Abstract:
An apparatus for providing a flat to stepped convex facing for grinding wheels used for the finished machining of parts, the apparatus including dressing wheels located only at the outer diameter of the Cubic-Boron-Nitride wheels to be dressed.

Description:
FIELD TO WHICH THE INVENTION RELATES  
         [0001]    This invention relates to an improved apparatus for dressing fine grinding wheels utilized to smooth machine surfaces together with a method for utilizing same.  
         BACKGROUND OF THE INVENTION  
         [0002]    Lapping and grinding machines have been utilized to manipulate the flatness of surfaces for subsequent use in mechanical and hydraulic mechanisms. The purpose of this manipulation operation is to make a surface of a part, typically metal, as smooth as possible. An example would be the opposing surfaces of the rotor utilized in the White Hydraulics, Inc. Motor as represented in White U.S. Pat. No. 5,135,369. In this example application, by flattening the opposing surfaces of the rotor, the volumetric and mechanical efficiency of the device can be increased by maintaining tighter spacing and tolerances between the flat surfaces of the rotor and adjoining surfaces of the motor housing.  
           [0003]    In prior art two wheel grinding devices, a parts carrier assembly is located between two iron lapping wheels (it is called ‘lapping’ because the fine grinding particles are located in a surry and not the actual movable wheels). An example is the Hahn and Kolb Model ZL801 lapping machine. In this machine a carrier assembly consisting of a fixed outer stator, a driven inner pinion and toothed planet wheels are located between two iron lapping wheels. The parts to be lapped are located in sockets in the toothed planet wheels.  
           [0004]    The iron lapping wheels themselves are initially dressed by a separate wheel dressing unit. The machine itself includes a source of the main cutting material, for example a silicon carbide surry, that accomplishes the actual lapping function. In the lapping operation, the devices typically operate under Rule 141 (New), double lap flatness of wheels. According to Rule 141, the flatness of the iron lapping wheels are periodically tested by the operator with a straight edge across the surface of each wheel. If one wheel in concave or low in the center, and the other wheel is convex or high in the center, then the wheels are run opposed to each other with the carrier run with the wheel which is low in the center. If both wheels are low in the center, both wheels and the center carrier are run in the same direction. If both wheels are high in the center, the wheels are run in the same direction with the carrier run in a direction opposed to the wheels. The actual rotational speed of the wheels is selected in consideration with the sizing of the work together with the amount of material to be removed.  
           [0005]    Operations under Rule 141 require significant operator involvement in the operation of the machine and, in addition, typically an assistant to aid in the testing of the wheels to determine whether the wheels are low in the center or high in the center. In addition, the surry takes the same amount of material off of the iron lapping wheels as the parts being operated on by the machine.  
           [0006]    The relative flatness of the lapping operation is thus normally interconnected with the tolerances of the machine together with the skill of the setup operator.  
           [0007]    In respect to fine grinding wheels, it is necessary to periodically remove such wheels to flatten their grinding surfaces. This interrupts production while subjecting the grinding wheels to the risk of damage.  
         SUMMARY OF THE INVENTION  
         [0008]    It is an object of this invention to improve the flatness of ground parts by dressing the grinding wheel.  
           [0009]    It is an further object of this invention to reduce the cost of dressing grinding wheels.  
           [0010]    It is another object of this invention to simplify the maintenance of grinding wheels.  
           [0011]    It is yet another object of this invention to lower the tolerances of dressed grinding wheels and the parts manufactured thereby.  
           [0012]    It is still a further object of this invention to increase the efficiency of manufacture of production parts.  
           [0013]    Other objects of the invention and a more complete understanding of the invention may be had referring to the drawings in which:  
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0014]    [0014]FIG. 1 is a top view of a carrier dresser assembly built in accord with the invention;  
         [0015]    [0015]FIG. 2 is a representational expanded perspective view of two Cubic-Boron-Nitride (CBN) wheels utilized in finishing the manufactured parts with the dresser assembly of FIG. 1 in operational position;  
         [0016]    [0016]FIG. 3 is a cross sectional side view of FIG. 2 taken generally along lines  3 - 3  therein;  
         [0017]    [0017]FIG. 4 is a top view of one of the planet dresser wheels of the dresser assembly of FIG. 1;  
         [0018]    [0018]FIG. 5 is an enlarged view of the end of a CBN wheel in FIG. 3 showing concave, tapered, and flat surfaces;  
         [0019]    [0019]FIG. 6 is a view like FIG. 1 of a part carrier assembly utilized in the manufacture of the manufactured parts;  
         [0020]    [0020]FIGS. 7 and 8 are views like FIG. 1 of alternate embodiments;  
         [0021]    [0021]FIG. 9 is a representational cross section of a multiple stepped convex surface grinding wheel dressed by the alternative of FIG. 7; and,  
         [0022]    [0022]FIG. 10 is a representational cross section of a classical curved convex surface grinding wheel, this example dressed by the alternative of FIG. 8.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0023]    This invention relates to an improved dressing wheel together with the method of use therefor.  
         [0024]    With grinding wheels, fine grinding particles (like CBN) are imbedded in the body of the wheels themselves. For this reason, it is called grinding not lapping. This has the advantage of eliminating the need for a cutting surry (although preferably a coolant such as oil is substituted for thermal stability). In addition, the fine grinding function occurs at room temperature while producing no sparks. However, using at least some grinding wheels (like CBN), the Rule 141 does not work making it necessary to totally remove the grinding wheels periodically, typically once a month or so, in order to separately dress them thus to compensate for any wear patterns which develop. This subjects the grinding wheels to the risk of damage (for example during removal and reassembly) as well as interrupting the production of finished parts on the machine.  
         [0025]    In the present invention, the grinding wheels are dressed in place utilizing parts of the production assembly to a flat or convex grinding surface. For clarity, the invention will be described utilizing a two wheel fine grinding machine for power and control of the various elements disclosed herein. It is to be understood, however, that the general principals of the invention can be utilized in other machines as long as the principals set forth herein are incorporated.  
         [0026]    In the present invention a dresser is differentially moved in respect to at least one grinding wheel, with the differential movement dressing the grinding wheel to have a flat to convex surface at least on the outer extent of the grinding surface for subsequent use in the manufacture of production parts. In general the faster the relative velocity between the dresser and the grinding wheel the quicker dressing will occur.  
         [0027]    The differential movement can be provided by movement of the wheel, the dresser, or both as might be appropriate for the particular application. As it is preferred that the dressing occur with the grinding wheel mounted in place on the production manufacturing machine utilizing same. The existing controlled production movements can then be used to establish base parameters for the dressing operation. In addition change over time from production to dressing and back to production is significantly improved.  
         [0028]    In the preferred embodiment utilized as an example herein, the production fine grinding machine has two fine grinding wheels  101 ,  111  with imbedded cutting materials, which wheels are each independently operationally interconnected to two motors  102 ,  112  (FIG. 3). The axis of rotation of the wheels  101 ,  111 , are aligned. A pinion  105 , driven by third motor  116 , is located between the wheels  101 ,  111 , for relative rotation. All are supported by bearings (not shown) to a unitary frame (also not shown). This orientation allows for each fine grinding wheel  101 ,  111  and the pinion  105  to be separately controlled in respect to both speed and direction of rotation. In certain other systems, differing drives and axis orientations could be utilized, for example a single motor for all moving parts in a dedicated machine, holding one grinding wheel stationary while moving the other, rotating the outer ring  110  instead of and/or in addition to the pinion  105 , or otherwise controlling the relative rotations of the parts therein.  
         [0029]    In the particular embodiment disclosed, the two fine grinding wheels  101 ,  111  are made of aluminum some 38″ in diameter having as cutting material CBN particles some 20 to 50 microns in diameter (the ISO 6106 DIN 848 nominal mesh is 180/150) suspended in a 3 mm thick plastic carrier at the surface of the wheels (FIG. 2). The matrix surface of the fine grinding wheels are interrupted by recessed slots  113  which, together with recessed inner edge  107  and outer edge  108 , facilitate the movement of coolant to the entire surface of the CBN wheels, and holes  114  which serve to help in draining off the coolant (the coolant shown is provided to the center of the upper fine grinding wheel  101  through a feed system  115  located generally thereat. Other coolant feeds could be utilized). The recessed inner edge  107  and outer edge  108  in addition create defined end locations for the actual CBN grinding surface, thus together with an over swept dressing action eliminating any inner and outer upwards extending lip problems (FIGS. 5, 9,  10 ) (i.e. the edges  107 ,  108  are the lowest points of the CBN grinding carrier  117 , although they could be coextensive with the slots  113  if desired. In addition the matrix could be segmented with edges  107 ,  108  coextensive with the aluminum backing.).  
         [0030]    This CBN fine grinding wheel is used by way of example and it is to be understood that other types of grinding materials (such as diamonds) and/or surfaces (such as a longitudinal planar surface) could be substituted.  
         [0031]    In the invention of this application, the two fine grinding wheels  101 ,  111  are dressed into a flat to convex shape, which shape has been ascertained to be the optimum for the flatness of resulting production parts and as having other advantages such as smoother production operation.  
         [0032]    In the particular example shown, the fine grinding wheels  101 ,  111  are dressed by a dressing wheel system  120  insitu on the fine grinding machine with the outer diameter of the fine grinding wheels corrected to produce a convex shape (see FIGS. 1 and 5).  
         [0033]    The dressing wheel system preferably includes certain operative parts of the grinding machine, in the example system  120 , parts of a planetary drive provide the dressing action. This envisions the use of the same pinion drive  105  and fixed outer ring  110  as the production part carrier assembly  150  utilizes, thus simplifying the assembly and disassembly of both the dressing wheel system and the production carrier system while interchanging between the two modes. Further, since removal of the grinding wheels  101 ,  111  is not necessary and since no specialty fixture is utilized, overall cost and manufacturing efficiencies are increased with dressing and change over time reduced.  
         [0034]    In the dressing wheel system  120 , the fixed outer ring  110  cooperates with the pinion drive  105  to operate the dressing wheel system  120 , in the preferred embodiment acting to provide for the double axis rotating motion of the planet dresser wheels  125 .  
         [0035]    In the preferred embodiment disclosed, an enlarged intermediate pinion wheel  121  is located immediately surrounding the pinion drive  105  between such drive  105  and the planet dresser wheels  125 . This causes the planet dresser wheels to operate on the outer 20-40% extent of the fine grinding wheels  101  and/or  111  (33% shown) to facilitate the formation of the convex surface. The enlarged intermediate pinion wheel  121  also provides for significantly faster rotational speeds and velocity for the planet dresser wheels  125  about their own respective axis, thus providing for the potential of a more aggressive dressing operation.  
         [0036]    Located immediately outward of the pinion extender gear  121  are the set of planetary dresser wheels  125 . These are preferably relatively small in size so as to increase their relative rotational speed or velocity in respect to a given rotational speed of the pinion drive  105 . In this respect note that due to the interaction of the parts of the system the relative velocity of the planet dresser wheels  125  can differ between the inside  123  and outside  124  of such wheels  125 . This allows for control of the nature of the shape of the fine grinding wheels  101 ,  111 . The small size of the planet dresser wheels  125  also ensures that primarily the outer extent of grinding wheels  101 ,  111  will be dressed thus to facilitate the convex shaping of the grinding wheels. The aggressiveness and the smoothness of the resulting surface is further facilitated by the optional use of later described inserts  126  spaced from the rotational center of the dresser wheels  125 , which inserts removes the plastic matrix allowing the CBN to break out faster during dressing.  
         [0037]    After the surface to be dressed is determined to have the required initial shape, dressing with the planet wheels  125  is accomplished. During dressing, the dressing wheels  125  differentially move about the grinding wheels  101 ,  111  to dress same. Note that in general, more surface dressed by the dresser wheels  125  per unit time, the quicker dressing will be finished. Due to this, the faster the planet dresser wheels  125  rotate in respect to a set length grinding surface, the faster dressing will occur. This is important in that in recognition of this, the differential movement does not have to be uniform between the two grinding wheels  101 ,  111 . For example, if wheel  101  needed less dressing than wheel  111  to meet production standards, running wheel  101  at a rotational speed about the axis of the pinion more similar to that of the planet dresser wheels  125  than that of wheel  111  would reduce the dressing of wheel  101  compared to wheel  111 . (Note the same differential operation is true of the inserts  126  as well.)  
         [0038]    Although this can be accomplished in many ways, it is preferred that the planet dresser wheels  125  move about the circumference of the fine grinding wheels  101 ,  111  while also rotating about their own individual axis. This provides for a relatively uniform dressed surface (by reducing the effect of any out of standard component). In the example herein, this differential is provided by rotating the two fine grinding wheels  101 ,  111  in the same direction as and at nearly the same speed as the pinion  105  (and thus also the extender  121 ) with a slight upwards or downwards speed difference. This provides for an even dressed surface.  
         [0039]    As the planet dresser wheels  125  pass over the fine grinding wheels, the fine grinding wheels  101 ,  111  are dressed to the desired shape. In the preferred embodiment, this is a taper shape  133  to convex shape  130 , this in contrast with a concave surface  131  (shown in representational form in FIGS. 5 and 10 respectively). Note that the convex shape  130  formed by the dresser of FIG. 1 has a taper  133  (approximately 0.001″ over 4″ shown). This initial taper convex shape  133  is thus between a classical curved convex shape  129  and a flat surface  132 . This is in recognition that a taper or stepped flat surface can provide a convex surface for purposes of this invention.  
         [0040]    Subsequent production operation of any embodiment will tend to blend this convex grinding wheel into a flatter and flatter shape to the surface determined by the user as a trigger redressing.  
         [0041]    The convex shape on both wheels is preferred in that this provides the flattest resulting production parts during the later manufacture thereof. It also has the advantage of not causing the planet dresser wheels  125  (nor the parts in the planet part carriers  151  of the production carrier assembly  150 ) to dig into the fine grinding wheels  101 ,  111  when passing towards the outer edge thereof.  
         [0042]    The dressing wheel system  120  can dress one, the other, or both of the fine grinding wheels  101 ,  111 . This selective operation is produced by either selecting a set of planet dresser wheels  125  having diamond coating or other dressing material on one axial end or having such on both ends of the planet wheels  125  or by controlling relative rotation of the parts (as later described). The selective dressing could be provided by a multiple series of unitary dressing wheels having with each series having one of the above attributes (two series total) or by centrally split dressing wheels with each individual half section having a cutting material end and a non-cutting material end (one series with twice the number of parts). To minimize complexity of changeover, two series of unitary dressing wheels are preferred. Intermediate attribute dressing wheels  125  could also be utilized if desired.  
         [0043]    In addition to the above, the movement of the fixed outer ring  110  upwards and downwards in respect to each individual fine grinding wheel  101 ,  111  provides an additional control parameter by increasing or reducing the pressure of the planet dresser wheels  125  on the respective fine grinding wheel. Note that this upwards and downwards motion is not impeded by the grinding wheels  101 ,  111  due to the fact that the inner circumferential edge of the outer ring  110  has a diameter greater than that of the grinding surface of the grinding wheels  101 ,  111  (and in the example embodiment, beyond the entire wheels). This diametrical difference also allows the dresser planets  125  (and production parts in apertures  152  of the production assembly) to sweep up to and, as preferred, past the outer edge of the fine grinding wheels  101 ,  111 .  
         [0044]    In the present preferred embodiment, the dressing of the outer diameter of the wheels  101 ,  111  and the speed of the dresser wheels  125  is provided by a single part, that of an intermediate pinion extender gear  121  which is located immediately outwards of the pinion drive  105 . This pinion extender gear  121  has the effect of markedly increasing the apparent diameter of the pinion drive  105  (over double—2.16 times), thus to locate the planet dresser wheels  125  at the outer extent of the grinding wheels, as well as increasing the amount of movement or velocity of the outer side  124  of the dresser gear  125  for a given speed of the pinion  105 . The pitch diameter of the extender gear  121  is selected in view of the desired convex shape for the dressed grinding wheels  101 ,  111 . In general the point where the pinion gear  121  meets the inside  123  of the planet dresser wheel  125  defines the beginning of the convex shape, with the exact nature of such shape depending on the relative speeds and direction of rotation of the moving parts. For example as later set forth with the grinding wheels  101 ,  111  and the pinion gear  121  running in the same direction at the same speed a taper convex shape is produced. The reason for the taper convex surface in the example is that the teeth at the inside  123  of the planet dresser  125  have substantially the same velocity of the interengaging teeth of the pinions gear  121  (and thus the CBN grinding wheels  101 ,  111 . This produces minimal dressing—V inner gear equals V planet dresser at this point. However the teeth at the outside of the planet dresser  125  have a much higher velocity. The reason for this is that the outside edge of the CBN grinding wheels have the highest velocity in the system. This in combination with the neighboring and engaged fixed ring gear  110  produces a more aggressive dressing operation for the planet dressers  125  at the outside  124  thereof, and thus the resultant taper.  
         [0045]    The flat to convex shape of the dressed grinding wheels can be adjusted and/or modified by altering the relative differential between the dresser and grinding wheel, for example running the pinion gear  121  in the opposite direction at the same speed would produce a stepped convex shape. Thus the speed and direction of parts and relative velocity of the dresser planets  125  are inter-related.  
         [0046]    The preferred taper  133  is created by the relative velocity of the planet dresser wheels  125  in respect to the CBN grinding wheels  101 ,  111 . For example with the intermediate pinion wheel  121  driven in the same direction at the same speed as the grinding wheels, the inside  123  of the planet dresser wheel  125  will have a slower relative velocity than the outside  124  of such dresser wheel  125 . The reason for this is again that the inside  123  of the dresser wheel  125  is moving at a relative speed substantially equal to the intermediate pinion  121  (and thus the CBN grinding wheels) while the outside  124  of such dresser wheel  125 , being engaged with the stationary outer ring  110 , will be moving at a relative speed much higher than the CBN grinding wheels  101 ,  111 . Due to this velocity difference the outside circumference of the grinding wheels is dressed more aggressively than inward thereof: hence the taper  133 .  
         [0047]    The angularity of the taper can be controlled by the speed differential between the intermediate pinion  121  and the grinding wheels  101 ,  111 . This controls the relative velocity of the dresser wheels  125  (and thus the aggressiveness of the dressing action). For example rotating the pinion  121  faster than the grinding wheels  101 ,  111  would tend to more equalize the velocity differential between the inside  123  and outside  124  of the dresser wheels  125 , thus producing a lesser angle taper  133  (albeit with a slight step on the area inside that swept by the dressing wheels  125  if run long enough). Additional example by running the pinion  121  in the opposite direction as the CBN grinding wheels  101 ,  111  the inside  123  will become as aggressive (if not more so) than the outside  124  of the planet dresser wheels  125 , producing a step convex shape  135  (FIG. 5). Note that if each CBN grinding wheel  101 ,  111  can be individually controlled, one can vary the aggressiveness of the dressing action differentially between such wheels. This is of benefit if one grinding has a more convex initial shape than the other grinding wheel (the former needing less dressing than the latter and thus a lesser velocity between the planet dresser wheel and the grinding wheel).  
         [0048]    The present invention utilizes planet dresser wheels  125  which rotate about the axis of the pinion drive  105  at speeds different than that of the CBN fine grinding wheels  101 ,  111  about their respective axis in order to provide for an aggressive cut. Further, this aggressive cut is accomplished primarily on the outside diameter of the CBN fine grinding wheels so as to provide for two convex wheels, thus eliminating the need to compensate for possible differing shapes (concave/convex) of two fine lapping wheels during production as was done under Rule 141 (previously described), while also eliminating the need to remove the CBN wheels to grind them flat (as previously required since Rule 141 does not satisfy the maintenance needs of fine grinding wheels).  
         [0049]    The particular fixed outer ring  110  has a pitch diameter of 38.97″ with 336 inner teeth, the pinion drive  105  has a pitch diameter of 13.46″ with 114 outer teeth, the enlarged pinion wheel  121  has a pitch diameter of 29.05″ with 246 outer teeth, and the planet dresser wheels  125  have a pitch diameter of 4.96″ with 42 outer teeth. (The production planet part carriers  151  have a pitch diameter of 12.76″ with 108 outer teeth and the apertures  152  therein are 4.63″ in diameter.) The inserts  126  are 2.5″ in diameter. The example dressing action occurs with both the pinion  121  and CBN grinding wheels  101 ,  111  rotating in the same direction at approximately 70 RPM. Dressing is complete in substantially three seconds producing a taper of some 4″ in length having a drop from 0.001 to 0.003″ from the outside of the CBN grinding wheels  101 ,  111  to the inside  123  of the planet dresser wheels  125 .  
         [0050]    The dresser wheels  125  may be used by themselves or in conjunction with one or more inserts  126 , which inserts  126  are utilized in the preferred embodiment to remove some of the matrix holding the cutting material to initially define a flat to convex shape.  
         [0051]    The dresser wheels  125  are used by themselves when a simple dressing is necessary to produce the desired convex shape. For example if in the preferred embodiment after dressing the plastic matrix and CBN have an acceptable length of usability for the subsequent production operation after dressing while still maintaining the preferred convex shape. For consistency, it is preferred that the standard for this “simple dressing” reflect a pre-established objective criteria such as number of parts able to be ground in subsequent production, matrix thickness drop over the convex shape, time of previous (or subsequent) grinding operation, etc. This would simplify dressing and subsequent manufacturing production by allowing a uniform procedure to be followed. This would tend to reduce operator error, tolerance deviances, and other problems.  
         [0052]    If the convex shape is less than a sufficient amount on the area to be dressed, for example the selected standard, inserts  126  are inserted into the dresser wheels  125 . The purpose of these inserts  126  is to initially remove the matrix and some of the cutting material, thus to initially shape the grinding wheels to a convex shape. To accomplish this, it is preferred that the inserts  126  have a height greater than that of the dresser wheels  125  together with a hardness greater than the matrix but less than that of the cutting material. These attributes would allow the inserts  126  to act on the matrix independently of the dressing material on the dresser wheels  125  (due to the height differential) while removing the matrix without substantive compromising harm to the cutting material like CBN embedded therein (due to the relative hardness). The number of inserts utilized preferably is selected dependent on the amount of matrix to be removed: The less material to be removed, the greater the hardness of the inserts and, the slower the speed of the inserts, the fewer the number of inserts need be utilized.  
         [0053]    The exact initial shape defined by the inserts  126  is dependant on the location and relative velocity thereof. In general, as previously set forth in respect to the planet dresser wheels  125  the higher the relative velocity of the inserts  126  in respect to the fine grinding wheels  101 ,  111  the more material will be removed per unit time. However, this should be tempered with a recognition of the more central location of the inserts  126  in respect to the planet wheels  125  as well as that the softer plastic matrix breaks out faster than the CBN grinding material. For this reason the inserts  126  tend to create more of a stepped surface than a taper in this initial shaping—i.e. the relative hardness overcomes velocity differential.  
         [0054]    The use of the inserts  126  can be before, after, or intermediate dressing by dresser wheels  125 . Further again, one or both wheels  101 ,  111  can be subject to the inserts  126  (having differing hardness between the axial ends of integral inserts  126 , or by splitting same into two differing hardness parts and/or differing relative velocities can be used to provide differential initial matrix removal between the grinding wheels  101 ,  111 .). As with the planet dresser wheels, two series of inserts are again preferred.  
         [0055]    In the preferred embodiment, the surface of the grinding wheels  101 ,  111  are preferably dressed at or before when such surface is flat and smooth. At this time, if necessary the inserts  126  of RC 66 aluminum oxide are utilized until approximately 30% to 66% of the diameter of the CBN cutting material is left exposed and the desired convex shape is initially produced in the plastic matrix. This gives a surface substantially equal to 100 grit sand paper prior to dressing by the dressing wheels  125 . After a sufficient amount of the cutting material is exposed, it is preferred that the inserts  126  be removed. This allows that height differential between such inserts  126  and the dresser wheels  125  be maintained for subsequent use of such inserts  126 . The grinding wheels  101 ,  111  are then dressed by the planet dressers  125  to the preferred convex shape.  
         [0056]    Upon completion of the dressing operation (i.e., preferably both fine grinding wheels  101 ,  111  being convex), the planet dresser wheels  125  and intermediate pinion extender gear  121  are removed from the machine and a production carrier assembly  150  substituted.  
         [0057]    In the example utilized herein this production carrier assembly  150  includes the pinion drive  105 , six intermediate toothed part carriers  151  and the fixed outer ring  110 . As the pinion drive  105  and fixed outer ring  110  are also utilized in the dressing wheel system  120  the change over is easily accomplished with minimal concern for tolerances.  
         [0058]    After the assembly of the production carrier  150 , the parts to be ground are inserted in the apertures  152  present in these part carriers  151  so as to pass them over the CBN dressing wheels in the double rotating manner inherent in a planetary type device. This production operation continues until dressing is again needed, at which time the dressing wheel system  120  is reassembled.  
         [0059]    Although the invention has been described in its preferred form with a certain degree of particularity, it is to be understood that changes can be made deviating from the invention as hereinafter claimed. For example, it is possible to utilize all of the production part carrier assembly  150  for dressing the grinding wheels, for example inserting dressing wheels in one or all of four (A, B, C, D) of the four apertures  152  (and if appropriate inserts) therein. This would, however, necessitate an additional step of warping the grinding wheel to produce a concave shape  131  (for example by temporarily shimming the outer circumference thereof downward during dressing) in order to dress the preferred outer radial extent of such grinding wheels to produce a convex surface. With stationary carriers  151  and rotating grinding wheels  101 ,  111 , alternately as the apertures  152  go outwards from C to B to A, more aggressive dressing materials could be utilized in the apertures. Without either of these options, a flat grinding surface would be produced due to the rotation of the carriers  151  in respect to the grinding wheels  101 ,  111 . Additional examples a pinion extender  140  having multiple pockets  141  can be assembled about the pinion  105  out of contact with the surrounding fixed ring gear  110  (FIG. 7). Dresser wheels  125 A would be inserted into the pockets  141  so as to dress the grinding wheels  101 ,  111 . With all pockets occupied, this alternative produces substantially the grinding wheel surface of FIG. 9. Note that the pockets  141  shown are arranged into three offset rows, with each row at least extending to touch the area swept by an adjoining row. By varying the number and location of the dresser wheels  125 A the amount and location of dressing can be adjusted. As previously set forth in respect to the preferred embodiment it is preferred that the outre radial extent of the grinding wheels be dressed to a convex shape, in general more dresser wheels  125 A would be inserted in the outer row  142  than any other. The middle and inner rows  143 ,  144  are preferably more for maintenance of the inner surface of the grinding wheels  101 ,  110  and would thus normally utilize a lesser number of dresser wheels  125 A (if any). A further alternative would be to make the dressing materials integral with a modified no-pocket pinion extender platter  140 B having two concave surfaces, one for each grinding wheel  101 ,  111 , again out of contact with the outer ring  110 —i.e. it is not necessary to use separate dresser wheels  125 A). This would form the preferred convex grinding surfaces utilizing a single additional member  140 A in combination with existing production assembly parts. This alternative would produce the grinding wheel surface of FIG. 10. The extent of the dressing materials would again be selected to provide the preferred dressing operation. Therefor many changes can be made without deviating from the invention as herein after claimed.