US2394344A - Drafting machine - Google Patents

Description

Feb. 5, 1946. 4 v. B. WALLACE 2,394,344 l DRAFTING MACHINE Filed Jan. 4, 1943 2 Sheets-Sheet l NWI/1.5m I MIIAIII :zu

member is moved in either direction from its normal or neutral position.

It is additionally an object of my invention to provide a drafting machine employing a'parallel motion mechanism of the endless band and pulley type in which guard members are provided for completely enclosing the endless bands and in which the guard members themselves serve as struts to hold the pulleys in the desired spaced a drafting machine in which the parallel motion mechanism is mounted for pivotal movement about a horizontal axis relative to the anchor or fixed portion of the machine and which includes alsov a. manually operable mechanism for selectively locking the drafting machine against pivotal movement about the horizontal pivot or releasing the machine to permit such pivotal movement.

In connection with a drafting machine` of the character referred to in the preceding paragraphs, it is another object of my invention to provide a novel method of assembling the endless bands within their enclosing guards vby means of permanent welded joints between the meeting ends of the bands which consists in so adjusting the effective length of the adjustable struts as to provide considerable slack in the endless band, bringing the adjacent ends of the endless band through an aperture in the guard for permanent welding of the ends to each other, restoring the length of the struts to place the band in its normal stressed condition, and employing a, name plate or other functional part of the apparatus as a cover for closing the aperture.

f Otherobjects and advantages of my invention will be apparent from a study of the following specifications, read in connection with the accompanying drawings, wherein:

Fig. 1 is a plan view illustrating the general form and appearance of a drafting machine constructed in accordance with my invention;

Fig` 2 is a longitudinal sectional view taken substantially along the line II 1I of Fig. 1 to illustrate the details of construction of the anchor portion of the drafting machine of my invention;

Fig. 3 is an elevational view with parts broken away and parts shown in section taken substantially along the line III-III of Fig. 2 of the anchor portion of the drafting machine and illustrating the details of construction of a manually releasable locking member controlling the pivota1 movement of the drating machine about a horizontally disposed axis;

Fig. 4V is an elevational view of the elbow portion of the drafting Imachine of my invention withfparts shown in section as indicated by the linerIV IV in Fig. 1;

Fig.v 5 is a longuitudinal vertical sectional vview of the protractor head assembly portion of the drafting machine cf my invention, Fig. 5 being taken substantially along the line V-V of Fig. 1;

Fig. 6 is a horizontal sectional view taken substantially along the line VI-VI of Fig. 5 and illustrating by means of dotted and solid lines two positions of the protractor notch plate mech- 'anism; and

Fig. '7 is a view similar to Fig. 6 but illustrating by means of dotted and solid lines two additional positions of the protractor notch plate mechamsm.

Referring'to the drawings, I have illustrated in Fig. 1 one form of drafting machine constructed in accordance with the preferred embodiment of my invention as being secured to a drawing board or table indicated generally by the reference character l. The drafting machine includes an anchor member indicated generally at 2 upon which one arm 3 including struts 4 and 5 is pivoted. The arm 3 is pivotally connected as by means of an elbow B to a second arm l which includes struts 8 and 9. The other end of the arm 'l' carries a protractor headassembly Il to which is affixed scales l2 and i3 or other conventional drafting tools.

-The anchor 2 and the protractor head assembly I l include, respectively, pulley guards or enclosures ld and i5, and the elbow similarly includes a pair of pulley guards or enclosures l and Il. Within the guard i4 there is enclosed a pulley lil about which extends an endless band l (see Fig. 2). a

As will be later described, the pulley l is fixed by the anchor mechanism 2 with respect to the drafting board I and the band l is extended about a pulley 2 secured within the pulley guards l5 and Il (see Fig. 4). The pulley ZS is pivotally supported, as will be hereinafter described, upon both of the guards i6 and ll and is encircled additionally by a second endless band 2l which passes also around a pulley 22 pivotally supported within the guard l 5 associated with thc protractor head assembly l i (see Fig. 5).

By this construction the arms 3 and l may be so moved as to permit the protractor head assembly l l to be moved to substantially any desired position over the surface of the drafting board l and the interconnection of the head pulley 22 with the fixed pulley ld through the medium of the bands 2l and l5 and the elbow pulley 20 serves to hold the protractor head assembly in fixed angui lar orientation relative to the drawing board l.

. As is best shown in Figs. 2, 3 and 4, the strut members v11i-5l and 8 9 associated with the arms 3 and l' serve the duel purpose of acting as struts to hold the respective pulleys in their proper spaced relation to each other and as guards completely enclosing the endless bands i9 and 2l.

Each of the pulley housings lll-H is provided with outwardly extending tubular extensions 23 disposed parallel to each other and positioned substantially tangentially with respect to the pulley with which they are associated. Each of the strut members 4 5, 8 l are at one end, as for example at the elbow end, secured as by means of welding 2i (see Fig. 4) or other suitable means of attachment to a nut member 25 which may be provided with a hexagonal cross section to permit ready application of a wrench or similar tool thereto.

The nut members 25 each include a reduced diameter shanl; portion 2 which is externally threaded to coact with internal threads provided in the bore within the tubular portions 23 assoelated-With the elbow 6. By this means the four strut members may each be immovably and nonrotatably secured to the pulley guards l5 and Il. The opposite ends of each o f the trusts 4 5, 8 9 are threaded externally as indicated at 2l in Fig. 3 to coact with similar internal threads formed on the interior of a tubular nut 23. The nut 28 is provided with a hexagonal cross section the corresponding tubular portions 25. Thus? byturning the tubular nuts 29, the strut members 4 5, 8 9 be caused to move inwardly or outwardly relative to the anchor 2 orr the pro tractor head assemblyl I'I- to thereby reduce orv increase the spacing between the pulleys carriedby the arms and I to provide for'thetightening' or loosening of. the bandsv I--andEI as desired'.

The anchor construction 2 is illustrated in dev tailV in Figs. 2 and 3 and includes a bed plate por=` tionv 3lY adaptedrtol engage the: upper surfaceof the drawing; board If. and be vsecu-red'theretoby means of suitable clamps. (notshown). adapted to engage the under surface ofthe drawing board; The bed plate 3l supportsor-has formed'integrally therewith an upstanding lug. or ear 52- which is adapted; toV be receivecl between a' pair of arms-'.33' and 34 formed integrally with or `secured toi the pulley Iii. The arms and' 34. each carry pivot screws 55 and 35. adapted: to engage suitable seats formed; inthe side surfaces ofthe lug. 3x2 tothereb'y. secure the pulley I8i tothe'bediplate'lr for pivotal movement. about ahorizontally extendingv axis while holding theI pulley t8" against pitotalmovement about its own vertical axis.

The pulley' I-S. is; as is. shown in: Fig. 2 fitted withsuitable antifriction lzieai'ings 3?? and;I 55' which in turn: support a; central. shaft 59. The shaft 35 may be headed as indicated. at' 401. and secured as by meansk of Welding to an upperplate i I whichis in turnA secured to the' pulley housing orenclosure t4 as by'meansfof'screws 42.5 or like attaching means. Accessto thescrews ft2 maybe? had throughaf removablef'cove'r plate 431.

By means; of. the construction. just' described; ther pulley housing or bracket I4 is journale'd for pivotal movement about' the airis of the pulley f8 andi for pivot-,alf movement along with the pulley i5 aboutthe-horizontal' axis defined by' the pivot screws35 and 35. Tonormally holdzthedr'afting machine against. pivotali movement about such horizontal axis theshaft 39'v is. extended downwardlyv below they pulley.` I8. andl is provided with a squared portion 44:.up'oni which. is positionediri succession aY cani. plate 455 aspa'c'e'r 46" aridi alflatchj plate 471, these three members being clamped againstA a shoulder' provided. on the` shaft 31.5 as by means ofa clampingnut 48.-

w The spacer 45 is provided with. a diametersomewhat less than the diameter of the latcl'ii'ng plate 41. so that a horizontally positionecl-f` latch member 492 may extend into the space between the'V latch plate 4f! and the cam plate 451. The latch member lisis. slidablymounted upon alsuitn ablelug i andY in apertures 55k and 521' formed upon and in. tliabasevplatel. The latch member 59 carries a collar 53 against Vvili-ii fr bear-sfa compression spring. 5111n surrounding thel latch member 49 and; bearing` also against'4 the bottom of a springl receiving socket fforrriedj in the lug 32'.

I'orwardl'yrv of thet collar- 55?; that:v is,` to'vsrardl the' shaft. 39, from the" collar53g. there is pos'i'ti'orreda latch operator 55= which may, asis illustrated in Fig. 3,. compriseV a lever 5,1: pivotedi tof. theA base plate 3-I as at-59- and. provided.with.` a bifurcated portionextending above and below the latch member 49..

With this construction pivotal movement of the lever 51 through engagement of thev bifur cated end thereof withl theV collar 5-3 serves to move the latch member 49 axially away from the shaft 39 and against the force of the spring 54 to position the latch member 49 a distance away from the` shaft 39 sufficient to permit the bracket I4, and pulley I 8l to be moved upwardly in a pivotal fashion about the horizontal pivot axis. This operation is, of course, prevented when the latch member 49 is inthe engaged position which is illustrated in Fig. 2.

In order tol provide for automatic relatching of the mechanism when the bracket I4 is; pivotally moved from a displaced position to the normal position illustrated in Fig, 2; the upper surface of the latch member 49 and the lov/er surface of the latch. plate 4l arepreferablybeveled as il.- lustrated in Fig. 2.

rIhe cam plate 45 hereinbefore referred tocomfprises: one element of a spring counterbalancing mechanism of the type described and claimedY in: a copending` application Serial No, 410,996-, filed by Russell I-I. Cole etal. on September 16, 1 941.,A and, entitled Spring counterbalance for drafting machines. The other part of this mechanism comprises a spring pressed follower 59 mounted within a tubular member 6I)` secured to the pulley I8 and carrying a cam engaging element 6I infaposition to engage the cam 45, which cam may 'beof circular form and positioned eccentrically relative to the axis of the pulley I8;

Referring now to Fig. 4^, it will. be noted that thepulley guards lliy and If'I included in the elbow 5 are each pivotally supported as by means ofantiefriction bearings 62 upon anV elbow shaft B3 to which the elbow pulley 20 issecured. A s is shown in Fig.- 4, each of the guards IB- and I'I is provided. with apertures 54` throughA which the bands I9 and 2lVl may be passed whenl the effective length of thestruts 4 5, 3 9 are reduced toa minimum.

In assembling the` drafting machine,l the strut and pulley assemblies are made up ashereinbeforev described and sufficient lengths of fiat tapelike` material are selected to form the endless bands I9 and fl.4 These are passed through the strutsf 4 5-. 8 9, respectively, and about the pulleys I8- andl 2.2. lIhe ends of the pieces which must be secured to each other to form an endless band are brought out through the apertures 6 4 and4 55' and are then butt or ashwelded tov each other to form a permanent jointy and provide truly endless bands for the bands Il9and 2-I.v The loops which` protrude through the apertures-64 and 55- are then. pushed-back into the interior of the housings I6 and Il, and the effective lengths of the struts are increased by manipulation of the adjustingnuts 28 totighten the bandsv about the pulleys. The apertures 64 andl 55 may then becovered by means of name plates or similar functionalcovers 55.

".ll'iel drafting machine construction of my inf Ventronl also includes' a novel pro'tractor head assembly.. This mechanism is illustrated in detail in Figs, 5-'7. As is shownin the' enumerated-v` fig'- the housing or bracket I5 carries anrantifriction bearing 5*.' which is in turn mounted upon a. central column-alike: structure Eli-'forming apart ofthe pulley y underside of the pulley" 2525 is c'utf away and turned; tor provide adownwardly extending cylin-` drical. portion 59: upon whichlisjo-urnaldifor ro` tation a protractor plate ;.v Between the protractor plate 10 and an under radial surface 1| of the pulley 22 there may be placed a thin layer of cushion or friction material 12 toY provide for smooth and somewhat restrained movement of the protractor ring 10 relative to the pulley 22.

The protractor ring 10 has inscribed on a sloping upper surface 13 thereof the customary angular graduations so that by virtue of the rotary mounting of the plate 10 upon the cylindrical portion 69 the plate 10 may be turned to any desired position of the zero or index of the angular scale with respect to the drawing board A clamping nut and screw arrangement 14 is provided for locking the protractor plate 10 in any adjusted position in which it may be placed.

Below the protractor ring 10 there is positioned a notch plate 15 secured to the protractor ring 10 as by means of screws 16, the notch plate 15 being held spaced from the under side of the protractor ring 10 as by means of interposed spacers 11. The notch plate 15 is provided with a series of peripherally spaced'notches 18 (see Fig. 6) adapted to be engaged'by the nose portion 19 of a pawl member 80 which is pivotally mounted as by means of a pivot 8| upon a scale supporting plate 82 to which the scale or drafting tools |2 and I3 are secured.

The plate 82 completely surrounds the protractor plate 10 as is illustrated in Figs. 6 and 'l and carries an under side cover plate 83 dening a housing space 84 within which certain control mechanism to be described hereinafter is positioned. The plate 82, though cut away at the location of the plane along which the section'of Fig. '5 is taken, extends inwardly into the interior space 84 and beneath the notch plate 15, that portion of the plate 82 being identified in Fig. 5 by the reference character 85. The inner portion ofthe plate part 85 is secured as by means oi screwsl or rivets 86. to a radially extending flange 81 formed on the lower end of a tubular member 88 which is passed upwardly through a suitable aperture formed in the column 68.

To the upper end of the sleeve 88 there is secured as by means of a clamping nut 89 a U- shaped yoke member 90. The upstanding arms oi the yoke 90 are received within a central interior recess 9| formed in a knob or handle 92 and are secured to the handle 92 as by means of pins, screws or rivets 93 so that the sleeve 88 and the scale supporting plate 82 secured thereto may be rotated by merely rotating the knob 92. This rotation of the scale supporting plate 82 is, of course, normally arrested and prevented by the engagement of the nose 19 of the pawl 80 with one of the notches 18, into which notches it is resiliently urged as by meansl of a hairpin spring 94.

' To provide for the proper control of the operation oi the latching pawl 80, I provide a circular control member 95which is`interposed between the knob 92 and the upper surface of the pulley guard I5. The member 95 is wheel-like in form and contoured to conform to the external contour ofthe knob 92 and may, if desired, be provided witha series of radial spoke-like webs 96 to assist the operator of the machine in turning the control member 95. The control member 95 is secured to a pivot shaft 91 which is extended upwardly through the interior of the tubular member 88,-the attachment to the shaft 91 being effected as by means of a cross-bar 98 secured to or molded into the control member 95 and affixed to the upper end of the shaft 91 as by means ol a clamping nut 99. y

The lower end of the shaft 91 carries a cam plate 00, the forni of which is best illustrated in Figs. 6 and 7. The cam plate |00 may be of substantially circular form but is provided with one flat side |0| adapted to be normally engaged by a relatively wide cam engaging shoe portion 02 of a sliding link |03. The sliding link |03 is mounted upon the plate portion and supported for sliding movement as by means of suitable projections |04 formed on the link |03 and received within guiding slots |05 formed in a lower guide member |06 which is in turn secured to the plate portion 85. The Outer end of the link |03 is pivotally attached as indicated at |01 to the pawl so that sliding movement of the link |03 may result in so moving the pawl B0 as to disengaee the nose 19 thereof from the notches 10.

The normal position of the parts is shown by solid lines in Fig. 6, in'which position it will be noted the shoe |02 engages the flat face |0| of the cam member |00 and the pawl 80 is in its inwardmost or notch engaging position.

A slight angular movement of the control member and the cam plate 00 in either direction, as for example in a clockwise direction through an angle of approximately 20 as is illustrated in Fig. 6, will cause the sliding link |03 to be moved outwardly from the center of the mechanism a distance suiicient to remove the nose of the pawl 19 from the notch 18, such positions of the parts being illustrated by dotted lines in Fig. 6.

It is to be noted that in this position, 'the device is automatically restoring since the pawl springV 94 acting upon the pawl and through the pivotal connection |01 serves to urge the sliding link |03 inwardly and since the point of engagement of the shoe |02 with the heel |08 of the cam plate |00 is to one side of the central thrust line extending from the center of the shaft 91 to the pivot 01. Thus the operator if he desires to change the angular orientation of the scales from one frequently used position to another, as -for example from 0 to 15, may by turning the control member 95 through a slight angle release the scale supporting plate 82 so that the knob 92 may be turned to shift the scales |2 and |3 to a new angular position.

This operation accompanied byV the release of the control member 95 will result in the nose of the pawl 80 riding along the peripheral surface of the notch plate 15 until it registers with the next notch 18, at which time the spring 94 will impel the pawl 80 to a notch' engaging position and rotate the control member 95 back to its original position.

In the event it is desired to prevent this automatic re-engagement of the pawl 80 with the notch plate 15, it is only necessary to turn the control member 95 Vthrough a somewhat greater angle, as for example 40, to dispose the parts in the positions illustrated by solid lines in Fig. 1.

It will be noted by h'aving reference to Fig. 7 that when the control member 95 is so rotated, the cam engaging shoe |02 engages the circular surface of the cam member |00 at a point beyond the heel |08 so that the thrust line along which the force oi' the hairpin spring 94 acts is positioned precisely along the line of centers joining the shaft 91 with the'pivot |01. The net result is, therefore, that no restoring force is applied to the cam |00 with the result that it will remain in the position in which' it is placed until it is anomali-i vmanually returned toward its voriginal position.

In order 'to provide 'for locking the protractor plate 82 ,at odd angular positions, I provide la vclamping screw |09 which is journaled for rotation in a suitable aperture provided in the plate portion V85. The upper end of the screw |09 is threaded 'to receive va clamping nut H0 which overlies the notch plate 'l5 so that when the nut and screw |09 are rotated with respect to each other,.the notch plate and the plate 'portion 85 will be clamped to each other between the nut 0 and a head-like portion of the screw |09.

The head-like portion is extended laterally 'to comprise a bell crank ||2 which is connected as `bymeans of a pivot projection H3 to one end of 'a drag link H4, the other end of which is pivotally secured as by means of a pivot projection H5150 the cam plate |00. The `positions of kthe parts are so adjusted as by initially disposing them in the positions illustrated by solid lines in Fig. 6 that whenever the cam plate |00 is rotated from its neutral position in either direction, the clamping screw |09 will be pivo-tally moved in a counter-clockwise direction. With this adjustment of the parts, increasing angular displacement of the control member 95 from its neutral position will result in an increasing pivotal movement of against, rotation by passing the screw through 'the appropriate one of the holes HB and re-engaging it with the adjacent portion |'8 of the scale supporting plate 82.

This adjustment is so 'eiected that angular displacement of the screw |09 to the extent indicated by the solid line positions of the parts in Fig. 7 will not result in clamping the notch plate 'l5 to the plate portion 85 but such lthat an appreciable increase in the angular displacement cf the controlrknob 95, as for 'example tothe dotted line position illustrated in Fig. '7, will result in the clam'pin'g ofthe notch plate 'I5 to the `scale supporting plate B2.

Thus, whenever it is desired to turn the scale supporting plate 82 to an odd angular position and secure it in such position, it is only necessary for the operator to rst Vrotate the control member 95 to a position corresponding to that illustrated l-by solid lines in Fig. 7 and then through use of the knob 9'2 to rotate the scale 'supporting `plate 82 tothe desired odd angular position. Thereafter further rotation of the control member 95 to Vmove the cam'plate |00 from the position illustrated in Vsolid lines in Fig. 7 to that illustrated by dotted lines in Fig. 7 will -result in the vclamping of the notch plate "I5 vto the vplate portion 385 of the scale supporting plate and so lock the scale supporting plate against displacement from the odd angular position in which it has been placed-by the operator. To restore the uscale supporting plate 92 to its original position, it is only necessary to reverse the above described operation.

-From the foregoing it will be noted thatI have provided a novel drafting machine construction and in that connection have provided a novel drafting machine head which is characterized by the employment of a single control member which (b) Vreleased from the protractor assembly vand held released until the control knob S5 Yis further manipulated, and

(e) locked in any odd angular position inwhich it may ybe placed.

It Will be further noted that provision is Amade for adjusting the clamping assembly A||l9-|'|"0 to provide Afor lthe 'desired 'clamping actionY in` response to substantially Aany 'amount of pivotal movement of the control Amember ,95.

"I have, in the foregoing, also disclosed a drafting rnaclfiiner construction which `provides for the 'pivotal movement of `the entire machine about a horizontally disposed axis, this construction being 'characterized particularly by the 'provision oiv -a manually operable Llatch 'which 'will eiec- 'tively resist 'such pivotal movement until such ntinreas the operator of the machiney intentionally releases'the kmachine for hit-ing movement away from the lsurfacel of the drawing board, thus avoiding 'the danger of injury Ato the machine kthrough inadvertent overturning of the machine when the 'parts thereof are so disposed as to overbalance the horizontal-pivot.

Attention -is directed to `the 'employment of 'thestrutmembers as guards 'for vthe endless 'bands and thearrangement which 'has been shown vand 'described 'for 4adjusting the effective length 'of struts to' provide for the tightening and loosen- 'ing ofthe-b-ands'as maybe desired.

Ifhave also disclosed a novel arrangement of parts and Ymethodsby means of which a permanent welded connection between the adjacent ends of the bandsmay 'be made, though the entire dband is arranged to be enclosed `within a Acompletely encompassingfguard arrangement.

While I have sho-Wn and `described the preferred embodiment of my invention, YI ado'not desire to be limited :to any of the details of 4construction shown or v'described herein, except as defined zin Y the appended claims.

VI claim: K

l. -ln afdraiting machine head including a 'rotatablesupport `and a spring :pressed pawl and notch -p'late mechanism v`for holding said support `in "a limited number of selected angular positions, the Ycombination of: a' control member `mounted on said head for pivotal movement in either direction through three successive angular'increments; coupling -means connecting-said control member to said pawl dior disengaging said .pawl in response to movement of said Ycontrol member throughithei-lrst of said angular increments, said coupling means Vbeing so arranged .thatlsaid spring pressing said pawl will be eiective yto .return said ,pavvl ltoholding position When saidy control member is moved through the first ofsaid `angular increments and released and ineiective -when said control 'member is moved through I'the :rst and second of said angular increments; clamping means :for clamping said support .againstrotationLregardless of the positionofsaid support; and means coactingwith said coupling means for operating said clamping means in response to angular movement of Vsaid ment of said cam member control means through all three of said angular increments.

.2. In a drafting machine head including a rotatable support and a spring pressed pawl and notch plate mechanism for holding said support in a limited number of selected angular positions, the combination of: a control member pivotally mounted on said head; a cam member coupled to be rotated by said control member and defining a cam surface comprising a segment of a circle; and a sliding link engaging said pawl and having a straight line cam engaging surface disposed substantially at right angles to the length of said link and having a length substantially equal to the chord of said segment, said cam member being movable by said control member in either direction through a predetervmined angle exceeding one-half of the central angle subtended by said chord.

3. In a drafting machine head including a rotatable support and a spring pressed pawl and notch plate mechanism for holding said support in a limited number of selected angular positions, the combination of: a control member pivotally mounted on said head; a cam member coupled to be rotated by said control member and defining a cam surface comprising a segment of a circle; a sliding link engaging said pawl and having a straight line cam engaging surface disposed substantially at right angles to the length of said link and having a length substantially equal to the chord of said segment, said cam member being movable bysaid control member in either direction through a predetermined angle exceeding one-half of the central angle subtended by said chord; clamping means for clamping said support against rotation regardless of the position of said support; and means interconnecting said clamping means with said cam member for operating said clamping means in response to movethrough said predetermined angle.

4. In a draf-ting machine head including a rotatable support, a, spring ,pressed pawl and notch plate mechanism for holding said support in a limited number of selected angular positions and a pivoted control member for disengaging said pawl and movable through a predetermined angle greater than that required to disengage said pawl, the combination of a screw and nut clamping means operable in response'to relative rotation of said screw and nut to lock said support against rotation; a bell crank aflixed to said screw; and a drag link interconnecting said bell crank with said control member, the length and angular disposition of said drag link being arranged to rotate said screw an amount suiiicient to clamp said support in response to movement of said control member in either direction through said predetermined angle. Y

In a drafting machine head including a rotatable support and a spring pressed pawl and notch plate mechanism for holding said support in a limited number of selected angular positions, the combination of: a control member mounted on said head for pivotal movement in either direction from a normal position; coupling means interconnecting said control member with said pawl and notch plate mechanism and operable in response to a predetermined angular movement of said control member to shift said pawl and notch plate mechanism to a released position permitting pivotal movement of said support relative to said head; and means coacting with said coupling means and said spring pressed pawl and notch plate mechanism to return said control member to its normal position in response to movement of said pawl and notch plate mechanism to said engaged position when said control member is subjected to said predetermined angular movement and released, said last-named means being responsive to pivotal' movement of said control member an amount exceeding said predetermined angular movement toI rendersaid pawl and notch plate mechanism inelective to restore said control member to said normal position.

. 6. In a drafting machine head including a rotatable support and -a spring pressed pawl and notch plate mechanism for holding said support in al limited number of selected angular positions, the combination of: a control member; a rotatable shaft ,pvotally mounted upon said head and coupled to be rotated by said control member; a cam member on said shaft defining a cam surface comprising a segment of a circle; a sliding link coupled to said pawl and notch plate mechanism for moving said mechanism from an engaged position to a released vposition in response to sliding movementv of said link; a cam engaging member fixed on said sliding link and defining a straight line cam engaging surface disposed substantially at right angles to the length of said link and having a length substantialy equal to the chord of said segment; a clamping means for securing said head and said support against pivotal movement relative to leach other and comprising a nut means disposed on one side of said head and support and a screw means engaging' said nut means and disposed on the opposite side of said head and support, Whereby relative rotation of said screw means and said nut means will immovably clamp said head and support to each other; a bell crank aiiixed to said screw means; a drag link interconnecting said bell crank with said cam member, the length and angular disposition of said drag link being arranged to rotate said screw means an amount suicient to clamp said head and said support to each other in response to angular movement of said cam member in either direction from a neutral position an amount exceeding one-half of the central angle subtended by the chord of said segment; and adjustment means carried by said head for holding said nut means in any angular position in which it may be placed.

7. In a draftingmachine head including a rotatable support and a spring pressed pawl and notch plate mechanism for holding said support in a limited number of selected angular positions, the combination of: a control member pivotally mountedV on said head;l coupling means connecting said control member to said pawl for disengaging said pawl in response to a predetermined angle of movement of said control member, said coupling means being so arranged that said spring pressing said pawl will be effective to return said pawl to holding position when said control member is moved to said predetermined angle and released; and means coacting with said coupling means and responsive to movement of said control member through an angle greater than said predetermined angle for holding said pawl disengaged against the pressure of said spring. f y

VARD B. WALLACE.

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