Drafting instrument

A circle drafting instrument (A) comprises a support member (12) rotatably mounting a slotted disk (10). A line forming point (50) is positioned in a slot (B) at a selected distance from a center point (36) of disk (10). Slot B has offset apexes (44a, 46a) incremental radially along disk (10) for precision circle drafting. Additional slots (C, D, E) may also be provided for a larger number of increments. The forming point and the disk are rotated in the support so as to form a circle of a desired size.

BACKGROUND OF THE INVENTION 
This invention relates to a drafting instrument for forming circles. 
The prior art provides several different circle drawing devices. U.S. Pat. 
No. 1,154,673 describes a drafting implement which is comprised of a 
triangle and protractor combination. The protractor is rotatably 
positioned within the triangle. A plurality of openings within the 
protractor enable circles of varying diameters to be drawn by inserting a 
pen or pencil into a hole and by revolving the protector within the 
triangle. U.S. Pat. No. 4,353,166 describes a device for drawing circles 
wherein a disk is rotatably carried on a sheet. The disk and sheet each 
have a series of holes spaced from a center point of the disc into which 
the tip of a suitable writing instrument may be placed to rotate the disc 
or sheet with respect to the other to inscribe a circle on a desired 
surface. U.S. Pat. No. 3,460,261 describes a drafting device which 
includes a support member and a rotatable member positioned within the 
support member. To draw circles, the rotatable member is removed from the 
support member. A pointed instrument may then be inserted into one of the 
apertures within the rotatable member to serve as the center of the circle 
and a pencil point may be inserted in any other selected aperture for 
drawing a circle by the rotating member. U.S. Pat. No. 3,465,445 describes 
a drawing and design apparatus for drawing circles and other geometric 
shapes. However, these devices do not allow intricate size selection, and 
are not suitable for a precision drafting instrument. Most of the devices 
require the use of a sharp center point which damages the surface to be 
marked. 
SUMMARY OF THE INVENTION 
The present invention provides a device for drawing circles which has no 
sharp center points. Intricate selection of circle size is provided by a 
calibrated and notched slot arrangement in a disk member. The slot extends 
parallel to a radii of the disk. The disk is mounted on bearing members in 
a position spaced from the marking surface. This allows for smooth and 
unrestricted rotation of the disk and eliminates smear marks on the marked 
surface. Diameter indicating lines, center opening, alignment cross lines 
and measuring indica are provided to assist in centering the device for 
use and for observing the line being drawn.

DESCRIPTION OF A PREFERRED EMBODIMENT 
A device for forming circles is generally shown in FIG. 1 as comprised of a 
rotatable template or disk member 10 and a support member 12. The support 
member is preferably formed of a plastic material in the shape of a ring 
having a circular opening of a given size. Support member 12 is comprised 
of a first plate member 14 and a second, annular plate member 16, as can 
best be seen in FIG. 3. First plate member 14 has an upper surface 18, an 
inner edge 20 and a lower support surface 22. An L-shaped bushing 24 is 
formed near inner edge 20 of plate 14 above support surface 22. Plate 14 
has a notch 28 formed in its upper surface 18. 
Plate 16 is formed of a transparent plastic material and measuring indica 
27, as can best be seen in FIG. 1, is arranged around a notch 28 of plate 
14 below plate 16. The measuring indica acts to assist the user when 
forming part circles. 
Disk member 10 is also preferably made of transparent plastic material. 
Diameter indicating lines 30 are arranged at spaced intervals on one 
surface of the disk, as can best be seen in FIG. 1 and 5. The lines may be 
spaced at intervals of between 1/16 inch and 1 inch with 1/16 inch 
intervals being preferred. Lines 30 allow the user to easily determine the 
path of the circle being drawn. 
Disk 10 is also provided with alignment cross lines 34. The cross lines are 
arranged at 90.degree. to each other and pass through a beveled center 
opening 36 of disk 10, as best seen in FIGS. 1 and 2. Protractor 
indicating lines 35 allow the user to divide a circle into a desired 
number of parts or to form part circles. Incremental circle guide slot B 
is formed in disk 10 to extend parallel to a radii thereof. Slot B begins 
at a point approximately 1/8 inch from the center point of disk 2 and may 
be up to 5 inches in length. Slot B terminates at a point adjacent the 
circumference of disk 10. Slot B consists of a first edge 40 and a second 
edge 42. Wedge-shaped notches 44 and 46 are formed in edges 40 and 42, 
respectively, at intervals of 1/8 an inch as can best be seen in FIGS. 4 
and 5. An apex 44a of notches 44 formed in edge 40 are offset relative to 
an apex 46a of notches 46 formed in edge 42, as can best be seen in FIG. 
4. Notches 44 and 46 are offset by 1/16 of an inch. Notches 44 begin at 
1/16 and notches 46 begin at 1/8 of an inch from center. While this 
arrangement is preferred, it is also possible to provide notches in only 
one of edges 40, 42. It is also possible to arrange slot B so that it 
extends from the center point at a slight angle relative to the radii of 
disk 10. Preferably, there are 3 other incremental circle guide slots C, 
D, and E, each having notches offset notches so that a full range of 1/64 
inch diameter increments is provided from 0 to 5 inches (or other radius 
as defined by the diameter of disk 10). 
Apexes 44a and 46a are offset in radial increments along a radial from the 
center 36 of disk member 10 to provide for drafting of circles in fine 
precision increments. A generally straight surface 44b and 46b projects 
circumferentially from respective apexes 44a, 46a to better confine 
writing point 50 during circle formation, as can best be seen in FIG. 5. 
Surfaces 44b, 46b make acute angles at apexes 44a, 46a. 
Edges 40 and 42 are spaced apart a distance which allows the point of a 
conventional line forming member to be placed therebetween. Notches 44, 46 
accurately and securely hold the forming point 50 (FIG. 5) at a desired 
distance from the center point of disk 10. To assist in selecting the 
desired location of the forming point measuring indica may be provided 
along edge 42 of slot B. 
As can best be seen in FIGS. 2 and 3, disk 10 is positioned on a ball 
bearing 54 in bushing 24 of plate 14 in a position which is elevated above 
support surface 22. This eliminates smear marks when the device is in use 
because disk 10 is spaced above a surface on which a circle is being 
formed. Such an arrangement also reduces friction between disk 10 and 
support member 22 thereby assisting in smooth and easy rotation of the 
disk. 
Second annular plate 16 is secured by any suitable means in notch 28 cut in 
support 12, and is of sufficient width so that an edge thereof extends 
above bearing 54 and secures disk 10 in bushing 24. 
In use, beveled center hole 36 of disk 10 is located over the axis of a 
circle to be formed. Line forming member 50 is positioned in a notch 44, 
46 at a selected distance from the center point of disk 10. The disk and 
forming member are rotated about the center point thereby forming an 
accurate circle. Guide slots C, D, and E include 1/8 inch incremental 
notches 50, 52; 54, 56; and 58, 60 (FIG. 1) which are offset 1/16 of an 
inch, and are identical in construction to notches 44, 46. Guide slots B, 
C, D, and E, in the template quadrants, provide a full range of circles 
with radii in 1/128 of an inch may be found. This provides a very accurate 
precision circle drafting tool. 
In an alternative form of the device, the exterior shape of support member 
1 may be configured as a usual triangle member (not shown) or any other 
geometrical shape whereby it may be encased. The arrangement of disk 10 
relative to the support triangle may be the same arrangement as in FIG. 3. 
While a preferred embodiment of the invention has been described using 
specific terms, such description is for illustrative purposes only, and it 
is to be understood that changes and variations may be made without 
departing from the spirit or scope of the following claims.