Patent Application
20050247569
The present invention relates generally to manufacturing processes, and, more specifically, to machining.
Precision machining is commonly effected using multiaxis numerically controlled (NC) milling machines. The cutting tool is suspended from a tool head which typically has three orthogonal axes of translation and one or more additional axes of rotation corresponding therewith. The workpiece or part to be machined is fixedly mounted to a bed which may impart additional axes of translation or rotary movement thereto.
During operation, the NC machine is programmed in software for controlling the machining or cutting path of the tool for precisely removing material from the workpiece to achieve the desired final dimensions thereof. The typical milling machine includes a rotary cutting tool having a controlled feedpath for removing material from the workpiece in successive passes finally approaching the desired machined configuration.
Great care must be exercised in programming and operating the NC machine to ensure that the intended precise machining of the workpiece is obtained. Damage to the workpiece during machining may require scrapping thereof, with an attendant loss in time and money corresponding therewith.
A particularly complex and expensive precision part is the typical bladed disk, or blisk, found in gas turbine engines. A gas turbine engine typically includes multiple stages of compressor rotor blades each mounted to the perimeter of a supporting disk. It is common to individually manufacture the compressor blades and mount them using suitable dovetails to the perimeter of the supporting disk.
Alternatively, the full row of compressor blades may also be manufactured integrally with the disk by machining slots in the perimeter of a disk workpiece resulting in a row of integral airfoils remaining after machining.
The initial blisk blank has a solid perimeter in which slots are machined for defining the resulting compressor airfoils extending radially outwardly from the supporting disk in a unitary, or one-piece component. The blisk material is typically a superalloy having enhanced strength, and is correspondingly expensive.
Blisk airfoils were originally manufactured from blanks using conventional NC machines with rotary milling tools for cutting the slots through the perimeter to form the airfoils. The resulting airfoils require substantially smooth and precisely configured surfaces, typically effected by additional machining processes on the initially formed blisk.
For example, electrochemical (ECM) machining is a conventional process in which cathode electrodes are specially built to achieve the desired final contours of the airfoils. An electrical current is passed through a liquid electrolyte in the gap between the electrodes and the workpiece for precisely removing small amounts of remaining material on the airfoils to achieve the desired final configuration thereof with substantially smooth surfaces.
The ECM process is effected in another form of multiaxis NC machine in which the electrodes undergo complex three dimensional (3D) movement as they approach an individual rough airfoil from its opposite pressure and suction sides.
The ECM process is particularly advantageous for quick removal of the superalloy material to the substantially final smooth finish required for the airfoil without undesirable damage thereto. Since the blisk workpiece requires multiple stages of manufacture and machining immediately prior to the forming of the airfoils therein considerable time and money are invested in the workpiece. And, as each of the multitude of airfoils around the blisk perimeter is machined, additional time and expense are invested which further increases the cost of the blisk.
Unacceptable damage to any one of the blisk airfoils or the supporting rotor disk itself during the various stages of manufacturing could render the entire blisk unusable for its intended use in a high performance gas turbine engine resulting in scrapping thereof with the attendant loss of time and expense.
In view of the considerable manufacturing time typically required in the production of blisks, manufacturing improvements are continually being developed for shortening the machining time and expense without increasing the chance of undesirable damage to the blisk during manufacturing.
For example, the ECM process may be specifically configured for initially forming rough airfoils in the blisk workpiece with a substantial reduction in time and expense over conventional milling machines. U.S. Pat. No. 6,562,227, assigned to the present assignee, discloses one form of plunge electromachining specifically configured for this purpose.
Furthermore, electrical discharge machining (EDM) is yet another process for machining material in gas turbine engine components, for example. In EDM machining, a dielectric liquid is circulated between the electrode and the workpiece and electrical discharges are generated in the gap between the electrode and workpiece for electrically eroding material. The EDM process is typically used for drilling the multitude of small film cooling holes through the surfaces of turbine rotor blades and nozzle vanes.
U.S. Pat. No. 6,127,642, assigned to the present assignee, is one example of an EDM machine having a slender electrode supported with lower and middle guides for reducing undesirable flexing thereof during the drilling process.
Both the ECM and EDM processes use electrical current under direct-current (DC) voltage to electrically power removal of the material from the workpiece. In ECM, an electrically conductive liquid or electrolyte is circulated between the electrodes and the workpiece for permitting electrochemical dissolution of the workpiece material, as well as cooling and flushing the gap region therebetween. In EDM, a nonconductive liquid or dielectric is circulated between the cathode and workpiece to permit electrical discharges in the gap therebetween for removing the workpiece material.
In both ECM and EDM the corresponding electrodes thereof are typically mounted in multiaxis NC machines for achieving the precise 3D feedpaths required thereof for machining complex 3D workpieces, such as the airfoils of blades and vanes. The NC machines include digitally programmable computers and include suitable software which controls all operation thereof including the feedpaths and the separate ECM and EDM processes.
In particular, in both processes electrical arcing between the ECM or EDM electrodes and the workpiece must be prevented to prevent undesirable heat damage to the workpiece surface. Electrical arcing is the localized release of high electrical energy which can undesirably burn the workpiece surface and adversely affect the mechanical and material properties thereof.
As indicated above, the exemplary turbine blisk is formed of a superalloy metal having high strength characteristics which can be degraded due to excess temperature. Electrical arcing during the ECM or EDM processes can result in a relatively large recast layer or heat affected zone (HAZ) on the machined workpiece in which the material properties can be undesirably degraded.
Accordingly, both the ECM and EDM machining processes include sophisticated electrical circuits for detecting arcing or incipient arcing and adjusting the machining process to prevent or eliminate undesirable arcing during machining. In this way, the recast layer or heat affected zone in both processes may be minimized for ensuring maximum strength of the finally machined workpiece.
Notwithstanding the various processes for machining material in the production of gas turbine engine blisks, the manufacturing process therefor still requires a substantial amount of time and expense which correspondingly increases the cost of the blisk.
Accordingly, it is desired to provide an electroerosion machining apparatus and process capable of achieving even higher machining rates without attendant undesirable damage to the workpiece.
An electroerosion apparatus includes a tubular electrode supported in a tool head in a multiaxis machine. The machine is configured for spinning the electrode along multiple axes of movement relative to a workpiece supported on a spindle having an additional axis of movement. A power supply powers the electrode as a cathode and the workpiece as an anode. Electrolyte is circulated through the tubular electrode during operation. And, a controller is configured to operate the machine and power supply for distributing multiple electrical arcs between the electrode and workpiece for electroerosion thereof as the spinning electrode travels along its feedpath.
The invention, in accordance with preferred and exemplary embodiments, together with further objects and advantages thereof, is more particularly described in the following detailed description taken in conjunction with the accompanying drawings in which:
Illustrated schematically in
Means in the form of a conventional direct current (DC) power supply 22 are provided for carrying electrical power through the electrode 12 and workpiece 20 during operation. The power supply includes suitable electrical leads 24 correspondingly joined to the electrode 12 as a cathode (−) and the workpiece as an anode (+) in one embodiment. In alternate embodiments, the polarity may be reversed with an anode electrode and a cathode workpiece.
Since the electrode 12 spins during operation, the electrical lead therefor may be suitably joined thereto using a conventional electrical slip ring or other connection as desired. And, the lead for the workpiece may be directly attached thereto or to the supporting spindle 18 as desired.
Additional means in the form of an electrolyte supply 26 are provided for circulating an electrically conductive liquid or electrolyte 30 through the electrode 12 during operation. The electrolyte supply includes various conduits 28 for supplying clean and cool electrolyte to the electrode while returning debris-laden electrolyte from the machining site. The electrolyte may be plain water, or oil, or other liquid having weak to strong electrical conductivity as desired.
Means in the form of a digitally programmable electrical controller 32 are operatively joined to the NC machine 14 for controlling its operation, and additionally joined to the DC power supply 22 for also controlling its operation, and coordinating relative movement between the electrode and the workpiece during the electroerosion machining process. The controller 32 may have any conventional form and includes a central processing unit (CPU) and all attendant memory and data handling systems which may be programmed using suitable software for controlling all operations of the apparatus. A monitor and keyboard are provided with the controller for use both by the operator in controlling the electroerosion machining process, as well as by the programmer for initially setting up the machine for specific forms of workpieces.
The electroerosion apparatus is illustrated in more detail in
As indicated above, the production of electrical arcs in conventional EDM and ECM processes is strictly prohibited therein due to the associated damage therefrom. In EDM and ECM processes, the corresponding electrical controllers thereof include circuits specifically configured for detecting arcing or incipient arcing, to thereby prevent or terminate arcing during operation.
In contrast, the electroerosion process illustrated schematically in
As indicated above, no-arcing operation is desired and achieved in conventional ECM and EDM electroerosion. And, persistent or continual arcing is undesirable in the ECM and EDM processes for the attendant thermal damage to the workpiece associated with a large recast or HAZ layer.
However, by both spatially and temporally distributing multiple electrical arcs between the spinning electrode and workpiece electroerosion material removal may be substantially enhanced, with a removal rate being substantially greater than that for both conventional EDM and conventional ECM, while minimizing the undesirable recast layer.
As shown schematically in
Accordingly, control of the power supply may be coordinated with the feedpath travel P of the electrode for effecting intermittent multiple electrical arcs 40 between the electrode tip and workpiece temporally alternating with electrical discharges between the electrode and workpiece without electrical arcing. In this way, the increase of material removal attributed to the multiple electrical arcs may be balanced with the resulting recast layer by alternating arcing with non-arcing electrical discharges. This balance may be determined for particular workpieces and particular machining processes empirically using both analysis and a series of test machining.
Key features of the spatially and temporally distributed multiarc electroerosion process illustrated in
Furthermore, clean and cool electrolyte 30 is channeled internally through the tubular electrode and out the orifice in the center of the electrode tip for providing clean and cool electrolyte in the machining gap G for promoting stability and distribution of the multiple electrical arcs. The electrolyte also flushes away the erosion debris from the machining process.
Quite significantly, a substantial increase in the electrical current may be used with the spinning electrode with a correspondingly lower peak current density due to the generation of the distributed multiple arcs, which combine to substantially increase the rate of material removal relative to conventional ECM and EDM machining processes.
The tool head 16 shown in
The tool head 16 may be mounted in the machine in any conventional manner for achieving these exemplary axes of movement, and is typically effected using suitable screw driven carriages powered by corresponding electrical servomotors. The various servomotors for the movement axes are operatively joined to the controller 32 which coordinates the movement thereof to in turn control the feedpath P of the electrode tip during operation. In this way, the electrode tip may follow a precise 3D feedpath through the workpiece as desired for machining complex 3D contours in the workpiece.
Correspondingly, the spindle 18 illustrated in
For example, the exemplary workpiece illustrated in
The controller 32 is correspondingly configured for driving the spinning electrode 12 along arcuate feedpaths P as illustrated in more detail in
Since electroerosion cutting is limited to the tip region of the electrode 12 as illustrated in
The rough airfoils 42 so machined include sufficient additional material thereon for undergoing a subsequent machining operation for removing the rough finish thereof and the thin recast layer for achieving the final dimension and smooth surface finish for the final airfoils of the blisk.
As the electrode 12 electroerodes material from the workpiece 20 as illustrated in
As shown in
The electrode 12 illustrated in
Accordingly, the tool head 16 illustrated in
Correspondingly, the multiaxis machine 14 further includes a rotary collet or chuck 50 suitably joined to an upper extension of the tool head 16 above the lower guide for supporting and rotating or spinning the opposite top or proximal end of the elongate electrode 12. In this way, the top of the electrode is mounted in the spinning chuck, and the bottom of the electrode is mounted through the lower guide for permitting spinning thereof during operation.
Since the electrode should be sufficiently long for allowing sufficient time for electroerosion machining prior to the consumption thereof, the tool head 16 illustrated in
The chuck 50 illustrated in
Accordingly, as the tip wears during operation, the electrode may be continually indexed lower as its length is reduced. When the electrode becomes too short for practical use, the machining process is temporarily interrupted for replacing the electrode with a new and longer electrode, and repositioning the chuck 50 to the top of its travel path.
The lower guide 48 is illustrated in a preferred embodiment in
The lower guide may be made from multiple parts, including a main body in which the ceramic bushing 54 may be mounted, and covered by a removable lid fastened thereto by bolts. A lower body extends downwardly from the main body of the lower guide through a corresponding aperture in the tool head 16 for retention thereon.
The lower guide may be formed of stainless steel to resist corrosion from the electrolyte, and has a center bore spaced suitably outwardly from the electrode to provide a small radial gap therebetween, with the electrode being radially supported by the close fitting ceramic bushing 54 disposed therearound.
The lower guide may have a length to diameter ratio greater than about 3 for ensuring stable support of the lower end of the electrode during operation. The middle guide 52 may be similarly configured with a trapped ceramic bushing therein for supporting the intermediate portion of the electrode during operation. The middle guide as illustrated in
As further illustrated in
In this way, additional electrolyte is channeled through the lower guide and around the tip of the spinning electrode for external flushing of the electrode tip directly above the slot being machined by the electrode tip itself.
As shown in
The electrolyte supply 26 illustrated in
The electrolyte supply preferably also includes a work tank 62 containing the spindle 18 and workpiece mounted thereto. The tank is sized for being filled with electrolyte 30 in a pool to submerge the workpiece 20 and the electrode tip during the electroerosion process. The bottom of the tank may be suitably connected to the rough filter 58 for removing the large debris particles from the electrolyte. The rough filter is in turn joined in flow communication with the fine filter 60 for removing even smaller debris particles. And, the upper portion of the tank 62 may be directly joined to the fine filter and bypassing the rough filter.
The rough and fine filters 58,60 may have any suitable configuration, such as a filtering conveyor belt in the rough filter 58, and rolled paper filters for the fine filter for effectively removing erosion debris from the electrolyte prior to return to the spinning electrode. Suitable cooling of the electrolyte may also be provided to remove therefrom heat generated during the electroerosion process.
The two stage filters 58,60 are preferably joined in flow communication with the electrode 12 for effecting both internal and external flushing thereof to enhance the stability of the intermittent multiple electrical arcs generated at the tip end of the electrode during operation. Internal flushing is provided by channeling a portion of the electrolyte through the center bore of the electrode and out its tip end. And, external flushing is provided by channeling another portion of the electrolyte through the lower guide 48 as indicated above, while also optionally bathing the entire workpiece in the bath of electrolyte contained in the work tank 62.
Significant features of the electroerosion apparatus disclosed above include the spinning electrode and its feedpath P coordinated with control of the electrical power provided thereto as illustrated schematically in
Correspondingly, the power supply is further configured for generating relatively high electrical current in the exemplary range of about 80 to 600 amps, with a correspondingly high average current density in the range of 1900 to 12,000 amps per square inches (295-1860 amps per square centimeter).
The relatively high current and average density thereof promote correspondingly large electroerosion material removal, with the additional advantage of relatively low peak current density of about 1000 amps per square inch (155 amps per square centimeter). The low peak current density is attributable to the multiple electrical arcs distributed over the entire cutting area of the electrode tip, as opposed to a single electrical arc. The low peak current density minimizes the production of the recast layer in the surface of the machine workpiece and prevents unacceptable heat affected damage thereto.
The low peak current density may be compared to the high peak current density of multiple orders of magnitude greater in conventional EDM machining in the event of the generation of an electrical arc therein. In EDM, a dielectric liquid is used between the electrode and workpiece and promotes a single electrical discharge or arc in which the entire electrical current is dissipated. That single high current arc has the potential to cause significant damage unless it is avoided or terminated in its incipiency.
The power supply 22 illustrated in
These pulse on and off times may be adjusted by the controller during the electroerosion process to control the generation of the intermittent multiple electrical arcs from the electrode tip alternating with electrical discharges without arcing. The alternating arcs and discharges may be balanced by maximizing the electroerosion removal rate while minimizing recast or heat affected surface layers on the workpiece.
The controller 32 may be preferentially configured for coordinating power to the spinning electrode 12 and the rate of movement or feedrate thereof across the workpiece for electroerosion machining the slot 36 at a machining rate exceeding about 1500 cubic millimeters per minute, without undesirable thermal damage or recast layers in the workpiece.
For example, testing indicates a substantially high material removal rate for the exemplary superalloy Inconel 718 blisk workpiece of 1500 cubic millimeters per minute for an electrode with 120 amp current and having a diameter of about 7.5 millimeters, with a corresponding frontal electrode area of 22 square millimeters. Testing additionally indicates a removal rate of about 2000 cubic millimeters per minute for an electrode having a 13 millimeter diameter with a corresponding frontal electrode area of about 52 square millimeters. And, testing further indicates a removal rate of about 3000 cubic millimeters per minute for an electrode having a 20 millimeter diameter and a corresponding frontal electrode area of about 80 square millimeters.
Compared with conventional electrical discharge machining, as well as electrochemical machining, these material removal rates attributed to the distributed multiarc electroerosion process described herein are orders of magnitude greater in a stable process without undesirable heat affected damage to the workpiece.
The introduction of distributed multiple electrical arcs between the spinning electrode and the workpiece in the presence of an electrolyte therebetween permits a substantial increase in the material removal rate of the electroerosion process which substantially exceeds the material removal rates of conventional EDM and ECM processes. Where those latter processes intentionally prohibit electrical arcing between the electrode and workpiece, the distributed arc process disclosed above preferentially introduces multiple electrical arcs with high average current density, yet low peak current density for maximizing material removal rate.
Accordingly, electroerosion of the workpiece may be effected more quickly than previously possible, without undesirable damage thereto, for reducing both the time and expense associated in the manufacture of the workpiece, which is particularly significant for complex and expensive workpieces such as the exemplary gas turbine engine rotor blisk disclosed above.
While there have been described herein what are considered to be preferred and exemplary embodiments of the present invention, other modifications of the invention shall be apparent to those skilled in the art from the teachings herein, and it is, therefore, desired to be secured in the appended claims all such modifications as fall within the true spirit and scope of the invention.
Accordingly, what is desired to be secured by Letters Patent of the United States is the invention as defined and differentiated in the following claims in which we claim:
