Abstract:
A method for heating surfaces includes heating the surface of a hole by inserting a magnet cylinder into the hole and rotating the magnet cylinder, heating the surface of a hole by inserting a magnet stack into the hole and reciprocating the magnet stack, or heating a selected area of a workpiece surface by positioning a magnet disc adjacent the selected area and rotating the magnet disc. In each case, eddy currents are produced, inducing heating of the surface.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/391,816, filed on Oct. 11, 2010, the entire content of which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    The heating of small holes, such as grease ports or holes in bearing components and the heating of selected surface areas of a workpiece is often necessary during the manufacturing of a product to impart desired properties to the product. Spot heating is sometimes required on a hard surface in order to maximize tooling life and improve the production rate. Spot heating is also used to help prevent, at great expense, carburizing. 
         [0003]    The surfaces of small holes and selected areas of larger surfaces can be heated by induction heating. However, to heat a small hole, such as a grease port or hole on a bearing component, is extremely difficult to achieve via induction heating. The power to the inductor is limited due to the fact that a short may occur if the power is at a level sufficient enough to cause eddy currents to be induced from one side of the inductor into the other side. Induction heating of a selected area of a workpiece surface is possible with a coil designed such that the heating is localized within a given geometric area. In such an application, the heating results from eddy currents generated within the material. By also utilizing conduction, deeper heat penetration is obtained for a given frequency utilized. The power supplies required for induction, although low in power output, are costly. 
         [0004]    Spot heating a selected area of a surface can also be accomplished by flame heating where the flames are directed by a nozzle. In flame heating, the material is heated by conduction which has a significantly slower heating rate as the thickness of the material increases. Flame heating requires a fuel source and can produce green house gases. Ventilation is required to ameliorate potential personnel safety issues. 
       SUMMARY 
       [0005]    Rotational Magnetic Heating (RMH) improves upon the prior art methods for spot heating and hole heating by using more economical and safer equipment. RMH is capable of more readily changing the frequency over a broader range as compared to conventional induction power supplies. Like induction heating, RMH produces eddy currents, however, without the need for a variable frequency power supply. RMH is safer and more environmentally friendly than flame heating. In carburizing, regions that will be machined post-heat treatment may be coated with paint to prevent carbon diffusion and remain soft. This paint must be manually applied, increasing cost. 
         [0006]    RMH can be used for hardening, tempering or other heat treatment of the surface of a hole. To accomplish hole heating via RMH, magnets arranged as a cylinder and with their poles alternating can be placed within the hole and rotated by a drive. Due to their high strength-to-size ratio, the inner diameter of small holes can be heated, and thus heat treated. In spot heating, the magnets are arranged annularly and are rotated by a spindle above or adjacent the desired location. 
         [0007]    By rotating the magnets at high rpm&#39;s, eddy currents are generated with ferromagnetic or paramagnetic materials placed in close proximity of the rotating magnets. The heat produced by the eddy currents is sufficient enough to anneal the material to a hardness suitable for machining For a constant heating time, the depth of penetration of the eddy currents is determined by the rotational speed of the spindle (and hence of the magnets). At lower rotational rates, a frequency suitable for deep penetration is produced; whereas at higher rotational rates, a frequency suitable for shallow penetration is produced. 
         [0008]    As an alternative to RMH for heat treating the surface of a hole, a stack or lamination of magnets defining regions of alternating polarity can be reciprocated translationally or oscillated within a hole to generate eddy currents and harden, temper, or otherwise heat treat the surface of the hole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic drawing of a heating apparatus used for heating the surface of a hole; 
           [0010]      FIGS. 2 and 3  are a schematic side elevational and plane views of a magnet cylinder of the device of  FIG. 1 , showing the orientation of the magnets of the magnet cylinder; 
           [0011]      FIG. 4  is a schematic drawing of a heating apparatus used for heating a selected area of a workpiece surface; 
           [0012]      FIG. 5  is a schematic plan view of a magnet disc for use with the device of  FIG. 4 , showing the orientation of the magnets of the magnet disc; and 
           [0013]      FIGS. 6 and 7  are schematic section views of a magnet stack being reciprocated translationally to heat the surface of a hole. 
       
    
    
       [0014]    Corresponding reference numerals will be used throughout the several figures of the drawings. 
       DETAILED DESCRIPTION 
       [0015]    The following detailed description illustrates the invention by way of example and not by way of limitation. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives and uses of the invention, including what I presently believe is the best mode of carrying out the invention. Additionally, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. 
         [0016]      FIG. 1  schematically shows a heating device  10  that is used to heat the surface HS of a hole H. The heating device  10  comprises a magnet cylinder  12  mounted to the bottom of a shaft  14  which in turn is connected to a drive  16 . The drive  16  can be an electric motor or any other type of drive which can impart rotational motion to the shaft  14  and the magnet cylinder  12 . As shown in  FIGS. 2 and 3 , the magnet cylinder  12  is comprised of a plurality of elongate permanent magnets M, each of which has a north pole N and a south pole S. The magnets M are configured such that the pole sides of the magnets form a cylinder of a desired axial length, preferably, the magnets M have a length generally equal to the depth of the hole (or the depth to which the hole surface is to be heated). Further, the magnets M are positioned in the cylinder  12  such that the poles alternate, thereby defining regions of alternating polarity. Thus, as seen in  FIG. 2 , the side surface of the magnet cylinder  12  presents elongate magnet surfaces of alternating poles. 
         [0017]    The magnets M are preferably rare earth permanent magnets capable of delivering a continuous flux density of greater than 1 Tesla. The illustrated embodiment uses neodymium-iron-boron (NdFeB) magnets of about 1.2 T and a Curie temperature of about 540 degrees Fahrenheit, however, other suitable rare earth magnets can also be used. In an alternative embodiment, ceramic magnets can be alternatingly positioned between every two NdFeB magnets. The orientation of the NdFeB magnets would be constant. The ceramic magnets can be electrically activated to create fields opposite in polarity to the NdFeB magnets. In other embodiments, the magnet cylinder  12  can be formed by starting with an unmagnetized cylinder of a desired size and shape and magnetizing it to have the desired magnetic characteristics (i.e., the desired regions of alternating polarity), such as those achieved by using the magnets M. As used herein an in the appended claims, the term “magnet cylinder” includes both a cylinder made from a plurality of individual magnets and a cylinder that is magnetized to have the desired magnetic characteristics. 
         [0018]      FIG. 4  schematically shows a heating device  20  for heating a selected area of a surface WS of a workpiece W. The heating device  20  comprises a magnet disc  22  mounted to the bottom of a shaft  24  which in turn is connected to a drive  26 . The drive  26  can be an electric motor or any other type of drive which can impart rotational motion to the shaft  24  and the magnet cylinder  22 . As shown in  FIG. 5 , the magnet disc  22  is comprised of a plurality of elongate permanent magnets M, each of which has a north pole N and a south pole S. The magnets have a length sufficient to define a circle of a desired diameter. Smaller length magnets will produce smaller discs, and hence, will heat smaller areas than longer magnets. The magnets M are configured such that the pole sides of the magnets form a lower surface of the magnet disc  22 . Further, the magnets M are positioned in the disc  22  such that the poles alternate, thereby defining regions of alternating polarity. Thus, as seen in  FIG. 5 , the bottom surface of the magnet disc  22  presents magnet surfaces which define the disc, the surface of adjacent magnets being of different poles. In other embodiments, the magnet disc  22  can be formed by starting with an unmagnetized disc of a desired size and shape and magnetizing it to have the desired magnetic characteristics (i.e., the desired regions of alternating polarity), such as those achieved by using the magnets M. As used herein an in the appended claims, the term “magnet disc” includes both a disc made from a plurality of individual magnets and a disc that is magnetized to have the desired magnetic characteristics. 
         [0019]    In operation, to heat a hole surface HS, the magnet cylinder  12  has a diameter that is slightly less than the diameter of the hole, such that the hole surface HS will be within a magnetic field produced by the magnets M. Similarly, to spot heat an area of a workpiece surface WS, the magnet disc  22  is positioned proximate the area of the surface WS to be heated, with the bottom surface of the disc  22  facing the surface WS. The disc  22  is positioned such that there is a gap between the disc  22  and the workpiece surface WS, but such that the workpiece surface is within the magnetic field produced by the magnets M of the disc  22 . In either device, the magnet cylinder  12  or disc  22  is rotated by the drive  16 ,  26 . The rotation of the magnets M produces eddy currents which heat the surface HS, WS. For a given amount of heating time, the depth of penetration of the heating is dependent upon the frequency of the eddy currents. The frequency, in turn, is dependent upon the number of poles in the cylinder  12  or disc  22  and the rate of rotation of the cylinder  12  or disc  22 . 
         [0020]    The formula equating the frequency (Hz), the number of poles (nP), and the rotational rate (RPM) is set forth as Hz=(nP*RPM)/60. The factor of 60 is to convert the RPM to revolutions per second (RPS), producing a frequency similar to that of a current from a power supply. The frequency is directly proportional to the number of poles and the rotational rate. Therefore, if the rotational rate of the magnet cylinder  12  or magnet disk  22  is reduced, the same frequency can be achieved by increasing the number of poles. 
         [0021]    In RMH, high magnetic flux frequency is generated even with low cost commodity industrial electric motors or other drive systems whose speed is often limited to a few thousands revolutions per minute. The rotation of the magnets M generates eddy currents within ferromagnetic or paramagnetic materials placed in close proximity to the tool piece. As the rotational speed increases, a progressively shallower region is heated. In the context of spot surface heating, the heat produced within the material by the eddy currents is sufficient enough to anneal the material to a hardness suitable for machining In some applications, the heating can be useful for hardening the surface of a hole or of a workpiece. Induced heating of the workpiece can be used to achieve a temperature in the austenitic range of the workpiece, resulting in hardening of the workpiece through a microstructural transformation after quenching. Such hardening could be useful in preserving threads or improving wear characteristics in the hole surface. 
         [0022]      FIGS. 6 and 7  schematically illustrate a heating device  30  that is used to heat the surface HS of a hole H. The heating device  30  comprises a stack or lamination of permanent magnets  32  mounted on a shaft  34 , which in turn is connected to a drive  36 . The drive  36  can be a linear actuator (e.g., a solenoid, etc.) or any other type of drive (e.g., rack and pinion arrangement, cam/follower arrangement, etc.) which can impart translational reciprocating motion or oscillation to the shaft  34  and the magnet stack  32 . As shown in  FIGS. 6 and 7 , the magnet stack  32  is comprised of a plurality of annular, disk-shaped permanent magnets M, each of which has a north pole N and a south pole S. The magnets M are configured such that a north pole of one magnet faces a north pole of an adjacent magnet in the magnet stack  32 . Likewise, a south pole of one magnet faces a south pole of an adjacent magnet in the magnet stack  32 . In other words, the magnetically opposing pole sides of the magnets face each other, resulting in regions of alternating polarity and resulting in the magnets M tending to repel one another. The magnets M are assembled in the magnet stack  32  using suitable securing means to hold the repelling magnets M together. In the illustrated embodiment, stop members  38  are provided to secure the magnets M together in the magnet stack  32  on the shaft  34  to form a cylinder of a desired axial length. In other embodiments, the magnet stack  32  can be formed by starting with an unmagnetized member or cylinder of a desired size and shape and magnetizing it to have the desired magnetic characteristics (i.e., the desired regions of alternating polarity), such as those achieved by using the magnets M. As used herein an in the appended claims, the term “magnet stack” includes both a stack made from a plurality of individual magnets and a stack that is magnetized to have the desired magnetic characteristics. 
         [0023]    Preferably, the magnet stack  32  has an axial length greater than the depth of the through hole H (or the depth to which the hole surface is to be heated). As seen in  FIGS. 6 and 7 , the hole H is a through hole and the magnet stack  32  has an axial length greater than or equal to three times the depth of the through hole H. In other embodiments, the hole can be a blind bore that can be heat treated via oscillation of the magnet stack  32  so that at least an portion of the hole surface HS adjacent the open end of the hole can be heat treated. 
         [0024]    In operation, to heat a hole surface HS using the heating device  30 , the magnet stack  32  has an outer diameter that is slightly less than the diameter of the hole H, such that the hole surface HS will be within a magnetic field produced by the magnets M. The magnet stack  32  is translationally reciprocated or oscillated along the axis of the hole between the positions shown in  FIGS. 6 and 7  by the drive  36 . The reciprocating translation or oscillation of the magnets M produces eddy currents which heat the surface HS. For a given heating time, the depth of penetration of the heating is dependent upon the frequency of the eddy currents. The frequency, in turn, is dependent upon the number of poles and the rate of reciprocation or oscillation of the magnet stack  32 . For a given amount of heating time, to heat to a deeper depth, a lower rate of reciprocation can be used, while a higher rate of reciprocation can be used to heat to a shallower depth. 
         [0025]    In the embodiment illustrated in  FIGS. 6 and 7 , the hole H has a diameter of about 0.455 inches and the magnet stack  32  includes twenty-eight ring magnets M of grade N42. The ring magnets M have an outer diameter of about 0.375 inches, an inner diameter of about 0.125 inches, and a thickness of about 0.0625 inches. This results in eight cycles per one inch of travel of the magnet stack  32  in a single direction, and sixteen cycles for each stroke of one inch movement (both up and down). An exemplary rate of 3,000 strokes per minute would therefore result in 48,000 cycles per minute, or 800 cycles per second. In other embodiments, the particular ring magnets M, stroke travel, and reciprocation rate can vary to suit the particular application. 
         [0026]    As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. Various features of the invention are set forth in the following claims.