US2713723A

US2713723A - Drafting machine

Info

Publication number: US2713723A
Application number: US277492A
Authority: US
Inventors: Ralph A Anderson
Original assignee: Individual
Current assignee: Individual
Priority date: 1952-03-19
Filing date: 1952-03-19
Publication date: 1955-07-26
Anticipated expiration: 1972-07-26

Description

3 Sheets-Sheet 1 Filed March 19, 1952 INVENTOR. RMA'MMM July 26, 1955 R. A.- ANDERSON 2,713,723

DRAFTING MACHINE Filed March 19, 1952 3 Sheets-Sheet 2 I N VEN TOR.

July 26, 1955 ANDERSON 2,713,723

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United States The invention relates to devices for making mechanical drawings. More particularly, it relates to a drawing board having a rotatable drawing surface and a parallel ruling straightedge adjustable thereover.

In the art of making detailed mechanical drawings, considerable skill is required of the draftsman in order to obtain good and accurate drawings within a minimum amount of time and with a minimum of effort.

Many devices have been provided heretofore for the purpose of reducing the effort and increasing the speed and accuracy of the mechanical draftsman. In the simplest form of drafting or drawing board, the draftsman uses a vertical edge of his board to guide a T-square for drawing parallel horizontal lines. Lines in any other direction are drawn along an edge of a triangle which is in turn held against the T-square. A further improvement is the well-known sliding parallel ruling straightedge drawing board which relieves the draftsrnan from holding the T-square against the boards edge. Another well-known and popular improvement is the so-called universal drafting machine which may be attached to a drawing board and has two calibrated straightedges at right-angles which may be adjusted to draw lines in any desired direction or angle, at any position on the board, thus combining the functions of parallel ruling straightedge, triangles, and protractor. However, the design and construction of the universal drafting machine normally provides straightedges which are only about one-half the width of the drawing board so that it is necessary to draw longer lines in two separate steps with the resultant possibility of misalignment.

In. the present invention, by rotating the drawmg beneath an adjustable sliding parallel ruling straightedge, the desirable advantages and functions of the regular parallel ruling straightedge drawing board and of the universal drafting machine, are incorporated in a single device. The drawing is mounted on a circular rotatable drawing board the periphery of which is graduated in degrees for adjustably rotating the drawing to various angular positions, and a horizontal sliding parallel ruling straightedge is vertically adjustable across the draw ngon the circular drawing board surface. Thus, by ad usting the vertical position of the straightedge and the angular position of the rotary drawing board, straight, parallel lines may be drawn in any desired direction or at any angle, at any position on the board. I

In other words, in the present invention having a sliding parallel ruling horizontal straightedge and a rotary drawing board, only horizontal lines need be drawn, straight lines in any other direction being drawn along the same horizontal straightedge after rotating the board the desired number of degrees. This construction eliminates the need for any triangles or protractor since, in effect, the entire rotary drawing board is itself a most precise protractor.

Other features of the present invention relate to the sliding parallel ruling straightedge and in particular to atent ice improved means for adjusting and positioning it across the drawing board.

Thus, the principal object of this invention is to provide an improved drafting machine which will enable a draftsman to make mechanical drawings more easily, rapidly, and accurately.

Another object of the invention is to provide a drawing board which may be rotated to any desired angular position beneath a parallel ruling straightedge.

Another object of the invention is to provide a circular rotary drawing board peripherally graduated in degrees so that a drawing mounted on the board may be adjusted rotatably the desired number of degrees.

Another object of the invention is to provide a sliding parallel ruling straightedge which may be adjusted across the rotary drawing board.

Another object of the invention is to provide a horizontal sliding parallel ruling straightedge which may be adjusted vertically across the rotary drawing board.

Another object of the invention is to provide a manual control knob connected to a rotary mechanical drive means for effecting an easily controlled Vernier adjustment of the parallel ruling straightedge.

Another object of the invention is to provide a simple and accurate means, actuated by aforesaid rotary drive means, for easily adjusting the parallel ruling straightedge through multiple small uniform increments of movement (for cross-hatching, shading, lettering, etc.).

A further object of the invention is to provide a nullbalance Wheatstone bridge electric resistance circuit for accurately locating the sliding parallel ruling straightedge and for easily making enlarged or reduced scale drawings.

A feature of the invention is the camber-ed, flexible, sliding parallel ruling straightedge which avoids the usual wiping, smearing action of conventional straightedges.

Another feature of the invention is the submounted, calibrated, horizontal straightedge scale which may be shifted left or right and locked in any adjusted position to align a major scale subdivision with any vertical line on the drawing.

Another feature of the invention is the vertical calihrated scale, vertically shiftable for aligning a major scale subdivision with the index line of the horizontal straightedge drive mechanism.

Further objects and advantages of the invention will be apparent to those skilled in the art and the novel features are described in the following specification and shown in the accompanying drawings which discloses a preferred embodiment of the invention.

In the drawings:

Fig. l is a top plan view of the improved drafting machine having a circular rotary drawing board and a sliding parallel ruling straightedge connected with a manual drive mechanism for moving the straightedge across the rotary drawing board.

Fig. 2 is a cross-section of the drafting machine generally along lines 11-11 in Fig. 1.

Fig. 3 is a top plan view in detail of the manual drive mechanism for moving the straightedge as well as illustrating a ratchet gear (in part only) and clicker for enabling audible incremental adjustment of the straightedge, sections being broken away for clarity. This view also shows the slide wire, and the slide wire contactor mounted for movement with the parallel ruling straight edge.

Fig. 4 is a side elevation view of the straightedge drive mechanism looking in the direction of arrows IV-IV in Fig. 3.

Fig. 5 is a side elevation view of the straightedge drive mechanism of Fig. 3, with the slide wire and straightedge removed, looking in the direction of arrows V-V in Fig. 3. The clicker control cam 31 is shown removed from the pin 32 in this View for better clarity.

Fig. 6 is a fragmentary section along lines VIVI of Fig. 1 looking in the direction of the arrows to show the support arrangement of the straightedge slide bars, the resistance slide wire, and the straightedge parallel motion pulleys and cables.

Fig. 7 is a fragmentary top plan view of the upper left-hand part of the machine with the slide bar 7 removed and the upper section of the slide bar support bracket 9 cut-off to show the construction and mounting of the parallel motion pulleys 11 through the panel 3.

Fig. 8 is a plan view from the rear of the machine showing the center bearing for the circular drawing board and the pulley and cable arrangement for maintaining parallel motion of the straightedge.

Fig. 9 is a diagrammatic view of a galvanometer and decade resistance box connected in a Wheatstone bridge resistance circuit with the straightedge slide bar 7 and the slide wire 43.

Fig. 10 is a wiring diagram of the machine, with a simplified illustration of the decade resistance part of the circuit.

Referring now to the drawings in detail, Figs. 1 and 2 show a circular drawing board I mounted rotatably on the bearing assembly 2 which is in turn securely mounted in a hole passing through the panel 3.

In order to provide a flush and level working surface, the

strips

4 and 5 on each side, and cut-out panel 6 surrounding the circular drawing board 1, are secured as shown to the panel 3. Sufficient clearance is allowed between the circular drawing board 1 and cut-out panel 6 so that the board 1 may be rotated without binding. Although Fig. 2 shows a relatively large separation between the bottom of the rotary drawing board 1 and panel 3, in practice, they are in sufficient frictional contact to prevent any unintentional displacement of board 1, but not sufficient to prevent deliberate manual rotation of the rotary drawing board. The rotary drawing board is peripherally graduated in degrees and alternate reference points on member 6 at ninety degree intervals enable adjusting the board through any desired number of degrees. Thus, any drawing which is mounted on the rotary drawing board 1 may be accurately adjusted to any desired angle.

In the form of the invention as herein described, support means are attached on the back of the machine for supporting it in a somewhat raised position and sloped slightly downwards towards the draftsman. However, such support means are of generally conventional design, forming no part of the invention, but are required only to provide clearance beneath and free operation of the bearing assembly 2 and the straightedge parallel motion pulleys and cables mounted through the back of the machine.

The parallel

straightedge slide bars

7 and 8 are supported by brackets 9 at their ends, in the open spaces between

strips

4 and 5 and cut-out panel 6, so that the

bars

7 and 8 are parallel and flush with the working surface of the drafting machine. The parallel straightedge 10 is supported at its ends by

slide bars

7 and 8 and is movable along these bars across the rotary drawing board 1.

The means for effecting parallel motion of the straightedge 10 is conventional in principle, comprising the wellknown system of pulleys and cables shown in Fig. 8. In Fig. 8, the pulleys are arranged to guide the cables away from the bearing assembly 2 which protrudes through the back of panel 3. It will be apparent that the cable ends w, x, and y, z, are attached to opposite ends of straightedge It) for eflecting the parallel motion thereof. In Fig. 8, if the connected straightedge 1G is moved up, it pulls on cable and x which in turn pulls on cable end y to move the oppositely connected end of straightedge 10 up correspondingly, in the well-known manner. Figs. 6 and 7 show in detail the arrangement and construction of the parallel motion pulleys shown in Fig. 8. The vertical pulleys 11 are mounted in the brackets 9 and pass through openings through panel 3 for carrying the parallel motion cables from the back to the front of the machine. Thus, in Fig. 6, pulley 11 carries the parallel motion cable to a position just beneath the slide bar 7, and the cable extends along and beneath bar 7 to be connected to the end of the straightedge 16. The other cable ends are similarly carried by other pulleys 11 to extend along just beneath the slide bars 7 and 8 to connections at the ends of the straightedge.

The devices supporting the parallel straightcdge 10 slidably along

bars

7 and 8 have not heretofore been de' scribed. In Fig. 2, the bracket 12. slidable along bar 8 is attached to the right-hand end of the straightedge 10 and supports the straightedge slightly above the drawing board surface. Cable ends w and x are attached to the bottom of bracket 12 for providing the parallel motion described. The other end of straightedge i0 is attached to a drive mechanism for moving the straightedge across the rotary drawing board 1, which is a principal feature of the invention.

The drive mechanism is shown in detail in Figs. 3, 4, and 5. A tube 13 is slidable on the bar 7, and a portion of tube 13 is cut-out to expose a large part of slide bar 7 at a point intermediate the ends of tube 13.

Bars

14 and 15 are counterbored to fit tightly on the ends of tube 13 without interfering with the sliding of tube 13 on slide bar 7. A flat member 16 is secured to the

bars

14 and 15 to hold and align them on tube 13. The lefthand end of straightedge 10 is connected to member 16 and thereby supported slightly above the drawing board surface. According to the construction thus far described, the straightedge 10 is supported just above the drawing board surface by the bracket 12 at one end and the tube 13 and

members

14, 15, and 16 at the other end so that 5 the straightedge 10 may be manually adjusted across the rotary drawing board 1 slidably along the slide bars 7 and 8, and the parallel motion pulleys and cables attached to the opposite ends of straightedge 10 enable a parallel motion thereof across the drawing board. A special friction drive pulley wheel 17 is mounted rotatably between the arms of a generally U-shaped bracket 18 and is registered in frictional driving connection with the slide bar 7 through the cut-out part of tube 13. The bottom or closed end of the U-shaped bracket 18 is connected to one end of a strip spring 19 and the other end of spring 19 is secured to the end of bar 14. Spring 19 is biased so that it normally tends to hold pulley 17 slightly away from driving connection with slide bar 7. However, in Fig. 3, by manually pressing the extended free arms of bracket 18 in towards slide bar 7, the pulley 17 may be forced tightly against bar 7, while the bracket 18 rocks on the pulley wheel shaft as an aXis against the spring tension of 19. In order to hold pulley 17 in this engaged frictional driving connection, a catch 20 slidably mounted through holes in bar 15 is provided.

By pressing catch 20 in against the tension of a spring 21, a transverse pin 22, secured between the arms of bracket 18, may become engaged by the cut-out part 23 of catch 20. Upon now releasing the free end of pulley bracket 18, the catch 20 (and pin 22) holds pulley 17 in tight frictional contact with slide bar 7 against the tension of spring 19. A knurled knob 24 on the upper free arm of bracket 18, and a similar knurled knob 25 secured to the member 16 provide a convenient gripping means for manually pressing the bracket 18 towards the slide bar 7. By placing the thumb and first finger of the left hand on opposite sides of

knobs

24 and 25 and squeezing, the bracket 18 swings pulley 17 in against bar 7 and by now pushing forward on catch 20, against spring 21, with the back of the middle finger, the catch 20 may be easily engaged with pin 22 to hold pulley 17 in driving connection with slide bar 7. Disengagement of pulley 17 from driving connection is accomplished simply by squeezing

knobs

24 and 25 together as before. Spring 21 immediately moves the catch 20 out of engagement and the head portion 26 limits the outward nve-- ment of the catch, while spring .19 moves pulley 17 slightly away from the slide bar 7.

The friction drive pulley 17 is shown in this embodiment of the invention as one drive mechanism for moving the straightedge 10 along

slide bars

7 and 8 and is described in detail in my Patent No. 2,692,748 on an adjustable support mechanism, so that for a detailed explanation not especially concerning the present invention, reference may be made to that patent. However, for the purposes of this invention, the pulley 17 is a grooved pulley wherein the pulley groove walls diverge at about a twenty degree angle. Furthermore, the slide bar 7 must be of such uniform cross-section that the divergent pulley groove walls and the bar 7 are registered in tangential contact near the periphery of pulley wheel 17 (see Fig. 2). Such construction. results in a wedging or binding of pulley 17 on bar 7 at the tangential points of rolling contact to provide an efiicient friction drive of the pulley 17 along slide bar 7. A manual control knob 27 secured to the drive shaft of pulley 17 (Figs. 1, 2, 4, and 5),. provides a means for rotating pulley 17 for effecting a Vernier adjustment of straightedge 10' along the slide bars 7 and 8 across the drawing board 1.

Although I have shown and described a friction drive mechanism for adjusting the straightedge, it will be apparent that other equally effective drive means can be provided to obtain the same result without changing the scope or principle of the invention. In one modification rack teeth could be formed along one side of the slide bar 7 and a pinion gear, meshed with the rack, substituted for the friction drive pulley 17, so that ro"- tation of the pinion gear by manual control knob 27' would eifect a gear drive of this drive mechanism along the slide-rack bar (7) for moving the straightedge across 3 the drawing board. Thus, in the claims, the expression manual drive means will include any suitable drive means of the type operated by manual rotation. But such modified drive means are not shown herein since they may be varied indefinitely, and except as otherwise set forth in detail, appear at this time to be primarily matters of design.

A novel arrangement is provided (Figs. 3, 4, and 5) whereby the Vernier adjustment of straightedge 10 by the drive mechanism can be easily made in uniform increments. In Fig. 3, the manual control knob 27 is removed to show a ratchet gear 28 (in part only) secured to the drive shaft of pulley 17. A spring metal clicker arm 29 is secured to the lug 30 and is lightly tensioned against the teeth of ratchet gear 28. When the manual control knob 27 is rotated, a distinct click is heard as each ratchet gear tooth moves past the end of clicker arm 29. Now it will be obvious that the number of teeth around gear 23, and therefore the number of clicks, provide an accurate and simple means for subdividing the movement of the straightedge 10 along

slide bars

7 and 8 by the friction drive pulley 17. In one example of this arrangement, the dimensions of the pulley 1'7 and the cooperative bar 7 are such that the straightedge I0 is moved exactly 3.00 inches along

slide bars

7 and 8 for each whole revolution of pulley 17. Thus, with a ratchet gear 28 having 48 teeth, each tooth, and therefore each click, would indicate an incremental advance or adjustment of straightedge 10 of A inch. In. the illustrated example of this incremental spacing device, the ratchet gear teeth and clicker arm are arranged for operation during the downwards adjustment of the straightedge. This is considered to be the most desirable direction of adjustment since successive lines drawn along the straightedge will thus not be covered by the straightedge as it is adjusted down across the drawing. The

value of such a simple and accurate spacing mechanism for making mechanical drawings is largely obvious. First, the incremental advance of the straightedge can be Controlled entirely by ear, avoiding the necessity for looking Lil 6 away from the drawing, and at the vertical calibrated scale. Second, the device provides a distinct and clearcut indication for the precise, uniform incremental ad- Vance of the straightedge and avoids any parallax error which might ordinarily result from the usual adjustment of thestraightedge along the vertical calibrated scale.

The device is primarily of value for laying out multiple small increments, usually over a short distance, such as for cross-hatching, shading, lettering, etc. It can, of course, also be used for laying out parallel lines short measured distances apart since, e. g., one clickinch, two clicks- /s inch, four clicks-34; inch, etc. A cam 31, pivotal on the pin 32 (Figs. 3 and 5), may be retatcd by the control knob 33 for camming the clicker arm 29 out of engagement with the teeth of ratchet gear 28 This disengagement permits the drive pulley 17 to be oppositely rotated for adjusting the straightedge 10' upwards by the control. knob 27 without clicker operation.

The preceding paragraphs have described in detail a drafting machine for making mechanical drawings where in a horizontal straightedge is adjustable across a rotatable drawing board by a manually operable drive mechanism.

A- feature of the invention is the submounted calibrated straightedge scale 34, Figs. 1 and 3, supported by the flush-headed member 35 and the knurled locking nut and screw 36 slidable in the slots 37 and 38 at opposite ends of the straightedge 10. By loosening the lock nut 36 and using it as a handle, the connected cali-- brated scale 34 may be easily shifted left' or right for aligning any major scale 34 subdivision with any vertical line onv a drawing mounted on the drawing board. Thus, for example, if a point or line is to be drawn say 3 from a given vertical line, the scale 34 is shifted to align the nearest full inch division, which might be say 9",- and the distance 3 is easily measured off from this point. In Fig. l a similar arrangement is made of the vertical calibrated scale 39 shiftable vertically and supported along the left-hand side of the cut-out panel 6 by the flush-headed member 40 and the locking nut and screw 41. An index pointer 42 on the member 16 (.Figs. 1 and 3) is used for locating the straightedge in the vertical direction by reference to scale 39.

Another feature of the invention relates to the support arrangement of the horizontal straightedge 10 and its submounted calibrated scale 34. In Fig. 2, because of the reduced scale of the drawing, only the straightedge 19 is shown supported at its ends by the bracket 12 and by the described drive mechanism. The straightedge 10 (and the scale 34) is flexible and is arched or' cambered across the drawing board so that the straightedge ltl (and scale 34) can not normally touch or wipe across the drawing surface while adjusting the horizontal straightedge. However, by merely pressing lightly down with the left hand at the desired drawing position, the line may be easily drawn along the straightedge in the usual manner. This arrangement is particularly advantageous when ink lines are being drawn, since the straightedge springs up and away from the inked (wet) line and avoids smearing it when the left hand is removed from holding the straightedge down. It should be understood that the term straightedge as used in this specification may in certain cases refer to the member 10 when that member alone is used for ruling parallel lines, and in other cases the term has reference to the described combination of straightedge 10 and scale 34 where parallel lines are, of course, ruled along the straightedge of scale 34.

In many cases, it is desired to make an enlarged or a reduced scale drawing. For such drawings the invention includes a null-balance Wheatstone bridge electric resistance circuit for precisely locating the horizontal straightedge in predetermined vertical positions. Referring now to Figs. 1, 3, 6, 7, and 9, a slide wire 43, comprising a winding of resistance wire insulated from but wound on the rod 44, is mounted parallel and adjacent to the slide bar 7 (see Figs. 1, 6, and 7). In Figs. 3 and 5, an electric contact member 45 is urged against the slide wire 43 by the conductive spring 46 which is in turn mounted on the drive mechanism bar 14, to provide a movable electric connection between the slide wire 43 and the metal slide bar 7. In the diagram of Fig. 9 and electric circuit of Fig. 10, electric connections are made with the lower ends of slide bar 7 and the resistance slide wire 43 and the amount of resistance put into the bridge circuit depends upon the position of the contact 45 along slide wire 43 which in turn depends upon the location of the straightedge 10. When the straightedge is adjusted to its lowermost or zero position along vertical scale 39 by the index mark 42, the contact 45 is at its zero position along the slide wire 43' corresponding to zero ohms resistance. This variable slide wire resistance is used as one arm of the bridge circuit while a four digit decade resistance circuit is used as the other arm. In the electric circuit of Fig. 10, the decade resistances are shown in a simplified illustration (to save space) as four variable resistances in series connection. The figures X10 by each variable resistance signify the adjustment of each resistance in ten steps, while the maximum value of each resistance and the number of steps to reach the maximum value are also indicated in the diagram. Thus, in Fig. 10, the values 0.9 ohm/9-X10, indicates that this resistance is adjustable in uniform increments (or steps) from 0.1 ohm to 0.9 ohm, while the tenth step (X10) is zero ohms. The diagram, therefore, refers to the conventional 10 position decade switching in of separate resistance values, but it will be apparent that ordinary variable rheostats adjustable in ten steps could be substituted with some sacrifice of accuracy. A 1000 ohm potentiometer is used for a coarse adjustment of the ratio of the two arms in conjunction with a 50 ohm potentiometer in series connection as a fine ratio adjustment.

A 250 ohm rheostat is connected between the galvanometer G and each of the measuring bridge arms for shifting a null-balance position upscale or downscale as desired. Several of the smaller flash-light type dry cells provide ample current for operation of the circuit. Four ten position switch dials are mounted on the control box (Fig. 9) for switching in the various decade resistances. On all dials, position zero indicates a zero resistance. The resistance entered by each switch is entered in series with the resistances entered by each of the other switches so that the values are totaled. The total decade resistance value is assumed to be 999.9 ohms in the present embodiment. The left-hand dial provides 900 ohms in 9 one hundred ohm steps, the next dial 90 ohms in 9 ten ohm steps, the next 9 ohms in 9 one ohm steps, and the last dial 0.9 ohm in 9 0.1 ohm steps. Thus, with the dials set to read, e. g., 7698, the actual resistance entered by this arm of the bridge circuit is 769.8 ohms.

To set the circuit for making a drawing, for example, one-half full scale, the straightedge is manually adjusted upsay 10" from the zero position of the slide wire 43 and the calibrated scale 39. Then the decade resistance dials are set to read 2000 (i. e., 20.00"), and with current on, the coarse and then the fine ratio potentiometers are adjusted to obtain the null-balance (no current) of galvanometer G. (It is well to point out that during all such ratio balancing adjustments the zero shift rheostats must be at zero resistance.) Then .the dials indicatea value of 20.00" and the straightedge a value of 10. If the dials are now set to read 3000 (30.00), the galvanometer G will swing off balance but by manually adjusting the straightedge in the direction for effecting a null-balance, it will be found that at the null-balance point the straightcdge will be at the 15" position on calibrated scale 39.

Other settings of the decade resistance dials will in an F obvious manner enable precisely locating the straightedge proportionately to the values set. The ratio of the actual straightedge adjustment to the dial values set may be widely varied to provide a broad range of reduction or enlargement of the scale of a drawing.

The zero shift rheostats provide means for adding resistance to one or both arms of the bridge and may be considered as an electrical analog of the system described earlier for shifting the calibrated scales 34 and 39 to align them with a major scale subdivision. Thus, assume that a line has been drawn along the straightedge at a position with the dials set at say 1176 (at scale 1/1 equals 11.76"). Now assume that a second, lower, parallel line must be drawn a certain indeterminate distance from line one and a third line drawn say 1.37" above and parallel to the second line. Instead of adjusting the dials to the balance position of the second line and then to the third line, after drawing the first line, the dials are set at 1100 (11.00"). The straightedge is then adjusted the indeterminate distance and the second line drawn. At this position the proper zero shift rheostat is adjusted to effect null-balance of the galvanometer G. Then the control dials are set at 1237 (11.00" plus 1.37" equals 12.37"), and the straightedge adjusted until the galvanometer G null-balance indicates the location of the straightedge at the required position, and the third line is drawn at this position. By simply adjusting the zero shift resistance back to zero, the original balance conditions again prevail.

Briefly, the invention provides an electric resistance Wheatstone bridge circuit and a slide wire mounted on the drafting machine and a null-balance indicator for accurately locating the straightedge vertically according to the preset potentiometer ratio arms and the preset decade resistance values entered in the circuit.

From the foregoing it may be gathered that a peripherally graduated circular drawing board may be adjusted rotatably to place a drawing mounted thereon in any desired angular position. A horizontal sliding parallel ruling straightedge may be manually adjusted across the rotary drawing board in the conventional manner or it may be manually adjusted with vernier precision through the use of a rotary drive mechanism connected therewith. An intermittent spacing mechanism associated with and actuated by the rotary drive mechanism may be used to enable adjusting the straightedge through multiple small increments of movement across the drawing board by means of intermittent signals actuated by said spacing mechanism. The sliding parallel ruling straightedge may be manually adjusted and electrically located along a resistance slide wire by means of a null-balance Wheatstone bridge resistance circuit according to the preset ratio arms of the bridge and the preset decade resistance values entered for making enlarged or reduced scale drawings.

The preceding specification has illustrated one form of the invention and described certain modifications thereof. It should be understood, however, that many other designs for this purpose can easily be made as required for adaptation to specific uses, and that mere change in the manner of supporting and adjustably driving the operating parts hereinbefore described will by itself in no way affect the operation and/or principles of the invention. It should also be understood that the mentioning of certain dimensions, electric resistance values, etc., are illustrative only and are in no way intended as limiting.

What is claimed is:

1. In a drafting machine, a revoluble drawing board, a straightedge parallelly adjustable thereover, an electric resistance member, means for adjusting said straightedge, means for varying the effective resistance of said member according to the adjustment of said straightedge, an electric circuit having two arms, means for connecting the effective resistance of said member in one of said arms, a plurality of manually operable denominational data entering devices, a number of graduated resistances adapted to be connected selectively in the other arm of said circuit,

means under control of said entering devices for selectively connecting certain of said resistances in said circuit according to the multidenominational data entered, and means in said circuit controlled by the current flowing in said circuit for indicating a condition of balance or unbalance of said circuit according to the adjusted position of said straightedge.

2. In a drafting machine, in combination, a revoluble drawing board, a straightedge parallelly adjustable thereover, a plurality of manually operable denominational data entering devices, a number of graduated resistances, means under control of said devices for selectively connecting certain of said resistances in series for providing a desired total resistance proportional to the entered datum value, a normally balanced electric circuit having two arms, means for connecting said total resistance in one of said arms, an elongated resistance member mounted at one side of said drawing board and substantially parallel to the direction of adjustment of said straightedge, means on said straightedge and engaging said member in sliding electrical contact therealong for varying the efiective resistance of said member according to the adjusted position of said straightedge, means for connecting said efiective resistance of said member in the other of said arms, a galvanometer in said circuit for indicating a condition of circuit balance or unbalance, and manually operable drive means connected with said straightedge for efiecting an adjustment thereof for balancing said circuit.

3. In a drafting machine, in combination, a base, a revoluble peripherally graduated drawing board mounted on said base, parallel guide bars at each side of said board on said base, a straightedge arranged for parallel sliding movement along said bars across said board, a manually operable rotary drive mechanism at one end of said straightedge and operatively engaging one of said guide bars for adjusting said straightedge across said board, said one guide bar being an electric conductor of substantially low resistance along its length, an elongated resistance member mounted on said base in adjacent and parallel relationship to said one guide bar, a contact element mounted on said one end of said straightedgeand movable therewith, said element being arranged for sliding electrical contact along said one guide bar and said resistance member for varying the elfective resistance of said member according to the adjustment of said straightedge, a Wheatstone bridge electric resistance circuit having two arms, one of said arms having said effective resistance connected therein, a number of sets of graduated resistances adapted to be connected selectively in the other-arm of said circuit, a corresponding number of manually operable denominational switches for selectively connecting certain of said resistances in said circuit, and a galvanometer in said circuit for indicating a condition of circuit balance or unbalance according to the adjusted position of said straightedge,

4. In a drafting machine of the type described, a bridge circuit for said machine comprising in combination, an adjustable tap potentiometer of relatively large resistance in said bridge, an adjustable tap potentiometer of relatively small resistance, the resistances of said potentiometers being series connected across a current source, a decade resistance circuit having a plurality of graduated denominational switches for entering certain resistance values in uniform increments in one arm of said bridge, means connecting the tap of said small potentiometer to one side of said decade resistance circuit, an adjustable tap linear slide wire connected to the current-corinected end of said large resistance potentiometer in the other arm of said bridge, a straightedge mechanically connected to the tap of said slide wire for rebalancing the circuit, a pair of series arranged variable resistors connected between the adjustable tap of said slide wire and the other side of said decade resistance circuit, and a galvanometer connected between the adjustable tap of said large resistance potentiometer and the juncture of said series arranged variable resistors for indicating the null balance of said circuit.

5. In a drafting machine, an electrical bridge, a decade resistance circuit in one arm of said bridge, for entering separately the denominational digits of a plural ordered number, an adjustable tap linear slide wire in the other arm of said bridge, a straightedge connected to the tap of the slide wire for rebalancing the circuit, means connecting said tap to one side of said decade resistance circuit, an adjustable tap potentiometer of relatively large resistance connected to said slide wire, another adjustable tap potentiometer of relatively small resistance connected in series with said large resistance potentiometer, means connecting the adjustable tap of said small resistance potentiometer to the other side of said decade resistance circuit, and means connected between the adjustable tap of said large resistance potentiometer and the means connecting said slide wire tap and said one side of said decade resistance circuit for indicating a diiference of potential therebetween.

6. In a drafting machine, an electrical bridge, a plurality of input devices connected in one arm of said bridge for entering separately the digits of a plural ordered number, said devices comprising a series of sets of graduated resistances, one set for each order, with manually operable graduated multi-tap switches, one switch for each order, for selectively connecting a certain one of each of said resistances in each of said orders in series with a certain one of said resistances in each of the other orders, the total resistance value selected varying according to the magnitude of the entered number, an adjustable tap linear slide Wire in the other arm of said bridge, an adjustable straightedge connected to the tap of the slide wire for rebalancing the circuit, means connecting said slide wire tap serially with said input devices, an adjustable tap potentiometer of relatively large resistance connected to one end of said slide wire, another adjustable tap potentiometer of relatively small resistance connected in series with said large resistance potentiometer, means connecting the adjustable tap of said small resistance potentiometer to the other side of said input devices, and a galvanometer connected between the adjustable tap of said large resistance potentiometer and the means connecting the slide wire with said input devices for indicating a difference of potential therebetween.

References Cited in the file of this patent UNITED STATES PATENTS Re. 11,498 Fiske June 4, 1895 550,413 Laughlin et al Nov. 26, 1895 663,065 Cox Dec. 4, 1900 836,919 Burbank et al Nov. 27, 1906 862,636 Hall Aug. 6, 1907 1,626,560 Schneider Apr. 26, 1927 1,843,824 La France Feb. 2, 1932 2,211,566 Henderson Aug. 13, 1940 2,614,327 Russell Oct. 21, 1952