Safety device for utility knives and the like

A safety attachment for a knife has a yoke member adapted to at least partially encircle the body of the knife a short distance behind the blade. A substantially planar guard plate is hingedly attached to the yoke member to freely swing about an axis located at or below the knife body. In a cutting position, the guard plate extends rearwardly from the axis and the blade, skimming the top of the workpiece. Upon contact with a sizable obstruction, the guard plate pivots about the obstruction to a guard position wherein the guard plate forms a solid barrier between the cutting edge of the knife blade and the obstruction, thereby preventing contact between the obstruction and the knife blade.

FIELD OF THE INVENTION 
The present invention relates to the field of cutting instruments and 
attachments therefor, and, in particular, to a safety device for utility 
knives and the like. 
BACKGROUND OF THE INVENTION 
For each cutting task, there is an appropriate knife. One of the most 
universally used knives is the all-purpose or utility knife wherein a 
two-half body, when clamped together, holds a razor or similar blade 
extending from one end thereof. Although these knives are used and abused 
for nearly every cutting task imaginable, probably the most common 
application is to make a cut in a planar workpiece (such as cardboard or 
Sheetrock.TM.) by inserting the blade downward into the workpiece and 
exerting downward and pulling forces thereby drawing the knife toward 
oneself. The user often holds the workpiece stationary by placing his free 
hand, palm down, on top of the workpiece or by positioning his body up 
against the workpiece. All too frequently, because of inattentiveness, a 
dull blade or an obstruction in the workpiece, the knife is pulled 
directly across the cutter's own hand or against his body causing 
potentially serious injury. Another problem is that the utility knife user 
often forgets, or intentionally fails (for convenience) to retract the 
blade of the knife during non-use. A retractable knife in this condition 
or a knife of the unretractable type poses an inherently dangerous risk to 
the inattentive or rushed worker or to a subsequent user. Many who use 
these knives, along with companies and their insurers whose employees are 
continually being injured in utility knife accidents, have been searching 
for a simple and inexpensive guard for these types of knives. 
Devices which have been developed to make these deceptively dangerous tools 
safer have been only moderately successful. Examples of such devices are 
disclosed in the following U.S. Pat. Nos.: 4,675,996 issued to DuBuque; 
4,587,735 issued Walters et al.; 4,531,286 issued to Vito et al.; 
4,091,537 to Stevenson, Jr.; 4,086,698 issued to Sparks; 3,781,988 issued 
to Jones and 2,376,887 issued to Walters. The devices disclosed in these 
patents, while providing some added degree of safety, often present an 
awkward tool which the user tires of due to the inconvenience. Despite the 
risk of injury, the user is tempted to abandon the awkward tool and return 
to the unguarded standard utility knife for its convenience. 
What is needed is a safety device for utility knives and the like which is 
simple, inexpensive, and does not interfere with the cutting routine.

SUMMARY OF THE INVENTION 
Generally speaking, there is provided a safety attachment for utility 
knives and the like which swings freely between a cutting position and a 
guard position. The attachment is activated both by gravity and by contact 
with an obstruction during a cutting stroke along a planar surface. 
The attachment includes a yoke member which is adapted to be firmly 
connected to the body of the utility knife just behind the blade. A 
generally rectangular guard plate is hingedly connected to the yoke member 
to pivot freely along an axis just at or below the knife body and through 
an angle of approximately 180.degree.. In a cutting position, the guard 
plate extends rearwardly from the axis and the blade, skimming the top of 
the workpiece. Upon contact with a sizable obstruction, the guard plate 
pivots about the obstruction to a guard position wherein the guard plate 
forms a solid barrier between the cutting edge of the knife blade and the 
obstruction, thereby preventing contact between the obstruction and the 
knife blade. 
Alternative embodiments are contemplated wherein the knife body provides 
the means for hingedly holding the guard plate. 
It is an object of the present invention to provide a safer cutting device. 
It is another object of the present invention to provide a safety 
attachment which can easily be added or removed to the standard utility 
knife. 
Other objects and advantages of the present invention will become obvious 
from the following description of the preferred embodiment. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
For the purposes of promoting an understanding of the principles of the 
invention, reference will now be made to the embodiment illustrated in the 
drawings and specific language will be used to describe the same. It will 
nevertheless be understood that no limitation of the scope of the 
invention is thereby intended, such alterations and further modifications 
in the illustrated device, and such further applications of the principles 
of the invention as illustrated therein being contemplated as would 
normally occur to one skilled in the art to which the invention relates. 
Referring now to FIGS. 1-3 there is shown a safety device 20 in accordance 
with the preferred embodiment of the present invention. In particular, 
safety device 20 is shown here adapted for use with a standard utility 
knife 10. Knives of this type generally include a pair of mating body 
pieces 12 and 13 which are held together by a screw 14 to form knife body 
11. A blade 15 having a downwardly facing cutting edge 16 is held for 
sliding movement within body 11. Movement of thumb piece 17 within track 
18 moves blade 15 (via a mechanism contained within body 11) between an 
extended position (FIG. 1) and a retracted position (not shown) wherein 
the entire blade 15 is encased within body 11. It is to be understood that 
knife 10 is merely representative of standard knives having an elongate 
handle with a blade extending outwardly from one end of the knife. Knife 
10 and safety device 20 are both substantially symmetrical about a 
vertical plane which cuts between body pieces 12 and 13 at joint 19. 
Safety device 20 includes a U-shaped yoke member 22, a guard plate 23 and 
an attachment strap 24. Yoke member 22 is integrally formed from a 
suitable plastic material such as polyurethane. Yoke member 22 defines a 
U-shaped collar 26 which at least partially encircles knife body 11. Yoke 
member 22 is molded to substantially conform to the outer body 
configuration of the standard utility knife at an area just forward of 
track 18. Collar 26 has a width 38 of approximately 3/16ths to one-quarter 
of an inch to provide additional stability to its connection to knife body 
11. Yoke member 22 also defines a support flange 27 which extends 
outwardly and downwardly from collar 26. At its bottom 30, support flange 
27 defines one-half of hinge 28. 
Guard plate 23 is substantially rectangular and is formed from a suitably 
strong but somewhat resilient material such as polyurethane. One edge 31 
of guard plate 23 forms the other half of hinge 28. A hinge pin 32 joins 
hinge halves 30 and 31 together. The sizing of hinge halves 30 and 31 and 
of hinge pin 32 is such that there is virtually no frictional inhibition 
to the pivotal movement between yoke member 22 and guard plate 23 due to 
hinge 28. The edge 33 of guard plate 23 which is opposite hinge half 31 is 
curved or bent slightly as shown in FIG. 2, thereby forming an arcuate 
lower surface 41. 
The means for securing yoke member 22 to knife 11 is provided in the 
present embodiment by attachment strap 24 and by a series of 
outwardly-opening notches 36 defined in both sides of support flange 27 
(best shown in FIG. 3). To attach safety device 20 to knife 11, knife 11 
is positioned downward within complementary-shaped collar 26. One end 37 
of attachment strap 24 is positioned within an appropriate notch 36 on one 
side of yoke member 22. The other end 38 of strap 24 is pulled and 
stretched over the top of knife body 11 and is positioned within an 
appropriate notch 36 on the other side of yoke member 22. There are a 
number of notches 36 provided at differing heights along both sides of 
support flange 27 in order to assure that knife body 11 may be held firmly 
within yoke member 22 and to provide for slight size variations among 
different brands of knives. 
As shown in FIG. 2, when safety device 20 is appropriately attached just 
forward of track 18, guard plate 23 may freely swing or pivot about an 
axis defined by hinge pin 32. Guard plate 23 may swing between a guard 
position (shown in solid lines) wherein plate 23 extends forwardly of 
hinge pin 32 and is disposed below blade 15, and a cutting position (shown 
in dashed lines) wherein plate 23 extends rearwardly from hinge pin 32 and 
from blade 15. With the pivoting axis (at 32) of guard plate 23 being 
substantially at or below the bottom of knife body 11, plate 23 is 
pivotable through approximately 180.degree.. 
Referring now to FIGS. 4-8, the operation of safety device 20 as adapted 
for use with utility knife 10 will now be described. 
Knife 10 is grasped in a normal manner. The size of safety device 20 as 
well as its location on the forward end of knife 10 is such that device 20 
does not get in the way or create an inconvenience to the grasping or use 
of knife 10. With cutting edge 16 pointing downward and with guard plate 
23 hanging vertically due to gravity, knife 10 is lowered until leading 
edge 39 of curved section 33 touches the workpiece 40 (a cardboard carton, 
for example). Just a slight forward motion of knife 10 will pivot guard 
plate 23 about edge 39 so that the arcuate lower surface 41 of curved 
section 33 will begin to contact the carton surface. Slight downward 
pressure of knife 10 translates through arcuate lower surface 41 causing a 
camming effect which causes curved section 33 of guide plate 23 to move 
rearwardly along the surface of carton 40. Knife blade 15 is pushed 
downward against the surface of carton 40, puncturing it, and is then 
drawn rearwardly as shown in FIG. 5. When a slight imperfection 42 in the 
surface of carton 40 (such as a puncture or seam) is reached, arcuate 
lower surface 41 of curved section 33 cams over the imperfection 42 
permitting guide plate 23 to pass on by without interrupting the cutting 
stroke. However, when leading edge 39 of guard plate 23 meets an object 
(such as fingers 44) having a generally vertical height (measured from the 
surface of carton 40) greater than the height of leading edge 39 (measured 
from the surface of carton 40), arcuate surface 41 does not contact 
fingers 44 and there is therefore no camming effect by arcuate surface 41 
to lift guard plate 23. Instead, either the object (the workman's fingers 
44) is pushed out of the way by guard plate 23, or guard plate 23 is 
stopped at its curved section 33. In the latter case, as shown in FIG. 6, 
curved edge 33 becomes a pivot point for guard plate 23. The drawing or 
cutting stroke of the workman is usually very quick and the dynamic 
momentum associated with the pulling action strongly urges knife 11 to 
continue to be drawn rearwardly after plate 23 contacts fingers 44. With 
curved section 33 no longer in a sliding relationship, but in a pivoting 
relationship with carton 40, knife 11 will move rearwardly and slightly 
upwardly, pivoting both about curved section 33 and hinge pin 32 (FIG. 6). 
Referring to FIG. 7, at the end of this motion, safety device 20 has gone 
from the cutting position of FIG. 5 to the guard position wherein guard 
plate 23 extends forwardly of hinge pin 32 and rests below and adjacent 
blade 15. Under conventional circumstances, without safety device 20, the 
entire cutting stroke may last only about one second and it is usually too 
late for the workman to stop the pulling action of the knife in time to 
avert blade contact with fingers. As shown in FIG. 7, however, with safety 
device 20 attached, by the time knife 10 is drawn over the fingers, guard 
plate 23 is positioned completely between blade 15 and the workman's 
fingers, thereby avoiding injury due to blade 15. 
It should be noted that the length of guard plate 23 from its connection at 
hinge 28 to the opposite side at curved section 33 should be such that, 
when safety device 20 is properly attached to knife 10, guard plate 23 
will extend along the entire length of cutting edge 16 and form a solid 
barrier between cutting edge 16 and an object therebelow as shown in FIGS. 
2 and 7. 
As noted above, collar 26 is shaped to conform to the outer body 
configuration of a standard utility knife. Even with slight variations in 
the outer body configuration among differing brands of utility knives, it 
is believed that the present device will provide sufficient surface area 
contact between the collar and the knife body and will provide a tight 
enough grip due to the securement of attachment strap 24, to keep safety 
device 20 firmly attached to the standard utility knife 10. The present 
invention is particularly adaptable to short-bladed knives such as utility 
knives, but is also recommended for other specific-task knives such as 
tile cutters. In such cases, the yoke member 22 of the particular safety 
device would define a collar 26 which was appropriately shaped for the 
particular knife. 
Alternative embodiments are contemplated wherein the securing means would 
be other than the elastic attachment strap 24. For example, an attachment 
strap might be formed as an integral part with the yoke member and have a 
series of apertures therein which would correspond with one or more posts 
on the opposite side of the yoke member. The attachment means should at 
least provide a manner of firmly securing the device 20 to a knife so that 
the device 20 when plate 23 contacts a large object, the path of the knife 
is interrupted and the knife pivots about the guard plate and the object 
rather than having the device be pushed off the end of the knife. 
As shown in FIG. 8, another frequent type of cutting injury is caused when 
a heavy resistance to the drawing force of the cutting stroke is suddenly 
removed causing the knife to lurch rearwardly, whereby the blade may 
strike the workman. Because guard plate 23 swings freely, once knife 10 is 
pulled away from or behind the workpiece 40, gravity will cause guard 
plate 23 to swing and hang downwardly as soon as it rises above the 
surface or passes the end 45 of workpiece 40. By the time knife 10 reaches 
the workman's body, guard plate 23 is hanging between the workman's body 
and blade 15, preventing blade 15 from coming in contact with the 
workman's body. 
Another embodiment is shown in FIG. 9 wherein safety device 50 is 
integrally formed with the body of knife 51. In this embodiment, guard 
plate 53 is pivotally mounted to the body of knife 51 by an appropriate 
hinge member 54. The guard plate of this embodiment may be identical to 
plate 23 of device 20 in FIG. 1 with the body of knife 51 being formed to 
define the upper half of hinge member 54. 
While the invention has been illustrated and described in detail in the 
drawings and foregoing description, the same is to be considered as 
illustrative and not restrictive in character, it being understood that 
only the preferred embodiment has been shown and described and that all 
changes and modifications that come within the spirit of the invention are 
desired to be protected.