Punch pin configuration

A punch pin cutting edge configuration having a leading edge and a following edge is described. The leading edge engages a workpiece, such as paper, before the following edge. The following edge is at a steeper angle than the leading edge relative to the workpiece being punched. The cutting edge thus produced reduces the force necessary to punch through the workpiece.

FIELD OF THE INVENTION 
The present invention relates to elements for punching holes in thin 
material. More specifically, the present invention relates to pins for 
punching holes in paper. 
BACKGROUND OF THE INVENTION 
Various methods have been used to arrange papers together for storage and 
reading. Loose sheets of paper can be permanently bound by gluing, sewing, 
stapling, and the like. Papers can also be held together with readily 
removable fasteners and releasable binders, for example. Such fasteners 
commonly have an enlarged head with a stem that is inserted through a 
punched hole with the stem end then bent radially outward at the back of 
that hole. In addition, binders, such as two-ring and three-ring binders, 
have spring biased rings which hold loose sheets of paper together. Such 
releasable binders and removable fasteners permit the easy binding of 
loose sheets of paper, yet permit the ready removal of the papers for 
copying, for instance. 
Among the more important requirements in making a hole for paper binding 
purposes is that the hole be uniform, neat and properly aligned with other 
holes in papers to be bound. That is, the holes should look the same after 
every punch, with the holes made so that their edges are precise, and not 
jagged. 
Ordinarily a hole will be made through many sheets of paper at one time. As 
many as twenty to thirty sheets may be simultaneously placed in a 
manually-driven punching device, for example, such as a three-hole punch 
operated by hand force. 
Various punch pin configurations have been used to make sharp and uniform 
holes. These pins are typically cylindrical in shape, with a base having a 
circular cross section. The variations in the pins are generally found in 
the shape of the punch pin base. 
Originally, such punch pins had a flat circular cutting edge. However, it 
became apparent that these pins required large amounts of force to 
perforate the paper due to the large surface area of the paper being 
engaged at one time by the cutting surface of the pin. In addition, 
because the entire surface area of the base cut the paper all at once, 
higher shear forces were applied to the edges of the hole being created. 
This caused the paper to be "pulled" around the edges into the hole, 
resulting in a dull hole edge. 
Another punch pin cutting edge configuration is shown in U.S. Pat. No. 
3,714,857. The punch pin is cylindrical with a generally circular cross 
section. The cutting edge of the punch pin has a parabolic-shape when 
viewed in section. 
The punch pin pierces the paper along a smaller cross section of the edge 
at the base of the cylinder. The paper furthermore contacts a variably 
changing cutting edge, so that the entire edge of the base never comes 
into contact with the paper at once. This reduces the force necessary to 
cut the paper because of the reduced surface contact between the pin and 
the paper. Conversely, using the same amount of force to pierce the paper 
results in a greater pressure on the paper being cut because of the lower 
surface contact. This allows the user to cut more sheets of paper with the 
same force. This higher pressure over a smaller area also results in a 
sharper cut, because the "pull" on the edge of the hole is reduced. 
Other punch pin configurations have also focused upon a reduction in the 
force necessary to make the paper hole. Such configurations have included 
highly sloped piercing points as well as rippled or star-shaped patterns 
on the cutting edge. There is thus a desire in the industry to develop a 
cutting edge configuration for a punch pin which creates a sharp, clean 
cut with the least amount of force possible. 
SUMMARY OF THE INVENTION 
The present invention comprises an improved punch pin with a cutting edge 
configuration that yields the desired clean cut with reduced force. The 
punch pin of the invention has a stem with a generally circular cross 
section (i.e., across the pin diameter) at least at one end. This one end, 
or base, has a generally concave or bowl-shaped configuration defining a 
cutting edge with two initial points of cutting entry along the edge, such 
as on opposite sides of the cutting edge. A leading edge and a following 
edge follow respective smooth curves between these entry points. 
The leading edge of the punch pin has a fairly shallow curve, while the 
following edge has a steeper slope when viewed with respect to a plane 
perpendicular to the longitudinal axis of the stem, i.e. the plane of a 
piece of paper being punched. The curves of both the leading and following 
edges are preferably smooth inward curves reaching respective center 
points on opposite sides of the stem. The points of entry, which are the 
furthest extensions of the cutting edge, and the center of the leading and 
following edges are thus spaced about 90.degree. apart in alternating 
fashion. 
The center of the leading edge is at a point slightly "higher" along the 
cutting edge than the center of the following edge, as measured along the 
longitudinal axis of the pin cylinder. This allows the leading edge to be 
the first of the two edges to come into contact with the paper (or other 
workpiece being punched). 
When the cutting edge of the punch pin contacts the paper, it initially 
pierces at the two points of entry. More of the surface area on the 
leading edge than on the following edge then engages the paper. The 
greater part of the cutting force is thus initially concentrated on the 
leading edge. The following edge is more inclined, enhancing the cutting 
action of the following edge. What results is an overall reduction in the 
force necessary to punch a hole in the paper. With this reduction in 
force, more paper can now be pierced with the same force previously 
applied, or less force can be applied to punch the same amount of paper as 
similar prior art punch pins. 
Additionally, the method for making this punch pin comprises a unique 
solution to creating a sharp cutting edge having a leading edge, a 
following edge and a hollow bowl-shaped end configuration. Essentially, 
the element used to cut the generally cylindrical punch pin comprises a 
disk-shaped cutting or grinding wheel. The cutting surface of the wheel is 
rounded, i.e. roughly semi-circular in radial cross section along the 
wheel edge. This wheel rotates about an axis of rotation at the center of 
the wheel, which allows one viewing the spinning cutting element to 
imagine a rotating torus in place of the cutting wheel. 
The key aspect of making the cut is that the punch pin is positioned so 
that the longitudinal axis of its cylinder is angled relative to a 
diameter of the cutting wheel. Putting it another way, and with regard to 
the imaginary torus, the longitudinal axis of the pin cylinder is parallel 
to a major radius of the torus. This angulation or skew between the 
longitudinal axis of the pin and the wheel/torus results in the formation 
of the foregoing leading and following edges of the present invention. 
Because a toroidal-like cutting wheel is used, the punch pin edge also 
becomes sharper than similar pieces made using reciprocating elements. 
Such a sharper cutting edge also reduces the surface area coming into 
contact with the paper, and consequently reduces the necessary force for 
cutting. 
The cutting edge of the punch pin of the present invention thus makes a 
clean, uniform cut. The punch pin also cuts the paper so that pressure is 
applied to the paper substantially only along the cutting edge, and not 
the paper "within" the hole. Most significantly, the improved punch pin 
configuration reduces the force necessary to make a cut by an estimated 
25%. 
The invention, together with its attendant advantages, will be further 
understood by reference to the following detailed description taken in 
conjunction with the accompanying drawings, in which:

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
As seen in FIGS. 1, 2 and 3, the present invention is a punch pin 10 which 
is generally cylindrical in shape with a circular cross section (taken 
across its radius). A cutting edge 12 is formed at one end (the base), 
comprising two points of initial entry 14, a leading edge 16, and a 
following edge 18. This cutting edge 12 surrounds a bowl-shaped (concave) 
underside or end 20. 
The punch pin 10 is formed to make a circular cut in the paper for a 
circular hole. The points of entry 14 are diametrically opposed on the 
cutting edge 12 of the punch pin 10, which is circular in plan view (FIG. 
6). The points of entry form the furthest points of the pin end 20 (and 
thus of the cutting edge 12), measured relative to the longitudinal axis L 
of the pin cylinder. 
The leading edge 16 extends between the entry points 14 in a smooth curve. 
The curve is fairly shallow, as measured from a plane P which is 
perpendicular to the axis L, with the points 14 being coplanar with plane 
P. Plane P can be generally equated with the plane of a piece of paper to 
be punched by the pin 10. The curve of the leading edge 16 reaches a 
midpoint or center at 16a. The following edge 18 also extends between the 
entry points 14 on a smooth curve, but has a steeper slope than the 
leading edge 16 as measured from plane P. The curve of the following edge 
18 reaches a midpoint or center at 18a. Center points 16a and 18a are 
roughly 180.degree. apart. 
As seen in FIG. 3, the leading edge 16 and the following edge 18 yield a 
generally semi-circular cross section (along the axis L) to the underside 
20. The bowl-shaped underside 20 in the punch pin 10 allows paper to 
collect therein as the cutting edge 12 of the punch pin 10 is piercing 
through the paper. 
As seen in FIG. 2, more of the leading edge 16 is brought to bear against 
the paper than the following edge 18 during the initial cutting. The 
cutting force is thus concentrated on the leading edge 16. The steeper 
slope of the following edge also reduces cutting effort, much as it is 
easier to cut with a knife that is more angled relative to the object 
being cut. The result of this configuration is an approximately 25% 
reduction in the force required to punch a given sheet of paper from that 
of contemporary punch pins with a conventional unangled bowl-shaped 
cutting end. 
The cutting edge 12 of punch pin 10 is formed using a cutting or grinding 
wheel 30 (FIG. 4), having a semi-circular radial cross section to its 
surface. The cutting wheel 30 can be compared to an imaginary torus 40, as 
shown in FIG. 5. The torus 40 has a major radius r.sub.1 extending from an 
axis of rotation C. Torus 40 has a minor radius r.sub.2, as best seen in 
FIG. 6. 
The skew in the cutting edge 12 is created by angling the pin 10 relative 
to the wheel 30. That is, the cylindrical pin blank used to make pin 10 is 
angled for cutting relative to a radial line R.sub.1 on the wheel. For 
example, if the punch pin 10 were to be cut with longitudinal axis L 
colinear with a wheel radius R.sub.2, there would be no skew in the arc 
made in the pin end. However, by positioning the punch pin 10 such that 
its longitudinal axis L is angled relative to radial line R.sub.1 (or 
parallel to radial line R.sub.2), the resulting cut is made along an angle 
in the punch pin base. The leading edge 16 and following edge 18 are thus 
formed between the two points of entry 14. 
With reference to FIG. 5, the torus 40 is merely meant as a geometric 
representation of the wheel 30. The radial lines R.sub.1 ' and R.sub.2 ' 
correspond to the radial lines R.sub.1 and R.sub.2 of the wheel 30, 
respectively. It will be noticed that the diameter of the torus, like that 
of wheel 30, is greater than the diameter of the pin base. The punch pin 
10 would otherwise be ground with a cutting edge 12 containing an 
overhang. For the same reason, the diameter of the toric section, 
equivalent to twice the minor radius r.sub.2, must be larger than the 
diameter of the pin base. 
In addition, increasing the angle A between the radial line R.sub.1 and 
longitudinal axis L will result in a steeper following edge 18 and a 
shallower leading edge 16. It has been found that the skew resulting in a 
maximum efficient punch pin cutting edge 12 is about an angle A of 6.7 
degrees, where longitudinal axis L and radial line R.sub.1 ' intersect 
along a line d extending between entry points 14 (FIG. 6), and wheel 30 
has a radius of about 1.25 inches, a crown having a radius (e.g., r.sub.2) 
of 0.201 inches, and the pin cylinder has a radius of about 0.140 inches. 
While the invention has been described in connection with a presently 
preferred embodiment, it will be apparent to those skilled in the art that 
various changes and modifications to the structure, arrangement, portions, 
elements, materials, and components used in the practice of the invention 
are possible without departing from the principles of this invention. It 
is intended that the foregoing description be regarded as illustrative 
rather than limiting, and that the following claims are intended to define 
the scope of this invention.