Abstract:
Embodiments of the present invention provide methods for manufacturing an even-wall rotor or stator that do not suffer from drawbacks of the prior art. Even-wall rotors or stators produced according to those methods are also provided. In one embodiment, a method for manufacturing a rotor or stator for use in a mud motor is provided. The method includes providing a vacuum chamber; providing a metal electrode at least partially disposed in the vacuum chamber; providing a mold disposed in the vacuum chamber; and melting a portion of the electrode with a direct current arc, the molten metal flowing into the mold ring.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     Embodiments of the present invention generally relate to methods for producing even wall down-hole power sections and power sections produced according to those methods.  
         [0003]     2. Description of the Related Art  
         [0004]     In drilling a borehole in the earth, such as for the recovery of oil, it is conventional practice to connect a drill bit on the lower end of an assembly of drill pipe sections that are connected end-to-end so as to form a “drill string”. The drill string is rotated and advanced downward, causing the drill bit to cut through the underground rock formation. A pump on the surface typically takes drilling fluid (also known as drilling mud) from a mud pit and forces it down through a passage in the center of the drill string. The drilling fluid then exits the drill bit, in the process cooling the face of the drill bit. The drilling mud returns to the surface by an area located between the borehole and the drill string, carrying with it shavings and bits of rock from downhole.  
         [0005]     A conventional motor is typically located on the surface to rotate the drill string and thus the drill bit. Often, a drilling motor that rotates the drill bit may also be placed as part of the drill string a short distance above the drill bit. This allows directional drilling downhole, and can simplify deep drilling. One such motor is called a “Moineau motor” and uses the pressure exerted on the drilling fluid by the surface pump as a source of energy to rotate the drill bit.  FIG. 1A  is a sectional view of a prior art Moineau motor  100 . Motor housing  110  contains an elastomeric rubber stator  120  with multiple helical lobes  125 . The stator  120  of  FIG. 1A  has 5 lobes, although a stator for a Moineau motor with as few as two lobes is possible. Inside the stator  120  is a rotor  140 , the rotor  140  by definition having one lobe fewer than does the stator  120 . The rotor  140  and stator  120  interengage at the helical lobes to form a plurality of sealing surfaces  160 . Sealed chambers  147  between the rotor and stator are also formed.  
         [0006]     In operation, drilling fluid is pumped in the chambers  147  formed between the rotor  140  and the stator  120 , and causes the rotor to nutate or precess within the stator as a planetary gear would nutate within an internal ring gear. The centerline of the rotor  140  travels in a circular path around the centerline of the stator  120 . The gearing action of the stator lobes  125  causes the rotor  140  to rotate as it nutates.  
         [0007]     One drawback in such prior art motors is the stress and heat generated by the movement of the rotor  140  within the stator  120 . There are several mechanisms by which heat is generated. The first is the compression of the stator rubber by the rotor, known as interference. Radial interference is necessary to seal the chambers to prevent leakage and under typical conditions may be on the order of 0.005″ to 0.030″. The sliding or rubbing movement of the rotor combined with the forces of interference generates friction.  
         [0008]     In addition, with each cycle of compression and release of the rubber, heat is generated due to internal viscous friction among the rubber molecules. This phenomenon is known as hysteresis. Cyclic deformation of the rubber occurs due to three effects: interference, centrifugal force, and reactive forces from torque generation. The centrifugal force results from the mass of the rotor moving in the nutational path previously described. Reactive forces from torque generation are similar to those found in gears that are transmitting torque. Additional heat input may also be present from the high temperatures downhole.  
         [0009]     Because elastomers are poor conductors of heat, the heat from these various sources builds up in the thick sections  130   a - e  of the stator lobes. In these areas the temperature rises higher than the temperature of the circulating fluid or the formation. This increased temperature causes rapid degradation of the elastomer. Also, the elevated temperature changes the mechanical properties of the rubber, weakening the stator lobe as a structural member and leading to cracking and tearing of sections  130   a - e , as well as portions  145   a - e  of the rubber at the lobe crests.  
         [0010]     This design can also produce uneven rubber strain between the major and minor diameters of the power section. The flexing of the lobes  125  also limits the pressure capability of each stage of the power section by allowing more fluid slippage from one stage to the subsequent stages below.  
         [0011]     These forms of rubber degeneration are major drawbacks because when a downhole motor fails, not only must the motor be replaced, but the entire drillstring must be “tripped” or drawn from the borehole, section by section, and then re-inserted with a new motor. Because the operator of a drilling operation is often paying daily rental fees for his equipment, this lost time can be very expensive, especially after the substantial cost of an additional motor.  
         [0012]     Advances in manufacturing techniques have led to the introduction of even wall power section motors  150  utilizing thin tubular structures as shown in  FIG. 1B . Manufacturing techniques have been developed to produce tubular stator  160  and rotor  140  members that allow manufacturers to bond a thin elastomer material layer  170  on one of these surfaces (layer  170  bonded on stator  160  as shown). These units  150  provide more power output than the traditional designs above due to the more rigid structure and the ability to transfer heat away from the insulative material  170  to the external housing  160 . With improved heat transfer and a more rigid structure, the new even wall designs operate more efficiently and can tolerate higher environmental extremes. Although the outer surface of the stator  160  is shown as round in shape, the shape of outer surface may also resemble the shape of the inner surface of the stator. Further, the rotor  140  may be hollow.  
         [0013]     Several manufacturing techniques have been developed to produce these tubular members. Hydro forming has been used to produce rotor and stator geometry. This process involves forming a tube into a specific geometry by collapsing the tube onto an inner mandrel of predefined shape using external pressure. The mandrel is extracted and reused after forming. Explosive forming is done utilizing the same process as above with one exception. The external forming pressure is produced by detonating an explosive charge.  
         [0014]     Roller forming (Extruding) utilizes rollers and a series of rams to gradually form and shape the tube onto an inner mandrel. Another variation involves a series of consecutive dies and rollers to gradually reduce the tube to final shape. These two processes require precise control of the tube and rollers to create accurate geometry. Once formed, the inner mandrel is extracted and reused as above. Pilger forming is a process where the tube is formed using hydraulic presses that beat or push the material into shape over a preformed mandrel. Investment casting has also been used to create short stator sections. These sections are aligned and joined together to form the complete stator component.  
         [0015]     Forming operations require materials that can tolerate a large amount of deformation or cold work to produce the final geometry. Materials are usually low carbon or low strength alloys that are initially in the annealed condition. The part/material gains its final strength through cold work to final shape. The nature of this process excludes the use of high strength materials and limits the use of some non-magnetic materials. Formed parts have a non uniform stress distribution that is geometry-dependent based on varying degrees of cold work as mentioned above. This compromises overall part strength and affects secondary manufacturing operations such as welded end connections, or surface coating integrity.  
         [0016]     The length of a formed part is determined by its support equipment, i.e. pressure vessels, fixtures, molds, etc. A large capital investment must be made to produce each unique part. Forming operations are also limited by market driven tubing sizes. Designs, fixtures, etc. must be designed around existing tube stock. The inner mandrel used during forming operations must be extracted from the finished part. This requires additional manufacturing steps that can cause damage to the finished part.  
         [0017]     Therefore, there exists a need in the art for a method for manufacturing an even-wall rotor or stator that is economical and produces a rotor or stator that is durable and reliable in operation.  
       SUMMARY OF THE INVENTION  
       [0018]     Embodiments of the present invention provide methods for manufacturing an even-wall rotor or stator that do not suffer from drawbacks of the prior art. Even-wall rotors or stators produced according to those methods are also provided.  
         [0019]     In one embodiment, a method for manufacturing a rotor or stator for use in a mud motor is provided. The method includes providing a vacuum chamber; providing a metal electrode at least partially disposed in the vacuum chamber; providing a mold disposed in the vacuum chamber; and melting a portion of the electrode with a direct current arc, the molten metal flowing into the mold ring.  
         [0020]     In one aspect of the embodiment, the method further includes rotating the mold. In another aspect of the embodiment, the mold includes inner and outer members and the molten metal pours into a space between the inner and outer members. In another aspect of the embodiment, the mold has a non-circular profile formed on an inner or outer surface thereof. In another aspect of the embodiment, the mold has a substantially hypocycloid profile formed on an inner or outer surface thereof.  
         [0021]     In another embodiment, a method for manufacturing a rotor or stator for use in a mud motor is provided. The method includes providing a robot having a welding gun; depositing a layer of metal using the welding gun; moving either one of the welding gun or the layer away from the other; repeating the depositing and moving step until the rotor or stator is formed.  
         [0022]     In one aspect of the embodiment, the layer is deposited onto a base and the method further includes rotating the base. In another aspect of the embodiment, the layer has a non-circular shape. In another aspect of the embodiment, the layer has a circular shape. In another aspect of the embodiment, the layer has a substantially hypocycloid shape. In another aspect of the embodiment, the method is performed in a chamber flooded with an inert or reactive shielding gas. In another aspect of the embodiment, the method is performed in a vacuum chamber.  
         [0023]     In another aspect of the embodiment, the layer of metal is deposited by plasma-arc welding. In another aspect of the embodiment, the layer of metal is deposited by a step for pinch arc welding. In another aspect of the embodiment, the layer of metal is deposited by gas tungsten-arc welding. In another aspect of the embodiment, the layer of metal is deposited by flux-cored arc welding. In another aspect of the embodiment, the layer of metal is deposited by submerged arc welding.  
         [0024]     In another embodiment, a method for manufacturing a rotor for use in a mud motor is provided. The method includes rotating a mold having a substantially helical-hypocycloid profile formed on an inner surface thereof; and pouring molten metal into the mold, wherein centrifugal force caused by the rotation of the mold will press the molten metal under sufficient pressure so that the molten metal will substantially evenly fill the profiled inner surface.  
         [0025]     In another aspect of the embodiment, the mold is in a pressure chamber. In another aspect of the embodiment, a longitudinal centerline of the mold is substantially horizontal.  
         [0026]     In another embodiment, a method for manufacturing a rotor or stator for use in a mud motor is provided. The method includes providing a means for manufacturing the rotor or stator; and a step for manufacturing the rotor or stator, thereby producing the rotor or stator having a substantially helical-hypocycloid shape.  
         [0027]     In another embodiment, a rotor or stator made according to the method of the first embodiment and/or aspects thereof is provided. In another embodiment, a rotor or stator made according to the method of the second embodiment and/or aspects thereof is provided. In another embodiment, a rotor made according to the method of the third embodiment and/or aspects thereof is provided.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0029]      FIG. 1A  is a sectional view of a prior art Moineau motor.  FIG. 1B  is a sectional view of a prior art even wall power section motor.  
         [0030]      FIG. 2A  is a simplified schematic of a prior art vacuum arc remelting (VAR) process.  FIG. 2B  is a sectional-isometric view of either a rotor or stator being formed using a VAR process, according to one embodiment of the present invention.  
         [0031]      FIG. 3A  is an illustration of a typical robot welder  300  as may be used in an alternative embodiment of the present invention.  FIG. 3B ( 1 ) is a side view of two workpieces prepared to be joined by welding.  FIG. 3B ( 2 ) is a sectional view of the GMAW gun with a pinch arc power supply in use.  FIG. 3C  is a sectional view of the PAW gun in use.  FIG. 3D  is a sectional view of the GTAW gun in use.  FIG. 3E  is a sectional view of the SMAW gun in use.  FIG. 3F  is a sectional view of the SAW gun in use.  FIG. 3G  is an illustration showing a rotor or stator being formed according to an alterntative embodiment of the present invention.  
         [0032]      FIG. 4  is an isometric view of a finished even wall rotor or stator made using either the VAR or weld casting processes described with reference to  FIGS. 2 and 3 , respectively.  
         [0033]      FIG. 5  is a longitudinal sectional view of a centrifugal casting (CC) apparatus employing a CC process.  
     
    
     DETAILED DESCRIPTION  
       [0034]     A simplified schematic of a vacuum arc remelting (VAR) process  200  is shown in  FIG. 2A . A cylindrically shaped, alloy electrode  201  is loaded into a liquid-cooled, copper crucible or mold  202  of a VAR furnace, the furnace is evacuated, and a direct current (dc) electrical arc is struck between the electrode (cathode) and some start material (e.g., metal chips) at the bottom of the crucible (anode)  202 . Alternatively, the electrode  201  may be continuously fed into the mold  202  and the mold may be made from graphite or another conductive material. Preferably, the electrode  201  is made from a metal, such as steel. The arc heats both the start material and the electrode tip, eventually melting both. As the electrode tip is melted away, molten metal drips off, forming a part  203  beneath while the electrode  201  is consumed. Because the crucible diameter is larger than the electrode diameter, the electrode must be translated downwards toward the anode pool to keep the mean distance between the electrode tip and pool surface constant; this mean distance is called the electrode gap  204 .  
         [0035]     As the cooling water  205  extracts heat from the crucible wall, the molten metal next to the wall solidifies. At some distance below the molten pool surface, the alloy becomes completely solidified, yielding a fully dense part  203 . After a sufficient period of time has elapsed, a steady-state situation evolves consisting of a “bowl” of molten material situated on top of a fully solidified part base. As more material solidifies, the part grows. The other significant parts of a typical VAR furnace shown in  FIG. 2A  include vacuum port  206 , furnace body  207 , cooling water guide  208 , ram drive screw  209 , and ram drive motor assembly  210 .  
         [0036]      FIG. 2B  is a sectional-isometric view of either a rotor or stator  220  being formed using a VAR process  250 , according to one embodiment of the present invention. The VAR process  250  can be used to produce the even wall power section shapes as continuous cast products. A tubular mold is composed of inner  215   a  and outer  215   b  members. A substantially hypocycloid profile is formed on an inner surface of the outer mold member  215   b  and on an outer surface of the inner mold member  215   a . Alternatively, only the outer mold member  215   b  is used to form a solid rotor, the inner surface of the outer mold member may simply be round to make the stator  160  shown in  FIG. 1B , and/or various profiles may be used to form any desired shape, such as other non-circular shapes.  
         [0037]     The mold members  215   a,b  are rotated  225  during the melting process to produce helical-hypocycloid shapes for either rotors or stators  220 . As the mold members  215   a,b  rotate, a solidified portion (see  FIG. 4 ) of the rotor or stator  220  feeds out  230  of the mold rings, thereby resulting in a continuous casting process. Coordinating the material deposition rate with the rotational speed of the mold, any pitch (lead) can be produced with high accuracy mimicking a conventional machining process.  
         [0038]      FIG. 3A  is an illustration of a typical robot welder  300  as may be used in an alternative embodiment of the present invention. As used herein, the term “robot” includes any automated device. Robot welder  300  may be, for example, a Panasonic Industrial Robot Pana Robo Model AW-010A, manufactured by Matsushita Industrial Equipment Co., Ltd., Osaka, Japan. This particular model is specifically adapted for use in automatic welding operations. Alternatively, a simpler welding robot or arm, i.e. a two or three axis arm, may be used. Robot  300  has a base  301  and a turret  302 . The turret  302  is rotatably connected to the base  301 . A front arm  303  is rotatably connected to the turret  302 . A rear arm  304  is also connected to the turret  302 . The front arm  303  and the rear arm  304  are connected to the upper arm  305 . The front arm  303  and the rear arm  304  are independent so the rear arm  304  can be used to adjust the angle of the upper arm  305  after the front arm  303  has positioned the upper arm  305 .  
         [0039]     The upper arm  305  is rotatably connected to a wrist assembly  320 . The wrist assembly  320  can be extended or retracted. Further, the wrist assembly  320  is rotatably connected to a first member  321 . The first member  321  is rotatably connected to a second member  322 . Also, the second member  322  can be extended from or withdrawn to the first member. The second member  322  holds a gas metal-arc welding (GMAW) gun  323   b , which is fed by a wire feeder  324 . Alternatively, the gun may be a plasma-arc welding (PAW) gun  323   c , in which case the wire feeder  324  is not necessary; a gas tungsten-arc welding (GTAW) gun  323   d , in which case the wire feeder  324  may be replaced by a filler rod feeder (not shown); a flux-cored arc welding (FCAW) gun  323   e ; or a submerged arc welding (SAW) gun  323   f , in which case the wire feeder  324  may be replaced by flux feeder from a hopper. Each robot welder  300  may also include a microprocessor and a memory for storing a job (not shown).  
         [0040]      FIG. 3B ( 1 ) is a side view of two workpieces prepared to be joined by welding.  FIG. 3B ( 2 ) is a sectional view of the GMAW gun  323   b  with a pinch arc power supply in use. A consumable metal electrode  340 , fed through the welding gun  323   b , is shielded by an inert gas  342 . No slag is formed on the solidified weld  337   a  and several layers can be built up with little or no intermediate cleaning. Examples of suitable inert gasses  342  are argon, helium, a mixture of argon and helium, a mixture of argon and carbon dioxide, carbon dioxide, and carbon dioxide with small amounts of oxygen.  
         [0041]     One type of a GMAW process is known as pinch arc or Rapid Arc GMAW. (Rapid Arc was a trademark of Zues Corp., now believed to be out of business. RapidArc is a trademark of Lincoln Electric Co. Note, however, the two processes may not be the same.) Such a pinch arc welder is made under one or more of the following U.S. patents, incorporated herein by reference: U.S. Pat. Nos. 2,800,571, 3,136,884; 3,211,953; 3,211,990; 3,268,842; 3,316,381; 3,489,973; and 4,857,693. The former website of Zues Corp., available in an Internet archive service at http://web.archive.org/web/20040619211422/http://www.zuescorp.com, is herein incorporated by reference.  
         [0042]     These patents and the website disclose methods and apparatus for pinch arc welding wherein in general context the length of weld wire  340  is provided for deposition  341  in molten form  337   b  on the workpiece  330  by the steps of electronically coupling a capacitance  343  between the workpiece  330  and the length of weld wire  340 , inductively  342  charging the capacitance  343  when the end of the length of weld wire  340  is out of electrical communication with the workpiece  330 , discharging the capacitance  343  through the weld wire  340  to establish an arc between the end of the length of weld wire  340  and the workpiece  330  by bringing the end of the length of weld wire  340  into electrical communication with the workpiece  330 , whereby the weld wire  340  end is deposited  341  as molten weld metal  337   b  onto the workpiece  330  while pinching off the end from the rest of the weld wire  340 , and continuously feeding weld wire  340  into the arc while shielding the arc from surrounding air.  
         [0043]      FIG. 3C  is a sectional view of the PAW gun  323   c  in use. Gas  334  is injected through a constriction nozzle  332  and out an orifice  335 . In the space between a tip of a tungsten electrode  331  and the workpiece  330 , high temperature strips off electrons from the gas atoms; thus, some of the gas  334  becomes ionized. The mixture of ions and electrons is known as plasma. The plasma becomes hotter by resistance heating from the current passing through it. Since the arc is constrained by an orifice  335 , the heat intensity and, thus, the proportion of ionized gas increase and a plasma arc is created. This provides an intense source of heat and ensures greater arc stability. Since workpiece  330  is connected to a positive terminal, electrons flow to the workpiece and the method is known as plasma-transferred arc welding (PTAW).  
         [0044]      FIG. 3D  is a sectional view of the GTAW (also known as tungsten inert gas (TIG)) gun  323   d  in use. The arc is maintained between the workpiece  330  and a tungsten electrode  360  protected by the inert gas  342 . A filler  362  may or may not be used. To strike an arc  374 , electron emission and ionization of the gas  342  are initiated by withdrawing the electrode  360  from the work surface in a controlled manner, or with the aid of an initiating arc. High-frequency current superimposed on the alternating or direct welding current helps to start the arc and also stabilizes it. The weld zone is visible, and there is no weld spatter or slag formation, but electron particles may enter the weld.  
         [0045]      FIG. 3E  is a sectional view of the SMAW gun  323   e  in use. The arc  374  is struck between the filler wire or rod (consumable electrode)  372   a  and the workpieces  330  to be joined. The current may be either ac or dc. In the latter case, the electrode  372   a  may be negative (dc, electrode negative, DCEN or straight polarity) or positive (DCEP or reverse polarity). The coating  372   b  fulfills several functions: combustion and decomposition under the heat of the arc  374  creates a protective atmosphere; melting of the coating  372   b  provides a molten slag  337   d  cover on the weld  337   a,b ; the sodium or potassium content of the coating  372   b  readily ionizes to stabilize the arc  374 . Also, alloying elements may be introduced from the coating  372   b . During welding, the coating melts into the slag  337   d  which must be removed if more than one pass is required to build up the full weld thickness. Since the coating  372   b  is brittle, a variant called flux-cored arc welding (FCAW) is used for automated processes. In FCAW, the coating  372   b  is placed inside the electrode  372   a  (called flux instead of coating) so that the electrode  372   a  may be wire fed. Sometimes additional shielding is provided with a gas, and then the process resembles GMAW. A heat affected zone (HAZ)  337   c  of the workpiece  330  is also shown.  
         [0046]      FIG. 3F  is a sectional view of the SAW gun  323   f  in use. The consumable electrode is now the bare filler wire  340  fed through a contact tube  380 . The weld zone is protected by a granular, fusible flux  384  supplied independently from a hopper (not shown) in a thick layer  337   e  that covers the arc  374 . The flux shields the arc  374 , allows high currents and great penetration depth, acts as a deoxidizer and scavenger, and may contain powder-metal alloying elements. Tandem electrodes can be used to deposit large amounts of filler material.  
         [0047]      FIG. 3G  is an isometric view of an even-wall rotor or stator  320  being formed using a weld casting process  350 . Utilizing the robot welder  300  and any of the GMAW gun  323   b  with a pinch arc power supply, the PAW gun  323   c  (connected for a PTAW process), the GTAW gun  323   d , the FCAW gun  323   e ; or the SAW gun  323   f , a structure, such as the even-wall rotor or stator  320 , can be weld formed by following a substantially hypocycloid path  355  as the weld gun  323   a/b  deposits weld metal in a layer by layer fashion. After each layer  320   a  is deposited, the created structure  320  is rotated  325  for the next layer so that the helical-hypocycloid shape (see  FIG. 4 ) will be formed and either one of the weld gun  323   a/b  or the part  320  is moved away from the other so that the next layer may be deposited. The welding gun  323   b - f  continues following the path  355  and applying material until the part  320  is complete. Alternatively, the weld casting process  350  may be used to form layers of any desired shape, such as circular and other non-circular shapes.  
         [0048]     This process capitalizes on the rapid solidification of the weld material and the low energy imparted into the part  320 . Without these low temperature processes, the formation of a stable structure would be difficult. Geometric tolerances and material microstructure can be held within tight tolerances with this process. Part surfaces may require secondary machining operations to achieve a smooth surface finish.  
         [0049]     Preferably, to guarantee proper metallurgy, this process is done in an environment that provides adequate shielding from reactive elements in the atmosphere. Preferably, each part  320  is produced within a chamber or area  358  flooded with the inert or reactive shielding gas  342  as opposed to just shielding the weld by injecting gas through the weld guns  323   b - f . A reactive gas constituent has the advantage of reducing surface oxides that may be present. A vacuum chamber  358  and  358   a  may also be used to provide this protection. Less preferably, the inert or reactive shielding gas  342  may simply be injected through the welding guns  323   b - f , however, this may not provide the one hundred percent shielding potential necessary for certified metallurgy.  
         [0050]      FIG. 4  is an isometric view of a finished even wall rotor or stator  420  made using either the VAR or weld casting processes described with reference to  FIGS. 2 and 3 , respectively. Ends  420   a,b  may receive couplings (not shown) so that the rotor or stator  420  may be disposed in a drill string (not shown). Alternatively, the ends  420   a,b  may be formed with other useful features.  
         [0051]     Using Weld Casting or the VAR process to produce tubular shapes has many advantages over existing manufacturing techniques. The Weld Cast or VAR process allows the use of a wider range of base materials and higher strength alloys including the majority of non-magnetic materials. Weld Cast or VAR produced parts have uniform stress distribution. The Weld Cast or VAR process can produce parts of varying length with theoretically no length limitation since the Weld Cast or VAR process actually produces the stock. The Weld Cast or VAR process will produce a metallurgically superior part, free from internal stress, with good surface finish and no length limitations.  
         [0052]     Several companies offer VAR equipment that can be customized for specialty processes and shapes. Material surface finishes resulting from the VAR process are smooth and seamless. Another advantage of the VAR process is the rate of material deposition.  
         [0053]      FIG. 5  is a longitudinal sectional view of a centrifugal casting (CC) apparatus  500  employing a CC process to form a rotor. A crucible  515  and a mold  512 , having a substantially helical-hypocycloid inner profile formed on an inner surface thereof, are disposed within a chamber  517  assembled through coupling by means of a flange  519 . A molten material  520  melted in the crucible  515  is led to the tundish  513  by means of a sprue runner  514 . The molten material  520  in the tundish  513  is discharged through a number of hole portions  518  formed in the tundish  513  to thereby be deposited on the inner wall surface of the rotating mold  512 . The rotation of the mold  512  is driven by mold drive mechanism  508 . A tundish reciprocation mechanism  516  causes the tundish  513  to repeat reciprocation.  
         [0054]     The crucible  515  is adapted to melt a metal or an alloy into a liquid material through application of heat, thereby yielding the molten material  520 . Examples of melting processes include resistance heating, induction heating, arc melting, and plasma arc melting. Melting and casting are performed in, for example, the atmosphere, vacuum, or an inert gas. The mold  512  may be made of steel protected with a refractory mold wash, green-sand lining, dry-sand lining, or graphite.  
         [0055]     The mold  512  is set in rotation during pouring and the molten material  520  is pressed against the profiled inner surface by the centrifugal force under sufficient pressure to substantially evenly fill the profiled inner surface of the mold  512 . Solidification of the molten material  520  progresses from the outer surface inward; thus, porosity is greatly reduced and, since inclusions tend to have a lower density, they segregate toward the center which is of little consequence because the inner surface will require post-machining clean-up. Forced movement by shearing the molten material  520  results in grain refinement. Long and large rotors of very uniform quality and wall thickness may be cast. Surface quality is good on the outside of the rotor.  
         [0056]     Alternatively, the methods described above with reference to  FIGS. 2, 3 , and  5  could be used to form other parts having other cross-sectional shapes, such as circular, elliptical, oval, and polygon shapes.  
         [0057]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.