TURNING MACHINES

Abstract: A tailstock (20) for a turning machine (10) has a housing portion (40) adapted to be moved and clamped on the machine base (16), and the housing portion (40) supports a beam portion (29) which extends outwardly from the housing portion. The beam portion is non-symmetrical, and of generally rectangular cross-section wherein its neutral axis (28) is coincident with the centre axis. Thus, the non-symmetry of the centre support has a principle modes of compliance which are dissimilar and these modes, when added to the compliance of a cylindrical workpiece, tend to increase the dynamic stability of the tailstock end of the workpiece.

Description: The invention relates generally to machines for supporting shaft-like workpieces, and the invention relates specifically to tailstocks on turning machines for machining shaft-like parts of rotation. Research into the dynamics of machine tool structures, particularly that shown in the treatise Machine Tool structures, Vol. 1 by Koenigsberger and Tlusty, Pergamon Press, Copyright 1970, contains a compilation of analysis work of a variety of researchers. Tlusty has shown that for a variety of horizontal bed, manual lathes, the weakest portion of the machining area lies along a shaft held between a headstock and tailstock at the tailstock centre. This portion may be further weakened when a live centre is preferred over a dead centre. Additionally, Tlusty has shown that operations such a grooving are more difficult to perform at the tailstock end of a workpiece on a lathe. Tlusty's work shows that of all the lathes investigated, two significant modes of vibration or instability are connected with the workpiece axis. The one with the lower frequency is almost horizontal with a slight inclination, while that with the higher frequency is almost vertical with a slight inclination, i.e. approximately 90° with respect to the first one. Applicant has determined that, for example, on a horizontal bed machine, the turning process generates normal and tangential cutting forces on the workpiece, i.e. horizontal and vertical forces, (the resultant cutting force lying between the two). It has also been observed that most centre support structures on tailstocks embody at least a snout for supporting a centre quill and the snout is generally symmetrical (usually cylindrical) about the workpiece axis. Even on machines where a square snout is employed, the horizontal and vertical dimensions are identical. Therefore, these machines tend to have identical compliance of the snout/quill/centre system in the vertical and horizontal directions. Applicant has determined that a far superior dynamic stability can be achieved by the inventive design, which provides for non-symmetry between the tangential and normal cutting directions at the tool point. Specifically, applicant is determined that a massive, solid quasi-rectangular cross-section is most desirable for increasing machine stability at the tailstock end of the workpiece. Workpiece size plays a great part in the overall dynamics of the turning machine stability, and the applicant has considered that specific quill support cross-sections may be configured and sized for a given workpiece to be supported, but in commercial application, a machine must be capable of handling a variety of workpieces. Therefore, the range of workpieces to be handled has necessitated applicant's optimisation of a tailstock support system, wherein the modes of compliance of the tailstock are coupled with the compliance of the workpiece to provide an outstanding support system. Applicants are aware of GB-A-2 106438, which discloses a lathe consisting of a stationary headstock, a travelling tailstock and a plurality of machining units, said tailstock being of non-symmetrical construction, being carried by guide bars in a conventional manner, and supporting centre means directly. Applicants have configured the tailstock support system so that the machine may be configured as a two-axis lathe, i.e. where a tool slide sytem moves a tool axially and radially with respect to a workpiece. Optionally, the machine may be configured as a four-axis machine, where a substantially indentical slide system is used on the opposite side of the workpiece so that simultaneous machining operations may occur. It is therefore an object of the present invention to provide a tailstock support system having improved dynamic stability characteristics. According to this invention there is provided a turning machine having a base, a headstock carrying a headstock spindle rotatable about a spindle axis, a tool support means movably mounted on said base for advancing and retracting a tool along a radial feed vector passing through said spindle axis, a tailstock having a housing portion supported on said base, said to stock having a central axis colinear with said headstock axis and including centre means for supporting a workpiece on said central axis, characterised in that the tailstock comprises a beam portion carrying the centre means and extending from said tailstock housing portion, said beam portion having a neutral axis colinear with said tailstock central axis, a cross-sectional minor axis normal to said neutral axis and oriented substantially in line with said radial feed vector, a cross-sectional major axis normal to both said neutral axis and said minor axis, said beam portion being non-circular in crosssection, and having a major axis dimension which is substantially greater than said minor axis dimension. Preferably the radial feed vector is inclined to the minor axis at a small angle. Preferably the machine further comprises a second tool support means movably mounted on said base for advancing and retracting a second tool along a second radial feed vector passing through said spindle axis, said major axis extending transversely to said first and second feed vectors, and wherein said first and second feed vectors substantially oppose one another. Preferably the first a second feed vectors are inclined relative to one another at a small included angle. Advantageously one of said feed vectors is aligned with the minor axis and the other of said feed vectors in inclined to said minor axis at a small angle. Advantageously said beam portion comprises a substantially rectanglar cross-section centered about said neutral axis, the ratio of major to minor dimension of said rectangular cross-section conveniently being between 1.5:1 and 3.5:1, preferably about 2:1. Preferably the beam portion is cantilevered from said housing portion. Advantageously said small angle is between 5° and 20°, preferably between 8° and 15°, preferably about 10°. The invention is shown embodied in a turning machine having a head stock and tailstock for suporting a shaft-like workpiece, the tailstock including a centre means for engaging the workpiece on a central axis, wherein the tailstock embodies improved apparatus comprising: a beam portion extending from the tailstock main housing portion, the beam portion having a neutral axis colinear with the tailstock central axis, and the beam cross-section is non-circular and has a minor axis normal to the neutral axis and generally oriented in line with the radial feed vector of a tool approaching the workpiece. The beam cross-section also has a major axis extending normal to both the neutral axis and the minor axis. In the preferred embodiment, the configuration of the beam portion extending from the tailstock is quasi-rectanglar, i.e. the cross-section being of substantially greater dimension along the major axis in respect of the dimension along the minor axis. There will now be given a detailed description, to be read with reference to the accompanying dr awings, of two turning machines which are preferred embodiments of this invention, having been selected for the purposes of illustrating the invention by way of example. In the accompanying drawings: FIGURE 1 is a perspective view of a turning machine for turning shaft-like workpieces which is the first embodiment of this invention; FIGURE 2 is a perspective view of a turning machine for turning shaft-like workpieces which is the second embodiment of this invention; FIGURE 3 is a view taken along the line 3-3 of Figure 1; FIGURE 4 is a perspective view of the modal coupling tailstock of the invention; FIGURE 5 is a view taken along the line 5-5 of Figure 1; FIGURE 6 is a view taken along the line 6-6 of Figure 5; FIGURE 7 is a view taken along the line 7-7 of Figure 5; and FIGURE 8 is a sectional view taken along the line 8-8 of Figure 6. Figure 1 of the drawing depicts a two-axis slant bed turning machine 10. A representative turning tool 11 is depicted in a tool turret 12, which is indexable and capable of supporting multiple tools 11 for a variety of operations. The tool turret 12 is carried on a cross-slide 13 and saddle 14 and is capable of movements in radial and axial directions, respectively with respect to a workpiece 15. The machine 10 has a slanted bed, or base 16 and headstock 17, and the headstock 17 carries a rotable spindle 18 and workholding chuck 19. A shaft-like workpiece 15 is shown extending between the chuck 19 and a tailstock 20. The tailstock 20 is movable on a way system 21 mounted to the base 16 under the impetus of a tailstock positioning unit 22, which does not form part of the present invention. The tailstock 20 comprises, in part, a quill 23 which is movable in axial directions to clamp and unclamp the workpiece 15 between the tailstock 20 and headstock 17. The quill 23 carries a centre 24 configured to fit the workpiece 15 in a manner well-known in the art. Once positioned with respect to the workpiece 15, the tailstock 20 is clamped on the machine base 16 by way clamps (not shown). Figure 2 illustrates a four-axis turning machine 25 where a second tool turret and slide assembly 26 is positioned on the opposite side of the workpiece 15 for simultaneous turning operations. Referring to Figure 3, the tool 11 of Figure 1 is shown on a feed vector 27 which passes through the centerline 28 of the workpiece 15 and tailstock 20. Referring also to Figures 4-8 the beam portion 29 of the tailstock 20 is shown as having a cross-section which is generally rectangular, although, for reasons of style and the provision of draft angles to facilitate casting, the cross-section more closely resembles, at the bottom half a "keystone" shape, and at the top half an inverted keystone shape, as can be seen in Figure 3. The quill 23 is supported in a close-fitting bore 30 in the beam portion 29 and the quill 30, in turn, carries the support centre 24 as is well-known in the art. Here, although it was previously mentioned that Figures 1 and 2 refer to "slant bed" machines, well-known in automatic machine art, it may be appreciated that the configuration of Figure 3 is equally applicable to a horizontal machine, where the tailstock mounting plane and ways are horizontal. The beam portion 29 has a minor axis 31 passing through the centerline 28 of the workpiece 16 at the minimum dimension of the generally rectangular beam portion 29. A major axis 32 passes through the cross-section normal to the minor axis 31 and establishes the height of the cross-section. Through optimisation of the design, the height-to-width ratio of the cross-section is about two, although the actual design figures for the specific embodiment have been chosen to be a width of approximately 5 1/2 inches (137.5 mm) and height of 12 1/2 inches (312.5 mm). The tool feed vector 27 approaches the workpiece 15 at an ang le alpha of approximately 10°. The saddle 14 therefor is provided with an included angle of 10° between the cross-slide 13 and the machine base 16. In the case of a four-axis machine, the second tool 33 approaches on a feed vector 34 on the opposite side of the workpiece 15 and the feed vector 34 is directed along the minor axis 31 of the beam portion 29. Thus, an included angle of 170° is established between the first and second tools 11, 13. The reason for the included angle between the first and second tools 11,33 being other than 180° is that applicant has determined that, since the separating forces occur at the cutting point of the tool, geometric stability of the workpiece will be enhanced; i.e. deflection of a workpiece away from a tool contacting a high spot on the workpiece and the corresponding separating force, will not result in a corresponding low spot being directly cut on opposite side of the workpiece 15 - as would be the case if the tools 11, 33 were placed 180° apart. Therefore, any lobes on the workpiece 15 will tend not to be amplified, but rather, the lobing will be smoothed out in an averaging fashion. In Figure 3, the second tool cross-slide 35 and saddle 38 are movable on way systems which are parallel to one another and to the slanted plane 39 of the machine base 16. Figure 5 is an elevational view showing the tailstock 20, wherein the the housing portion 40 is configured as a cored casting, having bottom way bearing surfaces 41 and the beam portion 29 extends, in a cantilevered fashion, outward from the housing portion 40. Figures 6 and 7 show further details of the tailstock 20. As shown in Figure 8, the beam portion 29 is substantially solid and has a close-fitting bore 30 surround the quill 23. The quill 23 is advanced and retracted in accordance with a quill stroke cylinder (not shown). As stated previously, the invention is applicable to both two-axis and four-axis machines, and is equally applicable to manual machines as well as automatic. Further, the base plane 39 of the machine may be in a variety of orientations, including horizontal. Further, while the tailstock has been depicted as a cast member, the invention herein is applicable to fabricated structures, as well. Additionally, those skilled in the art will appreciate that the cross-section of the beam portion may have a quasi-oval shape, such as an ellipse, or combination of shapes may be employed to establish the non-circular attributes. Those skilled in the art will also appreciate that, while the beam portion has been depicted as substantially uniform in cross-section along the entire length of the beam, variances in size along the beam are possible in accordance with the principle of this disclosure.

Claims: A turning machine (10) having a base (16); a headstock (17) carrying a headstock spindle (18) rotatable about a spindle axis (28); a tool support means (12, 13, 14) movably mounted on said base (16) for advancing and retracting a tool (11) along a radial feed vector (27) passing through said spindle axis (28); a tailstock (20) having a housing portion (40) supported on said base (16), said tailstock having a central axis colinear with said headstock axis, and centre means (23, 24) for supporting a workpiece (15) on said central axis; characterised in that the tailstock comprises a beam portion (29) extending from said tailstock housing portion (40), said beam portion (29) carrying the centre means (23, 24) and having a neutral axis (28) colinear with said tailstock central axis; a cross-sectional minor axis (31) normal to said neutral axis (28) and oriented substantially in line with said radial feed vector (27); a cross-sectional major axis (32) normal to both said neutral axis (28) and said minor axis (31); said beam portion (29) being noncircular in cross-section, and having a major axis dimension (32) which is substantially greater than said minor axis dimension (31). A machine according to Claim 1 wherein the radial feed vector (27) is inclined to the minor axis (31) at a small angle (∝). A machine according to Caim 1 further comprising a second tool support means (26, 35) movably mounted on said base (16) for advancing and retracting a second tool (33) along a second radial feed vector (34) passing through said spindle axis (28), said major axis (32) extending transversely to said first and second feed vectors (27, 34) and wherein said first and second feed vectors (27, 34) substantially oppose one another. A machine according to Claim 3 wherein said first and second feed vectors (34, 27) are inclined relative to one another at a small included angle (∝). A machine according to one of Claims 3 and 4 wherein one of said feed vectors (34 or 27) is aligned with the minor axis (31) and the other of said feed vectors (27 or 34) is inclined to said minor axis (31) at a small angle (∝). A machine according to any one of the preceding claims wherein said beam portion (29) comprises a substantially rectangular cross-section centered about said neutral axis. A machine according to any one of the preceding claims wherein the ratio of major-to-minor dimensions of said rectangular cross-section is between 1:5:1 and 3:5:1, preferably about 2:1. A machine according to any one of the preceding claims wherein the beam portion is cantilevered from said housing portion. A machine according to any one of Claims 2,4 and 5 wherein said small angle (∝) is between 5° and 20°. A machine according to any one of Claims 2,4,5 and 9 wherein said small angle (∝) is between 8° and 15°, preferably about 10°.

Inventors: HASZ, JOHN ROBERT
Applicants: CINCINNATI MILACRON INC.
Application year: 1987
Main classification: B23B3
IPC codes: B23B23, B23B3