Systems and Methods for Repairing Compressor Blades Using Hybrid Manufacturing Techniques

Systems and methods are provided for repairing compressor blades using hybrid manufacturing. A mounting tool for use in repairing a turbine component includes a slot formed in the mounting tool. The slot is configured to receive the turbine component for securing the turbine component to the mounting tool. The mounting tool includes at least one fastener engageable with the turbine component and capable of retaining the turbine component within the slot; and at least a pair of support bases provided on the mounting tool. Each of the at least a pair of support bases is positioned adjacent a respect edge of the turbine component when the turbine component is retained in the mounting tool. Each of the at least a pair of support bases is configured for additively building a sacrificial support on each of the at least a pair of support bases for enabling repair of the turbine component.

FIELD OF INVENTION

The disclosure relates generally to the repair of blades. More specifically, the disclosure relates to a tool configured for retaining a compressor blade while it is repaired using additive and subtractive manufacturing processes.

BRIEF SUMMARY OF INVENTION

In some aspects, the techniques described herein relate to a mounting tool for use in repairing a turbine component, the mounting tool including a slot formed in the mounting tool, the slot configured to receive the turbine component for securing the turbine component to the mounting tool. The mounting tool includes at least one fastener engageable with the turbine component and capable of retaining the turbine component.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the mounting tool includes x, y, and z orientations, and the at least one fastener includes a fastener oriented in the y orientation of the mounting tool and a second fastener oriented in the z orientation of the mounting tool.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the at least one fastener includes a screw fastener.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the at least one fastener oriented in the y orientation of the mounting tool is a screw fastener and the second fastener oriented in the z orientation of the mounting tool is a screw fastener.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, further including an aperture configured for allowing the at least one fastener to be accessed.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein at least one of the at least a pair of support bases is releasably attached to the mounting tool.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein each of the at least a pair of support bases is releasably attached to the mounting tool.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein at least one of the at least a pair of support bases is permanently attached to the mounting tool.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, further including a mechanism for securing the mounting tool to a hybrid manufacturing machine.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the hybrid manufacturing machine to which the mounting tool is configured for securement includes each of additive manufacturing and subtractive manufacturing.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, further including at least one mounting feature configured for securing the mounting tool to a hybrid manufacturing machine.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the at least one mounting feature includes at least one releasable mounting feature to releasably secure the mounting tool to the hybrid manufacturing machine.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein at least one of the at least a pair of support bases includes a polyhedron configuration.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the at least a pair of support bases are configured to align with a curvature geometry of the turbine component.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the turbine component is a turbine compressor blade.

In some aspects, the techniques described herein relate to a mounting tool for use in repairing a turbine blade, the mounting tool including: a slot formed in the mounting tool, the slot configured to receive the turbine blade for securing the turbine blade to the mounting tool; at least one fastener engageable with the turbine blade and capable of retaining the turbine blade within the slot, the at least one fastener capable of being externally accessed with respect to the mounting tool for moving the at least one fastener toward and away from the turbine blade; at least a pair of support bases provided on the mounting tool, each of the at least a pair of support bases positioned adjacent a respective edge of the turbine blade when the turbine blade is retained in the mounting tool; and at least one mounting feature configured for securing the mounting tool to a hybrid manufacturing machine; wherein each of the at least a pair of support bases is configured for building a sacrificial support on each of the at least a pair of support bases for enabling repair of the turbine blade.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein each of the at least a pair of support bases is configured for building a sacrificial support on each of the at least a pair of support bases for supporting the turbine blade during repairing.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein the at least a pair of support bases are configured to align with a curvature geometry of the turbine blade.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein each of the at least a pair of support bases is releasably attached to the mounting tool.

In some aspects, according to any one of the preceding aspects, the techniques described herein relate to a mounting tool, wherein: the mounting tool includes x, y, and z orientations; the at least one fastener includes a fastener oriented in the y orientation and ae second fastener oriented in the z orientation of the mounting tool; and each of the fastener and the second fastener is a screw fastener.

Two or more aspects described in this disclosure, including those described in this summary section, may be combined to form implementations not specifically described herein.

DETAILED DESCRIPTION OF THE INVENTION

A gas turbine is comprised of a compressor section for compressing ambient air, a combustor section for mixing and burning fuel and the compressed air, and a turbine section driven by the expanding combustion gases for powering the compressor and turning an output shaft for ancillary devices or power generation or for providing thrust for propelling an aircraft.

Some aircraft engine components are subject to wear and tear during operation due to erosive substances, for example, sand, dust, and/or ice. In the front part of the gas turbine, the area of the compressor, erosion generally causes a flattening of the leading edges of the compressor blades. When the compressor blades are not yet worn down, they have an aerodynamically optimized profile that is detrimentally affected by the wear. As a consequence of the deterioration of the aerodynamic profile, the gas turbine loses efficiency, which means that the gas turbine needs more fuel to generate the same power. Therefore, from a financial point of view, it may be desirable to counteract the wear of compressor blades, especially a deterioration of the aerodynamically optimized blade geometries.

Repairing compressor blades, particularly ones having complex geometries, is not a simple task, however. In the prior art, larger compressor blades with simple geometries are typically repaired by first adding material to the blade (e.g., to the damaged leading edge, trailing edge, and/or the tip) using conventional techniques such as manual welding. Because the material added to the blade via such conventional techniques cannot be precisely controlled, more material than is required is typically added to the blade and the extraneous material is then removed to bring the blade within the required specifications. Such may be done when the compressor blade is large and/or has a simplistic shape. When the compressor blade is small, thin, and/or has a complex shape (e.g., curves in the X, Y, and Z planes), however, such removal of additional material to bring the blade within its required specifications is virtually impossible or at least very difficult. When material is removed from the blade, the small/thin compressor blade vibrates and precludes precise repair. Further, the complex shapes of such compressor blades make repair (e.g., milling of the excess material) unduly laborious. In view of such considerations, damaged or worn compressor blades are typically simply scrapped, especially where the blade is small in size and/or its geometry is complex.

Embodiments of dimensional compressor blade repair systems and methods disclosed herein may allow for compressor blades, including small compressor blades having complex shapes and/or geometries, to be repaired with relative ease.

Turning now toFIGS.1through4, an embodiment of a blade mounting tool100, such as but not limited to a blade mounting tool100(hereinafter “mounting tool100) for use with compressor blade repair equipment is shown. The blade mounting tool100may be used in a method200(FIG.5) of repairing a turbine blade50, including but not limited to a HPT blade, a LPT blade, an airfoil, or a compressor blade50(hereinafter referred to as a “compressor blade50”), as discussed in further detail below. The mounting tool100may retain (e.g., temporarily retain) a compressor blade50while the blade50undergoes repair. Portions of the blade mounting tool100may facilitate the construction of one or more sacrificial supports (e.g., sacrificial supports130and140, seeFIG.4) that are built up as the compressor blade50is repaired. These sacrificial supports may stabilize the compressor blade50such that the blade50may undergo both additive and subtractive manufacturing at roughly the same time without suffering from undesirable movement or vibrations. Once the repair is completed, the sacrificial supports may be disassociated from the mounting tool100and the compressor blade50, using, e.g., using electrical discharge machining or any other machining method now known or hereinafter developed.

The equipment used to add material to the compressor blade50may be an additive manufacturing machine, such as, but not limited to a laser sintering machine, or any other additive manufacturing machine now known or hereinafter developed. Repairing the compressor blade50with an additive manufacturing laser sintering machine may involve depositing a metal powder on a platform of the additive manufacturing machine, a surface of the blade50, and/or the tool100, and heating the metal powder together into a layer of solid material. Typically, the metal powder is heated using a laser. This additive manufacturing depositing and heating process may be repeated until a desired number of material layers are formed on the blade50.

The compressor blade50may undergo further processes to complete the repair operation. For example, the compressor blade50may undergo a milling process where material (e.g., excess material from the additive manufacturing step) is removed from the compressor blade50. As another example, the compressor blade50may undergo a surface finishing or polishing process, where material is removed from the blade50until a desired surface finish is reached.

In some embodiments, the step of adding material by additive manufacturing to the turbine blade50may occur generally at the same time as a milling process (sometimes referred to herein as “hybrid manufacturing”). This may be accomplished either with several pieces of different equipment, or with a singular piece of equipment. For instance, the compressor blade50may undergo repair via a piece of equipment configured for both additive manufacturing and subtractive manufacturing. An example machine that allows for such functionality is a Lumex series hybrid manufacturing machine produced by Matsuura Machinery USA Inc. of St. Paul, Minnesota. Significant time and costs may be saved by effectuating both additive and subtractive manufacturing generally simultaneously while the compressor blade is secured to the same mounting tool. Conventional compressor blade repair methods are unable to accomplish hybrid manufacturing effectively, typically because the vibrations/harmonics caused by the milling process displace the compressor blade being repaired and disrupt the additive manufacturing process.

To mitigate such displacement by vibrations/harmonics concerns, the compressor blade50may undergo repair while it is held by the blade mounting tool100. A base110of the mounting tool100may have a slot118in which the compressor blade50(or a portion thereof) is situated. The compressor blade50may be retained within the slot118by one or more screws or other fasteners114. For instance, a base of the compressor blade50may be secured within the mounting tool100by a screw114which displaces along a y-axis, and/or by a screw which displaces along a z-axis (not shown) of the mounting tool100. The screws114may be manipulated by a user via one or more apertures in the mounting tool100. For example, the y-axis screw114may be accessible with a tool (e.g., a screwdriver, a hex key, a pin, etc.) via an aperture115in the base110of the mounting tool100, while a z-axis screw114may be accessed via an aperture125(FIG.3) located on a bottom surface120of the mounting tool100. Using the screws114and the apertures115,125, the mounting tool100may be readily adjusted to secure (e.g., temporarily) compressor blades50of various sizes. In embodiments, the screws114may alternatively or additionally be used to adjust a position of the compressor blade50within the mounting tool100.

When secured in the slot118, the compressor blade50may be adjacent an upright portion112of the mounting tool100that extends upwardly (i.e., in a z-axis) from the mounting tool base110. The upright portion112may at least partially envelope a secured compressor blade50in both the x-axis and the y-axis. That is to say, the upright portion112may extend from the mounting tool base110to provide a surface113which is located further along the compressor blade50in the z-axis. In embodiments, the surface113may be level in the x-y plane.

The blade mounting tool100may have one or more mounting features122(FIG.3) for securing the mounting tool100to other equipment, such as a hybrid manufacturing machine. The mounting features122may be any suitable feature(s) now known or subsequently developed for securing the mounting tool100in place. For example, the mounting features122may comprise a plurality of apertures which correspond to fasteners or a platform of a hybrid manufacturing machine. The mounting features122may be releasable such that a user may selectively attach/detach the mounting tool100as desired.

The blade mounting tool100may have a plurality of support bases or nubs116(116A,116B) extending from the upright portion112(e.g., from the surface113thereof). These support bases116may protrude from the mounting tool100in the z-axis. In operation, the support bases116may provide a platform or surface upon which sacrificial supports (e.g., sacrificial supports130and140, seeFIG.4) may be built while the compressor blade50undergoes additive manufacturing. By virtue of the support bases116and the sacrificial supports, the compressor blade50may be precluded from moving undesirably during the hybrid manufacturing process, and specifically during subtractive manufacturing.

In embodiments, there may be a support base116adjacent each of the trailing edge and the leading edge of the compressor blade50. These support bases116may not be in contact with the compressor blade50when the blade50is secured within the mounting tool100. That is to say, the support bases116may be situated such that there is some space between the support bases116and the compressor blade50. For example, there may be a first support base116A located near one of the trailing edge and the leading edge of the compressor blade50with a first gap117A therebetween (FIG.1), and there may be a second support base116B located near the other of the trailing edge and the leading edge of the compressor blade50with a second gap117B therebetween. The gaps117A,117B may be traversed by bridges (e.g., bridges135and145, seeFIG.4) which are built during additive manufacturing and extend between the sacrificial supports and the compressor blade50. In embodiments, the gaps117A,117B may be about less than a hundred-thousandths of an inch wide.

The shape and geometry of the support bases116may, in embodiments, generally correspond to the shape and geometry of the compressor blade50. For example, as depicted in the figures, the support bases116may be polyhedrons whose form mimics or follows one or more curves (e.g., x-, y-, and/or z-axis curves) of the compressor blade50and the edges thereof. In some embodiments, the support bases116may be generally V-shaped such that sacrificial supports built thereon may use relatively less material while still maintaining structural integrity. In general, the form of the support bases116may factor into the resulting shape of the sacrificial supports built thereupon, in which the development of sacrificial supports which follow and stabilize the compressor blade50as the blade50is built up. Where the compressor blade is tall (e.g., in the z-direction), the support bases116may likewise be relatively tall to ensure the sacrificial supports built thereon desirably stabilize the compressor blade during repair.

Prior to material being added to a compressor blade50that is secured in the mounting tool100, a topmost edge or tip (e.g., in the z-direction) of the blade50may be made level with a topmost edge or surface of at least one of the support bases116(i.e., a top of the blade50may rest in the same x-y plane as a top of the supports116). This may be accomplished by, for example, removing material from the compressor blade50prior to installing the blade50in the mounting tool100. Alternatively, or additionally, the compressor blade50and support bases116may be made level by adjusting a position (e.g., an x-, y-, and/or z-coordinate thereof) of the blade50within the mounting tool100by manipulating the fasteners114. Ensuring the compressor blade50and the support bases116are level in this manner may facilitate the additive manufacturing step of the repair.

In a current embodiment, the blade50may be secured within the tool100and the tip of the blade50may then be together with the support bases116A and116B such that each of the blade tip and the support bases116A and116B are coplanar. Material may then be added layer by layer to the blade50and the support bases116A,116B in a planar fashion. By grinding the blade50while it is in the tool100ensures that the support bases116A and116B, which are ground together with the blade50, e.g., in a machining apparatus, are in the same plane after the grinding operation such that the tool100may thereafter be affixed to a hybrid manufacturing machine for carrying out the additive and subtractive processes.

In embodiments, one or more of the support bases116may be formed as part of or made integral with the mounting tool100(e.g., the upright portion112thereof). Alternatively, or additionally, in still other embodiments, one or more of the support bases116may be releasably attached to the blade mounting tool100. For instance, the support bases116may be selectively secured to the mounting tool100using one or more fasteners (e.g., a screw, a bolt, a nail, a clamp, a pin, etc.). The support bases116may be temporarily affixed to the mounting tool100in this way for the building of sacrificial supports thereupon. Once hybrid manufacturing of the compressor blade50is complete, a user may dissociate both the blade50and the support bases116from the mounting tool100. With the mounting tool100out of the way, trimming the sacrificial supports from the blade50may be much easier.

In embodiments where one or more of the support bases116are selectively attachable to the mounting tool100, a user may readily customize the mounting tool100for the repair of a given compressor blade50. That is to say, the user may swap out a first set of support bases116for a second set of support bases116which differs in geometry (e.g., shape, form, and/or size) from the first set. Such customizability may be desirable where the user wishes to repair a compressor blade50which has geometry that does not correspond to geometry of the first set of support bases116. In this way, the user may adapt a single mounting tool100to repair a plurality of compressor blades50having disparate geometries by swapping out support bases116. Furthermore, selectively attachable support bases116may be easy to replace when the original support bases116are damaged or otherwise become unsuitable for use. Attachable support bases116may be replaced without having to replace or repair the rest of the mounting tool100.

Turning now toFIG.4, a mounting tool100having an example built-up (i.e., repaired) compressor blade50′ is depicted. The dashed lines show the compressor blade50prior to the additive manufacturing step, together with the support bases116A and116B underneath sacrificial supports130and140, respectively. As can be seen, the sacrificial supports130and140may be built-up and attached to the leading and trailing edges of the compressor blade50as material is added to the blade50. Relatively thin bridges135,145of material may link the sacrificial supports130,140to the compressor blade50′. Specifically, a first bridge135may span the first gap117A and couple the first sacrificial support130to an edge of the blade50′, and a second bridge145may span the second gap117B and couple the second sacrificial support140to an opposing edge of the blade50′. In this way, the sacrificial supports130and140may stabilize the compressor blade50, enabling the blade50to undergo hybrid manufacturing which may not have otherwise been possible particularly where the shape of the blade50is complex. Once the repair of the blade50is complete, the sacrificial supports130,140may be removed by machining away the relatively thin bridges135and145.

In embodiments, the geometry of the sacrificial supports130and140may correspond to the geometry of the repaired blade50′. That is to say, the sacrificial supports130,140may be built up in the x-, y-, and/or z-direction to mimic or follow the various complex curves of the compressor blade50. This matching of blade50geometry may be facilitated by the geometry of the support bases116on which the sacrificial supports130,140are built.

FIG.5is a flowchart illustrating an example method200of repairing a compressor blade (e.g., a compressor blade having complex geometry) using the compressor blade mounting tool embodiments disclosed herein.

First, at step202, the compressor blade50may be coupled to the mounting tool100. The compressor blade50may be secured within the tool100by placing the blade50within the slot118and using the one or more fasteners114to hold the blade50in place. The position of the blade50within the mounting tool100may be adjusted here using the fasteners114to, for instance, align the blade50with the support bases116.

At step204, the compressor blade50may be prepped for repair by grinding the tip of the blade50and the support bases116A,116B together (e.g., via a machining apparatus or other means) such that the support bases116A,116B and the blade tip are in the same plane. In other embodiments, material may be removed from the blade50while it is outside the tool100, and the support bases may be ground separately if required, to ensure that the blade tip, once the blade50is secured within the tool100, is in the same plane as the support bases116A,116B.

Then, at step206, once the blade tip and the support bases116A,116B are coplanar, the mounting tool100and the blade50retained therein may be arranged in hybrid manufacturing equipment or machinery. The mounting features122of the mounting tool100may be used to affix the blade mounting tool100to the machinery.

At step208, additive manufacturing may be used to add material (e.g., in layers) to the compressor blade50. The additive manufacturing process used may be, for example, laser sintering, where metal powder is deposited on the blade50and heated with a laser to form a solid layer. This process may be repeated until a desired amount of material is deposited or a desired number of layers are formed. At step210, additive manufacturing may be used to build sacrificial supports130and140and link the supports130,140to the blade50via bridges135,145. The sacrificial supports130,140may begin on the support bases116, and the geometry the sacrificial supports130,140may correspond to the geometry of the blade50as the blade50is built-up. This step may occur generally at the same time as the step of adding material to the compressor blade50(i.e., the blade50, sacrificial supports130,140, and the bridges135,145may be built-up generally simultaneously). That is, layers of the blade50at step208, and layers of the sacrificial supports116A and116B and bridges135,145at step210, may generally be formed together.

Next, at step212, a subtractive manufacturing process may be used to remove material from the blade50. This may be accomplished via, for example, milling of the blade50(e.g., to remove a specific amount of material, to achieve certain blade50dimensions, to finish or polish a surface of the blade50, etc.). This subtractive manufacturing step may overlap the steps of adding material to the blade50, the sacrificial supports130,140, and the bridges135,145. That is to say, while material layers are being added to the blade50(step208), hybrid manufacturing equipment may remove material from preceding layers (i.e., the subtractive process “follows” the additive process). The steps of adding material to and removing material from the blade50may be carried out by a singular device configured for hybrid manufacturing. The step of removing material may finish after the steps of adding material have already been completed. The compressor blade50may be stabilized and precluded from undue movement during this process by the sacrificial supports130and140.

In embodiments, and as is clear from the flowchart (specifically, from the arrow looping back from step212to steps208/210), at least part of the subtractive process at step212may follow at least part of additive processes at steps208/210and at least part of the additive processes at steps208/210may likewise follow at least part of the subtractive process at step212. For instance, where two layers of material are to be added, layer 1 may first be formed in steps208/210and machined thereafter in step212, and the method200may loop back to step208/212for the formation of layer 2 and then to step212for the machining of layer 2. The additive steps208/210and the substrative step212may be repeated in like manner until the repair of the blade50is completed.

Next, at step214, the repaired compressor blade50′ may be removed from the hybrid machinery and dissociated from the mounting tool100. Then, at step216, the sacrificial supports130,140may be removed from the repaired compressor blade50′ via machining (e.g., electrical discharge machining). The relatively thin bridges135and145may be exploited to facilitate this process. Once removed, the repaired turbine blade50′ may undergo any suitable finishing processes, such as machining to clean up the areas where the blade50′ was attached to the sacrificial supports130,140.

The artisan will understand that the steps of the method200may be readily omitted, added to, and/or modified to implement the various embodiments of the compressor blade mounting tool system described herein. For example, the steps of replacing the support bases116with a second set of support bases116having a disparate geometry, and then using the second set of support bases116to repair a second turbine blade50may be added. As yet another example, the step of detaching the support bases116from the mounting tool100after the blade50is repaired may be added in embodiments having support bases116that are removably attached to the mounting tool100. As still another example, the steps of securing the compressor blade50in the mounting tool100and securing the mounting tool100to the hybrid machinery may be carried out in a different order.

In an embodiment, the tool100may be modified to allow for the repair of multiple blades simultaneously. For example, a tool having multiple slots each having associated therewith a pair of support bases may be provided to allow for the securement and repair of a plurality of compressor blades at the same time. In this embodiment, all the support bases and blade tips may first be ground such that they are coplanar (e.g., all the blades and the support bases may be ground such that the blades and support bases have a height equal to the height of the shortest blade). This tool, together with the plurality of compressor blades secured thereto, may then be associated with a hybrid manufacturing machine to allow for the additive and subtractive manufacturing to be employed for the simultaneous repair of each of the blades. Repairing multiple blades in the same tool at the same time (e.g., repairing multiple compressor blades from the same engine) may further reduce the costs and time associated with the repair relative to repairing only a solitary blade in each tool.

Thus, as has been described, the mounting tool100may facilitate repair of compressor blades having complex geometry (and other blades) by enabling the use of both additive and subtractive manufacturing processes to repair the blade50while the blade50is coupled to the same mounting tool100. Without the mounting tool100, and the support bases116thereof, the compressor blade50may undesirably move during hybrid manufacturing, thereby precluding effective repair. Furthermore, effectuating both subtractive and additive manufacturing generally at the same time may reduce process time significantly relative to conventional repair methods. A mounting tool100having releasably fixed support bases116may allow for a cost-effective mounting tool100which may be readily adapted for repairing compressor blades of varying geometry.

The artisan will understand that embodiments of the compressor blade repair mounting tool disclosed herein may include or have associated therewith electronics (e.g., hybrid manufacturing equipment). The electronics may be used to control, modify, and/or facilitate the operation of the blade repair mounting tool. In some example embodiments, processor or processors may be configured through particularly configured hardware, such as an application specific integrated circuit (ASIC), field-programmable gate array (FPGA), or the like, and/or through execution of software to allow the blade repair mounting tool and/or the hybrid manufacturing equipment to function in accordance with the disclosure herein.

While example applications (e.g., compressor blade repair) are used to illustrate the workings of the system and methods disclosed herein, the artisan will understand that the embodiments disclosed herein may be adapted to other repair applications, and that such adaptations are within the scope of the present disclosure. For example, embodiments of the disclosure herein may be adapted for turbine blade repair.

Many different arrangements of the various components depicted, as well as components not shown, are possible without departing from the spirit and scope of the present disclosure. Embodiments of the present disclosure have been described with the intent to be illustrative rather than restrictive. Alternative embodiments will become apparent to those skilled in the art that do not depart from its scope. A skilled artisan may develop alternative means of implementing the aforementioned improvements without departing from the scope of the present disclosure. It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. Not all steps listed in the various figures need be carried out in the specific order described.