METHOD AND APPARATUS FOR SURFACE GRINDING

Abstract: A surface grinding method and apparatus in a first embodiment of which a workpiece supported on a surface (56) is rotated about a vertical axis while being translated along ways (46, 47) past a cutting head (22) comprising an abrasive belt (71) driven about a drum (72) to have a working edge (17) skewed with respect to the direction of the ways. In other embodiments the workpiece (100) is fed axially into the belt (106), which may be oscillated transversely to the direction of feed, and the workpiece may be moved, in the direction of the working edge, out of engagement with the belt while the belt and workpiece are in motion.

Description: METHOD AND APPARATUS FOR SURFACE GRINDING Background of the Invention In the art of surface grinding, solid abrasive wheels are increasingly being replaced by endless abrasive belts passing around cylindrical driving drums, and the belts are very effective for removing metal efficiently. Where it is desired to produce a very flat surface of exact dimensions, however, in mass production of a number of identical objects for example, difficulty is encountered. In the movement of a succession of work- pieces past an abrasive belt, certain portions across the width of the belt encounter metal for longer working intervals than others, and consequently are worn away more rapidly. Under such conditions the first fifty or so units may be flat within required tolerances, but there¬ after the belt, while still usefully cutting metal, never¬ theless produces a surface which increasingly departs from flatness with successive workpieces. Accordingly the belt must be replaced, even though it would still be capable of grinding perhaps 150 more pieces to a less exacting flat¬ ness tolerance. The foregoing is true whether the work is fed past the abrasive belt linearly, or rotated and fed into the belt along the axis of rotation. While the situation can in some measure be alleviated by differently position¬ ing successive workpieces for grinding, this repeated jigging adjustment is not consonant with efficient mass production. Summary of the Invention The present invention resides in an improved system and apparatus for surface grinding by the use of abrasive belts, by which all parts of the belt are brought equally into contact with the object to be ground, so that the workpiece itself continuously "dresses" the belt, and ■ __! V-hA (7 * O PI_ it is possible to produce work which is flat within re¬ quired tolerances throughout the cutting life of the belt. This is accomplished by providing a complex relative motion between the workpieces and the belt, the motion having both linear and rotational components. Various advantages and features of novelty which characterize the invention are pointed out with particu¬ larity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the inven- tion, its advantages, and objects attained by its use, reference should be had to the drawing which forms a further part hereof, and to the accompanying descriptive matter, in which there are illustrated and described certain preferred embodiments of the invention. In the drawing, like reference numerals indicate corresponding parts throughout the several views. Brief Description of the Drawing In the drawing, FIGURES 1 and 2 diagrammatically show characteristics of prior art procedures; FIGURE 3 is a front elevation of a first surface grinder embodying the invention; FIGURE 4 is a plan view of the grinder of FIGURE 3 to a larger scale; FIGURE 5 is a side elevation of the grinder * to a still larger scale, parts being broken away for clarity of illustration; FIGURE 6 is a diagram showing motions available in the grinder of FIGURE 3; FIGURE 7 is a schematic showing in side eleva- tion of a second embodiment of the invention; FIGURE 8 is a fragmentary plan view of the grinder of FIGURE 7; FIGURE 9 is a fragmentary view in front eleva¬ tion of the grinder of FIGURE 7; FIGURE 10 is a diagram showing a control ar¬ rangement for the grinder of FIGURES 7-9; FIGURE 11 is a schematic showing in front eleva¬ tion of a third embodiment of the invention; and FIGURE 12 is a similar view of the structure of FIGURE 11 in side elevation. Description of a First Embodiment of the Invention Preliminary consideration should first be given to FIGURES 1 and 2, which are presented to illustrate the problem my invention is intended to solve. In FIGURE 1, reference numeral 11 identifies a hollow metallic work- piece having an upper surface to be ground. If this workpiece ' is moved past an abrasive belt in the direction of arrow 12, normal to the working edge of the belt, diagram 13 shows the relative lengths of time during which transverse elements A, B G of the belt engage the workpiece. It is obvious that elements A and G do over three times as much work as the other elements, and there- fore wear down much faster. After a number of workpieces have passed the belt, the finished surfaces will no longer be flat, edges 14 being higher than the rest' because the abrasive belt worn is lower in those regions. If the work is moved in the direction of arrow 15 against the working edge of the belt, diagram 16 shows that most of the transverse elements of the belt engage the workpiece for the same intervals, but that the two outermost elements do not do so. In this arrangement grinding will result in high corners 17 after belt wear has begun to take place. In FIGURE 2, the workpiece 11 is rotated with respect to the working edge of the belt, as shown by the arrow 18, and is fed axially to the belt. Here again, the different transverse belt portions are subject to different degrees of wear, as suggested by diagram 19, and the finish on workpieces after an interval will have high circular ridges where belt elements C and N have been worn down. The present invention minimizes these effects. Turning now to FIGURES 3-5, a surface grinder 20 according to my invention, comprises a bed 21, a cutting head 22, and a work holder 23. Bed 21 includes a base 24 and a pedestal 25 secured thereto or integral therewith so that a flat upper plate 26 of the pedestal is horizontal. OMPI Base 24 extends horizontally beyond pedestal 25 to support work holder 23 for vertical adjustment on a set of four screw jacks, three of which are shown at 27, 30 and 31, arranged for simultaneous operation by means including a handwheel 32, a mechanical interconnection 33, and driving rods 34, 35, 36, to raise and lower an elongated horizon¬ tal carrier 37 for a worktable 40. Table 40 includes a cross slide 41 arranged for linear transverse adjusting movement in dovetail ways 42 by a hand wheel 43. Table 40 is mounted for movement longitudinally on carrier 37 by a carriage 44. Carrier 37 is formed as a trough to receive lubricoolant, when such is used, and deliver it for recir- culation through a connection 45. The carrier includes a pair of horizontal ways 46, 47 on which carriage 44 is mounted for sliding motion past pedestal 25 by operation of a linear hydraulic motor 50 having a cylinder 51 secured to .carrier 37 at 52, and a piston 53 secured to carriage 44. By conventional hydraulic circuitry, not shown, motor 50 may be actuated to move carriage 44 hori¬ zontally along ways 46, 47 past pedestal 25 in either direction. ' Table 40 includes a rotator 54 having a base 55, fixed to the top thereof as by a magnetic clutch in cross slide 41, and a working surface 56 to receive jigs for holding workpieces to be ground. Suitable hydraulic or electric motor means 57 is provided for causing rotation of working surface 56 with respect to base 55 about a vertical axis, under the control of conventional circuitry not shown. Cutting head 22 comprises an angle plate 60 having a flat horizontal member 61 formed with a plurality of concentric arcuate slots 62 for securement to plate 26 of pedestal 25 by fasteners 63, the arrangment being such as to permit pivotal adjustment of the angle plate on the pedestal. A vertical member 64 is reinforced in angle plate 60 by gussets 65, 66 and 67, and supports a housing 70 containing an abrasive belt structure including an endless abrasive belt 71 driven around a lower contact drum 72 by conventional drive, centering, and tensioning means, not shown, so that the belt and drum project at the bottom of a housing 73 to present a cylindrical abrasive surface moving with a component of rotation about the axis of drum 72. That axis is horizontal, and is skewed with respect to the direction of linear movement of the table, the angle of skew being adjustable at slots 62 and fas¬ teners 63. When the depth of cut is only a few thousands of an inch, the belt may be said to have a "working edge", which is a straight line parallel to the axis of drum 72 where the travel of the belt is at its lowest. It will be appreciated that if wet grinding is to be carried on, suitable splash guards can be provided as suggested at 73, 74, 75, 76 to extend the full length of bed 21. Attention is now directed to FIGURE 6, in which adjustments available in setting up the apparatus are shown by broken lines, and motions occuring in operation of the grinder are shown by solid lines. Arrow 80 shows the pivotal adjustment available to head 22 by reason of slots 62. Arrow 81 shows the transverse movement made available for table 40 by hand wheel 43. Arrow 82 shows the vertical adjustment available to carrier 37 by hand • wheel 32. Arrow 83 shows the movement of belt 70. Arrows 84, 84 show the linear movement of table 40 along ways 46, 47. Arrow 85 shows the rotary movement available to table 40 by operation of motor 57. An illustrative embodiment of the invention has the following characteristics: Speed of belt 71 7000 s.f.p.m. (2133.6 m/min) Width of belt 71 3 to 6 inches (7.62-15.24 cm) Belt grit 70 to 220 Horizontal travel of carriage 44 60 inches (152.4 cm) Working lineal speed of infinitely vari- carriage 44 able — 0 to 21 inches/second (0-53.34 cm/sec) Diameter of surface 56 20 inches (50.8 km) Vertical movement of carrier 37 12 inches (30.48 cm) Minimum clearance between working surface 56 and belt 71 10 inches (25.4 cm) Speed of rotator 54 0 to 600 r.p.m. variable Operation of the First Embodiment The operation of the grinder is as follows. A jig or holder (not shown) appropriate to the configuration of pieces to be ground in mass production is secured to surface 56 magnetically or by suitable clamps. Such pieces are typically the product of a previous machining or grinding step, and are interchangeably insertable in a suitable jig so that a surface to be ground flat is upward and horizontal, and so that only a few thousands of an inch need to be removed from that surface. The machine is shown in FIGURE 3 with table 40 at a first "loading" station, where a workpiece is manually loaded into the jig. Hand wheel 43 is rotated to center the jig on the longitudinal path of the carriage. A belt of suitable width and grit is inserted, centered, and tensioned in head 22, and the latter is pivotally adjusted at slots 62 to give the desired angle of skew between the axis of drum 72 and the direction of linear carriage movement. Hand wheel 32 is rotated to set the depth of cut for the belt as desired, the belt drive is initiated, and motors 57 and 50 are set in operation. Carriage 44 is now being driven to the left as seen in FIGURE 3, moving quite slowly, and motor 57 is causing rotation of the workpiece about a vertical axis at a considerably higher peripheral speed. As the workpiece is displaced and rotated under the belt, any given point on the surface of the workpiece engages many different sites transversely of the belt in a complex pattern such that no belt site is cutting material for a longer inter¬ val than any other. When the table reaches the end of its stroke it ■ has reached a second "loading" station, where the finished piece can be unloaded and another added for movement past the belt in the opposite direction. This arrang ent is one in which the work itself "dresses" the abrasive sur¬ face, keeping it everywhere the same thickness, so that the workpiece is given a truly flat finish in which the scratch pattern is well confused and no extended scratches exist at the surface for later trouble with minor leaks past a sealing surface. It will be evident that successive pieces ground on this machine will all meet flatness tolerances because the abrasive belt is being worn uniformly. If dimensional tolerances are also critical, suitable small changes in the level of carrier 37 may be made to compensate for wear of the abrasive and to keep dimensions acceptable. Description of a Second Embodiment of the Invention A grinder as shown in FIGURES 3-5 has many pre¬ cision components and is costly to build and maintain. ' For many purposes the advantages of my invention can be obtained in a simplified embodiment shown, somewhat sche¬ matically, in FIGURES 7, 8, 9, and 10. Here a workpiece 11, which may, for example, be that shown in FIGURE 1, is mounted in any conventional fashion upon a table 100 capable of rotating about a vertical axis as suggested by the arrow 101 and rotated at either of two speeds by a rotary hydraulic motor 102 through a belt 103. Table 100 is mounted on a carriage 104 having vertical motion as in¬ dicated by the arrows 105, which may conveniently include a hydraulic feed for rapid action and a mechanical feed for fine action, suggested at 106 and 107. A cutting head 110, generally like head 22 of FIGURES 3-5, is positioned above table 100, and includes an abrasive belt 111 driven by suitable motor means not shown over upper and lower horizontal cylindrical drums 112 and 113. The belt is considerably narrower than the drums, and at least the lower drum is of steel or other hard durometer material. A simple splash guard 114 of - QRE length determined by the maximum belt width may be mounted as suggested in FIGURE 9, to collect the swarf resulting from the grinding process, and may include a water curtain arrangement 115. Conventional driving and tensioning means are provided, as is a centering arrangement modified as will now be described. FIGURE 8 shows that upper drum 112 is mounted for slight pivotal movement about a vertical axis by operation of actuators 116 and 117 which pivot the drum in opposite directions, as suggested by -the arrows 120 and 121: the extent of the motion is somewhat exaggerated in the figure. A first sensor 122 comprising a photocell 123 and a lamp 124 causes operation of actuator 116, when the edge of the abrasive belt reaches a limit of motion to the left, to pivot drum 112 in the direction of arrow 121, and a second sensor 125 comprising a photocell 126 and lamp 127 causes operation of actuator 117, when the edge of the abrasive belt reaches the limit of motion to the right, to pivot drum 112 in the direction of arrow 120. While this arrangement is satisfactory for centering a full-width belt in the cutting head 22 of the first embodiment, it has been modified for use with the narrower belts of the second embodiment by the provision of further sensors 130 and 131 comprising photocells 132 and 133 and lamps 134 and 135. These can be positioned to be spaced by slightly more than the actual width of belt 111, and centered with respect to the length of drums 112 and 113. Control of actuators ' 116 and 117 can be trans¬ ferred from sensors 122 and 125 to sensors 130 and 131, as is suggested in FIGURE 10, by operation of a simple selec¬ tor 132 connected between the sensors and the actuator controller 133. In this embodiment of the invention no component for moving table 100 in a direction orthogonal to its axis of rotation is necessary. Operation of the Second Embodiment of the Invention The operation of the grinder is as follows. The width of belt 111 is selected to be not much greater than the greatest transverse dimension of workpiece 11 taken through the axis of rotation, and sensors 130 and 131 are spaced accordingly. A workpiece 11 is mounted on table 100, which is then set in slow rotation. Belt 111 is placed in operation under the centering control of sensors 130 and 131, and therefore remains at substantially the same axial position along drum 113. Carriage 104 is raised until the workpiece almost engages the belt, and is then set into slow upward motion so that the belt makes a "plunge" cut directly into the top of the workpiece as the latter rotates. When the desired amount of metal has been re¬ moved from the workpiece, the vertical movement of the carriage is stopped, the rotary speed of the table is in- creased, and centering control of the belt is transferred to sensors 122 and 125. Belt 111 now moves back and forth across the full width of drums 112 and 113 as the work- piece rotates, so that every part of the belt comes into alignment with every part of the workpiece, and the latter is given a fine finish, at the same time "dressing" the belt so that the wear on it is everywhere the same. The travel of the belt axially of the drums is preferably equal to the sum of twice the belt's width added to the greatest transverse dimension of the workpiece normal to the axis of rotation. After the piece is finished the table is lowered, its rotation is stopped, and the work- piece is replaced so that the process can be continued. Description of a Third Embodiment of the Invention Referring now to FIGURES 11 and 12, a third preferred embodiment of the invention is shown to comprise a cutting head 200 mounted over a workholder 201. Cutting head 200 is preferably like head 110 of FIGURES 7-9, and includes upper and lower drums 202 and 203 traversed by an abrasive belt 204. Conventional mounting, drive, and belt centering means not shown are included. A workpiece is mounted in any conventional fash¬ ion on a table 205, which is mounted on a carriage 206 and is capable of rotation about a vertical axis by a hydraul- ic motor 207 through a belt 210. Carriage 206 and motor 207 are mounted on the outer slide 211 of a nest 212 of such slides, to enable horizontal linear movement of carriage 206 in a direction parallel to the axis of drum 203 and hence to the working edge of belt 204. The inmost slide 213 of nest 212 is carried by a suitable support 214, and hydraulic means suggested at 215 cause movement of carriage 206 as indicated by the arrow 216, when such is desired. As in FIGURES 7 and 9, support 214 may be provided with rapid and fine vertical motions, suggested by the arrow 217. Operation of the Third Embodiment The general operation of this embodiment of the invention is as that described for the second embodiment: two added factors are present, however, which must be men¬ tioned specifically. As a factor of convenience, table 205 may be withdrawn from under cutting head 200 for mounting work- pieces without interference, after which the workpiece can be moved under the cutter head by hydraulic motor 215. Thereafter the workpiece is raised and rough-cut as described above, while belt 204 remains centered. Verti¬ cal feed is then arrested. A special feature of the third embodiment lies in the fact that the workpiece can be withdrawn laterally from the belt while both are still in operation, this withdrawal being aligned with the working edge of the belt. It has been found that this final step increases both the flatness of the finished workpiece and the per- fection of the dressing of the belt. From the foregoing it will be evident that I have invented a new procedure for using abrasive belts for production grinding of surfaces of great flatness with unprecedented economy of belt usage, as well as three different structures by which the inventive procedure may be carried out. The procedure comprises the steps of rotating the workpiece about an axis and simultaneously bringing about linear relative motion between the work- piece and the belt having at least a component aligned with the working edge of the belt. In one of the embodi¬ ments the belt is narrower than the axial dimension of the drums, and the linear motion is accomplished by causing oscillation of the belt axially on the drum. Numerous characteristics and advantages of the invention have been set forth in the foregoing descrip¬ tion, together with details of the structure and function of the invention, and the novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrative only, and changes may be made in detail, especially in matters of shape, size, and arrange¬ ment of parts, within the principle of the invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.

Claims: WHAT IS CLAIMED IS: 1. The method of surface grinding with an abrasive belt which comprises the step of so feeding the work to the belt that all parts of the work engage all sites on the belt to cause uniform wear of the belt across the entire working transverse dimension thereof. 2. The method of surface grinding with an abrasive belt which comprises the step of moving a workpiece lin¬ early past the working edge of the belt and simultaneously rotating the workpiece about an axis orthogonal to the direction of linear movement. 3. The method of claim 2 in which the width of the belt is at least half the distance from the center of rotation of the workpiece to the portion thereof most distant from said center. 4. The method of surface grinding, using an abra¬ sive belt moving to define a linear working edge, which comprises feeding the work to the belt with a motion compounded of a linear component, in a direction skewed to the direction of movement of the belt at the working edge, and a component of rotation about an axis orthogonal to that direction. 5. The method of surface grinding, using an abra¬ sive belt moving to define a linear working edge, which comprises feeding the work to the belt with a motion compounded of a linear component, in a direction skewed to the direction of movement of the belt at the working edge, and a component of rotation about an axis orthogonal to that direction, the magnitudes of said components being such that the work makes several rotations while advancing linearly by one inch. 6. A method of surface grinding with an abrasive belt which comprises rotating a workpiece about an axis passing through the working edge of an abrasive belt, feeding the workpiece toward the belt, interrupting the feed of the workpiece, and causing transverse motion of the belt with respect to said axis so that the workpiece dresses the belt. 7. A method according to claim 6 including the further step of increasing the speed of rotation of the workpiece when its feed is arrested. 8. A method of surface grinding with an abrasive belt which comprises the steps of moving a workpiece linearly along the working edge of the belt and simultane¬ ously rotating the workpiece about an axis orthogonal to the direction of linear movement. 9. A method of surface grinding with an abrasive which comprises rotating a workpiece about an axis ortho¬ gonal to the working edge of an abrasive belt, feeding the workpiece toward the belt, interrupting the feed of the workpiece, and causing transverse oscillation of the belt with respect to said axis so that every point on the work- piece passes every point on the belt. 10. A method of producing a smooth flat surface, with an abrasive belt driven around a cylindrical drum to define a linear working edge, which comprises the steps of first rotating a workpiece about an axis orthogonal to said working edge and feeding the workpiece toward the belt along the axis, to perform a roughing cut, and then interrupting the axial feed of the work, increasing its speed of rotation, and causing transverse oscillation of the belt with respect to the workpiece, to perform a finishing cut. - * £U EX 11. The method of claim 10 in which the width of the belt is at least as great as the greatest transverse dimension of the workpiece taken through the axis. 12. The method of claim 10 in which the amplitude of the oscillation of the belt is at least as great as twice the sum of the belt width added to the greatest transverse dimension of the workpiece taken through the axis. 13. A method according to claim 10, including the further step of finally moving the workpiece with respect to the belt in a direction having at least a component aligned with said working edge, without interrupting the rotation of the workpiece or the drive of the belt. 14. The method of claim 13 in which said direction is substantially aligned with said working edge. 15. A surface grinder comprising, in combination, a cutting head and a work holder; said cutting head comprising an abrasive belt, means continuously driving said belt to define a linear working edge, and means mounting said head above said work holder; and said work holder comprising a work table for securement of a workpiece thereto, means mounting said table below said head for adjustment in a direction ortho¬ gonal to said working edge, means for causing linear movement of said work table past said cutting head in a plane to which said working edge is parallel, the direc¬ tion of said movement being aligned with said working edge, and means for simultaneously causing rotary movement of said work table about a second axis perpendicular to said plane. 16. A surface grinder comprising, in combination, a cutting head and a work holder; said cutting head comprising a contact drum mounted for rotation about an axis, an abrasive belt carried by said drum, means continuously driving said belt to define with said drum a cylindrical abrasive surface moving with a component of rotation about said axis, and means mounting said head above said work holder with said axis substantially horizontal; and said work holder comprising a work table includ¬ ing means for securing a work piece thereto, means mount- ing said table below said head for adjustment in a direc¬ tion orthogonal to said working edge, and means for simul¬ taneously causing rotary movement of said work table about an axis orthogonal to the axis of said drum. 17. A grinder according to claim 16 including means for causing linear movement of said work table past said cutting head in a plane to which said working edge is parallel, the direction of said movement being skewed with respect to said working edge. 18. A grinder according to claim 16 in which said head is mounted for pivotal adjustment about a vertical axis. 19. A surface grinder comprising a cutter head and a work holder; said cutter head comprising an abrasive belt of predetermined width, means including first and second drums of width greater than that of said belt continuously driving said belt to define a linear working edge, and means operative in a first condition to maintain said belt in a predetermined location along said drums, and in a second condition to cause oscillation of said belt along said drums; said work holder comprising a work table for securement of a workpiece, and means causing rotation of said worktable about an axis orthogonal to said working edge to bring a workpiece mounted thereon into engagement with said working edge; ^^X REΛT OM?I and means fixing said head with respect to said work holder. 20. A surface grinder according to claim 18 in which the amplitude of the oscillation of said belt is at least as great as the width of said belt. 21. A surface grinder comprising a cutting head, a work holder, and means mounting said head above said work holder; said cutting head comprising an abrasive belt of predetermined width, means including upper and lower drums of width greater than said belt continuously driving said belt to define a horizontal linear working edge, and means operative in a first condition to maintain said belt at a predetermined location along said drums, and in a second condition to cause oscillation of said belt along said drums from end to end thereof, and said work holder comprising a work table for securement of a workpiece, and means causing rotation of said work table about a vertical axis to bring a workpiece mounted thereon into engagement with said working edge.

Inventors: STEINBACK CLARENCE I, STEINBACK C
Applicants: ACROMETAL PRODUCTS INC, ACROMETAL PROD INC
Application year: 1980
Main classification: B24B1
IPC codes: B24B21