Patent Publication Number: US-6981909-B2

Title: Method for conditioning superabrasive tools

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to a method for conditioning abrasive tools and more particularly to a method of conditioning superabrasive wheels. 
   Precision grinding is a known process used to form machined parts by rotating or otherwise moving an abrasive tool (e.g. a grinding wheel) at a high surface speed and feeding it into a workpiece. Common abrasive tools include aluminum oxide grit dispersed in a binder, for example. “Superabrasives” such as diamond or cubic boron nitride (CBN) have also been developed, which are much longer wearing than conventional abrasives and allow higher feed rates in hard materials. 
   Strict metallography requirements (e.g. low cycle fatigue and residual stress) have hampered or even prohibited the use of superabrasive grinding wheels for certain gas turbine engine components, because of the adverse post-dress condition of the superabrasive grinding wheel. After dressing or wheel truing the binder material that holds the superabrasive particles of the superabrasive grinding wheel fills the spaces between the superabrasive crystals, inhibiting good coolant and chip flow. This condition causes part material “burning” (i.e. localized overheating) to occur until the bond material erodes sufficiently to allow space for coolant and chips. This condition is especially sensitive to softer materials such as stainless steel (e.g. AM355 and A286 alloys). Prior art methods exist for “opening” the grinding wheel surface to expose the abrasive particles, however they are typically manual methods which are difficult to perform in a repeatable manner. 
   Accordingly, there is a need for a simple and repeatable method of conditioning a superabrasive tool to avoid localized workpiece overheating. 
   BRIEF SUMMARY OF THE INVENTION 
   The above-mentioned need is met by the present invention, which according to one aspect provides a method for conditioning a grinding tool which includes a plurality of superabrasive particles dispersed in an abradable binder and a cutting surface. The grinding tool is trued by moving the cutting surface against a truing device such that the cutting surface of the grinding tool is shaped into a preselected profile. The cutting surface is then moved against a sacrificial member so that the binder is worn down, thereby creating a plurality of spaces between the superabrasive particles. 
   According to another aspect of the invention, a method of conditioning a grinding wheel includes the steps of: providing a grinding wheel comprising a plurality of superabrasive particles dispersed in an abradable binder, the grinding wheel including at least one cutting surface for contacting a workpiece made of a selected first material; truing the grinding tool by rotating the grinding wheel while moving the cutting surface against a truing device such that the cutting surface is shaped into a preselected profile; and rotating the grinding wheel while moving the cutting surface against a sacrificial member made from a second selected material similar to the first material so that the binder is worn down, thereby creating a plurality of spaces between the superabrasive particles at the cutting surface. 
   According to another aspect of the invention, a method of forming a feature in a workpiece includes: providing a grinding wheel comprising a plurality of superabrasive particles dispersed in an abradable binder, the grinding wheel including at least one cutting surface for contacting a workpiece made of a selected first material; truing the grinding wheel by rotating the grinding wheel while moving the cutting surface against a truing device such that the cutting surface is shaped into a preselected profile; and conditioning the grinding wheel by rotating the grinding wheel while moving the cutting surface against a sacrificial member made from a second selected material similar to the first selected material so that the binder is worn down, thereby creating a plurality of spaces between the superabrasive particles at the cutting surface. The grinding wheel is then rotated while moving the cutting surface against a workpiece such that a feature is formed in the workpiece. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
       FIG. 1  is a schematic side view of a grinding apparatus including a grinding wheel being dressed with a truing device; 
       FIG. 2  is a schematic cross-sectional view of a grinding wheel undergoing a conditioning process; 
       FIG. 3  is a schematic cross-sectional view of a grinding wheel and a workpiece undergoing a grinding process; 
       FIG. 4  is an enlarged view of a portion of a grinding wheel after a truing operation; 
       FIG. 5  is an enlarged view of a portion of a grinding wheel after a subsequent conditioning process; and 
       FIG. 6  is a schematic perspective view of an exemplary sacrificial member. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  illustrates an exemplary grinding apparatus  10  including a grinding wheel  12  mounted to the spindle  14  of a known type of grinding machine  16 . The grinding machine  16  rotates the grinding wheel  12  at a selected high speed. The grinding apparatus  10  also provides a holding fixture  18  for holding a truing device  20  or a workpiece (not shown). The grinding apparatus  10  includes known means for and selectively moving the holding fixture relative to engage the grinding wheel  12 , for example along mutually perpendicular “X”, “Y”, and “Z” axes. The entire grinding apparatus  10  may be controlled by a numerical controller or computer of a known type, illustrated schematically at “C”. Although the present invention is described herein with reference to a grinding wheel, it is noted that it may also be used with other types of abrasive tools. 
   As shown in  FIG. 4 , the grinding wheel  12  is composed of a plurality of superabrasive particles  22 , for example diamond or cubic boron nitride (CBN), dispersed in an abradable binder  24 . In the illustrated example, the binder  24  is a vitrified ceramic material. The grinding wheel  12  includes a cutting surface  26  which is formed in the profile of the surface feature to be manufactured. 
     FIG. 1  illustrates the process of truing the grinding wheel  12 . As used herein the term “truing” is used to mean the process of forming the cutting surface  26  to the desired profile. This is done by rotating the grinding wheel  12  and moving the truing device  20  into contact with the cutting surface  26 , so that cutting surface  26  is shaped into the desired profile. As used herein, the term “shaped” refers generally to the cutting action of the truing device  20  on the cutting surface, and not to any specific wear mechanism. The truing device  20  is a tool having a working surface  28  which is formed in the desired shape (i.e. the positive profile to be formed in a workpiece) and which is harder than the cutting surface  26  of the grinding wheel  12 . For example, if the superabrasive particles are CBN, the truing device  20  may have a diamond working surface  28 . 
     FIG. 4  illustrates the cutting surface  26  of the grinding wheel  12  after the truing process. As a result of this process, the cutting surface  26  matches with the desired profile; however the superabrasive particles  22  are not exposed and the binder  24  fills the spaces  30  between the superabrasive particles  22 . If the grinding wheel  12  were used in this condition, there would be no space for coolant flow or chip transport between the cutting surface  26  and the workpiece. Thus, “burning” or localized overheating of the workpiece would occur until the binder  24  was sufficiently worn away to expose the spaces  30 . This practice is not acceptable for components which have strict heat treatment or crystallographic requirements, such as the rotating components of gas turbine engines. 
     FIG. 2  illustrates an exemplary conditioning process carried out according to the present invention. A sacrificial member  32  is provided for this purpose. The sacrificial member  32  is constructed of a similar material to and has a dimensions similar to the anticipated workpiece, in order to most closely simulate the actual grinding conditions. In the illustrated example, the member  32  (see  FIG. 6 ) is made from stainless steel, for example an AM355 or A286 alloy, and has a length “L” of about 15.2 cm (6 in.), a width “W” of about 2.5 cm (1 in.), and a thickness “T” of about 0.95 cm (0.375 in.). Successive cuts  34  are made into the sacrificial member  32  until no signs of “burning” remain in the sacrificial member by rotating the grinding wheel  12  and bringing the cutting surface into contact with the sacrificial member. These cuts  34  simulate the teeth or slots to be made in the actual workpiece. The following parameters are exemplary, based on a grinding wheel  12  having a diameter “D” of about 15.2 cm (6 in.) to about 17.8 cm (7 in.): grinding wheel speed about 3000 to about 3500 RPM; and feed rate into the sacrificial member  32  of about 5.1 mm/min. (0.2 in./min.) at the start of the process, increasing to about 10.1 mm/min. (0.4 in./min.) or about 12.7 mm/min. (0.5 in./min.) at the finish. In the illustrated example a total of about 15 cuts are sufficient for this process. By using a known type of controller C (see  FIG. 1 ), this conditioning process can be automated. That is, the grinding wheel  12  is automatically rotated at the proper speed and positioned at a preselected alignment relative to said sacrificial member  32 . The cutting surface  26  and the sacrificial member  32  are brought into contact to form the first cut  34  and then separated. These steps are then repeated to form a second cut  34 , and so forth until the conditioning process is complete. The material and dimensions of the sacrificial member  32  and the conditioning parameters are chosen to simulate the actual grinding process, but also to promote localized overheating or “burning” of the sacrificial member  32 . For example, the thickness T of the sacrificial member  32  is chosen so that the chip path is at least as great of that of the workpiece. Therefore, once “burning” ceases in the sacrificial member  32 , the workpiece may be machined with high confidence that no workpiece “burning” will occur. 
     FIG. 5  illustrates the grinding wheel after the conditioning process. During this process, the binder  24  is naturally worn away at a faster rate than the much harder superabrasive particles  22 , leaving the superabrasive particles  22  at the cutting surface  26  exposed with open spaces  30  between them. These spaces  30  provide a flow path for coolant and cutting chips between the cutting surface  26  and the workpiece. 
   After the conditioning step described above is completed, the grinding wheel  12  is ready for use. As shown in  FIG. 3 , the sacrificial member  32  is removed from the holding fixture  18  and a workpiece  36  is mounted in the holding fixture  18 . The grinding wheel  12  is then rotated and the cutting surface  26  is brought into contact with workpiece  36  to form the desired features, shown by the representative cut  38 . There is plenty of space for coolant flow and chip transport between the cutting surface  26  and the workpiece  36 . Thus, “burning” or localized overheating of the workpiece  36  is avoided. This method offers significant gains over prior art processes. In particular, it allows the use of superabrasives in applications where it was previously prohibited. The substitution of superabrasives for conventional abrasives substantially reduces the labor, cost, material waste, and environmental impact of the grinding process. 
   The foregoing has described a method for conditioning a superabrasive grinding tool. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation, the invention being defined by the claims.