Abstract:
An apparatus for grinding a workpiece on a support surface includes a rotatable and vertically movable grinding wheel having an abrading surface and at least one cylinder coupled to the grinding wheel. The cylinder includes a piston, a stepper-motor coupled to the piston, and a converter coupled to the piston and to the stepper-motor for converting rotary movement of the stepper-motor to linear movement of the piston to vertically move the grinding wheel. A measuring device provides a representation of the distance between the support surface and the abrading surface. A controller is coupled to the measuring device and to the stepper-motor for receiving the representation and applying an activation signal to the stepper-motor to vertically move the grinding wheel to reach a predetermined distance between the support surface and the abrading surface.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/450,242, filed Feb. 25, 2003. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to a method and apparatus for grinding a workpiece to achieve a desired workpiece dimension, and more particularly to a thru-feed grinding apparatus utilizing an improved closed-loop, feedback control system resulting in enhanced size control.  
         BACKGROUND OF THE INVENTION  
         [0003]    There are many varieties of grinding machines; for example, horizontal-spindle, reciprocating-table surface grinders; double-disc grinders; and abrasive belt grinders. A thru-feed grinder is a very efficient apparatus for the high production surface grinding of workpieces because it requires little fixturing and set-up time and provides for the continuous loading and unloading of workpieces. That is, because thru-feed grinders employ a conveyor feed assembly, workpieces are fed to the grinder on a continuous basis thus permitting virtually continuous grinding.  
           [0004]    Certain traditional thru-feed grinders are equipped with hydraulic cylinders that support the grinding wheel. Such grinders, however exhibit certain shortcomings related to size control stability. That is, over time the hydraulic cylinders may drift resulting in changes in the distance between the chuck (i.e. the surface supporting the workpieces) and the working surface of the grinding wheel. The above described drift occurs for three primary reasons. First, it is extremely difficult to bleed all air from the hydraulic system. Second, the hydraulic cylinders are typically not completely leak-proof, and third, hoses coupled to the hydraulic cylinders are generally flexible and will expand with increasing pressure. Drift can result in dimensional variations in the processed workpieces, and if the drift exceeds a certain value, the system may lift the grinding wheel from the part in a relatively uncontrolled manner requiring a very precisely controlled subsequent downward movement of the grinding wheel to compensate for overshoot. Events such as this cannot be tolerated in the production of parts with high dimensional tolerances. Furthermore, the problem of achieving high-tolerance precision grinding is exacerbated by the wearing down of the grinding wheel with time as abrading material on the grinding wheel is consumed.  
           [0005]    Thus, it would be desirable to provide a precision thru-feed grinding apparatus employing a closed-loop feedback control system that substantially avoids the problems associated with the above described drift. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    According to an aspect of the invention there is provided an apparatus for grinding a workpiece on a support surface. The apparatus comprises a rotatable and vertically movable grinding wheel having an abrading surface and at least one cylinder coupled to the grinding wheel. The cylinder includes a piston, a stepper-motor coupled to the piston, and a converter coupled to the piston and to the stepper-motor for converting a rotary movement of the stepper-motor to linear movement of the piston to vertically move the grinding wheel. A measuring device provides a representation of the distance between the support surface and the abrading surface. A controller is coupled to the measuring device and to the stepper-motor for receiving the representation and applying an activation signal to the stepper-motor to vertically move the grinding wheel to achieve a predetermined distance between the support surface and the abrading surface.  
           [0007]    According to a further aspect of the invention there is provided an apparatus for grinding a workpiece on a support surface. The apparatus comprises a rotatable and vertically movable grinding wheel having an abrading surface, and a feedback control network for maintaining a predetermined distance between the abrading surface and support surface. The feedback control system comprises a stepper-motor-controlled hydraulic cylinder coupled to the grinding wheel for vertically moving the grinding wheel, a measuring device for indicating when the distance between the support surface and the abrading surface is different than a predetermined distance, and a controller is coupled to the measuring device and to the stepper-motor-controlled hydraulic cylinder for activating the cylinder to vertically move the grinding wheel to achieve the predetermined distance. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0009]    [0009]FIGS. 1 and 2 are isometric views of a thru-feed grinding apparatus in accordance with one embodiment of the present invention;  
         [0010]    [0010]FIG. 3 is a plan view of the conveyor assembly and grinding wheel utilized in the apparatus shown in FIG. 1;  
         [0011]    [0011]FIG. 4 is an isometric view of an pneumatic gauge assembly utilized in the apparatus shown in FIG. 1;  
         [0012]    [0012]FIG. 5 is an isometric view of the slide-posts, dampeners, and plates utilized in the apparatus shown in FIG. 1;  
         [0013]    [0013]FIG. 6 is a schematic diagram of a grinding apparatus in accordance with the present invention; and  
         [0014]    [0014]FIG. 7 is a schematic diagram of a pneumatic air gauge for use in conjunction with the apparatus shown in FIG. 6. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
         [0016]    [0016]FIG. 1 and FIG. 2 are isometric views of a thru-feed grinding apparatus  10  in accordance with an embodiment of the present invention. The apparatus comprises a lower housing  12 , an upper housing  14 , a continuous conveyor assembly  16  powered by a motor (not shown) within lower housing  12 , guide rails  18  and  20 , and a grinding wheel  22  mounted for both vertical and rotational movement to an upper carriage assembly (not shown) within upper housing  14 . Grinding wheel  22  may be a standard, inserted-nut, disc wheel mounted on a vertical spindle ( 65  in FIG. 6) over conveyor assembly  16 . Conveyor assembly  16  includes a motor-driven, continuous conveyor belt  24  (preferably an abrasive belt) which passes over a magnetic table  26  in a direction indicated by arrow  28 . Magnetic table  26  may comprise a variable-power, electromagnetic table that serves as a locating table and magnetic chuck.  
         [0017]    Conveyor assembly  16  is preferably a variable speed system and imparts translational movement to conveyor belt  24  that is of sufficient width (e.g. six inches) to carry workpieces  30 , placed on the belt by an operator  36 , beneath grinding wheel  22  and through the grinding operation until the workpiece is off-loaded at the downstream end of the belt. As can be seen in FIG. 2, grinding wheel  22  is configured for rotation as indicated by arrow  32  and for vertical translation as indicated by arrow  34  in a manner to be described herein below. In this manner, a workpiece  30  passes under grinding wheel  22 , and a surface of the workpiece is ground to a desired dimension. Guide rails  18  and  20  are provided to position the workpiece  30  on conveyor belt  24  and absorb the side thrust of grinding wheel  22 . The guide rails are adjustable and assist in orienting and directing the workpieces by allowing the torsional thrust of the grinding wheel to automatically position the workpieces against the rails while at the same time prevent the workpieces from being swept off the conveyor. The guide rails also prevent tipping of the workpieces as they pass under the grinding wheel thus permitting the operator to simply and continuously place workpieces on the conveyor.  
         [0018]    Referring to FIG. 3, workpieces  30  loaded onto conveyor belt  16  are carried under the working surface of grinding wheel  22  and ground as they are held in position by the magnetic table  26  and adjustable guide rails  18  and  20 . Finished parts  48  are discharged at the opposite end  40  of conveyor  24  into a suitable container or, if desired, to subsequent handling equipment. A longitudinal inclination of the magnetic table (e.g. 0.0015 inches per inch) permits the workpieces  30  to be ground only when entering the grinding wheel area and pass through the center  42  and trailing section  44  of the grinding wheel  22  untouched. This causes a slight taper  46  on the face of the grinding wheel proportional to the inclination of the magnetic table  26 . In this manner, grinding wheel  22  is continuously dressed by the workpieces. Workpiece dimension is determined by the distance  50  between the abrading surface of wheel  22  and the surface  52  upon which the workpiece is supported.  
         [0019]    In short, the grinding process is accomplished by four elements; (1) magnetic table  26  that serves as a locating surface and magnetic chuck, (2) conveyor belt  24  that moves workpieces  30  through the grinding operation, (3) rotating grinding wheel  22  that imparts a vertical force on workpieces  30  to keep them securely position on the locating surface or conveyor belt; and (4) adjustable guide rails  18  and  20  against which the workpieces are firmly held and which absorb the grinding torsional force of the grinding wheel.  
         [0020]    Housings  12  and  14  are made of any material (e.g. steel) of sufficient strength to withstand the strain of heavy stock material while at the same time provide for smooth operation. Referring to FIG. 5, a plurality (e.g. four) vertical slide posts  54  support grinding wheel  22 , motorized spindle  65  (FIG. 6) and a hydraulic feed mechanism ( 74  and  76  in FIG. 6). All sub-assemblies are mounted on heavy support plates (e.g.  56 ) which form an integral structural unit with posts  54 . Dampeners  58  under support plates  56  reduce machine-to-workhead vibrations  
         [0021]    [0021]FIG. 6 is a schematic diagram of a grinder apparatus in accordance with the present invention wherein like elements are denoted by like reference numerals. An operator places workpieces on conveyor  24  which moves in the directions indicated by arrows  60  to bring the workpieces beneath grinding wheel  22  as described above. Conveyor  24  is driven by a conveyor motor  62 , and grinding wheel  22  is rotated by motor  64  as indicated by arrow  66 . Grinding wheel  22  is coupled to a support plate  68  which is configured to slide vertically on posts  54  as is indicated by arrow  70 . Posts  54  are coupled at their upper ends to a top plate  72 .  
         [0022]    Mounted above support plate  68  are a plurality (e.g. two) of stepper-motor-controlled high-precision hydraulic cylinders  74  and  76 . Each precision hydraulic cylinder includes a cylinder shaft  78  housing a piston  80  coupled to a piston rod  82  that extends through openings  84  in top plate  72  so as to move wheel support plate  68  vertically on posts  54 . As can be seen, each cylinder includes a stepper-motor  86 , a spool and valve assembly  88  and a rotation-to-translation converter  90 . Spool and valve assemblies  88  are coupled to a source of pressure  92  and, via conduits  94  and  96 , to the inner regions of shaft  78  on opposite sides of piston  80 .  
         [0023]    Stepper-motors  86  are electrically coupled to a programmable logic controller  98  which is in turn configured to receive a measurement signal from pneumatic gauge  100 , to be more fully described below. Simply stated, as grinding wheel  22  grinds workpieces on conveyor  24 , a small amount of abrading material is lost on the abrading surface of grinding wheel  22 . Pneumatic gauge  100  monitors the distance between the abrading surface of grinding wheel  22  and the surface upon which the workpieces are resting (i.e. conveyor  24 ), and when this distance exceeds a predetermined value due to the loss of abrading material on grinding wheel  22 , controller  98  activates stepper-motors  86  which in turn causes pistons  80  to move downward and, via piston rods  82 , to move wheel support plate  68  downward so as to achieve a desired spatial relationship between the abrading surface of grinding wheel  22  and the upper portion of conveyor  24  upon which the workpieces are loaded. That is, the activation signal provided by controller  98  to stepper-motors  86  will cause the stepper-motor shafts to rotate in very precise increments. The stepper-motor shafts operate on an internal spool and valve assemblies  88  imparting rotary and linear movement to the spool and valve assemblies  88  and the appropriate closure and opening of valves to provide fluid pressure to cylinder shafts  78 . Rotation of the spool is translated to linear movement in rotary-to-translation converters  90  to move pistons  80  vertically in an appropriate direction. This may be accomplished by a ball nut attached to each piston  80  that rotates a ball screw directly coupled to the valve spool. In this manner, the speed of pistons  80  is positively synchronized to the rotational speed of the stepping motor. The piston continues rotating the spool until a shut-off position is reached (i.e. when the predetermined spacing between the abrading surface of grinding wheel  22  and the upper portion of conveyor  24  has been reached). Thus, the digitally operated rotating stepping motors  86 , cylinders  74  and  76 , grinding wheel  22 , pneumatic gauge  100  and controller  98  form a closed-loop feedback control system. Each step of the stepping motors  86  is very precise and therefore very accurate positioning of grinding wheel  22  over conveyor  24  results. Stepping motors  86  may be incrementally rotated in either direction depending upon the manner in which the signal from controller  98  is applied. In the absence of any activation signal from controller  98 , the cylinder is inherently braked and maintained stationary. A more detailed discussion of high-precision digitally, controlled hydraulic cylinders may be found in U.S. Pat. No. 3,457,836 issued Jul. 29, 1969 and entitled “DIGITALLY OPERATED ELECTROHYDRAULIC POWER SYSTEM” assigned to The Superior Electric Company, Bristol, Conn. Such devices are also commercially available from Fluid Power Technology, located in Charlotte, N.C.  
         [0024]    [0024]FIG. 7 is a schematic diagram of a pneumatic gauge suitable for use at air gauge  100  in FIG. 6. The pneumatic (air) gauge provides for continuous and automatic compensation for wear on grinding wheel  22 . It maintains a substantially constant dimension between the surface on which workpieces  30  rest (i.e. conveyor belt  24 ) and the abrading surface of the grinding wheel. This distance corresponds to the ground dimension of the finished part. Once established, workpiece thickness is maintained until all usable abrasive in the grinding wheel is consumed. At this point, a controller coupled to the air gauge automatically shuts the system down and generates an alert or warning (e.g. illuminates a light on a control panel). It operates on the principals of air flow at constant velocity and a pneumatic wheatstone bridge. The device may be considered an air to electric converter and is fed by a single air supply line  102  which is divided into two parallel paths  104  and  106 . Air lines  104  and  106  are separated by an extremely flexible, air-tight diaphragm  108 . Lower air line  106  is referred to as a measurement line and comprises a calibrated opening or measurement nozzle  110  and a measurement opening  112  which is the resultant opening produced when the calibrated air jets (or variable restricting device of a gauge) is combined with the workpiece or master. The upper airline  104  is an adjustment or balance line and comprises a calibrated opening or balance nozzle  106  and an opening  114  (e.g. an annular orifice) which permits air to escape to the atmosphere at a rate dependant upon the position of a tapered needle  116  with respect to outlet  114 . A balance or equilibrium condition exists when there is substantially equal pressure in both the balance line  104  and the measurement line  106 .  
         [0025]    Any increase in pressure in the measurement line  106  will propel the diaphragm  108  upward thus moving needle  116  upward until the annular outlet  114  around needle  116  is such that the pressure in both upper line  104  and lower line  106  is substantially equal. An opposite effect would occur if pressure were to drop in measurement line  106 . A distance measurement between the lower surface of the grinding wheel and the upper surface of the conveyor is related to the displacement of needle  116  acting on plunger  118  and indicator  120  in relation to the original position which was determined when the instrument was calibrated against a known dimension, part, or master. This measurement may be read by controller  98  (FIG. 6) via an electric probe. The displacement measurement provided by air gauge  100  to controller  98  is then converted in controller  98  to energize stepper-motors  86  to increment or decrement. As a result, cylinder  74  and  76  vertically move wheel plate  68  until the appropriate dimension has been reached at which point controller  98  terminates activation of stepper-motors  86 .  
         [0026]    Thus, there has been provided a precision thru-feed grinding apparatus that avoids the problems associated with drift and compensates for loss of abrading material on the grinding wheel through the use of high precision digital cylinders and a closed look feedback system.  
         [0027]    While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.