GB2116460A

GB2116460A - Fixed screw and motorized ball nut mechanism for machine tool slide

Info

Publication number: GB2116460A
Application number: GB08306751A
Authority: GB
Inventors: Richard Johnstone
Original assignee: Kearney and Trecker Corp
Current assignee: Kearney and Trecker Corp
Priority date: 1967-12-12
Filing date: 1983-03-11
Publication date: 1983-09-28
Also published as: GB8306751D0; GB1266132A; GB2116460B

Abstract

1,266,132. Drilling machine. A. P. L. THOMAS. 12 March, 1969 [12 Dec., 1967], No. 56308/67. Heading B3C. [Also in Division F2] A self-contained machining head comprising a threaded spindle 14 drivable by motor M and axially movable by a lead nut 11 drivable by motor F, the spindle and lead nut being rotatable in the same direction, is provided with a master switch which, in one embodiment (as shown), upon actuation, connects the motor F to a current supply, and two further switches subsequently operated in succession in response to changes in the axial position of the spindle, respectively to connect the motor M and one of the motors via manually adjustable thyristors or equivalent electronic devices to the supply, and disconnect the motor F from the supply. In a second embodiment (not shown), the hand of the threads of the lead nut and spindle are opposite to those of the first embodiment, the master switch connects motor M to the current supply, and the two further switches respectively operate to connect motor F and disconnect motor M. The motors may be mounted coaxially (Fig. 1) or side-by-side (Fig. 2). The spindle 14, which is splined to the rotor 6 of motor M is connected through a gear-box 17 to one or more guide bars 15 axially movable with the spindle, the two further switches being located adjacent one guide bar for actuation thereby. The positions of the switches are adjustable relative to the guide bar. The motor which is first disconnected from the supply may, at such time, be connected to a D.C. supply to stop it instantaneously. Means may be provided for inching each motor independently to permit correct positioning of the three switches relative to the guide bar 15. The gear-box may include one or more toolcarrying spindles 18. A clutch may be provided to cause simultaneous rotation of the spindle and lead nut at the same speed; the clutch engaging after disconnection of the one motor and before disconnection of the other. As shown in Fig. 2 the clutch comprises a part 37 secured to the lead nut and a part 36 rotatable with the spindle. Where the motors are arranged side-by-side, the motor offset from the axis of the spindle may be connected by toothed belt 32 either to the lead nut or to the spindle. When both motors are running the speeds are such that the spindle is fed axially slowly toward the work. A third switch can be provided for disconnecting the second-actuated motor from the supply.

Description

1

SPECIFICATION Motorized ball nut

This invention relates generally to machine tools and more particularly, this invention relates to a motorized nut for use in combination with a lead screw for precisely displacing a movable machine tool member on a fixed machine tool member along a straight path.

In the design of various types of machinery, and particularly machine tools, an important design criterion is the ability to precisely displace a movable member, such as an axis slide in the case of a machine tool, on a fixed member, such as the machine tool bed, along a straight path. Invariably, where precise movable member positioning is required, the combination of a lead screw, and a nut, and in particular, a ball type lead screw, and ball nut have been employed, the ball nut typically being secured to the movable member while the ball screw is rotatably journaled in the fixed member in threaded cooperation with the ball nut 85 for displacing the movable member along the fixed member responsive to ball screw rotation.

In present day machine tools which employ the combination of a ball screw and a ball nut for precisely displacing a machine tool axis slide on the machine tool bed, the ball screw is typically driven from either an electric or a hydraulic motor through a gear box. Driving the ball screw from the motor through a gear box yields superior machine tool performance in contrast to directly driving the 95 screw from the motor via universal coupling as the direct drive arrangement suffers from greater backlash due to the resiliency of the universal coupling. Further, use of a gear box to couple the motor to the ball screw affords greater rigidity to 100 the ball screw which is very desirable. The disadvantage of employing a gear box to couple the ball screw to the motor is that such gear boxes are costly to manufacture. Further, while employing a gear box to couple the motor to the ball screw reduces backlash as compared to coupling the motor directly to the ball screw via a universal coupling, still some backlash is inherent with the gear box which reduces positioning accuracy.

In comparison, the present invention concerns 110 a positioning system employing a motorized nut in combination with a rigid lead screw and thus, eliminates the above-enumerated disadvantages of the prior art lead screw nut positioning mechanism.

It is an object of the present invention to provide a positioning system employing a motorized nut in combination with a rigid lead screw so as to achieve minimum backlash between the screw and the nut.

It is yet another object of the present invention 120 to provide a positioning mechanism including a motorized nut and a rigidly fixed lead screw to assure rigid coupling between the movable member and the fixed member as the movable member is displaced thereacross.

GB 2 116 460 A 1 BRIEF SUMMARY OF THE INVENTION

Briefly, in accordance with the preferred embodiment of the invention, a positioning apparatus for displacing a movable member relative to a fixed member comprises a rigid lead screw secured to the fixed member parallel to the desired path of movable member movement and a motorized nut in threaded engagement with the lead screw and secured to movable member for displacing the movable member along the lead screw. The motorized nut is typically configured of an induction motor having a case or frame secured to the movable member by bolts or the like. The induction motor rotor, which extends through the case, is made hollow so as to be coaxial about the lead screw. A pair of nuts are each disposed in separate ends of the induction motor rotor to threadedly engage the lead screw so that as the induction motor rotor rotates about the lead screw, the induction motor and the movable member are thus displaced on the fixed member along the axis of the lead screw. Precise displacement of the movable member on the fixed member is facilitated by a feedback transducer which determines the actual movable member position. In practice, the feedback transducer may take the form of a resolver affixed to the induction motor frame so that the resolver shaft is driven from the induction motor rotor. The resolver output signal, which varies in accordance with resolver shaft position and hence, induction motor shaft position, is supplied to an N/C control circuit which controls induction motor stator excitation in accordance with the difference between desired movable member position, as represented by N/C commands entered to the N/C control circuit and actual movable member position, as represented by the resolver output signal.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth with particularity in the appended claims. The' invention itself, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:

Figure 1 is a longitudinal cross section of the positioning apparatus of the present invention; and Fig. 2 is a cut away end view of the apparatus of Fig. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figures 1 and 2 show a longitudinal cross section and an end view, respectively, of an improved positioning apparatus 10 constructed in accordance with the teachings of the present invention for positioning a movable member 12 such as the axis slide of a machine tool on a fixed member 14, such as the bed of a machine tool, along a linear path. Positioning apparatus 10 2 GB 2 116 460 A 2 includes a lead screw 16 which typically has Acme type threads and which is rigidly secured in machine tool bed 14 by a pair of fasteners 18 (only one of which is shown) so that the lead screw exteds parallel to the desired path of movement of axis slide 12 and bed 14. Unlike conventional axis slide positioning mechanisms in which the lead screw is rotatably journaled into the machine tool bed, in the present invention, lead screw 16 is rigidly secured in bed 14 so as to be non-rotatable.

The heart of the present invention is a motorized unit which includs a prime mover 20 fastened to slide 12 for driving each of a pair of Acme threaded nuts 22a and 22b which are in 80 spaced apart threaded engagement with the lead screw. In the presently preferred embodiment, primer mover 20 comprises and induction motor whose frame consists of a lefthand and righthand end caps 24a and 24b which are each secured to 85 slide 12 by a separate one of bolts 25 so as to move in unison with slide 12 on bed 14. The end caps which each have a hollow bore therethrough for receiving lead screw 16, are secured to each other by a pair of long bolts 26a and nuts 26b. 9o Induction motor 20 includes a multiphase wound stator 27 secured between end caps 24a and 24b and a hollow bore squirrel cage rotor 28 which is journaled to each of end caps 24a and 24b by a 30. separate one of bearings 30a and 30b, respectively, 95 so that rotor 28 is coaxial about lead screw 16.

When induction motor stator 27 is excited with an alternating current, the motor stator generates a rotating magnetic field which causes an oppositely- poled magnetic field to be induced in rotor 28. The 100 interaction between the rotating stator field and the induced rotor magnetic field causes the rotor to rotate within the stator. Each of nuts 22a and 22b is secured in an opposite one of the ends of rotor 28 so as to be threaded in unison along lead screw 16 as 105 rotor 28 rotates within stator 27 responsive to electrical excitation of the stator.

Accurate positioning of slide 12 on bed 14, that is to say, positioning of the slide to an exact location on the bed, is accomplished by controlling the excitation of stator 27 in accordance with the difference between the desired and actual position of the slide. To this end, a feedback transducer 32, is provided which, in the present embodiment, comprises a resolver which is secured to the induction motor frame, typically to frame end cap 24b, so that a gear 34 on the shaft of resolver 32 meshingly engages a complementary gear 36 secured to the outwardly exposed face of nut 22a by bolts 37. Thus, as induction rotor 28 and nuts 22a and 22b rotate in response to the application of electrical excitation on stator 27, gear 34 is driven by gear 36 to rotate the resolver shaft, causing the resolver to output an electical signal which varies in accordance with the resolver shaft orientation and hence, the angular orientation of rotor 28.

The electrical output signal from resolver 32, which varies in accordance with the induction motor angular orientation and hence, the actual position of slide 12 and bed 14, is supplied to a numerical control circuit 38 such as are well known in the art. Numerical control circuit 38 is responsive to numerical control commands generated by a tape reader or the like which are indicative of the desired position of slide 12 and, when supplied with such numerical control commands, the N/C control circuit generates and output signal for controlling stator 27 excitation in accordance with the difference between the desired and actual slide position, as determined from the difference between the slide position specified by numerical control commands received by the N/C control circuit and the actual slide position as represented by the output signal of resolver 32.

The foregoing describes an improved positioning apparatus comprising of a rigidly fixed lead screw and a motorized nut in threaded engagement with the lead screw, the motorized nut being affixed to a movable member for displacing the movable member along the fixed member parallel to the axis of the screw. Since the distance between nuts of the present invention, which are coaxial with, and directly driven by the motor rotor can be varied at the outset of operation, the motorized nut of the present invention has practically no backlash. Further, the rigid mounting of the lead screw to the fixed member assures a rigid coupling between it and the movable member.

Although the illustrative embodiment of the invention has been described in considerable detail for the purpose of fully disclosing a practical operative structure incorporating the invention, it is to be understood that the particular apparatus shown and described is intended to be illustrative only and that various novel features of the invention may be incorporated in other structural forms without departing from the spirit and scope of the invention as defined in the subjoined claims. For example, prime mover 20 could easily be configured of a hydraulic motor in place of the illustrated induction motor. Further, a linear position transducer, such as a digital encoder or an optical position transducer, could easily be substituted for the rotary feedback transducer shown as resolver 32. Also, while a conventional Acme threaded lead screw and nut have been disclosed, other types of lead screw-nut combinations, such as the well known ball nut, ball screw combination could easily be substituted therefor.

Claims

1. Apparatus for use in conjunction with a threaded member for displacing a slide or the like on a bed or the like along a path parallel to the axis along which said threaded member is rigidly secured in said bed comprising:

a prime mover having a frame fixedly secured to said slide and having a hollow rotary driven rotor journaled in said frame so as to circumscrube said threaded member; nut means disposed within said bore of said prime mover rotor so as to be in threaded n V.

3 GB 2 116 460 A 3 engagement with said threaded member for 55 displacement along said threaded member as said primer mover rotor is rotatably driven so as to displace said prime mover and hence, said axis slide along said bed; and control means coupled to said prime mover and 60 responsive to the position of said slide on said bed for controlling the rotation of said prime mover rotor in accordance with the difference between the desired and actual slide position.

2. The invention according to claim 1 wherein said threaded member comprises an Acme threaded lead screw and wherein said nut means comprises at least one Acme threaded nut for threadedly engaging said ball screw.

3. The invention according to claim 1 wherein said prime mover comprises an alternating current induction motor having a frame rigidly secured to said slide, a wound stator disposed within said frame and a rotor journaled to said induction motor frame so as to circumscribe said threaded member and being rotatable about said threaded member responsive to electrical excitation of said stator. 25

4. The invention according to claim 3 wherein said control means comprises: a feedback transducer for providing an electrical signal indicative of the actual disposition of said slide on said bed; and 30 a control circuit coupled to said feedback transducer and responsive to commands indicative of the desired axis slide position on said bed for controlling the excitation of said induction motor stator in response to the difference between the desired and actual position of said axis slide.

5. The invention according to claim 4 wherein said feedback transducer comprises a resolver affixed to said induction motor frame and having an ouput shaft rotatably driven by said induction motor rotor for generating and electrical signal indicative of induction motor rotor angular position.

6. A positioning apparatus for precisely displacing a movable member, such as an axis slide along a fixed member, such as a bed, comprising:

a threaded screw rigidly secured within said fixed member parallel to the desired path of movable member movement on said fixed member; a prime mover having a frame secured to said movable member and having a hollow bore, rotary driven motor journaled within said frame so as to circumscribe said threaded screw; nut means secured within the bore of said primer mover rotor for threaded engagement with said threaded screw for displacement along said screw as said primer mover rotor is rotatably driven;and control means for determining the actual position of said movable member on said fixed member and being responsive to commands indicative of the desired position of said movable member on said fixed member for controlling prime mover rotor rotation in accordance with the difference between desired and actual movable member position.

7. The invention according to claim 6 wherein said threaded member comprises an Acme threaded screw and wherein said nut means comprises at least one Acme threaded nut for threadedly engaging said Acme threaded screw.

8. The invention according to claim 6 wherein said prime mover comprises an alternating current induction motor having a frame rigidly securing to said axis slide, a stator winding disposed within said frame and a hollow bore rotor journaled to said induction motor frame so as to circumscribe said threaded member and being rotatable about said threaded member responsive to electrical excitation of said stator.

9. The invention according to claim 6 wherein said control means comprises:

a feedback transducer for providing an electrical signal indicative of the actual position of said slide on said bed; and a control circuit coupled to said feedback - transdu - cer - and responsive to-numerical -commands indicative of the desired axis slide position on said bed for controlling the excitation of said induction motor stator in response to the difference between the slide position as represented by said numerical commands and the actual position of said axis slide as determined from said feedback transducer output signal.

10. The invention according to claim 9 wherein said feedback transducer comprises a resolver affixed to said induction motor frame and having an output shaft rotatably driven by said induction motor rotor for generating an electrical signal indicative of induction motor rotor angular position.

11. A motorized nut constructed and arranged to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Printed for Her Majesty's Stationery Office by the Courier Press, Leamington Spa, 1983. Published by the Patent Office, 25 Southampton Buildings, London, WC2A IlAY, from which copies may be obtained.