Abstract:
An axially elongate hobby knife having an external housing defining an internal cavity is described. The hobby knife has structural features that permit the rotation of an rearward actuator. Such rotation causes a cover to advance over the blade at the same time as the blade withdraws into the cover, thereby to provide for enhanced safety and security of the blade.

Description:
BACKGROUND 
       [0001]    This application relates to hobby knives. In particular, the application relates to a hobby knife with a replaceable blade, in which the blade may be withdrawn into the housing of the knife for safety. 
         [0002]    Hobby knives are well known in the art. One of the problems with a hobby knife is to secure the blade so that it will not cause harm while the knife is not being used. Some hobby knives provide for a sliding block to hold the blade that the user manually retracts into the housing by sliding the block directly, using a button that moves axially in relation to the knife. Such knives still present a problem however because the button, extending from the profile of the knife, may be inadvertently bumped, causing the blade to extend and to present a hazard. 
         [0003]    Thus, there is a need in the art for a hobby knife that allows for easy retraction of the blade without providing an axially moving button on the knife. The present invention addresses these and other needs in the art. 
       SUMMARY OF THE INVENTION 
       [0004]    According to a preferred embodiment of the invention, there is described a hobby knife configured to address various disadvantages in the prior art. 
         [0005]    An axially elongate hobby knife, having an external housing defining an internal cavity, preferably includes a hollow cylindrical cover having a cover forward portion and a cover distal portion, the cover forward portion defining a slot for a blade, and the cover rearward portion having a first external diameter. A hollow cylindrical handle may be positioned distal the cover, the handle having a handle forward portion and a handle distal portion, the handle forward portion having a second internal diameter larger than the first external diameter, the handle forward portion being configured to receive the cover rearward portion, the handle distal portion having a third external diameter. A hollow cylindrical actuator may be positioned distal of the handle, the actuator having a fourth internal diameter larger than the third external diameter, the actuator being configured to receive the handle distal portion. A piston, configured to move axially within the housing cavity, is provided, the piston being axially fixed to the cover whereby axial movement of the piston causes equivalent axial movement of the cover. A first rod may be axially fixed to the piston at one end and extending distally into the actuator so that a distal end of the first rod terminates inside the actuator. A second rod with a distal end that terminates inside the actuator is provided, the second rod extending from said distal end to a forward end that terminates inside the cover, the forward end being axially fixed to a blade holder block positioned inside the cover. The actuator defines an inclined planar surface that is inclined at an angle to the elongate axis of the knife, the planar surface being positioned adjacent the distal end of the first rod and the distal end of the second rod. These features have the advantageous result that, rotation of the actuator causes the inclined planar surface, by contact, to advance and to retract the distal end of the first rod and the distal end of the second rod so that the distal ends axially reciprocate out of phase with one another, and further whereby, the blade holder block and the cover axially reciprocate out of phase with one another. 
         [0006]    In another aspect of the invention, the second rod extends through a bore in the piston, whereby the second rod may freely slide through the bore. In a further aspect, a first spring surrounds the first rod. Alternatively, a second spring surrounds the second rod. Further alternatively, a first spring surrounds the first rod, and a second spring surrounds the second rod. 
         [0007]    In yet another aspect of the invention, the handle distal portion includes grooves for supporting the first rod and the second rod. 
         [0008]    In an additional preferred aspect, the piston defines first external threads, and the cover defines second internal threads configured to mate with the first external threads. 
         [0009]    These and other advantages of the invention will become more apparent from the following detailed description thereof and the accompanying exemplary drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a perspective view of a hobby knife having features of the present invention. 
           [0011]      FIG. 2  shows a perspective exploded view of the hobby knife of  FIG. 1 . 
           [0012]      FIG. 3  shows a sectional perspective view of a component of the knife of  FIG. 1 . 
           [0013]      FIG. 4  shows a perspective exploded view of some components of the hobby knife of  FIG. 1 . 
           [0014]      FIG. 5  shows a sectional view of the hobby knife of  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    With reference to the drawings, which are shown for exemplification and not limitation, a preferred embodiment of a novel and useful hobby knife is described.  FIG. 1 . shows a perspective view of a preferred embodiment of the hobby knife, generally referenced by the numeral  10 . As may be seen, an outer housing of the knife preferably has a cylindrical form, and defines an inner cavity ( FIG. 5 ). It is preferably divided into three distinct and independent hollow cylindrical components. In the front is a blade cover  12 . Behind the cover is a handle  14 , and behind the handle is an actuator tube  16 . A blade  18  extends from the front end of the blade cover, through a slot  20  in the cover  12 . 
         [0016]    In further detail, as exemplified in  FIGS. 2 , and  5 , the cover  12  preferably has two outer cylindrical portions with different outside diameters. A forward portion  50  has a larger outside diameter than that of a rearward portion  52 . Similarly, the handle  14  has a forward portion  54  that has a larger outside diameter than that of a rearward portion  56 . The outside diameter of the cover rearward portion  52  is sized to fit within an inside diameter of the handle forward portion  54 , as may be seen in  FIG. 5 . The outside diameter of the handle rearward portion  56  is sized to fit within an inner diameter of the actuator  16  ( FIG. 5 ). 
         [0017]    Inside the housing cavity provided by the three external components  12 ,  14 ,  16  is situated the working mechanism of the hobby knife. The mechanism includes a piston  26  configured to slide within the cavity of the knife. The piston has an external threaded portion  58  configured to mate with an internal threaded portion  60  of the cover rearward portion  52  (seen in  FIG. 5 ) so that the piston and the cover move axially in unison. The piston  26  also includes two rearward rods  28 ,  30  extending rearwardly from the rear of the piston, configured to provide an axially reciprocating motion of the piston within the housing cavity, as described below. Extending from the front of the piston is a forward extending reciprocating rod  36 , which is a continuation of aligned rearward reciprocating rod  30 . Thus, rods  30  and  36  are one continuous rod that slidably extends through a cylindrical bore  37  in the piston ( FIG. 5 ). The adjacent rod  28  is axially fixed to the piston. The rearward portion  56  of the handle may include flutes  67  to stabilize the reciprocating rods  28 ,  30  ( FIG. 2 ). Each piston rod  28 ,  30  terminates in a distal end  102 ,  104 . 
         [0018]    Attached to the forward end of the forward  36  rod is a conventional blade holder block  24  ( FIGS. 5 and 2 ) that preferably comprises a cylinder split into four portions to be compressed by conventional means for holding the blade  18  securely. The blade holder block  24  may reside within an internal cavity of the blade cover  12  and may slide axially within said internal cavity. 
         [0019]    Turning now to  FIG. 3 , another feature of the invention is described.  FIG. 3  shows a perspective sectional view of the actuator  16 , in which one half of the actuator is shown. The other half, not shown, is substantially a mirror image of the half that is shown. The hollow cylindrical actuator  16  is preferably positioned at the rear of the knife and is configured to receive the rearward portion  56  of the handle  14  within an inner bore  62 . However, forward of this inner bore  62  is a gap portion  64  with a larger bore than that of the inner bore  62 . The gap portion is diametrically sized to receive both reciprocating rods  28 ,  30 , so that when the actuator  16  rotates in relation to the handle  14 , the actuator rods  28 ,  30  remain rotationally stationary within the gap portion  64 . An important feature of the invention is that to the rearward of the gap portion, an inclined planar surface  42  is machined or molded into the actuator  16 , so that rotation of the actuator  16  about the axis of the knife  10  causes the planar surface  42  to rotate in contact with the terminal ends  102 ,  104  of the reciprocating rods  28 ,  30 . It will be appreciated that rotating the inclined planar surface  42  about the axis of the knife will cause the terminal ends  102 ,  104  of the reciprocating rods (which are held rotationally stationary by the handle  14 ) to be forced axially backwards and forwards, thus to oscillate backwards and forwards, or reciprocate, along the length of the device  10 . Where the rods  28 ,  30  are positioned diagonally across the center line of the knife, during such reciprocation the axial movement of the rods will be 180 degrees out of phase with one another. Springs  32 ,  34  are preferably provided to surround the rods  28 ,  30  in order to assist in the return motion of the reciprocating action of the rods  28 ,  30 . 
         [0020]    It will be appreciated that because rod  28  is axially fixed to the piston  26 , and because continuous rod  30 ,  36  slides through the bore  37  in the piston  26  to connect to the rear end of the blade holder  24 , reciprocation of the distal ends  102 ,  104  of the rods  28 ,  30  will (a) cause the piston to axially reciprocate (oscillate) in unison with the rod  28 , and will (b) also cause the blade holder  24  to axially reciprocate in unison with the rod  30 . Because the piston is axially fixed to the cover, the cover will also reciprocate in unison with the rod  28 . 
         [0021]    In further preferred aspects of the hobby knife, the actuator  16  is configured to rotate in relation to the handle  14 , but the actuator  16  is also configured to remain the same axial distance from the handle  14  by virtue of a bulbous stub  66  on the handle that has an outer diameter sized to be rotatably trapped in an internal cavity  68  in the actuator. 
         [0022]    Thus, in use, the user may hold the handle  14  stationary with one hand, and rotate the actuator  16  with the other hand. This action causes the inclined planar surface  42  internal to the actuator to rotate about the knife axis while being positioned adjacent the distal ends  102 ,  104  of the reciprocating rods  28 ,  30 , thereby causing the rods to reciprocate axially, 180 degrees out of phase with each other. It will be appreciated that this will cause the block  24  to axially reciprocate, and will also simultaneously cause the piston  26  to axially reciprocate 180 degrees out of phase with the block, with the continuous rod  28 ,  36  sliding through the bore  37  in the piston. Reciprocation is assisted by the presence of two springs, a first spring  32  surrounding the first rod  28 , and a second spring  34  surrounding the second rod  30 , as seen in  FIGS. 2 and 4 . As seen in  FIG. 5 , the internal threads  60  on the cover  12  are mated with the external threads  58  on the piston  26 . It follows that the piston  26  and the cover  12  (being axially fixed to each other) will axially reciprocate in unison with each other, and accordingly, that the block  24  (with its blade  18 ) and the cover  12  will axially reciprocate  180  degrees out of phase with each other. 
         [0023]    The cover  12  is able to axially reciprocate in relation to the handle  14  because the rearward portion  52  of the cover is able to slide back and forth within the forward portion  54  of the handle without disengagement and without obstruction from the handle. This action has the result that, when a user holds the handle  14  with one hand, and rotates the actuator  16  with the other hand, the blade  18  and the cover  12  axially reciprocate out of phase with each other. This has the advantageous effect that, rotation of the actuator  16  causes the blade  18  to withdraw into the cover  12  through the slot  20  at the same time as the cover  12  is axially advanced over the blade  18 , effectively doubling the relative speed with which the blade would otherwise be covered. This action advantageously provides for greater security in storing the blade  18  within the housing cavity because a small manual action in which the actuator is rotated causes both retraction of the blade  18  into the cover  12 , and advancement of the cover over the blade at the same time. It will be appreciated that continued rotation of the actuator will cause the blade  18  to advance out of the cover  12  at the same time as the cover is withdrawn from the blade. 
         [0024]    In order to exchange the blade when its edge has become dull, the user may retract the blade  18  through the slot  20  into the cover  12  by rotating the actuator  16  an appropriate amount as described above. The user may then hold the handle  14  stationary with one hand, and twist the cover  12  with the other hand to disengage the external threads  58  of the piston  26  from the internal threads  60  of the cover  12 . The cover is removed from the knife  10  to expose the blade holder block  24 . By conventional means, the blade  18  may be released from being gripped in the block and may be replaced by another blade. The cover  12  is then reinstalled on the knife by reversing the steps above, and the cover threads  60  are screwed onto the threads  58  of the piston  26  by an amount that will permit the blade  18  to be aligned with the slot  20  so that the blade may extend from the slot when the actuator  16  is rotated. 
         [0025]    Thus, in use, the operation of the components of the knife may be summarized thus: 
         [0026]    When the handle  14  is held rotationally stationary and the actuator  16  is rotated, the cover  12  and the handle  14  are constrained by their geometry to not rotate in relation to one another. Rather, under this action, the cover  12  axially reciprocates back and forth in relation to the handle  14 , with the rearward end  52  of the cover sliding back and forth within the forward end  54  of the handle. 
         [0027]    Furthermore, when the handle  14  is held rotationally stationary and the actuator  16  is rotated, the handle  14  and the actuator  16  rotate in relation to one another, but are constrained by their geometry to be held axially fixed in relation to each other. Under this action, the rotation of the actuator causes the inclined planar surface  42  to rotate about the knife axis adjacent the terminal ends  102 ,  104  of rods  28 ,  30  and, by contact, forces the terminal ends to reciprocate, preferably 180 degrees out of phase with one another. Reciprocation is assisted by springs  32 ,  34  mounted on the rods to restore the rods to a position in contact with the inclined planar surface  42  as the surface rotates about the axis of the knife. Reciprocation of rod  28  causes reciprocation of the piston  26  within the housing of the knife, and this in turn causes reciprocation of the cover  12  in relation to the handle  14 , because the piston is threadably fixed to the cover. Reciprocation of rod  30  causes reciprocation of rod  36  (rod  30  and rod  36  are in fact one single continuous rod), and this reciprocation of combined rod  30 ,  36  causes the blade holder block  24  to reciprocate out of phase with the reciprocation of the assembly comprising rod  28 , piston  24 , and cover  12 . Notably, rod  30 ,  36  preferably slides through a bore  37  in the piston  26 , thereby allowing the two reciprocating actions to take place freely and out of phase. 
         [0028]    For removing and replacing the blade, after the blade is withdrawn into the cover, the handle  14  may be held rotationally stationary, and the cover  12  may be rotated to disengage threads  58  and  60  from each other so that the cover may be removed from the knife for replacing the blade. 
         [0029]    Thus, it is seen that the hobby knife of the present invention provides novel and useful features for overcoming shortcomings in the prior art. The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.