US3656237A

US3656237A - Lead holder drafting compass

Info

Publication number: US3656237A
Application number: US90757A
Authority: US
Inventors: Thomas O Killgrove
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-11-18
Filing date: 1970-11-18
Publication date: 1972-04-18
Anticipated expiration: 1989-04-18

Abstract

A frame held drafting compass for drawing circles wherein the compass pivot is located at the center of the circle to be drawn and the base of the frame contacts the drawing surface outside of the circle to be drawn, so that a contact point at the center of the circle to be drawn is unnecessary. The scribing structure includes a lead holder adjustable so that the angle of incidence of the lead to the paper can be varied.

Description

United States Patent 7 Killgrove [151 3,656,237 [451 Apr. 18, 1972 [54] LEAD HOLDER DRAFTING COMPASS [72] Inventor: Thomas 0. Killgrove, P.O. Box 395, Frazi er Park, Calif. 93225 [22] Filed: Nov. 18, 1970 21 Appl. No.: 90,757

[52] US. Cl. ..33/27 D [51] Int. Cl .....B43l 9/02 [58] Field of Search ..33/27 [56] References Cited UNITED STATES PATENTS 1,402,961 1/1922 Ratcliff ..33/27 D FOREIGN PATENTS OR APPLICATIONS 675,033 10/1929 France ..33/27D 1,030,194 6/1953 France ..33/27D Primary Examiner-William D. Martin, Jr. Assistant Examiner-Charles E. Phillips 57 ABSTRACT A frame held drafting compass for drawing circles wherein the compass pivot is located at the center of the circle to be drawn and the base of the frame contacts the drawing surface outside of the circle to be drawn, so that a contact point at the center of the circle to be drawn is unnecessary. The scribing structure includes a lead holder adjustable so that the angle of incidence of the lead to the paper can be varied.

5 Claims, 8 Drawing Figures strument of the character described above having a quick and easily changed adjustment for setting the length of radius of the circle to be drawn.

A still further object of this invention is to provide a device of the above type. for drawing circles that does not require punching a hole in the drawing surface at the center of the circle to be drawn.

Another object of this invention is to provide a device of the above type which will guide a lead holder lead point about a circular path while at the same time pennitting angular motion of the guided lead holder about the lead point.

Yet another object of the invention is to provide a device of the above type having a pivot at the center of the circle being drafted which is easily positioned by a supporting structure whose base rests at a location outside of the circle, such that a supporting point or resting surface at the pivot is unnecessary.

Other objects and advantages will become apparent from the following detailed specification and accompanying drawings, in which:

FIG. 1 is a perspective view of this invention;

a FIG. 2 is a plan view of the rotatable portion of the compass, taken on the line 2-2 of FIG. 1, looking in the direction of the arrows;

FIG. 3 is a fragmentary sectional view of the handle and base structure, taken on the line 3-3 of FIG. 1, looking in the direction of the arrows;

FIG. 4 is an enlarged fragmentary sectional view of the pivot, taken on the line 4-4 of FIG. 2, looking in the direction of the arrows;

. FIG. 5 is an enlarged fragmentary sectional view of the locking cam and rotating housing, taken on the line 5-5 of FIG 2, looking in the direction of the arrows;

FIG. 6 is a fragmentary sectional plan view of the locking cam and rotating housing, taken along line 6-6 of FIG. 5,

looking in the direction of the arrows;

FIG. 7 is an enlarged fragmentary sectional view of the lead holder guide, taken along line 7-7 of FIG. 2, looking in the direction of the arrows; and

FIG. 8 is a fragmentary sectional view of the pivot support structure boom, taken along line 8-8 of FIG. 1, looking in the direction of the arrows, and also showing the position of the lead holder guide, in broken outline, when that portion of the compass is rotated from the position shown in FIG. 1 to a position adjacent to the supporting structure boom.

Referring now to the drawings in detail wherein like reference numerals indicate like parts throughout the several figures, the reference character 9 generally designates one embodiment of the lead holder compass constructed in accordance with this invention.

The lead holder compass 9 includes a flat base plate 10 which is above and parallel to the drawing surface, not shown in this view, and is rigidly connected to a vertical plate member 11 which is in turn rigidly connected to the T-shaped boom portion 12 which is again in turn rigidly connected to the cylindrically shaped boom extension 13. This group of elements referenced as 10, ll, 12, and 13 are shown as being constructed from a single metal casting of a suitable material such as aluminum.

FIG. 3*shows in cross sectional view the upper portion 14 of the T-shaped section which extends across the curved top of plate 11 and which,as seen in FIG. 1, passes the plate 11 and continues on to form the upper part of boom 12. A thin sheet 15 is bonded or molded to the bottom surface of plate 10. Sheet 15 provides thesurface for base 10 which rests upon the drawing surface. The sheet material is a neoprene rubber or a similarly suitable rubberlike or plastic material which has a high coefficient of friction in combination with paper or drafting vellum.

Referring now to FIGS. 1, 2, and 4, the rotating housing 16 contains and joins other parts of the roatable portion of the compass. Two lower disc-like segments 17 of housing 16 extend on either side of the thicker portion 18, and are parallel to the drawing surface. A discontinuation between segments 17, at the side opposite guide base 19, produces a void region between edges 20. The thicker portion 18 is of a rectangular plate shape containing holes required to join or contain parts of the rotatable assembly, being parallel to the plane of the drawing surface, and with the lower surface 21 being common with lower surface of plate portions 17. Housing 16 is constructed from Lucite plastic or for a similarly suitable transparent material. Stud 22 connects housing 16 to the boom extension 13 by threads 23. Housing 16 is suspended by stud head shoulder 24 in a position such that the lower surface 21 is slightly above the drawing surface. A close running fit exists between stud 22 and the mating hole in housing 16.

In FIG. 1 the positioning rods 25 are shown suspended in the housing 16 by being passed through loose fitting holes 26. When viewed in FIG. 2, rods 25 with holes 26 are equally disposed about a centerline passing through the center of stud 22. Rods 25 are constructed by forming a small round rod of stainless steel or other suitable metal into a narrow U-shaped piece with parallel legs.

Referring now to FIG. 5, cam lock 27 fits into a round flat bottomed clearance hole midway between and partially intersecting the rod holes 26. With the interlocking of bottom edge 28 underneath rods 25, cam lock 27 is prevented from withdrawl by interference. The finger tab portion 29 projects vertically above the top surface of housing 16. Cam lock surface 30 is depicted as being rotated to the unlocked position, there being a clearance around rods 25. In FIG. 6, the profile of cam surface 30 shows that upon rotation of the cam lock within its socket by the rods 25 will be caused to spread aparttowards hole surfaces 31 within housing 16. Cam lock 27 is constructed from nylon plastic, or from a similarly suitable plastic material.

As viewed in FIGS. 1, 2, and 7, the guide base 19 is composed of an attaching pad 32, two stabilizing pads 33, and a disc-like hemispherical portion 34, all being a common piece of material suitably connected by contouring and blending of 32, 33, and 34 together. The attaching pad 32 is firmly attached with a suitable bonding material to the ends of rods 25 which are installed into holes 35. The vertical centerline of hole 36 in hemispherical portion 34 passes through the theoreticalcenter point of 34. The two pads 33 which rest upon the drawing surface are formed as rounded bumps and are the only surfaces of guide base 19 which contact the drawing surface. As constructed and thus positioned, the theoretical center point of hemispherical portion 34 lies on the drawing surface, midway between stabilizing pads 33, and when viewed in FIG. 2, it also lies on a theoretical centerline parallel to and midway between rods 25. Guide base 19 is constructed from a suitably clear plastic material such as Lucite.

, In FIG. 1, the reference 37 is a gauge mark visibly impressed across both sides of part of the upper surface of housing 16. A

similar gauge mark 38, parallel to mark 37, is shown upon the top surface of guide base 19. Gauge

marks

37 and 38 are also viewed in FIG. 2, which shows them as seen in the plan view to be perpendicular to the long axis of rods 25, with mark 37 being coincidental with a centerline of stud 22, and with mark 38 being coincidental with a centerline of lead holder hemispherical portion 34.

Pivot alignment marks 39, 40, 41, and 42 are visibly impressed onto the lower surface 21 of housing 16, these marks being at 90 and 45 with respect to each other, and with all of them being in coincidence with a transverse centerline of pivot 22, as viewed in the plan view FIG. 2. The lines 43 and 44 are visibly impressed onto the lower surface of guide base 19. As viewed, these lines are coincidental with centerlines of spherical portion 34, and at right angles to each other. Line 44 is perpendicular to the long axis of rods 25, and line 43 appears as an extension of line 39 of housing 16.

FIG. 7 shows the round tubular shaped lead holder guide 45 with the lower end contoured to match, in a coaxial fashion, the hemispherical portion 34, and with an internally threaded portion 46 at the lower end of the inside diameter. Lead holder guide 45 is constructed from Lucite plastic or from another suitable transparent material. The inner surface 47 provides a loose fitting socket receptacle for the lower portion of the lead holder, not shown. The lead holder guide retainer 48 is also constructed from Lucite plastic or other suitable transparent material, and it consists of an externally threaded portion 49, a small round hold 50, which provides a clearance fit for the drawing lead, not shown, a tapered entry 51, and a head 52 which iscontoured to match lower surface of hemispherical portion 34, and which also provides an extension of this lower surface, as indicated at reference 53, and thereby also provides an annular clearance around hole 36. Male thread 49 is shown installed into female thread 46, this being possible by having the central hole 36 the same diameter or slightly larger than the interfering diameter of thread 49. The spherically contoured and shaped surfaces around hemispherical portion 34 are loosened or tightened by the threads to provide a close sliding fit. After obtaining the correct fit, a plastic or metal set screw 54 is installed into the lower body of guide 45, the end of the set screw 54 pressing against the male threads 49. All hemispherical surfaces are constructed to have a common center point. The lead holder guiding surface 47 and the lead hole 50 have a common axial centerline which, if extended downward, passes through the common hemispherical center point.

In FIG. 8 a cross sectional view of the boom 12 shows an upper point 55 and a lower point 56. These are contacting points on the right hand side of boom 12 for the outer diametral surface of guide body 45, when the lead holder guide is rotated to the position shown by the broken lines. Corresponding points on the left hand side of the boom 12 become the contact points when the lead holder guide is rotated around to that side.

Having thus described in detail the construction and the various parts of the lead holder compass, I will now describe how, by using this compass together with a drafting lead holder with lead, not shown in the drawings, a full circle or an arc may be drawn.

Centerlines of the circle to be drawn are first laid out in the usual manner, crossing the desired center point on the drawing surface and at right angles to each other. Next, the radius is established. This may be accomplished in one of three ways. The first method uses the gauge marks 37 and 38 together with a suitable drafting scale, ruler or dividers, not shown in the drawings. Cam lock 27 is rotated within its socket by twisting finger tab 29 until the cam is in the position shown by FIGS. and 6. Rods 25 are now free to slide within the housing 16, and guide base 19 can now be freely positioned with respect to the pivot 22. When the desired radius is obtained, as indicated by measuring the distance between gauge marks with the scale or dividers, the cam lock finger tab 29 is twisted with the fingers by 90. Surface 30 now contacts rods 25, spreading them apart. Dimensions of the cam surface, the rods, and the clearance holes 26 are such that when the cam is in this position the outer surfaces of the rods touch the surfaces 31 of the housing. Friction between contacting surfaces on both sides of each rod effectively bind the rods in a locked position.

The second method of establishing the radius is the same as the first method, with the exception of using the gauge marks. Instead of direct measurement of gauge marks 37 and 38, the lines 41. and are used by aligning them with previously drawn parallel lines on the drawing surface. Visual alignment of the two previously drawn parallel lines spaced at the desired radius, with lines 41 and 44, will result in the desired radius when locked in position by. the cam lock.

The third method of establishing the radius is the same as the second method, with the exception that line 44 is not used. Instead, the lead hole 50 is used to sight a point on the drawing surface. With the lead holder removed, and the guide body held in a generally vertical position, sighting of a point or a line on the drawing surface will result in a correct positioning of the lead holder guide with respect to the pivot, when used in conjunction with line 41, as with the previous method. It is also possible to establish the radius in a similar fashion but with the lead holder installed, by sighting the lead point on the drawing surface, through 34.

The next step in drawing a circle is to center the pivot over the center of the circle. This is accomplished by visual alignment of either pair of lines 39 and 41 or pair 40 and 42 with the pro-drawn circle Centerlines. Either alternately or simultaneously, the roatable portion of the compass is turned to align the lines, while the compass is lifted or slid across the drawing surface to move it into position. A light downward pressure in the vicinity of the handle or base is sufficient to maintain position of the compass while drawing the circle, however, the friction of 15 against the drawing surface is not so great as to interfere with deliberate, slow sliding of the compass into position. After the pivot is aligned, the lead holder guide is rotated to the vicinity of the boom 12. The lead holder with lead, not shown in the drawings, is then installed with the lead fitting into hole 50 and the point touching the drawing surface. Using the lead holder body to manipulate the guide and rotatable portion of the compass, the guide body 45 is caused to touch the boom at the two points as shown by FIG. 8. Since the guide surface now touches two seperated points, there is a stabilizing effect. Also, when in this position, the lead point is at a point on the drawing surface which is directly underneath or slightly past a theoretical vertical centerline which bisects the boom cross section shown in FIG. 8. Now, with the normally used downward force applied to the lead holder, the circle is drawn by rotating the guide around with the lead holder. A natural motion, with no special regard for vertical positioning of the lead holder is possible, since spherical guiding of the lead point causes the lead point to follow a true circular path. Also, the pads 33 help to maintain vertical position of the spherical guide with respect to the drawing surface, as well as to prevent torsional motion of the spherical guide about a theoretical centerline parallel to and between the rods 25. When the lead holder guide is rotated around to the opposite side of the boom, points on the boom are again contacted. The position of the guide body 45 can be observed and felt by the stabilizing effect when contacting the two points on boom 12. Again, the lead point is at a point on the drawing surface directly under or slightly past the theoretical vertical centerline bisecting the boom cross section. Under the boom, the ends of the circle are joined, and the circle is completed.

It is obvious now, that a partial circle, or are, may also be drawn by this compass. Procedures for drawing an are are the same as for drawing a full circle, however, when the boom 12 is positioned over the drawing surface in such a location as to avoid the vicinity of the are being drawn, observance of the position of guide body 45 with respect to boom 12 is not necessary. To limit the arc to some specific angular excursion, any of the lines 39, 40, 41, or 42 may be used by employing visual alignment with either angular boundaries or with a radial line previously placed on the drawing surface. For example, if the arc is drawn beginning when line 41 is in coincidence with a radial centerline on the drawing surface, rotation of the housing 16 in drawing the arc until line 42 is in coincidence with the same radial line will cause an arc of 45 to be obtained. Line 43 may also be used to advantage, by bringing it into coincidence with the desired angular boundaries on the drawing surface, at the beginning and termination of the are being drawn. Also, the lead point itself may be viewed through the clear plastic guide elements, in order to establish location of the lead point. This is best accomplished by pivoting the guide body 45 away from the viewer, allowing the point to be sighted through hemispherical portion 34 only, rather than through

elements

34, 45 and 48 in combination.

Another useful aspect of this compass is the capability of drawing an are when the center lies off of the drawing surface, or when, for other reasons, contact with a surface at the center of the arc is either not possible or not desireable. The procedure for drawing such an arc is the same as for drawing any are, excepting that locating of the compass pivot is accomplished by using line 43 in conjunction with a line on the drawing surface. Several means are available for positioning the pivot. With the pivot off of the drawing surface, the base and the rotatable portion of the compass are moved until coincidence with line 43 and a pre-drawn line is obtained. This pre-drawn line must be a projection from the theoretical center point of the desired are. For the drawing of an arc whose center lies off of the drawing surface, it is generally necessary to have first established the desired radial distance by one of the previously described methods. Alignment of the pivot may also be facilitated by having on the drawing surface a second line which is at right angles to the projected radial line and which crosses the radial line at the desired point of tangency. Again, line 43 is brought into coincidence with the radial line, but line 44 is also brought into coincidence with the second line. Another possibility for pivot alignment and placement is to place on the radially projected line, a mark at the desired point of tangency. Either the lead hole 50 or the lead point is then used for locating the lead to this mark, and after the line 43 is brought into coincidence with the radial line, the arc is struck with the lead holder and lead in the usual manner. Where great accuracy is not required, the rods 25 may be used for alignment, since a radial line will lie between these when viewed in the plan view. Placement of the compass such that rods 25 are equal distance and parallel to a radial line on the drawing surface, will result in radial positioning of the pivot which is sufficiently accurate for the larger arcs.

At this time it should be obvious that the guide 45 may be disengaged from retainer 48 after loosening set screw 54, and these parts may be easily removed. The hole 36 is then readily accessible and provides a mounting locator for other accesso- 40 ries, such as inking pen adapters, scribing tools, or cutters. The disassembly of the rotatable portion of the compass by removal of screw 22 is also obvious. The compass is easily assembled from or disassembled into the seperate items of: base and frame, shoulder screw, rotating housing and guide base, lead holder guide, and lead holder guide retainer. The ease of assembly and disassembly into the various parts facilitates packaging for transportation and also makes the replacement of parts a simple operation.

From the previous descriptions it will be apparent that this invention provides a novel compass which is simple and rugged in construction, which may be economically manufactured, and which has a long service life.

Having thus described the preferred embodiment of the invention, it should be understood that numerous structural modifications and adaptations and changes in the combination and arrangement of parts or elements as heretofore set forth in the specification and shown in the drawings may be made without departing from the scope of the invention as defined in the following claims.

What is claimed is:

l. A lead holder drafting compass comprising:

a flat base with a bottom friction surface, said base also including an upright handle member having an extending boom projecting over a drawing surface, the end of said boom containing in the vertical axis a screw attachment hole;

a transparent rotating housing member, said housing having a vertical axis bearing hole with a counterbore on the bottom side through which a shoulder screw is inserted and installed into said vertical axis screw attachment hole, said housing being held suspended above the drawing surface upon which said base rests by the head of said shoulder screw, said housing having two parallel horizontal clearance holes equal distanced from said vertical axis bearing hole, said housing also having a flat bottomed vertical hole lying between and partially intersecting at the diameter of said flat bottomed hole said parallel horizontal clearance holes;

two rod members, longer than said housing, said rods being parallel to each other and held within and passing through said horizontal clearance holes in said housing, said rods being joined together at one end in one direction from said housing, and with the ends of said rods protruding from said housing in the opposite direction being attached to a lead holder guide base;

a cam lock member having a short vertical portion with a recessed cam surface in and around said cylindrical portion, and having a finger tab portion extending vertically upward, and with said short cylindrical portion installed into said flat bottomed vertical hole in said housing, and having said recessed cam surface contoured to cause alternately either looseness or a tightening of said rod members between said cam surface and outer surfaces of said horizontal clearance holes when said cam is rotated by said finger tab in a horizontal plane, and also having a portion of said short vertical cylindrical portion lying below said rod members and captured between said rod members and bottom of said flat bottomed hole;

a transparent lead holder guide base member having a hemispherically shaped guiding disc portion concaved upward with a round hole in the center and with a hemispherical center point positioned at the drawing surface, said hemispherically shaped disc being spaced off of the drawing surface by two attached guiding pads equally spaced on opposite sides of said hemispherically shaped disc which contact the drawing surface, and also positioned vertically by the joining of an attached pad to the end of said rods which extend out of said rotating housing member;

a transparent lead holder guide member of hollow cylindrical shape having an upper guiding bore of diameter slightly larger than a lead holder barrel, with the lower end of said lead holder guide having a surface contacting the upper surface of said hemispherically shaped disc and being concentric with and normal to said guiding bore, the inner lower end of said lead holder guide having an internal thread concentric with said bore, there also being a small set screw hole with a set screw passing through the wall of said lead holder guide in the threaded lower portion of said guide;

a transparent lead holder guide retaining member, having external threads matching said internal threads of said lead holder guide but also with an interfering diameter the same as or smaller than said center hole of said hemispherically shaped disc, and having a lead hole of slightly larger diameter than a drafting lead, said lead hole also having a tapered entry at the upper end, with said lead hole being concentric with said external threads, with said guide retainer also having a head at the lower end and being of larger diameter than said center hole of said hemispherically shaped disc, said head being contoured to mate with the concave surface of the hemispherically shaped disc and also having an annular slot with an inner diameter smaller than said center hole adapted to receive the edge of said center hole and thus providing inner clearance around said center hole when said retainer threads are installed from the concave side of said hemispherically shaped disc into said internal threads of said lead holder guide thus allowing angular rotation of the then joinedguide and retainer about the said hemispherically shaped guiding disc center point.

2. A lead holder drafting compass in accordance with claim 1 in which said boom extending over the drawing surface is provided with a cross sectional shape having symmetry about a vertical centerline and whose upper width is greater than its lower width, said cross sectional shape also presenting sur- 75 faces at the greater and smaller widths whose connection by a straight line projected downward will terminate on the drawing surface at a point beyond a point of intersection of the said vertical centerline of said cross sectional shape and the drawing surface such that when outer surfaces of said lead holder guide cylinder contact surfaces of said cross sectional shape, the point of the guided lead holder lead will fall upon said point of intersection of the vertical centerline of said cross sectional shape and the drawing surface, or will fall beyond said point of intersection in the direction towards the said straight line downward projection.

3. A lead holder drafting compass in accordance with claim 1 further including a gauge mark visibly inscribed upon the upper surface of said rotating housing in a direction normal to the axis of said rods and also including a second gauge mark visibly inscribed upon the upper surface of said lead holder guide base in a direction parallel to first said gauge mark, and with said gauge marks being a distance apart corresponding to the distance between the vertical centerline of said rotating housing vertical axis bearing hole and the vertical centerline passing through said hemispherically shaped disc center point.

4. A lead holder drafting compass in accordance with claim 1 in which said transparent rotating housing suspended above the drawing surface possesses a lower flat and horizontal surface upon which are visibly inscribed alignment marks coincidental with transverse centerlines of said vertical axis bearing hole, such alignment marks being positioned at 90, at 45,

' and at other convenient distances apart, with one of said marks being parallel to the axis of said rods.

5. A lead holder drafting compass in accordance with claim 1 in which the lower surface of said lead holder guide base possesses several visibly inscribed alignment marks, one of said marks being normal to the axis of said rods and being discontinuous in the center portions of said guide base while also projecting radially in either direction from a vertical axis of said hemispherically shaped disc center point, and another said mark being parallel to the axis of said rods and being discontinuous in the center portions of said guide base while also projecting radially in either direction from a vertical axis of said hemispherically shaped disc center point.

Claims

1. A lead holder drafting compass comprising: a flat base with a bottom friction surface, said base also including an upright handle member having an extending boom projecting over a drawing surface, the end of said boom containing in the vertical axis a screw attachment hole; a transparent rotating housing member, said housing having a vertical axis bearing hole with a counterbore on the bottom side through which a shoulder screw is inserted and installed into said vertical axis screw attachment hole, said housing being held suspended above the drawing surface upon which said base rests by the head of said shoulder screw, said housing having two parallel horizontal clearance holes equal distanced from said vertical axis bearing hole, said housing also having a flat bottomed vertical hole lying between and partially intersecting at the diameter of said flat bottomed hole said parallel horizontal clearance holes; two rod members, longer than said housing, said rods being parallel to each other and held within and passing through said horizontal clearance holes in said housing, said rods being joined together at one end in one direction from said housing, and with the ends of said rods protruding from said housing in the opposite direction being attached to a lead holder guide base; a cam lock member having a short vertical portion with a recessed cam surface in and around said cylindrical portion, and having a finger tab portion extending vertically upward, and with said short cylindrical portion installed into said flat bottomed vertical hole in said housing, and having said recessed cam surface contoured to cause alternately either looseness or a tightening of said rod members between said cam surface and outer surfaces of said horizontal clearance holes when said cam is rotated by said finger tab in a horizontal plane, and also having a portion of said short vertical cylindrical portion lying below said rod members and captured between said rod members and bottom of said flat bottomed hole; a transparent lead holder guide base member having a hemispherically shaped guiding disc portion concaved upward with a round hole in the center and with a hemispherical center point positioned at the drawing surface, said hemispherically shaped disc being spaced off of the drawing surface by two attached guiding pads equally spaced on opposite sides of said hemispherically shaped disc which contact the drawing surface, and also positioned vertically by the joining of an attached pad to the end of said rods which extend out of said rotating housing member; a transparent lead holder guide membeR of hollow cylindrical shape having an upper guiding bore of diameter slightly larger than a lead holder barrel, with the lower end of said lead holder guide having a surface contacting the upper surface of said hemispherically shaped disc and being concentric with and normal to said guiding bore, the inner lower end of said lead holder guide having an internal thread concentric with said bore, there also being a small set screw hole with a set screw passing through the wall of said lead holder guide in the threaded lower portion of said guide; a transparent lead holder guide retaining member, having external threads matching said internal threads of said lead holder guide but also with an interfering diameter the same as or smaller than said center hole of said hemispherically shaped disc, and having a lead hole of slightly larger diameter than a drafting lead, said lead hole also having a tapered entry at the upper end, with said lead hole being concentric with said external threads, with said guide retainer also having a head at the lower end and being of larger diameter than said center hole of said hemispherically shaped disc, said head being contoured to mate with the concave surface of the hemispherically shaped disc and also having an annular slot with an inner diameter smaller than said center hole adapted to receive the edge of said center hole and thus providing inner clearance around said center hole when said retainer threads are installed from the concave side of said hemispherically shaped disc into said internal threads of said lead holder guide thus allowing angular rotation of the then joined guide and retainer about the said hemispherically shaped guiding disc center point.

2. A lead holder drafting compass in accordance with claim 1 in which said boom extending over the drawing surface is provided with a cross sectional shape having symmetry about a vertical centerline and whose upper width is greater than its lower width, said cross sectional shape also presenting surfaces at the greater and smaller widths whose connection by a straight line projected downward will terminate on the drawing surface at a point beyond a point of intersection of the said vertical centerline of said cross sectional shape and the drawing surface such that when outer surfaces of said lead holder guide cylinder contact surfaces of said cross sectional shape, the point of the guided lead holder lead will fall upon said point of intersection of the vertical centerline of said cross sectional shape and the drawing surface, or will fall beyond said point of intersection in the direction towards the said straight line downward projection.

4. A lead holder drafting compass in accordance with claim 1 in which said transparent rotating housing suspended above the drawing surface possesses a lower flat and horizontal surface upon which are visibly inscribed alignment marks coincidental with transverse centerlines of said vertical axis bearing hole, such alignment marks being positioned at 90*, at 45*, and at other convenient distances apart, with one of said marks being parallel to the axis of said rods.