high-force linear motor assembly using tandem axes

0001This invention relates to positioning assemblies utilising electrical linear motors and, more particularly, to assemblies that accelerate a movable tooling element approaching or exceeding one G.

0002Moving a heavy-duty cutting tool to sequential positions for carrying out a machining task has been conventionally carried out by use of three-axis positioners actuated and controlled by ball-screw drives (see US-A 4 102 035 and 4 359 814). If electrical linear motors were to be substituted for such ball-screw drives, the disadvantageous slow wind-up characteristic of the ball-screw drive would be eliminated and unprecedented acceleration/deceleration would be possible. A linear motor assembly according to the preamble of claim 1 is disclosed in DE-A- 2 358 421.

0003Unfortunately, accuracy of positioning of the tooling would be diminished by mere substitution, without uniquely controlling the force generated by the motors. Moreover, power control alone is insufficient to assure position preciseness if the assembly flexes or distorts variably in reaction to the thrust force of each motor. Such distortion is related not only to the natural flexing frequency of the supporting structure but also to the type of bearing contact with the movable elements.

0004Heretofore, table positioners using linear motors have generally employed air bearings (see US-A 4 392 642; 4 571 799; 4 985 651; and 4 761 876). Air bearings are made feasible by use of ultra-flat, inflexible granite surfaces and by limiting the movable element to a very light element such as needed for assembly or machining of light electronic components. Air bearings are undesirable because they require special support and guide surfaces that cannot be maintained in a heavy-duty, mass-machining environment for automotive components, and are undesirable because they are insufficiently dimensionally stiff when deployed to move large tooling at high accelerations.

0005Magnetic loading to increase stiffness of a linear motor assembly has been used in conjunction with sliding or roller bearings (see US-A 4 505 464 and 4 985 651). Magnetic loading of bearings does little to enhance stiffness because it is imprecise and weak; magnetic loading is primarily suited to a use that assists in following more closely a guide track and therefore does little to promote stiffness of the linear motor assembly.

0006GB-A 2 165 171 discloses an X-Y table assembly in which the table body is moved by linear motors and is mounted on a base through a saddle supported on linear bearings comprising recirculating ball bearings in which the balls run in grooves in the ends of the saddle. To reduce play or chatter between the saddle and the base the linear bearings are preloaded along the span of the saddle.

0007According to the present invention a linear motor assembly having enhanced mechanical stiffness has the combination of features set forth in claim 1.

0008Further features of the invention are set forth in the dependent claims.

0009In particular an assembly in accordance with the invention may be incorporated in a rapid positioning drilling machine as set forth in claim 10, which may be used in a method of increasing flexible productivity for drilling a plurality of openings in sequence in a workpiece, comprising indexing and feeding and sequentially reindexing, in a predetermined order, a drilling spindle over a plurality of opening sites in a single workpiece, said indexing and feeding being carried out at an acceleration/deceleration of at least about one G and the drilling spindle being rotated at speeds between 10,000 and 40,000 rpm.

0010The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>Figure 1 is a perspective view of a rapid positioning machine embodying the principles of this invention useful for drilling automotive components such as an engine cylinder block or head;</li><li>Figure 2 is essentially a side elevational view of Figure 1;</li><li>Figure 3 is an enlarged sectional view of one of the movable frames and its associated structure adapted for movement along the X-axis;</li><li>Figure 4 is an enlarged perspective view of a preferred bearing assembly for a frame showing the architecture of roller circulation and preloading;</li><li>Figure 5 is an elevational view of one structure used to fixedly interconnect the movable X-axis frames;</li><li>Figure 6 is a rear elevational view of the structure of Figure 5 with the ram and slide superimposed; and</li><li>Figure 7 is a cycle chart illustrating the degree of movement of the spindle along the respective X, Y, or Z axes to carry out drilling of a series of openings in an engine block workpiece.</li></ul>

0011The rapid positioning machine 10 equipped with a single spindle 11, shown in Figures 1-6, is capable of carrying out ultra-rapid sequential machining (i.e., drilling of a series of openings) in or on a workpiece 12 (such as an engine block shown in Figure 1) adjustably supported on a worktable 13. Although not shown in Figure 1, the worktable should deploy a fixture 90 (see Figure 2) for clamping the workpiece 12 in a rigid fixed condition that prevents deflection of the workpiece under the thrust forces of the tool while carrying out machining, and eliminates the spring effect of any hydraulic gripper or clamps. The machine has freedom to move the spindle 11 in three mutually perpendicular axes (X, Y, and Z) for effecting positioning of the spindle, although it is possible that additional degrees of movement freedom can be incorporated, such as rotation about any of the linear axes, without departing from this invention. The Z-axis permits linear feeding of the drill bit carried by the spindle into the workpiece, the Y-axis provides for vertical movement of the spindle 11, and the X-axis allows for movement in a horizontal plane.

0012At least one pair of tandem rails 14, 15 define the X-axis of movement for the spindle 11, such rails being spaced apart along the Y-axis by a distance preferably in the range of 101-203 cm (40-80 inches). The rails are rigidly interconnected by a fixed structure 16 which is desirably made of heavy steel plate and filled with sand in its base portion 17 to dampen vibrations from any applied forces. The interconnecting structure 16 has an upright column portion 18 standing on the bed portion 17. A cantilevered upper portion 19 carries rail 15, and bed 17 carries rail 14. The upright column portion 18 may be constructed as a boxed weldment, as shown in Figures 1-2, or as a trussed frame 29, as shown in Figure 5. The upper portion 19 may be an open weldment having a number of webs 30, and the bottom portion 17 may be a weldment with a plurality of bulkheads 31.

0013Alternatively, the tandem rails may define a Z-axis of movement for spindle 11, such rails being spaced apart along the X-axis. The rails would be interconnected by a horizontally extending bed. The frames would movingly sit on the rails interconnected by a trussed superstructure, preferably triangular in elevations. A movable Y-axis slide and a movable Z-axis ram could be carried by the trussed superstructure.

0014Returning to Figures 1-6, movable lower frame 20 wraps around rail 14 and movable upper frame 21 wraps around rail 15. Each frame has a C-shaped cross-section and the frames are oriented opposite to each other. Controlled movement of the frames is made possible by a plurality of roller bearing units 22 between bearing surfaces 23 on the rail exterior 24 and frame interior 25. Linear motors 26 are disposed between the rails and the frames to provide selective actuation or movement along the X-axis.

0015A column 27 fixedly connects the pair of frames 20, 21; such column supports other structure 28 which facilitates movement of the spindle 11 along or about other axes (i.e., Y and Z axes). Such column and other structure 28 are preferably constituted of lightweight metal such as aluminium alloy. Such intermediate structure may comprise an upright column 27, which has upright rails 9 thereon to define a Y-axis of movement, and a slide 72 adapted to wrap around such rails similar to that of the C-shaped cross-sectional frames. A saddle 74 is joined to the slide 72 and presents a C-shaped frame effective to cooperate in defining a Z-axis similar to that provided by the C-shaped cross-sectional frames for the X-axis. A ram 75 is movable within the saddle 74, the ram carrying a rotary-driven spindle 11 at one end thereof. The mass of the column and other structure 28 is substantially reduced by making the column 27 hollow and using a cellular skeleton therein. The use of C-shaped slide and saddle members and the use of a webbed skeleton for the ram reduce mass. The combination of the cellular skeleton for each of the components of the column and other structure 28, use of lightweight materials to reduce weight therein, and the use of rigidly-tied tandem axes makes possible an increased natural frequency for such structure which is in the range of 100-200 Hz (about 116 Hz for the preferred embodiment), and optimally at or above 180 Hz.

0016As shown in Figures 3 and 4, each frame 20, 21 is comprised of an aluminium alloy material having a web 50, integral arms 51, 52, and integral fingers 49, 53. Each frame presents three pairs of opposed inwardly-facing, parallel bearing surfaces, and the two frames together present an additional pair of parallel bearing surfaces (one surface of each pair being on each frame about the midsection of a span in the X-directions and at about the midsection for the span in Z-directions). More specifically, each frame presents a first pair of bearing surfaces 54, 55, a second pair 56, 57, and a third pair 58, 59. The bearing surfaces engage a plurality of recirculating roller bearings in units 60 which have roller bearings recirculating within the unit. A number of units may be spaced along each bearing surface, as shown in Figure 4. Frame 20 will present the mere image of the same bearing surfaces and bearing units as for frame 21. Frame 21 provides a bearing surface 61, and frame 20 provides an opposing bearing surface 62 to form yet still another opposed pair of surfaces. Pair 61, 62 is useful when the width of the frame exceeds a certain dimension, such as 90 cm (36 inches). More than one pair of surfaces 61, 62 may be deployed as needed for the bearing units shown in Figure 4.

0017The rails (14, 15) extend into the interior of one of such frames, the rails presenting pairs of outwardly-facing bearing surfaces complementary to the bearing surfaces on the frame; the rail surfaces are provided principally on attached tongues 63, 64 extending outwardly from the rail sides. As shown in Figure 2, rail 14 has a first pair of outwardly-facing, opposed bearing surfaces identified as 65, 66, a second pair of surfaces 67, 68, and a third pair 69, 70. For rail 15, the mere image of such bearing surfaces is provided.

0018Between such pairs of inwardly-facing bearing surfaces on the frames and the outwardly-facing bearing surfaces on each of the rails there is disposed the roller bearing units 60, each having a recirculating track with at least one roller bearing adapted to bear against the inwardly-facing surface of the frame and other roller bearings adapted to bear against the outwardly-facing surface of the rail.

0019The contact between the rollers of the roller bearing units and the frame and rail is with a controlled interference fit through increased preloading. Such fit is defined to mean that the bearing races or cage, and possibly the rollers, rail, or frame, deform below the yield point of the metal to accommodate roller movement. This deformation enhances the accuracy of positioning without increasing the weight of the structure. Such interference fit is desirably in the range of 10-20% of the maximum bearing loads to achieve rigidity comparable to that of 900 kg (2000 pounds). The preloading for an interference fit can be obtained by either adjusting the bearing preloaded blocks 63, 64 or precisely machining the gap between drums 51 and 52. Such tight-fitting bearing provides a dimensional interference fit in the range of 0.005-0.00125 cm (0.0002-0.0005 inches) when using steel alloy rollers and steel rails and aluminium alloy frames.

0020The linear motors deployed between the support elements of the positioner are high-force, brushless DC motors having 500-2000 pounds peak force. Such motors consist of an elongated stator, preferably coil packs, and a linear moving element, preferably rows of supported permanent magnets. Permanent/magnets and coil packs can be interchanged between stator arid movable element in any specific application. In the embodiment illustrated herein, power coil packs form the stator between the rails 14, 15 and the movable frames 20, 21. A pair of coil packs 26 are mounted on the top of rail 14 and a pair of coil packs 26 are mounted on the bottom of rail 15. Coil packs also form the stator between the saddle 74 and the movable ram 75, as shown in Figure 6. However, the permanent magnets form the stator for linear motors between the slide 72 and the column 27. Coil packs are mounted on the column itself to cooperate with the magnets of the slide.

0021Each coil pack is constructed of ferromagnetic plates laminated in an epoxy material to provide slots in which coil windings are deposited therearound prior to encapsulation. Potting techniques are used throughout the lamination so that the coils are embedded in the material devoid of bubbles or contaminants, the latter inhibiting heat transfer and thereby decreasing the life of the coils. The coils may he connected in series within a pack and the packs connected in parallel to a power control. Such control may supply the coils with two, three, or more phases; suitable commutation is used to change the polarity of the coils to keep a constant electromagnetic thrust force in one desired direction to promote linear motion. The permanent magnets are preferably a rare earth type, such as NdFeB type, providing an unusually high flux density.

0022Cooling channels are incorporated into a ferromagnetic platform that is in intimate contact with each of the ferromagnetic cores for the coil windings. Cooling fluid is forced through the channels to dissipate high heat generated by the motor induction, such cooling supplementing any convective cooling achieved by circulating a gas about the motor coil packs.

0023Bearing assemblies cooperate with the linear motors for the intermediate structure 28 in the same fashion as the bearing units were deployed for each of the frames 20, 21. For example, the slide 72 has a C-shaped cross-section, and the saddle 74 has structure which simulate the rails 14, 15 so that clusters of recirculating bearing units may be deployed in a similar fashion. Similarly, saddle 74, in cooperation with a closure plate 76, defines a C-shaped cross-section in which the ram is free to move; the ram has extensions which simulate the rails 14, 15 so that they would be surrounded by clusters of bearing units in a similar fashion as that used for frames 20, 21 (see Figure 6).

0024Spindle 11 is adapted for being driven by a motor at speeds of 10,000-40,000 rpm. A bearing assembly supports the spindle and comprises a ceramic roller with thin, dense chromium-plated races. The spindle is supported on the ram 75 which is essentially a cast member, preferably comprised of a series of bulkheads joined at a common side wall. The linear motors 83, 84 operate to actuate the ram along Z-axis by movement of the ram tongues in bearing clusters 80, 81, and 82, such tongues and bearing units defining tracks parallel to the Z-axis. The linear motors 83, 84 can actuate the spindle toward and away from the workpiece 90 at controlled rapid feed (i.e., 600-6000 cm (240-2400 inches) per minute).

0025When such machine is operated, it carries out a method that increases productivity in drilling a plurality of openings in sequence. Such method particularly comprises indexing, feeding, and sequentially indexing, in a predetermined order, over a plurality of drilling sites in a single workpiece, a single rotary-driven cutting tool by use of a linear motor assembly having at least one of the X or Y axes of movement stiffened by the use of two rigidly interconnected parallel rails spaced apart along such axes and having interference-fitted bearings supported by such rails to carry the supporting structure for such linear motor assembly. The positioning and feeding of such cutting tool is carried out at an acceleration of at least 0.5 G, but can approach two G's, with the drilling spindle being rotated at speeds between 10,000-40,000 rpm and the controlled feed being at speeds of 600-600 cm (240-2400 inches) per minute. The stiffness of the positioning apparatus allows the assembly to be used to machine cast iron within an accuracy of plus or minus 0.00127 cm (0.0005 inches). There is no need to use a bushing plate to guide the tools for accuracy. The apparatus may be modified to deploy the worktable with linear motors for carrying out Z-axis movement while limiting the mass-machining assembly to movement in the X and Y directions.

0026A duty cycle using the machine of this invention to drill 10 openings in a workpiece is shown in Figure 7. The rate of movement in inches/seconds is shown along the vertical coordinate and a time to carry out the movement is shown along the bottom coordinate. A typical detail for each hole drilling consists of feeding the drill into the workpiece at 600 cm (240 inches) per minute for a depth of 5.6 cm (2.20 inches), retracting the drill out of the drilled opening, free of the workpiece, and indexing the axis of the drill over to the next drilling site (such as by movement solely along the X-axis). This procedure is repeated for the first five holes to provide one row of drilled openings. At the end of the completion of drilling for the fifth hole, the drill is carried along the Y-axis (with no X-axis movement) to start a new row of openings and the same sequence is then followed for the remaining 6-10 openings. After the 10th opening is drilled, the drill is retracted to its start position, first along the Z-axis and then concurrently along the Y-Z axes to its zero starting status.

0027The total cycle time for drilling all of the 10 openings was 12.407 seconds or 2.068 minutes. One single opening is drilled and the drill indexed in a time period of 1.14 seconds.

0028A significant use advantage of the disclosed assembly is that its footprint (the area occupied by the assembly as it sits on a platform) is relatively small, i.e., 1.5 m x 2.5 m (5' x 8'), and is readily substitutable for conventional or existing machining units in existing machining facilities while providing enhanced productivity and machining accuracy.