US2531636A - Instrument for making perspective drawings - Google Patents

Description

Nov. 28, 1950 E. E. M CULLOUGH 2,531,636

INSTRUMENT FOR MAKING PERSPECTIVE DRAWINGS Filed April 2, 1949 4 Sheets-Sheet 1 Nov. 28, 1950 E. E. MOCULLOUGH' 2,531,636

INSTRUMENT FOR MAKING PERSPECTIVE DRAWINGS Filed April 2, 1949 '4 Sheets-Sheet z W I 63 2 L9 26 3/ Il 52 NOV. 28, 1950 MOCULLQUGH 2,531,636

INSTRUMENT FOR MAKING PERSPECTIVE DRAWINGS Filed April 2, 1949 4 Sheets-Sheqt 3 IN V EN TOR.

Nov. 28, 1950 E. E. M CULQQUGH ,6

INSTRUMENT FOR MAKING PERSPECTIVE? DRAWINGS Filed April 2, 1949 LSheetS-Sheet 4 IN V EN TOR.

ga/W/ 61/66/4144 Patented Nov. 28, .1950

INSTRUMENT FOR MAKING PERSPECTIVE DRAWINGS Edward E. McCullough, Minneapolis, Minn.

Application April 2, 1949, Serial No. 85,216

10 Claims.

My invention relates to drawing instruments and, more specifically, is a device for making drawings of three-dimensional views in perspective. The objects of my invention'are, first, to facilitate the rather involved and costly process of making perspective drawings and to insure greater accuracy therein; second, to provide a device which may employ as many vanishing points as are desirable in a drawing and which will make possible the predeterminability of said vanishing points by the operator; and third, to provide a device, by the use of which, the image of an object can be quickly drawn in perspective as viewed from a nearly unlimited variety of angles.

I attain these objectives by the use of the mechanism illustrated in the accompanying drawings, in which Fig. 1 is a perspective view of the device;

Fig. 2 is a view in side elevation;

Fig. 3 is a fragmentary vertical section taken on the line 3-3 of Fig. 2;

Fig. 4 is a fragmentary sectional view taken on the line 4-4 of Fig. 3;

Fig. 5 is a fragmentary plan view showing the joint or juncture of the two sets of parallel arms;

Fig. 6 is a sectional view taken on the line 66 of Fig. 5;

Fig. 7 is a sectional view taken on the line 1-1 of Fig. 2;

Fig. 8 is a fragmentary vertical section taken on the line 3-8 of Fig. 4;

Fig. 9 is a vertical section, with some parts shown in front elevation, taken on the line 9-9 of Fig. 2;

Fig. 18 is a fragmentary horizontal section, with some parts shown in plan, taken on the line l8-l8 of Fig. 16;

Fig. 19 is a horizontal section taken on the line l9l9 of Fig. 16;

Fig. 20 is a fragmentary perspective view showing a modified form of the device in which at least one of the movable straight edges is replaced by a fixed straight edge;

Fig. 21 is a vertical section taken on the line 21-45 of Fig. 20; and

Figs. 22 and 23 are merely fragmentary diagrammatic plan vi ws showing the relative positions of the movable straight edges when the base plate is located at the upper left-hand and the lower left-hand regions of the drawing board, respectively.

It may be here stated that Figs. 1 and 2 are drawn to a common scale; Figs. 3, 4, 5, 6, 7, 8, 9, 10, i4 and 15 are drawn to a common scale which is approximately twice as large as that of the former group of figures; Figs. 11, l2, 13, 16, 17, 18, 19, 20 and 21 conform to a common scale, which is still larger than the scales of the preceding groups of figures; and that Figs. 22 and 23 are not drawn to scale.

Fig. 10 is a fragmentary vertical section taken at the transverse center'of the device;

Fig. 11 is a vertical section taken on theline ll--H of Fig. 2;

Fig. 12 is a vertical section taken on the'line iZ-i of Fig. 11;

Fig. 13 is a fragmentary perspective view of one of the three rectangular projections of the base plate member, with the strai ht edge removed, showing the calibrated scale by which the distance, from the center of the drawing to the imaginary vanishing point, may be determined;

Fig. 14 is a vertical section taken on the line I l-44 of Fig. 10;

Fig. 15 is a partial plan view of a special straight edge which is to be used when the vanishing point, toward which the vertical lines of the drawing converge, is located above the horizon line of the drawing;

Fig. 16 is a fragmentary section, with some parts shown in side elevation and showing the sliding portion in a partially extended position, taken on the line lit-453 of Fig. 15. Also, this view shows the straight edge as applied t the device;

Fig. 17 is a vertical section taken on the line 11-" of Fig. 16;

Like characters refer to like parts throughout the several views.

The device is equipped with a clamp member 24 by means of which it may be firmly anchored to the edge of a drawing board or table top :17. ihis clamp member 2d consists essentially of three parts which comprise the integral whole: i. e. a long vertical body member 25 and two clamping members 26 and 21 which project in a horizontal plane, from the same side of said vertical body member. The clamping member 26 projects from what is approximately the vertical center of the body member 25 and ispenetrated near its end portion by a vertical threaded hole which accommodates a wing bolt 23. This wing bolt 28 is in an inverted position when in threaded engagement with said hole and has pivotally attached to its upper end portion and positioned in a hori ontal plane, a flanged disc 29 whose dance is projected upwardly from said disk.

Projecting from the upper end portion of the body member 25 is the clamping member 2?, in the form of a thick and substantially rectangular bar, which provides on its under side, and in opposing relation to the flange of the disk 29, a surface of resistance 3 Between this surface 3i! and the upper edge of the flange of disk 29, the edge of a drawing board or table top may be sec rely clamped.

The main body portion 25 of the clamp 24 is in the form of a long vertical boss 3!, in which is inserted a long bolt 32. This bolt 32 is threaded at its lower end portion and is equipped with a nut which engages the lower end of the boss 3|. At the upper end portion of the bolt 32, and integral therewith, is a cylindrical member 33 whose longitudinal axis lies in a horizontal plane. This cylindrical member 33 has on its under side, a small shoulder 34 which provides a bearing surface; enabling said cylindrical member to rest on the upper end surface of the boss 3!. Through the longitudinal axis of th cylindrical member 33 is a hole which accommodates with rigid engagement a pin '35 which also passes through aligned holes in the two prongs 36 of the bifurcated end of a long rod 31, between which the cylindrical member 33 is interposed for pivotal movement therewith. Thus it may be seen that the members 3!, 32, 33;, 35, 36 and 3! constitute the elements of a universal joint 38; the rotary movement of the long bolt 32 in the boss 31 being in a horizontal plane and the two prongs E6 on the pin 35. which is rigidly held by the cylindrical member 33, providing pivotal movement in a vertical plane at point A.

The long rod 3? is normally upwardly inclined in relation to. its pivot point A and its diameter is somewhat enlarged at the upper end portion where it contains a deep groove which admits the eye-like ends of two long, parallel arms 49 and M which normally descend obliquely therefrom, see Fig. '7. The long arm 4-8 is the primary arm and is attached to the extreme upper end portion of the rod 31; theeye-like end of arm 43 being inserted in'the groove 33 and pivotally attached therein by a pin a2, which passes through the hole in the eye-like end of said arm isheld with rigid engagement in aligned holes in the sides of said groove. The arm ll is a secondary arm, somewhat smaller in diameter than the primary arm 158, andi pivotally attached in the groove 33 in the same manner as was described for the arm. it, by the pin 63, except that the secondary arm M; is attached in the groove 39 a short distance from the arm as toward the longitudinal center of the rod 37. Thus it may be seen that the parallel arms 4! and il are provided withpivotal engagement, in a vertical plane, tothe rod'ii'i at points B and C, respectively.

The lower end portion of the primary arm 48 is bifurcated and between the two prongs thereof is interposed the eye-like end id of a short crossbar' ld through which passes a pin 46, said pin being rigidly engaged in aligned holes in the two prongs of the bifurcatedheadof primary arm 49 and providing pivotal movement, therewith at point D. Theother end of this crossbar 45, which is in the formof a bifurcated eye or bearing All, is inserted between the two prongs of the bifur-. cate'd lower end portion 43 of the secondary arm 41, said prongs also being in the form of eyes or bearings. Interposed between the two prongs of the dichotomous bearing 47, is the vertical pro-v jection Of a swivel piece 49. Said vertical proiection is equipped with a hole which is to be aligned with the holes in the bifurcated bearings 48 and dB, and through this assemblage of aligned holes passes a pin 50, which is held with rigid engagement in the. hole of thevertical projection of the swivel piece 69 and provides pivotal movement withthe dichotomous bearings 43 and 49 at point E. See Fig. 2.

The two. prongs of the bifurcated end of primary arm are extended beyond point D and the end portions of said extensionsare penetrated by aligned holes to provide a'bea 'ing for the pin 5i, which is rigidly ailixed in an aligned hole in a vertical projectionsai d projection being integral with the top. of a shortv shoulder bolt 52 and interposed between the bearings on the extensions ofthe prongs of the arm 40. I'Ihus pivotal movement of arm 46 is provided with shoulder bolt 52 at. point F.

The shoulder bolt 52 is in threaded engagement with a cylindrical, internally threaded socket 53, which is also equipped with a shoulder, at its lower end portion, the distance between these two shoulders being determined by the length of the cylindrical portion of the socket 53. Between these two shoulders, the eye-like ends of three straight edges, which will be described later, are loosely confined for pivotal movement in a horizontal plane. The shoulder of the cylindrical socket 53 is in the form of a thick disc and has on its under side a slot, or groove by which it may be manually revolved into, a, tight, threaded en- E sement with the shoulder bolt 52.

The swivel piece 49 is comprised or the rather fiat vertical projection, which has already been described as being in pivotal engagement with the bifurcated bearings 47 and 48, and a fiat disc positioned in a horizontal plane and having a vertical ,hole at its center. The fiat, vertical projection is in a radial edgewise position on the circumference of the disc. and is integral there- I with.

Through the vertical hole in the disc-like, portion of the swivelpiece 49 passes. a nut-equipped shoulder bolt 5% which also passes through an aligned hole in the base. plate .575, engaging said swivel piece to said base plate for pivotalaction, therewith, in a horizontal plane. A recess is provided in the under side of the baseplate 55 to accommodate the slotted head of the shoulder bolt 5. 3, see Fig. 10.

The horizontally projecting clamping member 21, which has beendescribed as being an integral part of the clamp 24, is in the form of a rather thick bar, which normally rests upon top of the edge of a drawing board ortable top. In the 1. left-hand side of this bar 2,? and penetrating to approximately three-fourths of the width thereofp is a deep, groove, in the sides of which are two sets of vertically aligned holes. Inserted in said groove andaligned Withgsaid holes are the eyelike ends of two long parallel arms 56, through the eyes of which pass the pins 5'1; the pinsd'l being held with rigid engagement, in the aligne'd'holes in the sides of said groove and providin theparallel arms .56 with pivotal engagementflin a, horizontal plane, with respect tQpins-E'Lat pointsG. and H.

The arms "55 are normally obliquely engaged to the clamp 24. and are in the form of long rods, each of the end portions of which is in the form of a horizontally positioned eye, the axes of the hole thereof being vertical. The other ends, of these parallel arms 56. are also, as. has been said, in ,the form of eyes and through them pass-the pins 58.. The pins 58 also pass through identical eyes which constitute the end portions of a secondary pair of parallel arms 59, said arms being identical to the primary set of parallel .arms- 55. The, eyes. of arms 58. are arranged immediately-below the eyes of arms 59 on the pins 53 and the end portions. of said pinsare rigidly affixed in vertically aligned holes in two elliptical, horizontally positioned plates 60. In this manner, pivotal. motion in a horizontalplane' isv made possible for the arms 56 and 59, on thepins, 58 at points J and K.

' The other eye-like ends of the arms 59 are engaged, for pivotal motion in a horizontal plane at points L and M to a substantially triangular projection SI of the base plate 55 by two short, nut-equipped shoulder bolts 62, which pass through vertical holes in said projection. Recesses are provided in the under side of the projection Bl to accommodate the heads of said shoulder bolts.

The periphery of the base plate 55 is fundamentally circular but has three somewhat rec tangular projections 53, which are integral therewith and whose longitudinal axes are arranged radially from the center of said plate and at right angles to each other. Also integral with the base plate 55 is the triangular projection 6| described above as engaging the parallel arms 59. Coincident with the longitudinal axes of the projections 63 and extending well in toward the center of the base plate 55 are slots 64 which make possible the relative adjustments of three bolt-equipped thumb-screws 55, which represent the vanishing points. The thumb-screws 65 are of a knurled socket type, into the sockets of which extend, for threaded engagement therewith, short bolts 66 having cuboid heads 61, which fit into the slots 64. The sides of the slots 64 have short, opposingly-projecting horizontal flanges 68, which may be rigidly engaged between the thumb-screw 69 and the cuboid bolt heads 61. The cuboid bolt heads 61 fit snugly into the slots 64 and are thereby prevented from turning when the knurled thumb-screws 65 are revolved into threaded engagement therewith.

Also integral with the base plate 55 is a handle bar 69 which may be manually gripped when the device is being used.

As has been already mentioned, the cylindrical threaded socket 53 provides a focus point for three straight-edges 1|], H and 12 which radiate therefrom; the eye-like ends of which are loosely confined for pivotal movement therewith between the shoulder of said socket and that of its shoulder bolt. Each of these straightedges 10, H, and I2 is composed of three integral parts: an eye-like end IS, a fiat bar 14 and the wedge-shaped straight-edge-proper 15. The eye-like ends 13 are aligned in a vertical arrangement on the cylindrical portion of the socket 53 and said eye-like ends l3 are joined to the fiat bar portions 14 by a slight offset 16, which compensates for the thickness of the base or shoulder portion of the socket 53, or this thickness, together with that of one or two of the eye-like ends 13, enabling each flat bar portion M to make sliding contact with the surface of the plate 55. It is, of course, obvious that the offset 16, for each of the straight-edges, must be of a different height because of their vertical arrangement on the socket 53. At the transverse centers of the flat bar portions l4, and extending nearly the full len ths thereof, are slots 11, through which the thumb-screws 65 project. The lower portions of these thumbscrews 65 are of a smaller diameter than the upper, knurled portions; affording shoulders 18. Between these shoulders 18 and the base plate 55, the fiat bar portions are loosely confined for sliding pivotal movement. The thumb-screws 65 thus act as pivot or fulcrum points for the straight-edges H1, H and H. The lower portions of said thumb-screws are of a size which snugly fit the width of the slots 17.

The ends of the flat bars 14 are integrally joined to the upper portions of wedee-shaped straight-edges proper '15, which are necessarily quite thick at this juncture, to compensate for the thickness of the base plate 55, since the straight-edges proper must rest on the surface of the table or drawing board. After said junc- 6 ture, the straight-edges proper '15 gradually taper to a comparatively thin end and said straightedges are wedge-shaped, as viewed in transverse section, so that a relatively thin edge may be presented as a guide for a pencil or other drawing instrument. See Fig. 14.

Having thus described in detail the various parts of which the mechanism is comprised, its principles of operation may now be presented by the employment of the afore-mentioned points A. B. C., etc., as being the vertices of various geometrical figures, since this device is composed principally of a combination of triangles and parallelograms,

In referring to Fig. 2, it may be seen that the large imaginary triangle ABF is an isosceles triangle; the small imaginary triangle DFE is an isoceles triangle; and that the figure BCED is a parallelogram, havin opposite sides of equal length. It is necessary that the said triangles be isoceles, i. e. having two sides equal, inorder that the movement of point F in relation to the drawing board may be limited only by the combined lengths of the arms AB and BF.

In practice, the artist or draftsman will move the base plate 55, which is attached to point E, freely on the surface of the drawing board. This will constantly change the shape of the large triangle ABE. However, because of the action of the parallelogram BCED, the angle at B will always be copied at D; so that the triangles ABF and DEF, being of necessity isosceles triangles, will also always be similar triangles. Therefore, regardless of the position of the base plate 55 on the surface of the drawing board, the distance from point F to point E will always be in a constant ratio to the distance from point F to point A. It will be observed that the points AE and F lie in a straight line to permit unimpeded movements of the arms. Point A, as has been said, is attached to the edge of the drawing board and point E is attached to the corresponding edge of the base plate 55, so that the movement of point F in relation to the base plate 55 may be regarded as a miniature replica of the movement of point F in relation to the drawing board as a whole.

It will be observed that, although the base plate 55 is capable of being placed at any position on the drawing board (with n the reasonable radius allowed by the rod 31 and the arm its horizontal axes are kept oriented in para lel relation to the horizontal axes of the drawing board (or more specifically, to the clamp 24, which is rigidly attached to the drawing board) by two adjoining, variable parallelograms GI-IKJ and KJLM, wh ch will be described later. It may now be seen that the above analo y of regarding the base plate as a miniature scal mod l of the area of the drawing board, which lies within the functional radius of the device, may be carried further to show that since the axes of the base plate are always in parallel orientation with the axes of the drawing board. the point F must assume the ident cal position in relation to the base plate 55 as it assume-s in relation to the workable area of the drawin board. It follows, then, that the thumbscrews 55 are in the same position, relative to the base plate 55, as points V.P. are in relation to the drawing board, since the straight edges '15 are merely extensions of lines drawn from point F to the thumbscrews 65. In th s connection, it will also beobserved that the imaginary lines E- and A-V.P. are parallel (Fig. 1) and that'the imagi 7 nary triangles F-E-6,5 and F.-AV,l-f'. are always similar.

In referring to the two sets of parallel arms 56 and 59, as they are shown in Fig. 1, it may be seen that the figures GHKJ and KJLM describe two parallelograms which are joined together by a. common side KJ. The points G and H are rigidly affixed to the clamp 24 and therefore the sides KJ and LM must always be parallel to the side GH, regardless of the position of the. base plate 555 (to which point L and M are attached) upon the drawing board. "In this manner the horizontal axes of the base plate 55 are kept consistently parallel with those of the drawing board.

As has been said, a short shoulder bolt 52 depends from the pivot point P and is in threaded engagement with a cylindrical socket 53 whose base slides along the surface of the base plate 55 when the position of the base plate 55, on the drawing board, is changed. From this socket 53. radiate the three straight-edges lil, ll and i2, which are guided by the thumbscrews 65 in the mannerwhich has already been described.

In making perspective drawings, three vanishing points are usually considered: i. e. two vanishing points which are located on the horizon line of the drawing (one on'each side of the drawing) toward which the horizontal lines of the drawing converge, and one vanishing point which may be located either above or below the horizon line of the drawing, toward which the vertical lines of the drawing converge.

,In Fig. 1, the straight-edge i6 wiil point to the vanishing point located on the horizon line to the left of the drawing, the straight-edge '1! will point to the vanishing point on the horizon line to the rig-ht of the drawing (marked V.P. in Fig. 1) and the straight-edge 712 will point to the vanishing point which is located below the horizon line of the drawing.

In orthodox practice, when drawing in perspective, it is often necessary to place the vanishing pointsat quite, a distance from the draw ing, necessitating the use of a rather large drawing board. In this method, the vanishing points are often represented by tacks or pins to which strings are, tied and stretched taut to serve as guides for drawing the lines of perspective, or perhaps a very long straight-edge is guided by direct contact with these ins or tacks.

In the mechanism illustrated in the drawings, the surface of the base plate 55 upon which the cylindrical socket 5? slides may be regarded as a scale model of the'working surface of the drawing board. Therefore, the knurled thumb-screws 65 may be compared to the pins or tacks which represent the vanishing points, in orthodox practice, and the slotted portions of the straightedges may be compared to the strings which are stretched therefrom. The wedged-shaped straight-edges proper are merely extensions of these slotted, flat bar portions and actually point to imaginary vanishing points much farther out from the center of the drawing board, of which the thumb-screws represent scaled models.

It is possible to determine the distances of these imaginary vanishing points from the center of, the working surface of the drawing board by employing calibrated scales, which are printed or engraved on one side of the slots 6 to repre-' sent the linear measurement in feet and inches. See Fig. 13. The ratio of the size of this calibrated scale to an actual foot rule must be the' same as the ratio of the distance to the,; di'stance FA. This scale may be indexed by the front edge iii of afraised portion on the top. of the cuboid head 61 of the bolt 66. See Fig. 13. Of course, the flat bar'portions M of the straightedges are made of transparent material, so that this scale may be visible.

It is sometimes desirable that the third vanishing point (toward which the vertical lines of the drawing converge), be located above the horizon line of the drawing instead or" below it, as is provided for by the straight-edge 12. In such'an event, it is necessary to replace the straight-edge 12 with a special straight-edge 30, which is illustrated in Figs. 15, 16, 1'7, 18 and 19. This straight edge consists of an eye-like end 88 similar to those of the straight-edges 79, H and i2, which is arranged on'the cylindrical socket 53 in'the same manner as was. the eye like end of the straight-edge 12. This eye-like end is integral with a flat bar portion'82 which lies in the same plane therewith. The flat bar portion 82 projects through and is slidably engaged in a rectangular aperture 83 in a fiat, vertical, rectangular member 84 which is integral with a horizontally positioned half of a spur gear 85. On the end of the flat bar portion 82 a small, ver tioal, flange 86 which prevents the flat bar portion 82 from becoming disengaged with the vertical rectangu ar member 84. The partial spur gear 85 pivots on a pin 8?, whose ends are rigidly journaled in a pair of vertically aligned holes in one end of two oblong, horizontally-positioned plates 88 and 89; plate 33, being the upper plate and plate 89 being the lower plate. Another partial spur gear 94!, the teeth of which are meshed with those of the gear 85, is pivotaly engaged on a vertical pin Q! whose ends are rigidly journaled in a pair of vertically aligned holes in the other end of the two plates E38 and 89. Integral with this partial spur gear cc, and lying in the same plane therewith, is a short flat bar portion 92, the other end of which is integral with the upper portion of a wedgeshaped, straight-edge proper, which is identical with the straight-edges already described.

In the center of the under surface of the plate 88 and integral therewith, is a square oroiection 93 which contains a vertica threaded hole to receive, with threaded engagement, a short machine screw 94. The width of this square pro- .iection as is the same as the distance between the opposing flanges 68 on the sid:s of the slot 64 and its length is the same as the thickness of said flanges; so that the flanges 68 may be rigidly engaged between the head of the machine screw 94 and the bottom ofthe plate 89. Inthis manner, the plates 83 and 83 may be rigidly affixed to the rectangular projection 63 of the base plate 55. It may also be noted that pivotal movementof the plates 8?, and 89 on the projection 93 is prevented by the square shape of said projection, which fits between the flanges 63.

As the cylindrical socket 53 moves on the surface of the base plate 55, the flat bar portion 32 will slide in the aperture 83 of the vertical member 34 and will'also cause the member 8% to be moved from side to side in accordance with the changing position of the socket 53. This movement, of course, will cause the'partial gear 85 to pivot on the pin 8'! which, in turn, will cause the partial gear to pivot in the opposite direction. In this manner, the straight-edge as is made to move in a direction which is substantially the opposite of the action of the straightedge 12. However," because of the fact that they pivotal center to which the straight-edge 80 is directly attached is not the same as for the straight-edge 12, its motion will not be the exact opposite of the motion of the straight-edge 12. Nevertheless, it is believed that the motion of this straight-edge 35 will be sufficiently accurate to perform many useful purposes for the artist or draftsman.

In some instances, as in some types of architectural drawing, it may be desirable that the vertical lines of the drawing be perfectly parallel rather than converging toward a vanishing point. For such drawings, a modified form of the device is illustrated in Figs. 20 and 21. In this form of the device, the straight- edge 12 or 80 is replaced by a straight-edge 95 which is rigidly affixed to the rectangular projection 63 of the base plate 55 by four small machine screws 85. The four small machine screws 96 penetrate four vertical holes in the opposing flanges B8 of the slot 54 and are in rigid, threaded engagement with four ertical threaded holes in a rectangular block 91, which is integral with the straight-edge proper. Said rectangular block is designed to fit the space of the slot 64 below the flanges 68. See Fig. 21. The straight-edge proper of this modification, of course, is identical to that corresponding part of the other straight-edges previously described.

What I claim is:

1. In an instrument of the class described, a member having a swive led bearing applicable to a relatively fixed support, a slidable base plate having a swivelled bearing, a floating connector on the base plate, a long upwardly and forwardly inclined rod hinged at its lower end to the first noted bearing, a primary arm pivoted at one of its ends to the outer end of the rod and hinged at its other end to the connector, a secondary arm substantially parallel to the primary arm and pivoted at its upper end to the rod inwardly of the primary arm and hinged at its lower end to the bearing on the base plate, an oblique pivoted link connecting the primary arm to the bearing on the base plate, three straight-edges attached at one of their ends to the connector to turn about a vertical axis, two of the straight-edges projecting in opposite directions from the plane of the rod and the primary and secondary arms, the third arm being in said plane, a fulcrum guide on the base plate for each straight-edge, and two pairs of parallel arms in the form of a horizontally disposed toggle joint, the arms of one of pairs being pivoted at their outer ends to the member and the arms of the other pair being pivoted at their outer ends to the base plate, the arms of said pairs being pivotally connected at their inner ends.

2. The structure defined in claim 1 in which the fulcrum guides for the straight-edges are adjustable toward or from the connector.

3. In an instrument of the class described, an anchor member applicable to a fixed support, a base plate arranged to slide in a horizontal plane, a connector mounted on the base plate to float parallel to the plane thereof, said member, the base plate and the connector each having a vertically disposed pivot member, a long vertically disposed rod attached at its lower end to the pivot member on the anchor member to swing in a vertical plane about a horizontal axis, a parallelogram consisting of a long primary arm, a long secondary arm, a lower short member and a short upper member, said arms being vertically disposed, said parallelogram being attached at its upper end to the upper end of the rod for swinging movement in the plane thereof, said secondary arm being attached at its lower end to the pivot member on the base p ate to turn about a horizontal axis, means connecting the primary arm at its lower end to the pivot member 0n the connector to turn about a horizontal axis, the operative lengths of the rod and the primary arm and the means connecting the same to the pivot member on the connector being equal, a straight-edge attached at one end to the pivot member on the connector to turn about the axis thereof, a fulcrum guide on the base plate for the straight-edge, and parallel motion means connecting the base plate to the anchor member.

4. The structure defined in claim 3 in which the fulcrum guide is adjustable from the center of the base plate and further including means for securing the fulcrum guide where adjusted on the base plate.

5. The structure defined in claim 3, further including a second straight-edge attached at one end to the swivel on the connector to turn about the axis thereof independently of the other straight-edge, a fulcrum guide on the base plate for the last noted straight-edge, adjustable from the center of the base plate, and means for securing the last noted fulcrum guide where adjusted on the base plate.

6. The structure defined in claim 3 in which the fulcrum guide is adjustable from the center of the base plate and further including means for securing the fulcrum guide where adjusted on the base plate, and a sca e on the base plate for positioning the fulcrum guide different distances from the center of the base plate.

7. The structure defined in claim 3 in which the parallel motion means is two pairs of parallel legs in the form of a horizontally disposed toggle joint, the legs of one of said pairs being pivotally attached at their outer ends to the anchor memher, the other pair of legs being pivoted at their outer ends to the base plate.

8. The structure defined in claim 3 in which the base plate has a slot extending radially from the center of the base plate, said straight-edge having a longitudinal slot, and in which structure the fulcrum guide is a head mounted in the slot in the base plate and having sliding interlocking engagement with the base plate, said head having an upstanding screw stud extending through the slots in the base plate and the straight-edge, and a nut-acting member applied to the screw stud and having a head overlying the straight-edge at the slot therein and operable to clamp the head and the straight-edge relative to the base plate.

9. The structure defined in claim 3' in which the upper member of the parallelogram is an extension of the rod and in which the means connecting the primary member to the pivot member of the connector is an extension of said primary member.

10. The structure defined in claim further including a handle on the base plate for manipulating the same.

EDWARD E. MoCULLOUGI-I.

REFERENCES GITED UNITED STATES PATENTS Name Date Upton Oct, 11, 1949 Number

Images