Device for sharpening knives

A device for sharpening knives, comprising a frame and a grinding element rotatably mounted in the frame. The grinding element is provided with two cone-shaped grinding surfaces turned towards each other. The cone angle of the grinding surfaces varies in the peripheral direction of the grinding element, with at least one portion with a small cone angle having a rougher surface than the portion having a wide cone angle.

The present invention relates to a device for sharpening knives, comprising 
a frame, a grinding element rotatably mounted in the frame, which grinding 
element comprises two grinding surfaces both having the shape of the 
surface of a truncated cone and forming an angle with each other, and a 
device for steering a knife into contact with said grinding surfaces so 
that, during the sharpening process, it bears against both said grinding 
surfaces. 
Devices of this type of construction are, in general, known. For example, 
U.S. Pat. No. 1,360,998, issued to H. E. Barber on Dec. 7, 1920, describes 
a sharpener wherein the knife is directed in such a way that the angle 
between the longitudinal axis of the knife and the axis of rotation of the 
grinding element is about 70 degrees. Respective grinding surfaces are 
provided, one for each side of the knife edge, so that both sides of the 
knife edge are treated simultaneously. When the knife is passed to and fro 
over the grinding element, the grinding element is rotated, causing the 
grinding surfaces to grind the edge of the knife. Although the rotation is 
relatively slow, the grinding is very effective, since the edge of the 
knife bears against the grinding surfaces substantially over their whole 
width. The high friction between the edge of the knife and the grinding 
surfaces allows at the same time use of a rotating grinding element, which 
may be cleaned with water in a trough arranged below the grinding element. 
Other devices having a disclosed in U.S. Pat. No. 1,342,275, issued to A. 
Cornell on June 1, 1920, and U.S. Pat. No. 4,050,197, issued to Thompson, 
et al, on Sept. 27, 1977. 
A common feature of these known sharpening devices is that they grind both 
sides of the edge at a certain angle in relation to the central plane of 
the knife blade. Thus a knife blade is obtained which has a sharp 
transition between the sides of the knife blade and the sides of the edge. 
This shape of the edge is not the best possible, since the edge angle must 
thereby be made relatively small. For many applications and considering 
the durability of the edge, it would be advantageous to have an edge with 
convex sides which smoothly adjoin the sides of the blade. In this case, 
the edge angle can be larger than in edges with planar sides. 
U.S. Pat. No. 3,461,616 discloses a sharpening device for razor blades and 
the like which produces a convex edge. The razor blade is passed through a 
groove between two screw-shaped rotating grinding elements which are 
positioned partly within each other (interdigitated). The thread height of 
the screw-shaped elements increases in the direction of one end of the 
grinding element. Thus, the angle between the grinding surfaces becomes 
wider, and the edge of the razor blade is ground at various angles as the 
razor blade is displaced in the longitudinal direction of the screws. This 
device, however, has a complicated construction and, therefore, is suited 
primarily only for factory applications. 
SUMMARY OF THE INVENTION 
The present invention provides a sharpening device for domestic use, which 
grinds convex edges on both sides of a knife edge. According to the 
present invention, the cone angle of the grinding surfaces varies in the 
peripheral direction of said grinding element; that portion or those 
portions of the grinding surfaces having a small cone angle are provided 
with a rougher surface than that portion or those portions having a wide 
cone angle. 
Because the cone angle of the grinding surfaces varies, different portions 
of the grinding surfaces grind different parts of the side surfaces of the 
edge in such a way that the rough grinding surfaces grind those parts of 
the edge surfaces which are situated further away from the edge point 
(apex of the edge) while the portions with a smoother grinding surface 
grind the sides of the edge close to the edge point. Because of the 
varying roughness of the grinding surfaces, more material is removed at a 
distance from the edge point than in the vicinity of the edge point, which 
provides favorable knife edge shape. Convex knife edge sides are obtained 
by varying the cone angle evenly from a minimum value to a maximum value, 
whereby the portion of the grinding surface which bears against the knife 
edge is continuously relocated between a first end position in the 
vicinity of the edge point (apex) and a second end position at a distance 
from the edge point. Thus, a knife edge with convex side surfaces is 
obtained, which surfaces smoothly adjoin the side surfaces of the blade of 
the knife. 
Since that portion of the grinding surface which has a rough grinding 
surface works the edge of the knife blade more than the portion having a 
smoother grinding surface, it has been found to be sufficient if the 
portion having the rough grinding surface has an angle length of about 90 
degrees, with the smooth grinding surface extending over an arch of about 
270 degrees. 
In order for the knife to stay at a substantially constant distance from 
the axis of rotation of the grinding element, it is advantageous that the 
rough portion or portions of one grinding surface be positioned opposite 
the smooth portion or portions of the other.

DETAILED DESCRIPTION OF A PREFERRED EXEMPLARY EMBODIMENT 
Referring to FIGS. 1 to 4, a sharpening device in accordance with the 
present invention comprises a substantially semi-cylindrical frame 1 
having end surfaces 2 and 3, and a fastening ear 4 having an opening which 
enables the hanging of the device on a nail or the like. All portions of 
the frame are preferably made of a rigid plastic. 
In the vicinity of end surface 3, frame 1 is provided with a recess (best 
seen in FIG. 4), defined by two walls 5 and 6 parallel with end surfaces 2 
and 3, two low side walls 7 and 8, and a bottom 9 which connects walls 5-8 
with each other. Walls 5-8 and bottom 9 together form a waterproof trough, 
the upper edge of which is formed by the upper edges of the side walls 7 
and 8. 
A grinding element 10 is arranged in the recess between the walls 5, 6. The 
grinding element consists of a wheel which is rotatably mounted with an 
asix of rotation 12, in bearings 11 disposed on those sides of walls 5, 6 
which are turned towards each other. 
The grinding element 10 is formed in such a way that its peripheral surface 
forms two grinding surfaces 13 and 14. Grinding surfaces 13 and 14 are 
turned towards each other, and both correspond in shape to a truncated 
cone. Grinding element 10 is preferably made of a ceramic material, for 
example, aluminium oxide or tungsten, or of metal. 
With reference to FIGS. 1-3 and 5, and particularly to FIG. 5, the recess 
in which the grinding element is positioned is covered with a transparent 
plastic lid 15 having the same arch shape as frame 1. Longitudinal edges 
of lid 15 bear against a groove (not shown) in the outer surface of the 
frame. Lid 15 extends from the upper edge of side wall 7 up over grinding 
element 10 and down again to the upper edge of side wall 8. The 
longitudinal edges of the lid, i.e., the edges which are parallel with the 
end surfaces of the frame, are provided with protruding rounded protective 
flanges 16 and 17, which extend down over side walls 7 and 8 and which 
contribute to the snap-locking of the lid on frame 1. 
Lid 15 is provided with a slit 18, extending over a part of the length of 
the lid and disposed transversely with respect to the longitudinal axis of 
frame 1. Slit 18 is adapted to receive a knife 19 to be sharpened (FIG. 
6), and is sufficiently wide to receive knives of a normal thickness. As 
is shown in FIG. 1, the slit ends are preferably positioned at a point 
slightly above the horizontal plane of the axis of the grinding element. 
As best seen in FIG. 2, the axis of rotation 12 of the grinding element 10 
is inclined with respect to the longitudinal axis of the frame, with the 
result that knife 19, when inserted into slit 18, does not follow the 
contact line (i.e., juncture) between grinding surfaces 13 and 14; rather, 
knife 19 contacts one of the grinding surfaces on one side of axis 12, and 
the other grinding surface on the opposite side of axis 12. The angle 
between grinding surfaces 13 and 14 and the angle between slit 18 and axis 
of rotation 12 are chosen so that the contours of the grinding surfaces, 
seen in the longitudinal direction of the knife or the slit 18, cross each 
other, i.e., the grinding surface positioned further back disappears 
partly behind the grinding surface positioned in the front. 
FIGS. 7 to 9 show schematically how knife 19 bears against grinding element 
10. Especially, FIG. 7 shows that, due to the overlapping of the grinding 
surfaces, the knife does not contact the common periphery (i.e., juncture) 
of the grinding surface; instead, it contacts the grinding surfaces above 
the periphery. 
FIGS. 10 and 11 show on an enlarged scale how grinding element 10 according 
to the invention is constructed. The angle of the grinding surfaces 13, 14 
with respect to a plane perpendicular to the axis 12 varies evenly between 
a minimum value .alpha..sub.1, to a maximum value .alpha..sub.2. Minimum 
value .alpha..sub.1 may be 20 to 25 degrees, preferably 27 degrees. That 
portion of the grinding surfaces which has the smallest angle to the plane 
is, according to the invention, provided with a relatively rough surface 
which effectively works the side surfaces of the knife edge. This portion 
is indicated with the reference number 20, and it has an angle length of 
about 90 degrees. From this portion, in the middle of which the 
inclination angle is .alpha..sub.1, the inclination angle grows 
continuously in the peripheral direction of the surface until it reaches 
the value .alpha..sub.2. That portion 21 of the grinding surface which is 
positioned outside portion 20 is smoother than portion 20 and, thus, does 
not work the knife edge as much as portion 20. 
As is shown in FIG. 12, portion 20, having a small inclination angle 
.alpha..sub.1, works a portion of the knife edge positioned at a distance 
from the edge point (apex), whereas portion 21, with a wider inclination 
angle, works the portion close to the point of the edge. 
As is shown in FIG. 10, grinding surfaces 13, 14 are relatively disposed so 
that portion 20 on one of the grinding surfaces is positioned as far away 
as possible from portion 20 on the other grinding surface. Thus, both the 
portions 20 are positioned opposite a portion 21. 
As an alternative to the above, the grinding surfaces 13, 14 may be 
provided with two or more mutually spaced rough surface portions 20, 
whereby the grinding surfaces obtain a wave shape. The angle length of the 
portion 20 may naturally differ from the length shown in the drawings.