Beam compass

A beam compass including a base having a center pivot pin and a bore, and including a portion extending from the base to a center pivot reference point. An elongated beam member is mounted within the bore such that the base is slidably and revolvably adjustable along the length of the beam member. A pencil or pen guide is carried by an outer end of the beam member for guiding a marking instrument. The guide includes a first portion extending from the beam member to a first reference point. The first portion defines at least one aperture. A marking instrument positioned to project through the aperture will engage a drawing surface. An arc or circle traced by rotating the marking instrument relative to the center pivot on the base will have a radius equal to the distance between the first reference point and the center pivot reference point.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a beam compass used in drafting. 
2. Description of the Prior Art 
Many types of beam compasses have been employed in the past. A typical beam 
compass includes a center pivot, a marking instrument (pen or pencil) 
holder and a beam connecting the marking instrument holder and the center 
pivot. The radius of the arc to be drawn may be set by adjusting the 
distance between the marking instrument holder and the center pivot along 
the beam. 
When using a beam compass, it is frequently necessary to use marking 
instruments having points of varying hardness and thickness. However, many 
beam compasses, such as U.S. Pat. No. 3,002,280 to Bennett, Jr., U.S. Pat. 
No. 2,943,392 to Attridge and U.S. Pat. No. 3,797,118 to Yamamoto, do not 
provide a marking instrument holder, through which the marking pen or 
pencil projects, designed for marking instruments having points of 
different sizes. 
U.S. Pat. No. 3,513,548 to Itano discloses the use of apertures of varying 
diameters to accomodate marking instrument points of different sizes or 
thicknesses. However, Itano provides these apertures along the 
longitudinal axis of the beam. Thus, it is necessary to readjust the 
position of the marking instrument holder relative to the center pivot, to 
trace the original arc through the different apertures. This realignment 
of the desired aperture with the desired arc radius must be performed 
prior to using the new marking instrument. These adjustments can be very 
time consuming and tedious. 
With prior art beam compasses, it has also been difficult to properly 
adjust the compass to draw an arc or circle of desired radius. Because 
those marking instrument holders literally cover the desired point of 
contact on a drawing surface, it is difficult for the draftsman to locate 
the center pivot. Thus, the draftsman may have to align the marking 
instrument by trial and error. This can prove disastrous if a mark is 
incorrectly made with ink or other indelible material. 
SUMMARY OF THE INVENTION 
The present invention includes apparatus used for drawing arcs or circles. 
The apparatus includes a pivot base for adjustably receiving a beam member 
and having a center pivot pin for defining the pivot axis or center of the 
circle or arc to be drawn. The base is adjustable along the length of the 
beam member. The base further includes a portion extending from the base 
to a center pivot reference point, for precisely locating the pivot axis 
when adjusting the compass to draw an arc of desired radius. 
A marking instrument guide is attached to an end of the beam member for 
retaining a pen or pencil point used to draw the desired arc or circle. 
The guide includes a generally planar surface portion which is formed on a 
first plate-like wall portion and lies parallel to the working surface. 
The wall portion extends to a first reference point along its outer edge 
and further includes at least one first portion aperture sized to receive 
a marking instrument point of corresponding diameter. The instrument point 
projects through the first portion aperture for engagement with a working 
or drawing surface. The distance between the first reference point and the 
center pivot reference point reflects the radius which the compass is set 
to draw. 
In use, the desired arc or circle radius is precisely set by spacing the 
first reference point from the center pivot reference point at a distance 
equal to the desired radius. The center pivot is then positioned over the 
center of the arc to be drawn. Rotation of the pen or pencil extending 
through the first portion aperture, relative to the center pivot on the 
base, will trace the desired arc or circle. 
The marking instrument guide may also have a second plate-like wall portion 
extending from the beam in opposite direction from the first portion and 
offset from the first portion so it is not on the working surface when the 
first portion is in use. When the marking instrument guide is rotated 
180.degree. the second portion is in usable position adjacent the working 
surface. The second portion extends to a second reference point along its 
outer edge. This second reference point, when the second portion is in its 
usable position, is at the same radius from the center pivot reference 
point as was the first reference point prior to rotation of the beam. The 
second portion further includes at least one aperture, different in size 
from the first portion aperture, and of size to receive the point of a 
second marking instrument of corresponding diameter. The second marking 
instrument point projects through the second aperture for engagement with 
a working or drawing surface. The radius of the arc or circle to be drawn 
is set in the manner described above, using the second reference point and 
the center pivot reference point. The center pivot is then positioned over 
the center of the circle or arc to be drawn and the point of the 
instrument extending through the second portion aperture, when rotated 
relative to the center pivot on the base will trace the arc or circle at 
the desired radius.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A beam compass made according to the present invention is indicated 
generally at 10 in FIG. 1. Beam compass 10 includes a base 12 having a 
longitudinal bore 14 for slidably, adjustably receiving an elongated beam 
member 16. Base 12 has a pair of depending lugs at its opposite ends 
through which bore 14 is made. The bore 14 thus forms two bore portions 
spaced in direction along the beam for receiving the beam 16. The beam 16 
is cylindrical and has a cross section which permits it to be rotated in 
the bore 14. The base 12 is frictionally held or clamped to the beam 16 
with a clamp screw 46 that is threaded in a cross bore 48 so an end of the 
screw 46 will bear against the beam 16 to hold it securely, in a first 
form of the invention. 
The base 12 further has a cross bore 22 for mounting a center pivot pin 18 
having a point 20 projecting from the base 12 for engagement with a 
drawing or working surface, to define a pivot axis perpendicular to the 
drawing or working surface. Base 12 further includes a portion extending 
to a center pivot reference point 15, for precisely locating the pivot 
axis when setting the compass to draw an arc or circle of desired radius, 
as will be shown. 
Projecting outwardly from the bottom of base 12 are a plurality of bosses 
13 and a leg 11, for spacing base 12 from and stablizing it on the drawing 
surface. This arrangement further enhances the ease with which base 12 may 
be rotated about center pivot pin 18 on the drawing surface. 
A pencil or pen holder or guide 24 is attached to an outer end portion 16B 
of beam member 16 and is used for guiding a marking instrument such as a 
pen or pencil. Guide 24 has a center block 25 that fits on the beam 16 and 
a pair of parallel, but offset, flat plate-like wall portions 28 and 34 
extending from opposite sides of the center block 25. The plate-like wall 
portions 28 and 34 are offset in direction of extension from the working 
surface of the pivot axis defined by the pivot pin 18. 
First wall portion 28, as shown in FIGS. 2 and 4, is triangular shaped and 
tapers from the center block 25 to a first reference point 30. At least 
one aperture, such as aperture 32, is formed through wall portion 28 and 
its center is at the same distance from the center of cross bore 22 (from 
the pivot axis) as the reference point 30 is from center pivot reference 
point 15. Thus, the radius of an arc or circle to be drawn may be set by 
adjusting the distance between the reference point 30 and the primary 
first reference point 15. The point 20 of center pin 18 is then positioned 
over the center of the arc to be drawn. A point of a pencil or pen is 
projected through the aperture 32 for engagement with a drawing surface 
and when kept in the aperture may be moved to trace an arc by movement of 
the pencil and guide about the center pivot pin 18. 
Second wall portion 34 also is triangular shaped and tapers from the center 
block 25 to a second reference point 36 on the opposite side of the beam 
16 from the first reference point 30. If beam 16 is rotated in the block 
180.degree. to its "usable position", second reference point 36 will be at 
the same radius from center pivot reference point 15 as was the first 
reference point 30 before beam 16 was rotated. 
Second wall portion 34 has at least one aperture therethrough, such as 
aperture 38, with its center at the same radius from the pivot axis as the 
second reference point 36 is from the center pivot reference point 15. 
Again, the radius of an arc to be drawn may be set by spacing the second 
reference point 36 at a distance from center pivot reference point 15 
equal to the desired radius. Center pin 18 is then positioned over the 
center of the arc to be drawn. A point of a marking instrument positioned 
in and projecting through aperture 38 will engage a working or drawing 
surface on which the compass is placed and when moved with the guide 24 
will trace an arc at the set radius about the center pivot pin 18. 
Aperture 38 of second wall portion 34 and aperture 32 of first wall portion 
28 are different size. When pencils or leads of different size are used, 
and consequently a larger or smaller aperture is required for a close fit, 
the draftsman may simply rotate the beam 16 in the block 180.degree. and a 
different, appropriately sized aperture will be in position for use. 
Because the position of the point 20 of the pivot pin 18 does not have to 
be readjusted relative to the guide 24 a significant time savings may be 
realized. 
A close fit between the apertures and their corresponding leads also 
eliminates the need for the draftsperson to make an allowance for the lead 
diameter. Since the center line of a lead defines the radius of the arc or 
circle drawn, the larger the lead diameter, the further its center line 
will be from a wall or edge against which the lead is held. Thus, with the 
present invention, the close fit of the first and second portion apertures 
with their corresponding leads assures that a large lead and small lead 
will draw an arc or circle at the same radius. 
As stated before, it is also preferred that first wall portion 28 and 
second wall portion 34 are offset and spaced from each other in direction 
perpendicular to the drawing surface (in direction along the axis of pivot 
defined by pivot pin 18) so the one wall portion clears the drawing 
surface when the other wall portion is being used for guiding a marking 
instrument. First wall portion 28 and second wall portion 34 have planar 
surfaces 40 and 42, respectively, which are oppositely facing and which 
face and lie adjacent to the drawing surface when in use. The surfaces 40 
and 42 extend across the center block 25, as shown. The surfaces 40 and 42 
each have a plurality of bosses 44 and legs 45 projecting outwardly 
therefrom for spacing the surfaces from and stabilizing the surfaces on 
the drawing surface. This prevents smearing the lines being drawn due to 
contact between the drawing surface and surfaces 40 or 42, or smearing ink 
if a tubular ink pen is being used. 
When an arc or circle is to be drawn, the planar surface of the portion 
having the desired aperture is positioned adjacent the drawing surface. As 
an example, if the size of the drawing instrument requires that aperture 
38 be used, the guide 24 would be moved so surface 40 should be disposed 
adjacent the drawing surface. 
To properly center the marking instrument point and aid in holding it 
perpendicular to a drawing surface, apertures 32 and 38 may be 
countersunk, as shown in FIG. 4, with the widest part of the aperture 
facing upwardly when the aperture is being used. 
A sectional view of a portion of a second form of the present invention is 
shown in FIG. 5 and includes a base 50 having a longitudinal bore 52 for 
frictionally slidably, adjustably receiving an elongated beam member 54. 
Beam 54 of FIG. 5 is undulated with wave-like corrugations along its 
length so that the outer surface portions of the corrugations frictionally 
engage the periphery of the longitudinal bore 52. By proper design, this 
frictional loading will provide sufficient holding force for adjustably 
securing the position of base 50 relative to beam member 54. 
Base 50 further has a cross bore 56 for mounting a tapered center pivot pin 
58, having a point 60 projecting from the base 50 for engagement with a 
drawing or working surface to define a pivot axis perpendicular to the 
drawing or working surface. Pivot pin 58 may be a standard, commercially 
available "push pin". This frictional mounting of beam member 54 and use 
of a commercially available "push pin" 58 represent a relatively 
economical manufacturing method for uses where this is important. 
Cross bore 56 may also partially intersect longitudinal bore 52 so that 
tapered center pivot pin 58 may be wedged against and frictionally engage 
beam 54 to secure base 50 thereto. Thus, pin 58 may serve as both a 
secondary clamp and a center pivot. 
It will also be apparent to those skilled in the art that a number of other 
modifications and changes can be made without departing from the spirit 
and scope of the present invention. Therefore, it is to be understood that 
the invention is not to be limited except by the claims which follow.