FIELD OF THE INVENTION
The present invention relates generally to a device for describing
arcs of generally constant radius, and particularly to a device having two legs
pivotably attached at one end and including features which make it safe and easy
to use for children.
BACKGROUND OF THE INVENTION
Devices for describing arcs are commonly known. Such devices often
are referred to as compasses and include two legs pivotably attached at one end.
The free ends of the legs can thus be pivoted towards or away from one another.
By attaching a marking instrument, such as a pencil, to one leg and holding the
other leg at a fixed pivot point, the marking instrument can be rotated about
the pivot point to form an arc of generally constant radius.
One problem with this type of device is the difficulty of maintaining
one leg at a fixed pivot point while the other leg and attached marking instrument
are rotated about that point. Some compasses include a sharp, needle-like point
at the free end of one leg. However, as one might imagine, these compasses can
be extremely dangerous, particularly in the hands of younger children. Other compasses
have a suction cup or pad to hold the pivot leg in place. Problems also arise in
maintaining the compass at the fixed pivot point and in locating the precise center
point of the desired arc or circle. Some devices have a pointer within a suction
cup to help a user locate the desired center point of the arc with greater precision.
The suction cup, however, must either be made of transparent material or have some
type of window so the pointer can be seen. These configurations tend to be complicated
and difficult to use, particularly if the devices are to be used by children learning
about shapes and basic geometry.
Another problem with existing compasses is maintenance of the constant
radius while the arc or circle is formed. Many compasses simply use a pivot joint
between the legs which has a certain level of friction to limit unintentional movement
of the legs with respect to each other. However, if the user provides excess pressure
on the compass while describing an arc or circle, the legs will tend to spread,
resulting in an unsatisfactory arc. Again, this is particularly a problem with
younger children who may be learning to use a compass and inadvertently apply more
pressure than the frictional pivot joint can withstand.
Other compasses include a threaded bar extending through threaded
bores in each of the legs to maintain the legs at a fixed distance with respect
to each other. The threaded bar typically includes threads of opposite direction,
i.e. threads turned in one direction at one leg and threads turned in the opposite
direction at the other leg. This is necessary so both legs can either be expanded
or drawn together by turning the single threaded rod. Such a system is more expensive
to manufacture and is not particularly susceptible to the use of plastic components.
Existing compasses are also often awkward to hold and somewhat difficult
to use because of their design. There are no large, comfortable surface areas
for grasping by a user. Again, this is particularly disadvantageous for young children
learning to use compasses since their hands may lack the dexterity required to
manipulate conventional devices.
The present invention addresses the drawbacks of current compass
SUMMARY OF THE INVENTION
The present invention features a device for describing arcs. The
device includes a pair of legs pivotably connected at one end by a pivot joint.
Each leg also includes a free end opposite the pivot joint. A rack is affixed
to one of the legs and oriented to extend through an opening in the opposite leg.
The rack includes a plurality of teeth along at least one of its edges. A wheel
is rotatably disposed in the opening and includes threads which cooperate with
the plurality of teeth on the rack. Thus, by rotating the wheel, the threads act
against the teeth of the rack to draw it through the opening in one direction or
the other. An instrument holder may be attached to or built into one of the legs
and is configured to hold an instrument for describing arcs. Examples of such instruments
include a pencil for marking arcs and a cutter blade for cutting arcs.
According to a further aspect of the invention, a shoe is rotatably
connected to the free end of the leg opposite the leg bearing the instrument holder.
This allows the device and attached instrument to be moved in an arc or circle
about the shoe. The shoe may also include alignment marks to facilitate centering
over a specific point.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will hereafter be described with reference to the accompanying
drawings, wherein like referenced numerals denote like elements, and:
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Figure 1 is a front view of a device for describing arcs according to a preferred
form of the present invention;
- Figure 2 is a side view of the device of Figure 1 including partially cut-away
- Figure 3 is a lengthwise sectional view taken generally along line 3-3 of Figure
- Figure 4 is a cross-sectional view taken generally along line 4-4 of Figure
- Figure 5 is a cross-sectional view taken generally along line 5-5 of Figure
- Figure 6 is a front view of an alternate embodiment of the invention showing
a cutting disc for cutting arcs;
- Figure 7 is a side view taken generally along line 7-7 of Figure 6 showing
the cutting disc;
- Figure 8 is a cross-sectional view taken generally along line 8-8 of Figure
- Figure 9 is a front view of an alternate embodiment of the cutting disc; and
- Figure 10 is a front view of another alternate embodiment of the cutting disc.
Referring generally to Figures 1 and 2, a device 10 for describing
arcs is illustrated. Device 10 generally includes a pair of legs which can be designated
as first leg 12 and second leg 14, a pivot joint 16 connecting first leg 12 and
second leg 14, an instrument holder 18, a shoe 20, and an adjustment mechanism
22 to adjust the position of first leg 12 with respect to second leg 14.
More specifically, first leg 12 includes a distal or free end 24
and a connector end 26, and second leg 14 similarly includes a distal or free end
28 and a connector end 30. Connector ends 26 and 30 are affixed to pivot joint
16 to allow first leg 12 and second leg 14 to be pivoted about pivot joint 16.
Free ends 24 and 28 move towards or away from one another as legs 12 and 14 are
pivoted about joint 16.
Instrument holder 18 is designed to hold an instrument 32 for describing
arcs, but its exact configuration will depend on which type of instrument 32 is
desired. In the embodiment illustrated in Figure 1, instrument holder 18 is designed
to hold an instrument such as a pencil or pen. A pair of arcuate flanges 34 are
preferably sized and disposed to apply pressure to instrument 32 and hold it in
place as an arc is drawn on a surface 36. Arcuate flanges 34 can include a plurality
of apertures 38 extending therethrough, or they can be solid.
Shoe 20 preferably includes a plate 40 having a non-slip surface
42 disposed for contact with surface 36. In the most preferred embodiment, plate
40 is generally cylindrical and non-slip surface 42 is created by adhesively applying
a thin non-slip pad 44, such as a rubber pad, to plate 40. Shoe 20 also includes
a socket 46 opposite non-slip surface 42. Socket 46 rotatably cooperates with
a ball 48 affixed to free end 28 of second leg 14. Socket 46 is sized to resiliently
hold ball 48 while allowing it to rotate or pivot within socket 46. Thus, when
device 10 is operated, shoe 20 is placed in one location over a fixed center pivot
point 50, and second leg 14 is rotated at that location to move first leg 12 and
instrument 32 in an arc about point 50.
Shoe 20 is also designed to assist the user in precisely centering
device 10 over center point 50. A plurality of alignment marks 52 are disposed
on a visible surface 54 of shoe 20, preferably on the top and side surfaces of
plate 40 over non-slip surface 42. If four alignment marks 52 are used, they are
preferably placed at approximately 90° intervals around plate 40. In this manner,
a user of device 10 can simply draw two perpendicular lines on surface 36 intersecting
at center point 50. By aligning alignment marks 52 with the intersecting lines,
the user is assured that shoe 20 is centered precisely over center point 50 and
that all arcs or circles are described about that point.
Adjustment mechanism 22 includes an arcuate rack 56 extending from
one leg generally towards the opposite leg. In the illustrated embodiment, rack
56 is attached to or integral with first leg 12 generally between free end 24
and connector end 26. Rack 56 extends towards second leg 14 and through a cross
shaped opening 58 disposed in or along second leg 14. Opening 58 preferably includes
a longitudinal channel 60 generally aligned with rack 56 and a transverse channel
62 oriented transversely to intersecting channel 60 and having rounded walls.
An adjustment wheel 64 is rotatably disposed in transverse channel
62 and rotates within a pair of opposed rounded grooves 66 of opening 58. Adjustment
wheel 64 includes an opening 68 disposed generally at its radial center. Adjustment
wheel 64 is located in a plane 74 which is generally transverse to the movement
of rack 56.
As illustrated more clearly in Figure 3, an inner wall 70 defines
center opening 68 and includes threads 72. Threads 72 are oriented at an angle
with respect to plane 74, as shown in Figure 3, and are configured to cooperate
with a plurality of teeth 76 disposed along an edge 78 of rack 56. As adjustment
wheel 64 is rotated, threads 72 act against teeth 76, moving rack 56 either forward
or backward with respect to longitudinal channel 60 of opening 58. This, of course,
moves legs 12 and 14 either closer together or father apart, depending on the
direction in which adjustment wheel 64 is rotated.
Preferably, rack 56 extends through center opening 70 to cooperate
with threads 72. However, threads 72 can also be formed on a peripheral or outside
surface 80 of adjustment wheel 64. Rack 56 would then extend through opening 58
external to adjustment wheel 64 and cooperate with the external threads. Additionally,
opening 58 can have a variety of other configurations. For instance, longitudinal
channel 60 can be formed through the center of second leg 14 as illustrated or
through an edge of second leg 14. Longitudinal channel 60 can also be formed through
a portion extending outwardly from second leg 14. Similarly, adjustment wheel
64 can turn within second leg 14, as shown, or it can be mounted alongside second
leg 14. Opening 58 and adjustment wheel 64 can also be disposed in first leg 12
while rack 56 is affixed to second leg 14.
As illustrated in Figures 1 and 4, pivot joint 16 is preferably an
enlarged, easy to grasp structure extending outwardly from connector ends 26 and
30 of first leg 12 and second leg 14, respectively. Pivot joint 16 can be a variety
of shapes, but is preferably generally spherical, having a first hemisphere 82
and a second hemisphere 84 which are defined by an outer grasping surface 86.
Outer surface 86 includes a plurality of indentations or projections 88 to facilitate
gripping of pivot joint 16. Indentations or projections 88 may include a variety
of configurations, such as dimples, crevices, bumps or ridges.
First hemisphere 82 and second hemisphere 84 are preferably connected
by an internal press-fit connection 90 having an insert portion 92 and a socket
portion 94 which are pressed together for interlocking but rotatable engagement.
Insert portion 92 preferably includes resilient prongs 96 having a rounded, outwardly
directed flange 98 which resiliently interlocks with an inwardly directed annular
flange 100 on socket portion 94 to hold the first and second hemispheres together.
However, numerous other types of interlocking mechanisms could also be used with
pivot joint 16 to hold legs 12 and 14 in pivotable engagement. In the embodiment
shown, leg 12 is integrally formed with hemisphere 82, whereas leg 14 is integrally
formed with hemisphere 84. At the point of connection, the legs may partially overlap
both hemispheres, but each leg is only integrally formed with one of the hemispheres
to allow pivotable motion of the legs.
In its preferred embodiment each component is made of plastic although
a variety of other materials could be used. Preferably, components which have sliding
contact are made of dissimilar plastics or other materials to reduce wear. For
example, the legs and rack may be made of nylon while the shoe and wheel are made
of polycarbonate or vice versa. In another example, the legs and rack could be
made of polyester while the shoe and wheel are made of styrene or vice versa.
In Figure 6, an alternate embodiment of the preferred invention is
illustrated. In this embodiment, most features of the alternate device 10' remain
the same as those illustrated in Figures 1-5 and are numbered accordingly. However,
in this embodiment, an alternate instrument holder 109 is designed to hold a cutting
disc 110 having a cutting edge 111. Instrument holder 109 is generally disposed
at leg end 24 of first leg 12 as with instrument holder 18. A hub 112 is integrally
formed in free end 24 and includes a bore 114 extending therethrough. (See Figure
8). Bore 114 is sized to receive a retaining axle pin 116 which extends through
a center aperture 118 of cutting disc 110 and into bore 114 to secure cutting
disc 110 proximate hub 112 without inhibiting the rotation of cutting disc 110.
As the device is pivoted about center point 50, cutting disc 110 rotates along
surface 36 of, for instance, a sheet of paper or cloth, and cuts an arc of generally
constant radius through the sheet. Cutting disc 110 is disposed in a plane 120
tangent to the arc being cut at the point of contact between cutting disc 110 and
As illustrated, pin 116 may be combined with a retaining ring 122
having a center hole 124 through which retaining pin 116 extends. Retaining pin
116 includes an enlarged head 126 which holds retaining ring 122 adjacent cutting
disc 110, supporting cutting disc 110 between hub 112 and retaining ring 122. Cutting
disc 110 may be designed either to rotate with retaining ring 122 or to rotate
independently of retaining ring 122 intermediate hub 112 and retaining ring 122,
Preferably, retaining pin 116 is held in bore 114 by a fastener,
such as cap 132 disposed on an opposite side of hub 112 from cutting disc 110.
Cap 132 includes a threaded hole 134 configured for engagement with a threaded
end 136 of retaining pin 116. A spring washer 137 is preferably disposed intermediate
cap 132 and hub 112. Depending on the desired design of pin 116, cap 132, and
bore 114, pin 116 can either remain stationary while cutting disc 110 rotates about
it or pin 116 can rotate with cutting disc 110.
As illustrated in Figure 7, instrument holder 109 preferably includes
a safety flange 138 extending over cutting edge 111 for at least a portion of its
length. Preferably, safety flange 138 extends at least approximately 90° along
cutting edge 111 to prevent injury to the user of device 10.
In Figures 9 and 10, alternate cutting discs 110 are illustrated.
In each of these designs, cutting edge 111 is formed in a different configuration.
For instance, Figure 9 shows a wave edge 140 designed to provide an arc of generally
constant average radius, but having an undulating appearance. Similarly, Figure
10 illustrates a pinking edge 142. Pinking edge 142 is designed to cut an arc
of generally constant radius having more frequent undulations than obtained with
the wave edge 140. With either edge 140 or edge 142, the arc of generally constant
radius would actually be the average arc taken through the undulations.
It will be understood that the foregoing description is of preferred
exemplary embodiments of this invention and that the invention is not limited to
the specific form shown. For example, different instrument holders and instruments
for describing arcs may be used, the rack and wheel may be reversed, various materials
may be used, and various leg configurations can be constructed. These and other
modifications may be made in the design and arrangement of the elements without
departing from the scope of the invention as expressed in the appended claims.