US2820162A - Contour controlling tracing head - Google Patents

Description

CONTOUR CONTROLLING TRACINQ HEAD Filed April 11, 1957 FIG] .0 4 "'6 120 40o CYCLE 4 Moron GENERATOR CONTROL INVENTORZ J. KIRK SNELL,

HIS ATTORNEY.

the ball nut, of rotation in two directions.

concluding portion of this specification.

2,820,162 CONTOUR CONTROLLTNG TRACING HEAD 1. Kirk Snell, Waynesboro, Va., assignor to General Electric Company, a corporation of New York Application April 11, 1957, Serial No. 652,209 4 Claims. (Cl. 310-75) This invention relates to an improved means for use in the reproduction of contours by material-working or forming devices. More specifically, it is concerned with a novel tracing head producing electrical signals usable for the control of material-working and forming devices in order that a contour may be reproduced automatically.

Contouring control systems have been provided in which a tracing head is maintained in engagement with a template or form whose conformation is to be reproduced by a material-working or forming machine. Such arrangements have taken the form of mechanical linkages between the tracing head engaging a template and the machine, such as pantographs, whereby the tracing head duplicates all the motions of the work-engaging tool. Hydraulic tracing heads which control a valve or valves controlling fluid feed motors as the stylus of the tracing head is displaced by the template have also been used for this purpose. Such devices require a mechanical feedback loop. For machines of larger size, it has been found desirable to utilize remote electrical feedback control systems for this purpose in order to eliminate complicated and bulky mechanical linkages. The use of 7 electrical tracing heads also permits the use of hydraulic control systems without the necessity of excessive lengths of fluid conductors. Tracing heads of the electrical remote variety used heretofore have required complicated and costly gear arrangements to convert the motion of a stylus engaging a template into a usable electrical signal. Such devices have inherently a large error therein due to backlash or other imperfections in the gear train. Such devices are also subject to the objection that their cost is relatively high due to the precision with which they must be made.

Therefore, it is an object of this invention to provide a novel tracing head for use in contour reproduction systems which, by virtue of its construction, is capable of a high degree of accuracy in converting the motion of a stylus into an electrical signal and yet is relatively inexpensive to construct.

In the use of remote tracing heads it has been a practice to mount a plurality of them in a side by side relationship each engaging a separate template. When gear arrangements are utilized, the tracing heads have transverse dimensions which utilize an excessive amount of space when they are so mounted.

It is still another object of this invention to provide a novel tracing head for use in contour reproduction systems which utilizes a ball nut and screw shaft means for converting the reciprocating motion of a stylus into a rotary motion in order to drive an electrical signal generating device and, therefore, requires a minimum of traverse space.

Briefly, in one embodiment of my invention I provide a ball nut secured to and movable with a template-engaging stylus. A screw shaft passes through the ball nut and is capable, by virtue of the relatively low friction of The screw shaft is coupled to an electrical signal generator, such a selsyn device to furnish usable electrical control signals. Also provided are braking means and means for eliminating undesirable backlash in the device.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the My invention, however, both as to its structure and manner of operation tent G ice together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a view partially in section of an embodiment of my invention; and

Fig. 2 is a schematic illustration of one form of a contouring control system incorporating my invention.

Referring now to Fig. 1, I show a tracing head designated generally by reference numeral 2. The tracing head is constituted by a casing 4 on which apertured brackets 6 are provided and by which the casing may be mounted to any suitable support such as a traveling toolsupporting member of a machine tool. Axially movable in the casing 4 is a ram 8 which has provided at the lower end thereof a cap 10 secured therein as by means of the pin 12. A plate 14 is bolted to the cap as shown at 16. Any suitable work-engaging stylus may be provided on the plate 14 as desired. It is customary to provide in certain operations a stylus having substantially the same configuration as the workpiece engaging member of the materiahforming machine being controlled by the tracing head.

Secured in the upper end of the ram 8 is a ball nut 18 which is movable therewith. Also provided on the ram 8 is a key or projection 20 engaging in a keyway 22 provided in the casing 4 in order that relative rotary motion between the ram 8 and easing 4 is prevented. If desired, bearings may be provided between the outer wall of the ram 8 and the inner wall of the casing 4 in order to eliminate friction and to stabilize the motion of the ram. Also, it may be found desirable to provide lubricant in the casing 4 and ports may be provided therein for this purpose. Passing through the ball nut 18 is a screw shaft 24 which has provided thereon a projection 26 of lesser diameter than the diameter of the screw shaft and a threaded upper end 28. Mounted on the projection 26 are a pair of bearings 30, the inner races of which are rotatable with the screw shaft 24. A collar 32 engages the outer races of the bearing and its lower end rests on an offset 34 in the casing 4. An inner annular flange 36 of the collar 32 engages between the bearings 30 for a purpose to be brought out hereinafter. Also provided on the collar 32 is a sealing means such as an O ring 38. The seal 38 functions to prevent the passage of lubricant from the ram 8 to the area of the collar. Secured as by an adhesive to the upper surface of the collar 32 is an annular member 40 of a suitable high friction material such as cork. A rectangular nut 42 is threaded onto the portion 28 of the screw shaft 24 and engages in a rectangular hole in a disc 44 resting on the member 40. A lock nut 46 is threaded on to the extension 28 in order to hold the disc 42 in its assembled position. A coil 48 is wound on a spool 50 and is encompassed by a magnetic material 52 so that a solenoid structure is formed. Mounted between the solenoid and the disc 42 there is provided a brake disc 54 which has provided thereon an upwardly extending angular flange shown at 5.6. In the embodiment illustrated, the brake disc 54 is normally held in an upward position displaced from the disc 42 against the action of a coil spring 58 by energizing the coil 48 of the solenoid.

Secured to the upper end of the screw shaft 24 is a coupling 60 engaging a matching coupling 62 mounted on the rotating shaft 64 of a motor 66. The motor 66 rests on a motor mounting plate 68 which is fastened to the upper end of the casing 4 by any suitable means such as screws (not shown). When the motor mounting plate 68 is secured to the casing 4, it is noted that the portion 69 which extends radially inward overlies the magnetic member 52 of the solenoid. As the mounting plate 68 is tightened down, the portion 69 exerts a force on the collar '32 to pinch it between the portion 69 and offset 34 to thereby prevent it from rotating with the screw shaft 24. The housing of the motor 66 maybe provided with a boss 70 to seat it on the motor supporting plate 68 and with which the upper end of the coil spring 58 is engageable. The other end of the motor shaft 64 is provided with a coupling 72 engaging in a matching coupling "74 on the shaft 76 of an electrical signal generating device such as a selsyn '78 mounted on any suitable supports such as those shown .illustratively at 80. A cover 32 may be provided to protect this structure. The electrical signal generator may also take the form of apotentiom'eter having voltage output proportional to the position of a rotor or a digitizing device providing an output in the form of pulses.

In the operation of this device, a stylus or templateengaging member is secured to the lower end of the ram 3 and the casing 4 supporting the entire assembly is mounted on a portion of a material-forming device so as to provide for relative movement between it and the surface of the template. In response to changes in the contour of the template, the ram 3 moves in an axial dire-ction in a reciprocating manner prevented from rotating movement by the engagement of the key 2t) in -the keyway 22. Since the screw shaft 2.4 is relatively fixed, the effect of the ball not moving axially relatively thereto is to impart a rotating motion thereto which is transmitted by the couplings 60 and 62 to the motor shaft 64 and thcrethrongh to the shaft 76 secured to the rotor of the electrical signal generator 78. The electrical signal generator 78 being a device wherein the output is proportional to the relative position of its rotor and stator (not shown), an electrical signal proportional to the axial displacement of the ram 8 will result. This electrical signal may be utilized in control devices such as the one disclosed in Fig. 2 to be described in detail hereinafter .to control the motion of the material-forming machine.

in order to eliminate backlash in the ball nut 18 and to maintain the stylus attached to the ram 55 in engagement with the template, the motor 66 is energized to have .a low torque output in a direction to rotate the screw shaft 214 downwardly when a relatively light stylus is .used. The motor 66 may also be used to rotate the screw shaft 24 in the opposite direction at a higher torque in order to raise the ram 8 from the work when it is desired. When a relatively heavy stylus is used, the motor [1 6 is energized in the opposite direction to maintain the stylus just engaging the template and when it is desired .to raise the ram 8 the motor is operated at a higher torque for this purpose.

When it is desired to apply a braking action to the screw shaft 24. the solenoid .coil 43 .is de-energized permitting the coil spring 58 to force the brake disc 54 downwardly against the disc 42 and thereby against the member 40 in order to apply a braking action. It has also been found that a film of oil between the brake disc 54 and disc 42 during normal operation of the tracing head is an effective means of viscously damping the rotation of the screw shaft 24.

In Fig. 2 of the drawing, I show a control system utilizing a tracing head incorporating my invention. The tracing head at 2 is provided with a stylus 100 engaging a template 102. In the illustration, the electrical signal generator comprises a selsyn, the rotor of which is excited by a 400 cycle generator 104. The output voltage of the selsyn stator which is a function of the relative position of the rotor is furnished via the conductors 106 to a feedback selsyn, the rotor of which is coupled to the shaft of a feed motor 110 which serves to position a work-engaging tool 112 carried on any suitable support 114 on the workforming device. The output of the 400 cycle generator is furnished to a discriminator 11.6 which also receives signals via the conductors 118 from the rotor of the feedback selsyn 108, effects a comparison therebetween and 4 develops "therefrom an error signal proportional *tothe displacement of the cutting tool 112 from the position corresponding to that indicated by the tracing head 2. The error signal is amplified by suitable means 120 and furnished to a motor control system 1 22 so that the motor 11d may be actuated to correctly position the tool 1H2. In the embodiment illustrated, the template 1 02 and the workpiece 124 are connected to each other as indicated by the dotted line 126 by being supported on a movable part the work-forming device and move relative to the tracing head 2 and cutting tool 112. .It is obvious that, ,if desired, the template 102 and workpiece 124 may be stationary while the tracing head 2 and cutting tool .112 are connected so .as to be movable relative thereto.

the foregoing description, it is seen that the traning head 2 is responsive to changes in contour in a single dimension and will accordingly control a work-forming device in this dimension. if desired, two or more tracing heads may be provided, each engaging a template rep, resentative of the desired shape of the workpiece in one of a plurality of dimensions and used in conjunction with each other and associated control systems to direct the motions of the work-forming device in more than one dimension.

Although in accordance with the provisions of the patent statutes this invention is described as embodied in concrete form and the principle has been explained together with the best mode in which it is now contemplated applying that principle, it will be understood that the apparatus shown and described is merely illustrative and the invention is no-t lirnited thereto since alterations and modifications will readily suggest themselves :to per.- sons skilled in the art without departing from the true spirit of the invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A tracing head comprising a casing, a ram mounted for reciprocating movement in said casing, .a ball out secured to said ram, a screw shaft mounted in said casing for rotating movement only and passing through said ball nut, motor means mounted on said casing, a shaft having portions extending from each end of said motor :rneans, means coupling one portion of shaft to said screw shaft, an electrical signal generating device having in rotatable element and means coupling the other portion of said shaft to the rotatable element of said electrical signa-lggenerating means.

2. A tracing head comprising a casing, a ram mounted for reciprocating movement in said casing, a ball not secured to said ram, a screw shaft mounted in said casing for rotating movement only and passing through said ball nut, motor means mounted on said casing, a shaft haying portions extending from each end of said motor means, means coupling one portion of shaft to said screw shaft, an electrical signal generating device having a rotatable element, means coupling the other portion of said shaft to the rotatable element of said electrical signal generating means, a brake element secured to said screw shaft and rotatable therewith, a cooperating tbrake element-engageable with said brake element, means maintaining cooperating brake element normally disengaged withsaid brake element and means urging said brake element and said cooperating brake element rintmengagement with each other.

3. A tracing head as defined in claim 2 wherein said brake element comprises a disc and said cooperating brake element comprises a brake disc overlying said saline and wherein said maintaining means comprises asolenoid and said urging means comprises a spring.

4. A tracing head as defined in claim 3 wherein afilm of lubricant is provided between :said disc and said brake disc whereby relative movement therebetween is wiscously damped.

No references cited.

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