Patent Description:
Wind turbines are used to produce electrical energy using a renewable resource and without combusting a fossil fuel. Generally, a wind turbine converts kinetic energy from the wind into electrical power. A horizontal-axis wind turbine includes a tower, a nacelle located at the apex of the tower, and a rotor having a central hub and a plurality of blades coupled to the hub and extending outwardly therefrom. The rotor is supported on a shaft extending from the nacelle, which shaft is either directly or indirectly operatively coupled with a generator which is housed inside the nacelle. Consequently, as wind forces the blades to rotate, electrical energy is produced by the generator.

In recent years, wind power has become a more attractive alternative energy source and the number of wind turbines, wind farms, etc. has significantly increased, both on land and off-shore. Additionally, the size of wind turbines has also significantly increased, with modern wind turbine blades extending between <NUM> to <NUM> meters in length, and is expected to further increase in the future. As blade length continues to increase, the stresses experienced at the joint between the wind turbine blade to the rotor hub also increases. Conventional joints between wind turbine rotor blades and the rotor hub include threaded stud bolts coupled to and extending from the root end of the wind turbine blade, which are in turn coupled to a pitch bearing associated with the rotor hub. Wind turbine blades are typically made from one or more composite materials formed from fibrous material and resin. Such materials generally do not have the structural integrity to provide a secure fixing mechanism into which the threaded stud bolts may be directly inserted. A hole or bore, for example, may be tapped into the composite material at the root end of the rotor blade to provide a complementing thread upon which the stud bolt may achieve a connection. However, the composite material has insufficient shear strength to transfer the loads between the blades and hub via the stud bolts and deterioration of the composite material at the interface would readily occur.

For this reason, it is generally known to utilize internally threaded metal inserts at the interface between the threaded stud bolts and the composite material at the root end of the wind turbine blade. In this regard, tapped bores are typically formed along the circumference of the root end of the wind turbine blade. The metal inserts are then positioned within the tapped bores and adhesively bonded therein to essentially embed the metal inserts within the composite material of the rotor blade. The stud bolts are then threadably engaged with the metal inserts. The forces acting between the rotor blade and rotor hub act through the stud bolts, and thus are transferred via the metal inserts, which operate to more uniformly distribute the forces over the interface area with the softer composite material. The force distribution characteristics provided by the metal inserts in turn provide a connection joint with a structural integrity sufficient to provide a secure connection between the rotor hub and rotor blade during use.

The bores formed along the circumference of the root end of the wind turbine blade are typically formed through a milling process using a milling machine and one or more mill bits operatively coupled to the milling machine for rotating the mill bit during use. One of the final mill bits used in the formation of the bores is referred to as a finishing mill bit. Conventional finishing mill bits have a hollow construction and include a fluted exterior surface and an interior surface, both of which are completely covered by an abrasive particle coating, such as an industrial diamond coating applied through an adhesive wash process, for example. A distal end of the finishing mill bit is configured to be removably coupled to a castellated milling tip, and a proximal end of the mill bit is configured to be coupled to the milling machine. During operation of the finishing mill bit, the abrasive coating on the exterior and interior of the mill bit produces a fine powder of fiber and resin particles from the composite material from which the wind turbine blade is formed. The interior of the mill bit is operatively coupled to a vacuum system for pulling a vacuum along the interior of the mill bit. In this way, air and powder (e.g., fine fiber and resin chips) flows downwardly along the exterior of the mill bit via the flutes, through the openings in the castellated tip, upwardly along the interior of the mill bit, and out of the mill bit adjacent its proximal end. The finishing mill bit not only conforms the bore to its final shape, but also activates the surface of the bore so as to provide an improved bonding interface with the metal insert that is received and secured in the bore through a suitable adhesive.

While such finishing mill bits are successful for their intended purpose, there remain a few drawbacks with existing mill bit designs. In this regard, conventional finishing mill bits have to be replaced fairly regularly due to excessive wear and clogging. In this regard, it is believed that current finishing mill bits become excessively hot during operation such that the resin particles in the composite powder produced from the milling process essentially reactivates, causing the powder to stick to the mill bit. This, in turn, causes a reduction in air flow over and through the mill bit, such that the temperature further increases and more of the resin reactivates to increase the stickiness of the powder. Through this process, the mill bit eventually becomes clogged and inoperable for removing the milled composite material from the bore. When this occurs, the mill bit must be replaced. Current finishing mill bits have an operating life of between about <NUM> and about <NUM> bores. Frequent replacement of the finishing mill bit increases the overall costs to manufacture wind turbine blades. <CIT> discloses a fluted rotary tool, which may be a reamer, in which the rake face of the tool base material is brazed with a polycrystalline diamond film to form a cutting edge. <CIT> discloses a rotary milling or reaming or routing tool comprising a tool body on which a plurality of cutter assemblies in the form of teeth are arranged spaced azimuthally, each cutter assembly comprising a plurality of super-hard cutter segments attached to the tool body in an axial arrangement, each cutter segment having a helical or straight cutting edge; the tool body and the cutter segments being configured such that the cutting edges define a cut length of at least <NUM> times the mean cut diameter. <CIT> discloses a cutter element which may be an end mill, a routing tool or an edge-trimming tool for a rotary machine tool, the cutter element comprising a plurality of cutting edges defined by at least one cutter structure comprising superhard material, and at least three consecutive, azimuthally spaced apart, fluted cutting edges which may be bi-directionally fluted.

Accordingly, there is a need for an improved finishing mill bit for forming the bores in the root end of wind turbine blades that overcomes the drawbacks in conventional mill bit designs and extends the operating life of the mill bit in order to reduce the manufacturing costs of wind turbine blades.

A mill bit for the manufacture of a wind turbine blade includes an elongate base body having a proximal end, a distal end, an outer surface, and an internal bore that defines an inner surface; one or more flutes formed on the outer surface that defines one or more teeth; and an abrasive coating on at least a portion of the outer surface, wherein the one or more flutes are free of the abrasive coating. The one or more flutes on the outer surface may have a surface roughness less than or equal to about Ra = <NUM> microinches (<NUM> micrometers). The abrasive coating may include industrial diamonds or cubic boron nitride (CBN). Additionally, in one embodiment, the grain size of the abrasive coating may be about D501 or greater. Furthermore, the one or more flutes on the outer surface may be helical flutes.

The inner surface of the mill bit also includes an abrasive coating. The abrasive coating may be selectively applied to the inner surface so as to define one or more void strips that are free of the abrasive coating, thereby defining one or more flutes on the inner surface. An interface between the abrasive coating and the one or more void strips defines a cutting edge that removes material through a cutting action rather than a grinding action. The one or more void strips may be finished so as to have a surface roughness less than or equal to about Ra = <NUM> microinches (<NUM> micrometers). In one embodiment the one or more void strips define one or more linear flutes on the inner surface of the mill bit.

In a further aspect, the mill bit may include one or more porting bores extending through a wall of the base body to fluidly connect an external environment with the internal bore. In an exemplary embodiment there are a plurality of porting bores that are longitudinally and/or circumferentially spaced from one another in, for example, a regular or irregular pattern. By way of example, the plurality of bores may be arranged as a first ring of circumferentially spaced porting bores, a second ring of circumferentially spaced porting bores, and a third ring of circumferentially spaced porting bores, wherein the first, second, and third rings are longitudinally spaced from one another. The number of porting bores in each of the first, second, and third rings may correspond to the number of flutes on the outer surface of the mill bit (e.g., four). Each of the one or more porting bores may be open to a flute on the outer surface of the mill bit in order to enhance airflow. Alternatively or additionally, the one or more porting bores may be open to a void strip on the inner surface of the mill bit.

In another embodiment a method of making a mill bit for the manufacture of a wind turbine blade includes providing an elongate base body having a proximal end, a distal end, an outer surface, an internal bore that defines an inner surface, and one or more flutes formed on the outer surface that defines one or more teeth; and selectively applying an abrasive coating on the outer surface such that the one or more teeth include the abrasive coating and the one or more flutes are free of the abrasive coating.

The method may additionally include finishing the one or more flutes on the outer surface to have a surface roughness less than or equal to about Ra = <NUM> microinches (<NUM> micrometers). Moreover, the method includes applying an abrasive coating on the inner surface. In one embodiment, this includes selectively applying the abrasive coating to the inner surface to define one or more void strips that are free of the abrasive coating, thereby defining one or more flutes on the inner surface. The one or more void strips on the inner surface may further be finished so as to have a surface roughness less than or equal to about Ra = <NUM> microinches (<NUM> micrometers).

In yet a further aspect, the method further includes, forming one or more porting bores through a wall of the base body to fluidly connect an external environment with the internal bore. This forming step may include, for example, forming the porting bores such that each porting bore is open to a flute on the outer surface of the mill bit. This forming step may additionally include forming the porting bores such that each porting bore is open to a void strip on the inner surface of the mill bit.

In a further embodiment, a method of recycling a mill bit used in the manufacture of wind turbine blades includes providing a worn mill bit including an elongate base body having a proximal end, a distal end, an outer surface, an internal bore that defines an inner surface, one or more flutes formed on the outer surface that defines one or more teeth, and an abrasive coating on the outer surface; removing the abrasive coating on the outer surface; and reapplying another coating to selective portions of the outer surface. In one embodiment, reapplying another abrasive coating further includes selectively applying an abrasive coating on the outer surface such that the one or more teeth include the abrasive coating and the one or more flutes are free of the abrasive coating.

In yet another embodiment, a mill bit for the manufacture of a wind turbine blade includes an elongate base body having a proximal end, a distal end, an outer surface, and an internal bore that defines an inner surface; one or more flutes formed on the outer surface that defines one or more teeth; an abrasive coating on at least a portion of the outer surface; and one or more porting bores extending through a wall of the base body to fluidly connect an external environment with the internal bore. In an exemplary embodiment there are a plurality of porting bores that are longitudinally and/or circumferentially spaced from one another in, for example, a regular or irregular pattern. The plurality of porting bores may be longitudinally spaced from one another by a substantially equal amount. By way of example, the plurality of bores may be arranged as a first ring of circumferentially spaced porting bores, a second ring of circumferentially spaced porting bores, and a third ring of circumferentially spaced porting bores, wherein the first, second, and third rings are longitudinally spaced from one another. The number of porting bores in each of the first, second, and third rings may correspond to the number of flutes on the outer surface of the mill bit (e.g., four). Each of the one or more porting bores may be open to a flute on the outer surface of the mill bit in order to enhance airflow. Moreover, the one or more porting bores may be open to a void strip on the inner surface of the mill bit.

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.

With reference to <FIG>, a wind turbine <NUM> includes a tower <NUM>, a nacelle <NUM> disposed at the apex of the tower <NUM>, and a rotor <NUM> operatively coupled to a generator (not shown) housed inside the nacelle <NUM>. In addition to the generator, the nacelle <NUM> houses miscellaneous components required for converting wind energy into electrical energy and various components needed to operate, control, and optimize the performance of the wind turbine <NUM>. The tower <NUM> supports the load presented by the nacelle <NUM>, the rotor <NUM>, and other components of the wind turbine <NUM> that are housed inside the nacelle <NUM> and also operates to elevate the nacelle <NUM> and rotor <NUM> to a height above ground level or sea level, as may be the case, at which faster moving air currents of lower turbulence are typically found.

The rotor <NUM> of the wind turbine <NUM>, which is represented as a horizontal-axis wind turbine, serves as the prime mover for the electromechanical system. Wind exceeding a minimum level will activate the rotor <NUM> and cause rotation in a plane substantially perpendicular to the wind direction. The rotor <NUM> of wind turbine <NUM> includes a central hub <NUM> and at least one rotor blade <NUM> that projects outwardly from the central hub <NUM> at locations circumferentially distributed thereabout. In the representative embodiment, the rotor <NUM> includes three blades <NUM>, but the number may vary. The blades <NUM> are configured to interact with the passing air flow to produce lift that causes the central hub <NUM> to spin about a central longitudinal axis.

The wind turbine <NUM> may be included among a collection of similar wind turbines belonging to a wind farm or wind park that serves as a power generating plant connected by transmission lines with a power grid, such as a three-phase alternating current (AC) power grid. The power grid generally consists of a network of power stations, transmission circuits, and substations coupled by a network of transmission lines that transmit the power to loads in the form of end users and other customers of electrical utilities. Under normal circumstances, the electrical power is supplied from the generator to the power grid as known to a person having ordinary skill in the art.

As is well known in the industry, for certain wind turbine designs, the rotor blades <NUM> are coupled to the rotor hub <NUM> in a manner that allows the blades <NUM> to rotate or pitch about a longitudinal axis of the blades <NUM>. This is achieved by coupling the root end <NUM> of a blade <NUM> to a pitch bearing (not shown) operatively coupled to the rotor hub <NUM>. The pitch bearing generally includes a ring rotatable relative to the hub <NUM> to which the root end <NUM> of the blade <NUM> is coupled. Pitch bearings are generally well known in the art and thus will not be described in further detail herein.

As illustrated in <FIG>, a connection joint <NUM> between a rotor blade <NUM> of the wind turbine <NUM> and the rotor hub <NUM> includes a plurality of inserts <NUM> coupled to the rotor blade <NUM> at the root end <NUM> thereof, and a plurality of stud bolts <NUM> configured to be coupled to the inserts <NUM> in the rotor blade <NUM> and further configured to be coupled to the rotor hub <NUM> (<FIG>), such as through the pitch bearing. As illustrated in <FIG>, the inserts <NUM> may be circumferentially spaced about an end face <NUM> at the root end <NUM> of the blade <NUM> and embedded within the material of the blade <NUM> such that a connecting end of the insert <NUM> slightly protrudes from the end face <NUM> of the blade <NUM> (<FIG>). The number of inserts <NUM> along the circumference of the root end <NUM> of the blade <NUM> depends on the size of the blade, among potential other factors, but may be anywhere from <NUM> to <NUM> inserts for blades between <NUM>-<NUM> in length. It should be realized that more or less inserts may be used depending on the specific application.

As illustrated in <FIG>, the metal inserts <NUM> for the wind turbine blades <NUM> include a generally cylindrical main body <NUM> and a tubular extension <NUM> projecting from one end of the main body <NUM>. The main body <NUM> includes an internal bore <NUM> including a threaded portion <NUM> for receiving and threadably coupling to a stud bolt <NUM>. The main body <NUM> defines an interface 40a with the composite material <NUM> of the wind turbine blade <NUM> at the outer surface of the main body <NUM>. The tubular extension <NUM> extends from the main body <NUM> and defines an outer interface 40b with the composite material <NUM> of the blade <NUM> at the outer surface of the tubular extension <NUM>, and an inner interface 40c with the composite material <NUM> of the blade <NUM> at the inner surface of the tubular extension <NUM>. As noted above, the inserts <NUM> are positioned within bores in the root end of the blade and bonded to the composite material using a suitable adhesive, such as an epoxy, as illustrated in <FIG>.

The stud bolts <NUM> are generally cylindrical elongate members having a threaded blade end <NUM> and a threaded hub end <NUM>. As illustrated in <FIG>, during assembly of the wind turbine <NUM>, the stud bolts <NUM> are threadably engaged with the inserts <NUM> at the root end <NUM> of the wind turbine blade <NUM> such that the threaded hub end <NUM> of the stud bolts <NUM> extends away from the root end <NUM> of the blade <NUM>. The stud bolts <NUM> are then aligned with corresponding holes in the pitch bearing on the hub <NUM>, inserted therethrough, and secured to the pitch bearing via a threaded fastener or the like. Through the connection joint <NUM>, a wind turbine blade <NUM> may be securely coupled to the rotor hub <NUM> of the wind turbine <NUM> and generally accommodates the loads applied to the blades <NUM> during the operational life of the wind turbine <NUM>.

As discussed above, the plurality of inserts <NUM> are circumferentially spaced about the end face <NUM> at the root end <NUM> of the rotor blade <NUM> and embedded in the composite material <NUM> that forms the root end <NUM> of the blade <NUM> (<FIG>). Thus, after the wind turbine blade <NUM>, or at least the root end <NUM> thereof, is formed, a plurality of circumferentially spaced bores <NUM> may be formed in the end face <NUM> of the root end <NUM> of the blade <NUM>, as illustrated in <FIG>. The bores <NUM> are generally configured to correspond in size and shape to the size and shape of the inserts <NUM> so that the inserts <NUM> may be received therein as described above. In this regard, in an exemplary embodiment each bore <NUM> may include a first generally cylindrical cavity portion <NUM> that extends inwardly from the end face <NUM> and terminates at a second end <NUM>. The width (e.g., cross dimension, diameter, etc.) of the bore <NUM> is just slightly larger than the inserts <NUM> configured to be received within the bores <NUM>. Moreover, the length of the first cavity portion <NUM> is configured to generally correspond to the length of the main body <NUM> of the insert <NUM>.

The bore <NUM> further includes a second annular cavity <NUM> having a first end <NUM> at the second end <NUM> of the first cavity portion <NUM> and extending inwardly therefrom and terminating at a second end <NUM>. In this way, the second annular cavity <NUM> is open to the first cavity portion <NUM>. The configuration of the annular cavity <NUM> generally corresponds to the configuration of the tubular extension <NUM>. Thus, in an exemplary embodiment, the annular cavity <NUM> may have a generally cylindrical configuration or a generally conical configuration (e.g., a <NUM>° taper) to match that of the tubular extension <NUM>. Furthermore, the length of the annular cavity <NUM> generally corresponds to the length of the tubular extension <NUM>. As can be appreciated, the first annular cavity <NUM> should be slightly larger than the first tubular extension <NUM> and slightly longer than the first tubular extension so as to accommodate the first tubular extension and surrounding adhesive.

In one embodiment, the bores <NUM> may be formed by a milling process using a milling machine, schematically shown at <NUM>, and one or more mill bits operatively coupled to the milling machine <NUM> (<FIG>). Milling machines <NUM> used for making bores in the root end of wind turbine blades are generally well known in the industry and will not be further described herein. When forming bores <NUM>, one or more mill bits may be used to form the first cavity portion <NUM> and the annular cavity <NUM>. As noted above, one of the final steps in formation of the bores <NUM> is to finalize the bore profile and activate the surfaces of the bores <NUM> using a finishing mill bit. <FIG> illustrates an exemplary embodiment of a finishing mill bit <NUM> in accordance with aspects of the present invention.

As will be described in more detail below, finishing mill bit <NUM> overcomes many of the drawbacks of convention finishing mill bits so that the operating life of the mill bit may be extended, and the costs associated with the manufacture of wind turbine blades may be reduced. To this end, the finishing mill bit <NUM> includes one or more features that enhance cooling of the mill bit such that the temperature of the mill bit during operation is reduced. More particularly, the finishing mill bit <NUM> includes one or more features that keep the temperature of the mill bit <NUM> below a threshold operating temperature that prevents the resin particles in the milled composite powder from reactivating and sticking to the mill bit. By way of example and without limitation, the finishing mill bit <NUM> may be configured to have an operating temperature below about <NUM>°F (<NUM>). Below this threshold temperature the resin particles will not generally reactivate and start sticking to the mill bit. In this way, air and the milled composite powder move more easily over the outer and inner surfaces of the mill bit, and out of the mill bit under vacuum pressure. In other words, a buildup of milled composite powder on the surfaces of the mill bit <NUM> is prevented or significantly reduced such that the operating temperature is reduced and the operating life of the mill bit is extended. A description of a mill bit <NUM> in accordance with embodiments of the invention now follows.

Mill bit <NUM> includes a generally elongate cylindrical base body <NUM> having a first proximal end <NUM> configured to be operatively coupled to the milling machine <NUM>, such as via a mill chuck or the like (not shown), and a second distal end <NUM> configured to be removably and selectively coupled to a milling tip <NUM>. For example, the distal end <NUM> may include an annular flange <NUM> that slidably receives the milling tip <NUM> in a tight friction fit manner. A terminating end <NUM> of the milling tip <NUM> may have a castellated configuration defined by a plurality of openings <NUM> (e.g., four openings around the circumference). The terminating end <NUM> may further include an abrasive coating <NUM> that facilitates grinding or cutting of the composite material during use. As the milling tip <NUM> is at the forefront of engagement with the composite material, the wear rate of the milling tip <NUM> may be higher than that of the remainder of the mill bit <NUM>. Accordingly, when a milling tip <NUM> reaches the end of its operating life, it may be removed from the mill bit <NUM> and replaced with a new milling tip <NUM>.

The cylindrical base body <NUM> includes an outer surface <NUM> and an inner surface <NUM> defined by an internal bore <NUM> that extends along at least a portion of the length between the first and second ends <NUM>, <NUM>. For example, the internal bore <NUM> may extend substantially along the entire length between the first and second ends <NUM>, <NUM>. At least a portion of the outer surface <NUM> extending from the annular flange <NUM> towards to first end <NUM> defines a cutting and/or grinding portion <NUM> configured to engage with the composite material that forms the root end <NUM> of the blade <NUM>. In an exemplary embodiment, the cutting/grinding portion <NUM> may extend along a majority of the length of the base body <NUM>. For example, the cutting/grinding portion <NUM> may extend between just above <NUM>% to about <NUM>% of the length of the base body <NUM>. However, the invention is not so limited and other lengths may be possible and within the scope of the present invention.

The cutting/grinding portion <NUM> includes one or more helical flutes <NUM>, which defines one or more cutting/grinding teeth <NUM>, and an abrasive coating <NUM> on the outer surface <NUM>. In an exemplary embodiment, for example, the mill bit <NUM> may include four helical flutes (and four helical teeth) extending along the cutting/grinding portion <NUM>. However, the mill bit may include more or less flutes/teeth along its outer surface in various alternative embodiments, and the invention is not limited to any particular number of flutes/teeth. The teeth <NUM> are configured to engage with the composite material <NUM> and cut and/or grind material away from an interface, while the flutes <NUM> are configured to provide air flow and particle removal from the milled interface. In one embodiment, the width of the flutes <NUM> may be generally constant along the length of the mill bit <NUM>. In an alternative embodiment, however, the width of the flutes <NUM> may vary along the length of the mill bit <NUM>. More particularly, the width of the flutes <NUM> may decrease in a direction from the first end <NUM> toward the second end <NUM>, and the width of the teeth <NUM> may correspondingly increase in that direction. Such a tapering of the flutes <NUM> effectively compresses the air flowing along the flutes to increase flow velocity as the air moves toward the second end <NUM>. Other configurations of the flutes <NUM> and teeth <NUM> may, however, also be possible.

In one aspect in accordance with the invention, the abrasive coating <NUM> does not cover the entire surface of the cutting/grinding portion <NUM>, but only covers a select portion of the outer surface <NUM> along the cutting/grinding portion <NUM>. More particularly, in an exemplary embodiment, the helical flutes <NUM> may be free of the abrasive coating <NUM>, such that only the surfaces <NUM> of the teeth <NUM> are covered by the abrasive coating <NUM>. In one embodiment, the abrasive coating <NUM> may be formed from an industrial diamond coating similar to existing mill bits. In an alternative embodiment, however, the abrasive coating <NUM> may be provided by cubic boron nitride (CBN). In this regard, CBN may have improved grain face wear durability as compared to industrial diamonds and thus may be more desirable. Other abrasive coatings may also be possible.

The abrasive coating <NUM> may be applied according to known processes. In one embodiment, the abrasive coating <NUM> may be applied through an electroplating process using, for example, nickel (II) nitride. In this regard, a nickel layer of about <NUM> microns may be deposited on the selected portions of the mill bit <NUM>. The portions of the mill bit <NUM> that are to be abrasive free (i.e., the helical flutes <NUM>) may be masked during the electroplating process. Subsequently, the abrasive elements may be immersed into the nickel layer. Grain immersion may be between about <NUM>% to <NUM>% of the grain diameter. In this regard, the grain size (e.g., diameter) of the abrasive elements may be, for example, D426 (FEPA standard) elements (e.g., industrial diamonds or CBN elements). However, due to the lower operating temperatures afforded by aspects of the present invention, thermal expansion of the base body <NUM> may be decreased, and thus larger grain sizes may be used in the abrasive coating. For example, a grain size of D501 or even larger may be used in the abrasive coating <NUM>.

In addition to omitting the application of abrasive coating <NUM> along the helical flutes <NUM>, the surfaces <NUM> of the helical flutes <NUM> may be honed, buffed, polished or otherwise finished so as to be very smooth. In this regard, the surfaces <NUM> that form the helical flutes <NUM> may have a surface roughness equal to or less than about Ra = <NUM> microinches (Ra = <NUM> micrometers). By omitting the abrasive coating <NUM> from the helical flutes <NUM> and finishing the helical flutes <NUM> to have a very smooth topology, particle capture along the flutes <NUM> is reduced and air flow (i.e., increased air speed and/or air flow volume) is significantly improved. As a result, the temperature of the mill bit <NUM> during operation may be reduced due to improved heat transfer from the mill bit <NUM>. The deduction in operating temperature, in turn, reduces the reactivation of the resin particles in the milled powder, and reduces clogging of the mill bit. Accordingly, the operating life of the mill bit <NUM> may be extended.

Similar to the above and with reference to <FIG>, in one embodiment at least a portion of the inner surface <NUM> extending from annular flange <NUM> towards the first end <NUM> defines a cutting/grinding portion <NUM> configured to engage with the composite material that makes up the root end <NUM> of the blade <NUM>. In an exemplary embodiment, the cutting/grinding portion <NUM> may extend along a majority of the length of the base body <NUM>. For example, the cutting/grinding portion <NUM> may extend between just above <NUM>% to about <NUM>% of the length of the base body <NUM>. However, the invention is not so limited and other lengths may be possible and within the scope of the present invention. The lengths of the cutting/grinding portions <NUM>, <NUM> on the outer and inner surfaces <NUM>, <NUM>, respectively, may be substantially the same or may be different from one another depending on, for example, a particular application.

In one embodiment, substantially the entire cutting/grinding portion <NUM> may include an abrasive coating <NUM>. The abrasive coating <NUM> may be an industrial diamond coating similar to existing mill bits. In an alternative embodiment, however, the abrasive coating <NUM> may be provided by cubic boron nitride (CBN) similar to abrasive coating <NUM> on the outer surface <NUM> of the mill bit <NUM>. Other coatings may also be possible. The abrasive coating <NUM> on the inner surface <NUM> may be applied by the process described above, for example.

In accordance with another aspect of the invention, the abrasive coating <NUM> may not cover the entire surface of the cutting/grinding portion <NUM>, but only cover a select portion of the inner surface <NUM> along the cutting/grinding portion <NUM>. More particularly, the cutting/grinding portion <NUM> may include a plurality of generally linear void regions or strips <NUM> which may be free of the abrasive coating <NUM>. These void strips <NUM> effectively form a plurality of linear "flutes" along the cutting/grinding portion <NUM> of the inner surface <NUM> (i.e., extend in the longitudinal direction of the mill bit <NUM>). In an exemplary embodiment, for example, the mill bit <NUM> may include four void strips <NUM> extending along the cutting/grinding portion <NUM>. However, the mill bit <NUM> may include more or less void strips <NUM> along the inner surface <NUM> in various alternative embodiments.

The void strips <NUM> may extend from the annular flange <NUM> toward the first end <NUM> of the mill bit <NUM> for a length between about <NUM>% and about <NUM>% of the length of the cutting/grinding portion <NUM> (and/or between about <NUM>% and about <NUM>% of the length of the helical flutes <NUM>). Other lengths may also be possible within the scope of the invention. In one embodiment, the width of each of the void strips <NUM> may be between about <NUM>% and about <NUM>% of the inner circumference of the mill bit <NUM> at the annular flange <NUM>. In one embodiment, the width of the void strips <NUM> may be substantially constant along the length of the strips <NUM>, but in an alternative embodiment may vary along their lengths, such as having a tapered or conical configuration (not shown). Additionally, and similar to the helical flutes <NUM>, in addition to omitting the application of abrasive coating <NUM> along the void strips <NUM>, the surfaces <NUM> of the void strips <NUM> may be honed, buffed, polished or otherwise finished so as to be very smooth. In this regard, the surfaces <NUM> that form the void strips may have a surface roughness equal to or less than about Ra = <NUM> microinches (Ra = <NUM> micrometers).

The inclusion of the void strips <NUM> (or linear flutes) provides a relief along the inner surface <NUM> that allows for improved air flow and particle evacuation in the mill bit <NUM>. As a result, the temperature of the mill bit <NUM> during operation may be reduced. The reduction in operating temperature, in turn, reduces the reactivation of the resin particles in the milled powder, and reduces clogging of the mill bit. Accordingly, the operating life of the mill bit <NUM> may be extended. Additionally, and in another advantageous aspect, the linear flutes <NUM> define a leading edge <NUM> and a trailing edge <NUM> at the junction between the abrasive coating <NUM> and the void strip <NUM>. The leading edge <NUM> provides an abrupt cutting face at the interior of the mill bit <NUM> that more efficiently removes material through a cutting action as opposed to a grinding action. The removal of material through a cutting action produces less heat compared to grinding, resulting in lower operating temperatures. It should be understood that the linear flutes <NUM> on the interior surface <NUM> may be used alone or in combination with the abrasive-free flutes <NUM> on the outer surface <NUM> as described above.

In another aspect in accordance with the invention, the mill bit <NUM> may include one or more porting bores <NUM> that extend through the sidewall of the mill bit <NUM> so as to fluidly connect the external environment <NUM> of the mill bit <NUM> and the internal bore <NUM>. For example, the porting bores <NUM> may between the outer surface <NUM> and the inner surface <NUM>. Should the porting bores be located in positions that include one or both of the abrasive coatings <NUM>, <NUM>, then the porting bores <NUM> would also extend through these coatings as well. The porting bores <NUM> are configured to establish an air flow between the external environment <NUM> of the mill bit <NUM> to the inside of the mill bit <NUM>. The air flow may be generated, for example, at least in part by a vacuum system <NUM> operatively coupled to the internal bore <NUM> of the mill bit <NUM>, similar to existing systems (<FIG>). The vacuum system <NUM> may be part of the milling machine <NUM> or a separate system. In any event, the vacuum system <NUM> is configured to pull a vacuum within the internal bore <NUM> of the mill bit <NUM> and thereby create an air flow through the porting bores <NUM>. In an exemplary embodiment, the porting bores <NUM> may have a diameter between about <NUM> and about <NUM>. For example, the porting bores <NUM> may have a diameter of about <NUM>. The size of each of the porting bores <NUM> may be substantially the same or the size of the porting bores <NUM> may have varying sizes. In one embodiment, the total cross-sectional area of the plurality of the porting bores <NUM> may be between about <NUM>% and about <NUM>% of the cross-sectional area of the internal bore <NUM> at the proximal end <NUM> (i.e., the discharge port area). More preferably, the total cross-sectional area of the plurality of the porting bores <NUM> may be about <NUM>% of the cross-sectional area of the internal bore <NUM> at the proximal end <NUM>.

In an exemplary embodiment, the mill bit <NUM> includes a plurality of porting bores <NUM>. By way of example, in one embodiment a plurality of porting bores <NUM> may distributed circumferentially about the mill bit <NUM> at, for example, substantially the same longitudinal distance from the second end <NUM> of the mill bit <NUM>. In an alternative embodiment, a plurality of porting bores <NUM> may be distributed longitudinally along the mill bit <NUM> at, for example, substantially the same circumferential position (e.g., longitudinally aligned). In a preferred embodiment, however, a plurality of porting bores <NUM> may be distributed both circumferentially and longitudinally about the mill bit <NUM>. The arrangement of porting bores <NUM> may have a regular pattern or an irregular pattern in various embodiments.

By way of example, in an exemplary embodiment, a pattern <NUM> of porting bores <NUM> may include a first ring <NUM> of bores <NUM> circumferentially distributed at a first longitudinal distance 128a from the second end <NUM> of the mill bit <NUM>; a second ring <NUM> of bores <NUM> circumferentially distributed at a second longitudinal distance 130a from the second end <NUM> of the mill bit <NUM>; and a third ring <NUM> of bores <NUM> circumferentially distributed at a third longitudinal distance 132a from the second end of the mill bit <NUM>. While three rings <NUM>, <NUM>, <NUM> are described, it should be recognized that the pattern <NUM> may include additional/fewer rings. Each ring <NUM>, <NUM>, <NUM> may include the same number of bores <NUM> or different number of bores. For example, each ring <NUM>, <NUM>, <NUM> may include the same number of bores <NUM>, and that number may coincide with the number of flutes <NUM> on the mill bit <NUM> (e.g., four porting bores <NUM> per ring). The longitudinal spacing between adjacent rings <NUM>, <NUM>, <NUM> may be substantially constant in one embodiment (i.e., rings <NUM>, <NUM>, <NUM> may be equally spaced longitudinally). For example, the rings <NUM>, <NUM>, <NUM> may be longitudinally spaced between about <NUM> and about <NUM> apart. Preferably, the rings <NUM>, <NUM>, <NUM> may be spaced about <NUM> apart. Additionally, the porting bores <NUM> may start in a region between about <NUM> to about <NUM> from the second end <NUM> of the mill bit <NUM>. The porting bores <NUM> in the rings <NUM>, <NUM>, <NUM> may be longitudinally aligned in one embodiment. In an exemplary embodiment, however, the porting bores <NUM> in adjacent rings <NUM>, <NUM>, <NUM> may be circumferentially offset from one another such that the porting bores <NUM> of the rings are not longitudinally aligned. This is best illustrated in <FIG>.

In an exemplary embodiment and as illustrated in <FIG>, each of the plurality of porting bores <NUM> may be open to one of the flutes <NUM> on the outer surface <NUM> of the mill bit <NUM>. In this way, the faster moving air flowing along the flutes <NUM> may access the porting bores <NUM> and inject high velocity air into the internal bore <NUM>. In addition, each of the porting bores <NUM> may be open to one of the linear flutes or void strips <NUM> on the inner surface <NUM> of the mill bit <NUM>. This is best illustrated in <FIG>. By locating the porting bores <NUM> in the helical flutes <NUM> and/or the linear flutes <NUM>, particle movement and airflow may be significantly improved.

The pattern of porting bores <NUM> in the mill bit <NUM> is configured to generate an air flow within the mill bit <NUM> that enhances heat transfer away from the mill bit <NUM> in order to reduce the operating temperature of the bit. In this regard, the pattern of porting bores <NUM> may be particularly configured to induce vortices within the interior of the mill bit <NUM>. By organizing the flow pattern within the internal bore <NUM>, the air flow velocity and volume throughput may be significantly increased as compared to convention designs. In the three-ring pattern described above, the first, lowermost ring (e.g., closest to the distal end <NUM>) may be configured as the primary vortex inducer. The second and third rings may be configured as charger ports that increase the terminal air flow and further organize the fluid moving through the internal bore <NUM>. By inducing internal vortex generation, it is believed that the volumetric air flow may be increased between about <NUM>-<NUM><NUM>/ min, which represents about a <NUM>% increase in the volumetric air flow through the mill bit <NUM> as compared to a conventional design.

It should be recognized that the porting bores <NUM> may be used alone or in combination with one or more of the other features described above to reduce the operating temperature of the mill bit <NUM>, and thereby extend its operating life. For example, in an exemplary embodiment, a finishing mill bit includes the smooth helical flutes <NUM> on the outer surface <NUM>, the linear flutes <NUM> on the inner surface <NUM>, and the porting bores <NUM>. Alternate embodiments, however, may include any one or various combinations of these features and remain within the scope of the present invention. It is believed that by incorporating one or more of these features, the operating life of a mill bit may be significantly extended. By way of example, it is believed that a mill bit that incorporates all of the features described above may have an operating life of well over <NUM>,<NUM> bores, which represents an improvement of over <NUM>%. Due to reduced operating temperatures, which may be just slightly higher than ambient temperature (and well under a threshold temperature of about <NUM>°F (<NUM>)), fewer mill bits will have to be provided for wind turbine blade manufacturing. Accordingly, the costs for wind turbine manufacturing may be reduced.

In a further aspect of the present invention, at least a portion of the mill bit <NUM> may be recyclable. In this regard, after a mill bit <NUM> has been worn and rendered unusable, what remains of the abrasive coatings <NUM>, <NUM> may be removed from the underlying base body <NUM>. For example, the worn mill bit may first be blasted using, for example, glass bead media (i.e., glass bead blasting). Once the mill bit <NUM> has been blasted, the mill bit may be subjected to chemical stripping or debriding. In this process, the desired parts of the mill bit are dipped or otherwise subjected to a suitable stripping agent that dissolves the abrasive coating on the base body <NUM>. For example, acidic immersion stripping, alkaline cyanide immersion tripping, and electrolytic stripping may all be used to remove the abrasive coatings from the base body <NUM>. Once the abrasive coatings have been removed from the base body <NUM>, the base body <NUM> may be cleaned and prepared for another abrasive coating on the outside and/or inside of the base body <NUM>. For example, the electroplating process described above may be used to apply a new abrasive coating to select portions of the outer surface <NUM> and/or select portions of the inner surface <NUM>.

Claim 1:
A mill bit (<NUM>) for the manufacture of a wind turbine blade, comprising:
an elongate base body (<NUM>) having a proximal end (<NUM>), a distal end, an outer surface (<NUM>), and an internal bore (<NUM>) that defines an inner surface (<NUM>);
one or more flutes (<NUM>) formed on the outer surface (<NUM>) that defines one or more teeth (<NUM>); and
an abrasive coating (<NUM>) on at least a portion of the outer surface (<NUM>), wherein the one or more flutes (<NUM>) are free of the abrasive coating (<NUM>), and wherein said inner surface (<NUM>) includes an abrasive coating (<NUM>).