WO1998046385A1

WO1998046385A1 - Turret-mountable live center for lathes

Info

Publication number: WO1998046385A1
Application number: PCT/US1998/006719
Authority: WO
Inventors: Michael E. Hoffmann
Original assignee: Kennametal Inc.
Priority date: 1997-04-16
Filing date: 1998-04-03
Publication date: 1998-10-22
Also published as: AU6883998A

Abstract

A turret-mountable live center for rotatably supporting one end of a workpiece in a lathe having at least one toolholding turret is provided. The live center includes housing assembly, a live center body rotatably mounted within the housing assembly, and a mounting mechanism for detachably connecting the housing assembly to one of the tool-holding turrets of the lathe. The housing assembly includes a threaded preload ring which can be easily screwed over an opening in the housing in order to facilitate a quick replacement of not only the live center body, but the bearing assemblies which rotatably support the body within the housing assembly.

Description

TURRE -MOUNTABLE LIVE CENTER FOR LATHES

Background of the Invention:

This invention generally relates to lathes for rotating and machining a workpiece, and is specifically concerned with a turret-mountable live center for replacing the tail stock center that is normally present in such a lathe.

Lathes for rotating and machining a workpiece are well known in the prior art. Such lathes are used to perform precision machining operations on a workpiece, and include a driven chuck at one end for engaging and rotating one end of the workpiece, and a live center rotatably mounted in a tail stock at the other end for rotatably supporting the workpiece. The tail stock is axially movable toward and away from the driven chuck in order to accommodate workpieces of different lengths. In precision lathes, the tail stock is typically computer driven, and includes a live center having a spindle with a conical end designed to engage a recess in the workpiece. In contrast to a "dead" center, a live center is rotatably mounted within the tail stock, and rotates along with the workpiece when the workpiece is turned by the driven chuck. The co-rotation of the live center and workpiece eliminates friction where these two components engage each other. Such lathes also typically include one or two turrets for holding cutting tools that perform a particular machining operation on the rotating workpiece (i.e., grooving, cutting, shaving, etc.). Each of the toolholding turrets is precision-movable both axially and radially with respect to the workpiece via rails and a lead screw mechanism so that the machining operation may be executed with precision. While such prior art lathes are capable of satisfactorily carrying out their intended purpose, the applicant has noticed several areas where the structure and operation of such devices can be improved. For example, the axial movement of the programmable tail stock in such machines is limited along the axial length so that the lathe is capable of machining workpieces only within a limited range of lengths. In particular, the lengths of the shortest and longest workpieces that the lathe can machine is arbitrarily limited by the minimum and maximum distances that the tail stock may be moved with respect to the driven chuck. Even more seriously, the relatively large diameter of such programmable tail stocks limits the envelope of space available for a system operator to extend and retract a tool with respect to the workpiece, particularly when the diameter of the workpiece is smaller than the diameter of the tail stock. This is a significant drawback, as such tail stocks are typically on the order of six inches in diameter and there are many workpieces that are substantially less than this diameter.

These problems are exacerbated when the system operator attempts to machine not only a small diametered workpiece, but a short workpiece as well. Under such circumstances, the available envelope for manipulating tools around the workpiece is even smaller and is substantially reduced by the relatively large diametered programmable tail stock and drive chuck. The small operating envelope is not just an inconvenience; it can actually interfere with the accuracy of the machining operation since tools supported by the turrets must extend past the block supporting the live center and therefore be cantilevered over long distances, thereby adding to the amount of chatter and vibration that results when the cutting end of the tool engages the workpiece. Finally, the applicant has noted that such programmable tail stocks are expensive components of the lathe, and are difficult to repair when the spindle point of the tail stock wears out or when the bearings between the live center and the surrounding housing become worn.

Clearly, there is a need for a live center device for use in such a lathe which extends the range of the length of the workpieces which can be machined, and which provides a larger envelope of space for the system operator to manipulate cutting tools. Ideally, such a device would be reliable, accurate, and less expensive than the programmable tail stocks now in use. Finally, such a device would facilitate the replacement of worn spindle points and bearings so that with proper maintenance the same device could be used for long periods of time.

Summary of the Invention:

Generally speaking, the invention is a turret- mountable live center for rotatably supporting one end of a workpiece on a lathe having at least two toolholding turrets. The live center of the invention comprises a housing assembly, a live center body rotatably mounted within the housing assembly, and a means for detachably mounting the housing assembly onto the toolholding turret of the lathe.

The housing assembly includes both a housing enclosure, and a pair of bearing assemblies for rotatably mounting the live center within the enclosure. Each of the bearing assemblies includes bearing races that are free floating within the enclosure to facilitate both the assembly of the apparatus and the replacement of the bearing assemblies. The housing assembly further contains a preloading means, which may take the form of a coil spring, for preloading the free floating bearing assemblies against the live center body. A preload ring is also provided that not only functions to preload the coil spring against one of the bearing assemblies, but also serves as a means for easily installing or replacing the live center body from the housing enclosure when its spindle point becomes worn. To this end, the preload ring includes a plurality of screw threads around its periphery which are engagable with mating threads disposed around an opening in the wall of the housing enclosure .

The threadedly engagable preload ring of the housing assembly not only allows the system operator to replace a worn live center body or worn bearing assemblies, but also allows the operator to select a particular type of live center body that is best adapted for the job at hand (i.e., a conical type center body, a bull nose center body, etc.) . The centering apparatus of the invention further advantageously extends the range of workpieces that may be machined on the lathe, as the turret can move the apparatus either closer to or farther away from the work-holding chuck, thereby allowing the lathe to work on shorter or longer workpieces than the lathe was originally designed for. When the operator desires to machine a longer workpiece than the lathe could normally handle, the tail stock center that normally forms a part of the lathe is completely removed from the lathe, and replaced by the centering apparatus of the invention. Brief Description of the Several Figures:

Figure 1 is a perspective view of a prior art lathe of the type that the turret-mountable live center of the invention may be applied to, wherein both of the turret assemblies are shown on one side of the lathe for the purpose of simplification; Figure 2 is a partial cross-sectional side view of the turret-mountable live center of the invention;

Figure 3 is a front view of the lathe of Figure 1 as it would appear with the tail stock removed and the live center of the invention detachably mounted onto one of the turret assemblies of the lathe illustrating in particular how the invention increases the range of lengths of workpieces;

Figure 4 is a partial plan view of the chuck head and tail stock of the prior art lathe of Figure 1, illustrating the length of cutting tool shank necessary for the lathe to machine a workpiece whose diameter is less than the diameter of the tail stock; and

Figure 5 is a plan view of the lathe of Figure 3 from the same perspective as Figure 4, only with the live center of the invention replacing the live center present in the tail stock of the lathe, illustrating how the shank of the cutting tool may be shortened due to the easier access afforded to the workpiece by the smaller width of the invention.

Detailed Description of the Preferred Embodiment :

With reference now to Figure 1, the invention finds particular application in a lathe 1 of the type used to turn and machine a cylindrical workpiece 3. Such a lathe 1 includes a base assembly 4 which supports, on one side, a driven chuck assembly 5 having a chuck head 7 with a plurality of jaws 9 disposed around its periphery for grasping one end of the workpiece 3. The chuck assembly 5 is slidably mounted on a pair of rails 11 so as to be movable along the axis of rotation of the workpiece 3 to accommodate workpieces of differing lengths. On the other side, the base assembly 4 of the lathe supports a tail stock 13 having a live center 15 projecting toward the chuck head 7. The live center includes a spindle with a conical tip 17 engagable within a bore 19 machined in the workpiece 3. Similar to the driven chuck assembly 5, the tail stock 13 is likewise slidably mounted on a pair of rails 21. The movement of the tail stock 13 is computer controlled so that the distance between the conical tip 17 of the live center 15 and the chuck head 7 may be precision controlled.

Lathe 1 further includes a pair of turret assemblies 23a, b, each of which has a toolholding arm 24a, b. The ends of each of the arms 24a, b include a socket 25a, b for receiving the shank of a tool coupling such as the frusto-conical, quick-release shank 36 illustrated in Figure 2. In the preferred embodiment, the sockets 25a, b and shanks 36 of the tool couplings are Model KM^® quick-change tool couplings of the type manufactured and distributed by Kennametal Inc., located in Latrobe, Pennsylvania. Such a tool coupling system is disclosed and claimed in U.S. Patent Nos . 4,708,040; 4,747,735; 4,836,068, and 4,932,295, each of which is assigned to Kennametal Inc., and is expressly incorporated herein by reference. The shank 36 shown in Figure 2 and the associated coupling mechanism may be one of a number of commercially available mounting assemblies and the subject invention is operable with any coupling that will secure the live center 30 to a toolholding arm 24. Each of the toolholding arms 24a,b is mounted on a turret 26a, b which in turn is rotatably movable on its respective turret assembly 23a, b. Rails 27 afford movement along the axis of rotation of the workpiece 3, while other rails (not shown) afford movement of the toolholding arms 24a, b toward and away from the workpiece 3. For the purpose of simplifying the drawings, both turret assembles 23a, b and axial rail 27 are illustrated as being on the same side of the lathe 1. However, in most such prior art lathes, the turret assemblies 23a, b and rails 27 are located on opposite sides of the lathe 1. In the prior art, the toolholding arms 24a, b were used to support various cutting tools designed to perform a particular cutting operation on the rotating workpiece (i.e., grooving, slotting, shaving, etc . ) . With reference now to Figure 2, the live center

30 of the invention is designed to replace the live center 15 of the tail stock 13 in order to achieve the advantages set forth with specificity hereinafter. To this end, the live center 30 includes a housing assembly 32, a live center body 34 rotatably mounted within the housing assembly 32, and a shank 36 that is detachably connectable to a socket 25a, b of one of the toolholding arms 24a, b. The shank shown in Figure 2 and the associated coupling mechanism is one of a number of commercially available mounting assemblies, and the subject invention is operable with any coupling that will secure the live center 30 to a toolholding arm 24.

The housing assembly 32 has an upper wall 40, a lower wall 42, and a back wall 44 as shown. Sidewalls (not shown) form a complete, fluid-tight housing enclosure 47 for receiving one end of the live center body 34 of the invention 30. Within the upper wall 40 is disposed an oil inlet bore 48 for admitting liquid lubricant to the interior 49 of the housing portion of the assembly 32. Bore 49 is capped by a screw-type plug 50. The front wall 46 of the housing portion of the assembly 32 includes a circular opening 52 for receiving an annular preload ring 5 . The circumferential peripheries of the opening 52 and ring 54 include mutually engagable screw threads which form a threaded joint 55 when the ring 54 is screwed into the position illustrated in Figure 2. On its inside surface, the preload ring 54 includes an annular recess 57 for receiving a preload spring 56 whose function will be described in more detail hereinafter. On its outer surface, the preload ring 54 includes a pair of opposing dead-end bores 58a, b for receiving the prongs of a special wrench designed to screw or unscrew the ring 54 into or out of the front wall 46. In order to maintain a fluid-tight relationship between the live center body 34 and the interior 49 of the enclosure formed by the housing assembly 32, annular grooves 60,62 housing resilient sealing rings 64,66 are provided in the locations illustrated in Figure 2.

Disposed within the interior 49 of the housing of the assembly 32 are a pair of opposing bearing assemblies 68a, b. Each of these assemblies includes a bearing race 70 whose outer periphery dimensionally conforms to the inner periphery of the walls of the housing assembly 32, and whose inner periphery houses a plurality of rotatably mounted roller bearings 72. The bearing races 70 of each of the bearing assemblies 68a, b is free floating within the interior 49 of the housing of the assembly 32. Such a free-floating relationship facilitates the assembly of the live center 30 of the invention by allowing the manufacturer merely to drop the bearing assembly 68b into the housing assembly 30 in the position shown in Figure 2. This step of assembly is followed by the insertion of the rotatably mounted end 76 of the live center body 34, whereupon the bearing race 70 of the opposing bearing assembly 68a is dropped into the position illustrated. In the final steps of assembly, the preload spring 56 is disposed in contact with the front face of the race 70 of the bearing assembly 68a, and the preload ring 54 is screwed into the position illustrated in Figure 2, thereby compressing the spring 56 against the front face of the race 70 of bearing assembly 68a. In addition to facilitating both the assembly and the replacement of the bearing assembly 68a, b, the free-floating relationship between the races 70 of these bearing assemblies 68a, b and the interior walls of the housing assembly 32 allows the roller bearings 72 of the front bearing assembly 68a to constantly apply a load onto the rotatably mounted end 76 of the live center body 34, thereby eliminating all "slack" movement of the body 34 along the axis of rotation of the workpiece 3 during the operation of the lathe 1. The live center body 34 includes the previously mentioned, rotatably mounted end 76 disposed within the interior 49 of the housing of the assembly 32, and spindle having a conical end 78 for engaging a complementarily-shaped recess in a workpiece. The conical end 78 is connected to the rotatably mounted end 76 by way of an integrally formed, spindle shaft 80. The rotatably mounted end 76 has a pair of opposing, frusto-conical portions 82a, b whose conical angle conforms to the orientation of the roller bearings 72 captured within the races 70 of the bearing assembly 68a, b. These portions 82a, b are interconnected via an annular, integral midsection 84.

Figure 3 illustrates the increase in the range of workpiece lengths that is made possible by the application of the invention 30 to a conventional lathe 1. As is illustrated in phantom, the relatively large size of the tail stock 13 limits its longitudinal movement along the rail 21 to a length LI. By contrast, the relatively smaller size of the live center 30 of the invention, coupled with greater mobility of the toolholding turret assembly 23a, allows the live center to be moved along a much longer range L2 as shown. Hence, the replacement of the live center 15 of the tail stock 13 via the mounting of the live center 30 of the invention onto the arm 24a of the turret assembly 23a allows the lathe 1 to turn and machine both shorter and longer workpieces .

The contrasts between Figures 4 and 5 illustrate how the installation of the live center 30 onto a lathe 1 allows for the use of cutting tools 86 with shorter shanks 88a, b. In Figure 4, when a very short workpiece 85 is installed between the chuck head 7 and live center 15, the shank 88a of a cutting tool 86 mounted on the end of arm 24a of the turret assembly 23a must be of a certain minimum length in order for the cutting insert 90 to reach the workpiece 85, particularly when it is desired to perform a machining operation near the end of the workpiece 85 closest the conical tip 17 of the center 15. By contrast, as shown in Figure 5, the smaller diameter of the live center 30 of the invention allows a cutting tool 86 having a much shorter shank 88b to be mounted onto the arm 24b of the turret assembly 23b. The ability to use a shorter shank 88b increases the overall rigidity of the support provided for the cutting insert 90, thereby minimizing unwanted chatter and vibration when the insert 90 engages and cuts the workpiece 85. The substantial reduction in vibration significantly improves the accuracy and smoothness of the machining operation performed by the insert 90.

While this invention has been described with respect to a preferred embodiment, various equivalences, additions, modifications, and variations of the invention will become evident to those of skill in the machine arts. For example, while the quick-release mount has been disclosed as a KM^* type mount, any detachable mount could be used to implement the invention. And while the term "toolholding turret" has been used throughout the specification and claims, the invention is applicable to any type of toolholder used in connection with a lathe. All such additions, modifications, and variations are intended to fall within the scope of this patent, which is limited only by the claims appended hereto.

Claims

WHAT IS CLAIMED IS;

1. A turret-mountable live center for rotatably supporting one end of a workpiece on a lathe having at least one toolholding turret, comprising: a housing assembly; a live center body rotatably mounted within said housing assembly; and means for detachably mounting said housing assembly onto one of said toolholding turrets.

2. The turret-mountable live center of claim 1, wherein said housing assembly includes a housing enclosure, and means for installing and removing said live center body with respect to said housing enclosure.

3. The turret-mountable live center of claim 2, wherein said housing assembly includes bearings for rotatably mounting said live center body within said housing enclosure .

4. The turret-mountable live center of claim 3 , wherein said bearings include bearing races that are free floating within said housing enclosure to facilitate the installation and replacement thereof.

5. The turret-mountable live center of claim 4, wherein said housing assembly includes means for preloading said bearings against said live center body.

6. The turret-mountable live center of claim 5, wherein said preloading means is a spring means for applying a biasing force against one of said bearing races .

7. The turret-mountable live center of claim 6, wherein said live center body installing and removing means is a preload ring for compressing said spring means against one of said bearing races .

8. The turret-mountable live center of claim 7, wherein said preload ring is threadedly engaged around an opening present in said housing enclosure.

9. The turret-mountable live center of claim 1, wherein said means for detachably mounting said housing assembly onto one of said toolholding turrets includes a frusto-conical member receivable into a complementary-shaped socket in said toolholding turret.

10. The turret-mountable live center of claim 9, wherein said frusto-conical member of said detachable mounting means includes at least one aperture for receiving a reciprocally movable locking sphere.

11. A turret-mountable live center for rotatably supporting one end of a workpiece on a lathe having at least one toolholding turret, comprising: a live center body; a housing assembly including a housing enclosure for receiving and rotatably mounting said live center body, and means for installing and removing said live center body with respect to said housing enclosure; and means for detachably mounting said housing assembly onto one of said toolholding turrets.

12. The turret-mountable live center of claim 11, wherein said housing assembly includes bearings for rotatably mounting said live center body within said housing enclosure, said bearings including races that are free floating within said enclosure to facilitate the installation and replacement thereof.

13. The turret-mountable live center of claim 11, further comprising a plurality of different live center bodies, each of which is installable and removable with respect to said housing enclosure via said installation and removal means.

14. The turret-mountable live center of auxiliary centering apparatus of claim 13, wherein said housing assembly further includes a coil spring for preloading said bearings against said live center body.

15. The turret-mountable live center of claim 14, wherein said live center body installing and removing means is a preload ring that also functions to compress said coil spring against one of said bearing races, said ring being threadedly engaged around the periphery of an opening in said housing enclosure.

16. The turret-mountable live center of claim 11, wherein said means for detachably mounting said housing assembly onto one of said toolholding turrets includes a frusto-conical member receivable into a complementary-shaped socket in said toolholding turret, and wherein said frusto-conical member of said detachable mounting means includes at least one aperture for receiving a reciprocally movable locking sphere.

17. A method of rotatably supporting a workpiece in a lathe having a workpiece-holding chuck and at least one toolholding turret by means of a turret- mountable live center, comprising the steps of: detachably connecting said centering apparatus to said turret; manipulating said centering apparatus into a workpiece centering position with respect to said workpiece-holding chuck by moving said turret; and rotatably supporting a workpiece between said centering apparatus and said chuck.

18. The method as described in claim 17, further comprising the step of removing a center-bearing tail stock from said lathe so that said centering apparatus replaces said center-bearing tail stock.

19. The method as described in claim 17, wherein said turret-mountable live center includes a means for installing and removing a live center body from a housing assembly.

20. The method as described in claim 19, further comprising the step of selecting a live center body from a plurality of such bodies for use in said centering apparatus, and installing said body in said housing assembly.

21. A turret-mountable live center for rotatably supporting one end of a workpiece on a lathe having at least one toolholding turret, comprising: a housing assembly; a live center body rotatably mounted within said housing assembly; and a quick-release mount for detachably mounting said housing assembly onto one of said toolholding turrets.