been adapted for use with knife  300 . Despite its superficial resemblance to a traditional, sliding-blade-type utility knife, the knife  300  comprises a blade  308  that is fixed and immobile relative to the knife handle, like the blade  208  of knife  200 . Knife  300  further comprises a handle comprising casings  310 ,  312 , a hinged cover piece  314  and clamp  316  for clamping the blade, a plunger  303 , and a shield  302 . Shield  302  comprises an L-shaped bar. It is connected to plunger  303  by a reversing-type movement mechanism, similar to the movement mechanism of knife  200 .  FIG. 12  shows knife  300  with the plunger  303  and shield  302  in an extended position. Depressing the plunger causes the shield  302  to retract into the handle, to a position as shown in  FIG. 13 , in which a toggle latch deploys to maintain the position until the plunger is again depressed. The plunger also retracts into the handle, although a small portion (not visible in this view) should remain protruding from the handle. 
   Certain comparisons between knives  200  and  300  may be helpful. Repositioning or replacement of blade  308  may be accomplished similarly to knife  200 . Clamp  316  may be similar to clamp  216  of knife  200 . Blade  308  is of a single-edged type, and does not contain a central opening. The handle of knife  300  is straighter than the handle of knife  200 . Accordingly, knife  300  includes components that are adapted to accommodate these and other differences from knife  200 . Materials and construction of knife  300  may be generally similar to knife  200 , with principle differences relating to the shape and arrangement of certain components. Like knife  200 , knife  300  may include a similar finger grip  306  along the underside of the handle 
   Components of knife  300  are shown in FIG.  14 . Briefly, knife  300  comprises a right handle casing  310  attached to a left handle casing  312  by fasteners such as fastener  322 . A pivoting cover piece  314  is pinned by pin  360  to casing  310 . Cover  314  includes a circular recess  354  in which clamp  316  is positioned. Clamp  316  is fastened to cover  314  by a fastener  358 . The clamp includes tapered edges  356 , which cooperate with recesses  326  in the left handle casing to compress the cover  314  against blade  308  when the clamp is rotated. Clamp  314  may be grasped by bar  318 . The operation of the cover and clamp may be generally similar to the cover and clamp for knife  200 , previously described. 
   A movement mechanism, comprising loop  338  and pulleys  340   a-c , is disposed in casing  312 . Plunger  303  may be attached at opening  390  to fastener  344 , and is shaped to slide in a corresponding channel of the left casing  312 . A spring  342  may also be attached to loop  338 , and connected to casing  312  by pin  350 . Shield  302  is attached to loop  338  at fastener  348 . A toggle latch  334  may be biased by spring  380  and attached to the casing by a pin  392 . The movement mechanism is of a reversing type, and provides linear motion over a range substantially equal to the distance by which blade  308  extends from the handle. 
   Lip  378  of shield  302  is disposed adjacent to and covers the cutting edge  364  of blade  308 . Blade  308  includes cut-outs  362  along an upper edge. Blade  308  is disposed on retainer  324 , with one of cut-outs  362  engaging the semi-cylindrical retention feature  372 . Coyer  314  clamps blade  308  to a flat bearing surface of the retainer  324 . On an opposite side of the retainer, stand-offs  370  engage positioning holes  326  in the left casing  312 . Shield  302  slides in channel  328 , free and clear of the retainer. 
   Partition  388  may be used to separate the plunger  303  from a sliding tray  320 . The partition may also assist guiding of the plunger and sliding tray in their motion into and out of the handle. Tray  320  may include a recess  384  for holding replacement blades, such as spare blade  386 . 
   Having thus described a preferred embodiment of the utility knife, it should be apparent to those skilled in the art that certain advantages of the within system have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. For example, a utility knife with a shield attached to a reversing mechanism has been illustrated, but it should be apparent that the inventive concepts described above would be equally applicable to non-reversing mechanisms. The invention is further defined by the following claims.