WO1994011795A1

WO1994011795A1 - Method for cnc machining

Info

Publication number: WO1994011795A1
Application number: PCT/US1993/010906
Authority: WO
Inventors: Gregory F. Putira; Edward Marchitto; George W. Leistensnider; Alfred J. Albetski
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1992-11-19
Filing date: 1993-11-10
Publication date: 1994-05-26
Anticipated expiration: 1995-05-19

Abstract

A workpiece is machined to preselected dimensions and tolerances using a computer numerically controlled machining system by probing the workpiece in the area to be machined to generate data representative of the actual position, generating a polynomial equation representative of the region of the workpiece to be machined using the measured data and preselected data indicative of the desired after machining dimensions of the workpiece generating machine tool instructions based on the generated equation, and using the instructions to machine the workpiece to the desired configuration. The method is also useful for machining features not directly measurable but relatable to the measured feature, and for machining parts where the acceptable tolerance of the mated assembly in less than the normal capability of the machining system to produce close-tolerance parts.

Description

METHOD FOR CNC MACHINING

DESCRIPTION

1. Technical Field

This invention relates to the computer numerically controlled (CNC) machining of workpieces and, more particularly, to the machining of workpieces whose actual configurations deviate from their nominal dimensions.

2. Background Art

It is desirable to reduce the cost of manufacturing by automating manufacturing processes to the greatest extent possible. For instance, in airfoils for gas turbine engines, the shape of the leading and/or trailing edge of the airfoil (i.e., its radius of curvature or other shape when the edge is viewed in cross-section perpendicular to the airfoil stacking length) is important to efficient performance of the airfoil and such edges therefore need to be very accurately formed. Since highly accurate machining is generally a very expensive process, any improvements in efficiency will be reflected favorably in the cost.

As is well-known, airfoils are designed with a variety of shapes. For example, some airfoils are fairly flat such that their trailing and leading edges are nominally straight. Other airfoils may have a degree of twist about the stacking line such that the trailing and/or leading edge follows a spiral-like path which may have a small or large degree of curvature, depending upon the degree of twist. The airfoil may also be designed with an edge EH-9084 having variable thickness or variable radius of curvature along its length.

Often it is desirable or necessary to form the airfoil edge to its final shape in a separate operation after the body of the airfoil has been at least partially formed or machined. For example, hollow airfoils are sometimes formed by bonding pressure and suction side metal skins to opposite sides of a supporting rib structure. Bonding may be accomplished by electron beam welding, resistance welding, diffusion bonding or the like. The airfoil shape may be formed simultaneously with the bonding operation or thereafter by coining, for example. In one manufacturing technique, a bead of weld material is partially machined away to form the final desired shape of the edge. If an unfinished airfoil edge were known to be accurately located, oriented and dimensioned in accordance with its nominal engineering design, then the airfoil edge could be readily machined by current numerical control machining methods. However, if the location, orientation, thickness or other features of the edge vary significantly from part to part (although still within acceptable tolerances) , finishing the edges by prior art numerical control machining methods has not been possible. In fact, such edges have sometimes had to be finished by hand, which is very expensive and does not produce consistently reliable results.

It is desirable to be able to automatically and relatively inexpensively and accurately finish machine various components. While the example of machining the edges of airfoils embraces the concept of a number of parts having similar characteristics, it is important to provide a method which is not so restricted.

In U.S. Patent No. 5,055,752, to Leistensnider et al., which shares inventors and is of common assignee herewith, a computer numerically controlled machining system is used to machine the elongated edge of a workpiece, such as an airfoil. The numerically controlled machining system probes the surface of the workpiece along the length of the machine to determine edge dimensions and/or its actual position and orientation at preselected locations relative to a cutting tool holder and to the workpiece fixture.

Data are generated and stored and the edge of the workpiece is machined under the direction of a machine program which accesses that data and other known preselected part design data which have been stored. The computer processes the measured data and modifies a generalized cutting tool control program such that the computer causes a cutting tool to follow the edge of the actual part, cutting the edge to preselected dimensions as the cutting tool moves along the edge. During the cutting operation the workpiece and the cutting tool are constantly reoriented relative to each other as the tool moves along the edge to maintain the tool in appropriate angular and positional relation to the workpiece over the length of the edge.

What is needed is a method for machining a workpiece where the workpiece is of generally known configuration, but the workpiece varies from the nominal configuration in a measurable manner, but unpredictable, manner. SUMMARY OF THE INVENTION

A workpiece is machined to preselected dimensions and tolerances using a numerically controlled machining system by probing the features of the workpiece to be machined to determine their actual position and orientation at preselected locations relative to a cutting tool holder and to the workpiece fixture, generating and storing data indicative thereof, processing the measured data using a statistical analysis method to curve-fit the data to a polynomial equation which is descriptive of the workpiece, and machining the workpiece under the direction of a machine program which causes the machine tool to move in accordance with the polynomial equation generated.

The method of this invention is suited to the machining of single workpieces, as well as to the machining of nominally identical parts. In either case the individual characteristics of the workpiece result in the requirement that the machine tool be capable of producing a workpiece which is within blueprint tolerances even though the workpiece does not satisfy the nominal blueprint requirements. When such workpieces are placed in a fixture in an automated machine system, the portion to be machined is located and oriented differently from a blueprint nominal part relative to fixture reference points used to locate the part in the fixture. Measuring to determine the true location of the workpiece, and processing the measured data according to the method of this invention, provides machine control instructions which cause a cutting tool to follow the actual workpiece, cutting the desired portion of the workpiece to preselected dimensions as the cutting tool travels relative thereto, the workpiece and the cutting tool being reoriented relative to each other as the cutting tool moves to maintain the tool in appropriate angular and positional relation to the workpiece over the region of the workpiece being machined.

Thus an airfoil may have its leading edge accurately machined and positioned relative to the airfoil stacking line. However, when that leading edge and stacking line are accurately positioned in the fixture of an automated machining system, the unmachined trailing edge will be located, oriented or shaped differently for each part simply due to manufacturing tolerances.

The present invention uses a computer numerically controlled machining system which produces a machine control program unique to each workpiece which is to be machined. Using an airfoil as an example, each airfoil workpiece is fixtured into the machine apparatus and the feature to be cut, such as the unfinished trailing edge, is probed to generate data which are stored in a memory. The machine program accesses the data to determine exactly where the feature is located and how it is oriented relative to a cutting tool holder and the fixture. One or more equations describing the unfinished trailing edge are generated using the measured data. The equations are then used to generate a series of instructions to control the machine apparatus to move the part and cutting tool relative to each other in a manner resulting in the correct cut. More particularly, in a multi-axis closed loop numerically controlled machining system having a workpiece fixture and cutting tool spindle controllably movable relative to each other, the method of machining a leading or trailing edge of an airfoil workpiece disposed within the fixture includes the steps of placing a probe within the cutting tool spindle and probing reference surfaces on the fixture to generate machine offset data indicative of the relative positions of the spindle, the fixture and the probe; sorting the offset data; and then probing points on the surface of the airfoil workpiece along and adjacent to the length of the edge to be machined to generate airfoil data indicative of the actual position and characteristics of the edge; storing such airfoil data; generating a polynomial equation using the stored data which represents the configuration of the airfoil workpiece edge; generating a machine control program based on the measured data; removing the probe from the spindle and replacing it with the cutting tool; and machining the length of the edge of the airfoil workpiece under the direction of the machine program generated from the stored data by causing the cutting tool to follow along the actual airfoil workpiece edge, cutting the edge as it travels relative thereto, and causing the airfoil and the cutter to be continuously reoriented relative to each other as the cutter moves along the edge to maintain the cutting tool spindle axis and the cutting tool in appropriate angular and positional relation to the workpiece. By this technique the actual configuration of the airfoil is used to create a machining program which is then used to machine the edge of the airfoil automatically, efficiently and accurately.

Thus a single computer numerically controlled machining system is used to machine a variety of workpieces. The location, orientation and configuration of the features of the workpieces to be machined may vary significantly. The probing provides specific data (i.e., actual hardware data) which are used to generate an equation which represents the workpiece feature to be machined. A machine control program then generates instructions for controlling the movements of the workpiece fixture and cutting tool in an interactive manner. These control instructions establish the correct position and orientation of the cutter and the workpiece during machining of each specific workpiece.

The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying figures.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a perspective view of a compressor airfoil having an unfinished trailing edge which is to be machined by the method of the present invention.

Figure 2 is a sectional view taken along the line 2-2 of Figure l. Superimposed on the section is a phantom outline of the airfoil at that section, per nominal engineering design dimensions.

Figure 3 is a simplified, illustrative view of machining apparatus used in practicing the method of the present invention, with the part to be machined fixtured therein and the probe in operative position.

Figure 4 is a view taken in the direction 4-4 of Figure 3, with certain elements of the apparatus not shown or broken away to more clearly show other elements located behind them in this view.

Figure 5 is an enlarged view of a portion of Figure 3.

Figure 6 is a sectional view taken along the line 6-6 of Figure 4. Figure 7 is a sectional view taken at the same vertical location as the view of Figure 6, but with the probe replaced by the cutter and the apparatus reoriented to the appropriate cutting position for the section of airfoil shown, in accordance with the teaching of the present invention.

Figure 8 is a sectional view taken along the line 8-8 of Figure 6 showing the probe about to contact the airfoil surface. Shown in phantom is the cutter in operative position in place of the probe, with the same airfoil section having been appropriately reoriented relative to the cutter in accordance with the teaching of the present invention.

Figure 9 is a block diagram illustrating the interrelationship between various portions of the machining system of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The method of this invention is applicable to any workpiece for which the portion to be machined can be described by a polynomial equation. For purposes of illustration, the invention will be described as it applies to the machining of an unfinished trailing edge on a compressor airfoil. Figure 1 shows a compressor airfoil 10 at an intermediate stage in the manufacturing process. The airfoil 10 comprises a finished leading edge 12, an unfinished trailing edge 14, a pressure surface 16, and a suction surface 18. Integral with each end of the airfoil are tabs 20, 22, respectively. The tabs are manufacturing aids and are not part of the finished airfoil. They are machined off during a later manufacturing step. In this airfoil the trailing edge is nominally of constant thickness. In Figure 2 the solid line depicts a typical section taken through the airfoil 10 in a plane perpendicular to the airfoil stacking line 24. The point 24' where the stacking line intersects the section plane is referred to as the stacking point of that airfoil section. The stacking point of a typical airfoil section is the point about which the airfoil section is positioned. The stacking line contains all the stacking points of all the airfoil sections. In this embodiment, the stacking line 24 is a straight line, but it may also be a curved line. In the airfoil section shown in Figure 2 the stacking point falls within the confines of the airfoil section; however, that is not necessarily true for every section along the length of the airfoil.

The trailing edge 14 of the airfoil 10 is to be machined by the method of the present invention. In Figure 2 the planes E and F intersect along the stacking line 24 and represent the coordinate system used on the engineering drawing to define the nominal size, shape and position of each airfoil section relative to other airfoil sections. The dotted line in Figure 2 represents the outline of the airfoil at the section shown, in accordance with the nominal engineering drawing dimensions of the airfoil. The airfoil in this example is designed such that the center points 26' of the leading edge at every airfoil section fall within a common plane which is parallel to the stacking line 24. However, the leading edge center points 26' do not form a straight line in that plane. K' is the distance between the point 26* and the stacking line 24 at each section. In machining the leading edge 12, the distance K was maintained to close tolerance at the appropriate nominal (per engineering design) value K' for each section.

As is indicated in Figure 2, unfinished trailing edge 14, like the leading edge 12, is not located accurately with respect to the engineering nominal position. More importantly, the relative position between the trailing edge 14 of the actual partially manufactured airfoil 10 and the nominal engineering design position may vary randomly from section to section, and might fall on one side of the nominal position in one section and on the other side in another section. What has been determined to be important to the performance of this particular airfoil is that the engineering design nominal distance D¹ (which can vary from section to section) between the stacking point 24 and the center point 28 of the trailing edge at each airfoil section be maintained to close tolerance, and that a tangent to the mean chordline of each airfoil section at the trailing edge is close to coincident with a radial line of the trailing edge radius. These requirements will become more clear as the method of the present invention is further described hereinafter.

Reference is now made to Figures 3 and 4 which show a programmable numerically controlled machining system generally represented by the reference numeral 100. The particular machining system used in this exemplary embodiment is a Model HN63B numerically controlled machining center manufactured by Niigata Engineering Company, Ltd. , of Tokyo, Japan. In the drawing the machining system 100 is shown and described illustratively, in simplified fashion, and only with sufficient detail to explain its operation in conjunction with the method of the present invention. It will become clear that the method of the present invention does not require the use of a specific type or model of numerically controlled machining system. The one described herein happens to be a five axis system, but three and four axis systems may be suitable depending upon the requirements of the part being machined. With reference to Figure 3, the machining system 100 comprises a stationary bed 102, a fixture support bed 106, a workpiece fixture 108, and a cutting tool support column 120. The stationary bed 102 has a channel 104 therein. Disposed within the channel 104 is the fixture support bed 106 which slides within the channel 104 along an axis perpendicular to the plane of the paper and which is herein referred to as the X axis. Mounted on the slidable fixture support bed 106 and rotatable relative thereto about a vertical Y axis parallel to the plane of the paper is the workpiece fixture 108. The fixture 108 comprises a base 110, a support frame 112, a rotatable plate 114, workpiece holding apparatus generally designated by the reference numeral 115, and a gage block 116. The support frame 112 is fixedly secured to the base 110. Rotatably mounted on the frame 112 is the circular plate 114. The plate 114 rotates about an axis herein designated as the C axis which is perpendicular to the face 118 of the plate 114.

The tool support column 120 slides in a channel 121 in the stationary bed 102 in the direction of an axis herein referred to as the Z axis, which is perpendicular to the X and Y axes. Within the column 120 is a vertically extending spindle track 122. Disposed for movement in the Y direction within the spindle track 122 is a spindle 124. The spindle axis 125 is parallel to the Z axis.

Disposed in the spindle 124 is a probe 152. The probe axis is essentially coincident with the spindle axis 125. The probe includes a stylus 154 extending along the axis 125 and terminating at a spherical tip 156, best shown in Figure 6. In this example, the probe is a Renishaw Model MP7 touch trigger probe with an optical transition system, made by Renishaw, Inc. of Schaumburg, 111.

Referring, now, to Figures 3, 4 and 5, the workpiece holding apparatus 115 is secured to the face 118 of the plate 114. The holding apparatus 15 comprises a pair of spaced apart knife edges 128, 130 precisely located a predetermined distance from the face 118. The face 118 is a precise, known distance from the Y axis. A locating member 132 includes a locating surface 134 at a known distance from the C axis. Also secured to the plate 114 are lateral locators 136, 138 that, in conjunction with the knife edges 128, 130 locate the airfoil such that its stacking line 24 is parallel to a known distance from the Y axis. The locator 138 is best shown in Figure 6.

The airfoil 10 is positioned into the workpiece holding apparatus by urging the leading edge 12 against the knife edges 128, 130, and positioning the outer edge 140 of the end tab 122 on the locating surface 134. The surface 134 locates the airfoil in the Y direction. A lower hydraulically operated rocker arm 142 urges the lower portion of the suction surface of the airfoil against the locating feet 146 of the lateral locator 138. Similarly, a hydraulically actuated plunger 144 urges the upper portion of the suction surface of the airfoil against the locating feet of the upper lateral locator 136.

After the airfoil is secured in its appropriate position by the locating means just described, four additional sets of hydraulically actuated plungers 148 are moved into position against opposites sides of the central portion of the airfoil to provide additional support for the workpiece. Hydraulic lines are designated by the reference numeral 150 throughout the figures.

With reference to Figure 9, the machining system 100 is depicted schematically as encompassing the machining hardware described above as well as the electronic hardware which controls the operation of the machining hardware. The box 200 represents the machining hardware and is labeled "machine tool". A machine control 202 sends a variety of signals 203 to the machine tool 200 to move and rotate the hardware in a particular manner. The system 100 also includes a computer 204, storage means 206, and the probe 152. For discussion purposes, the computer and storage means are shown as separate from the machine control; however, they may also be considered part of the machine control.

The storage means 206 is simply a memory which is accessible by the computer 204. In the method of the present invention a computer program, which is also referred to herein as the machine program, is input into the storage means 206. The machine program includes certain preselected nominal engineering design dimensions of the part to be machined. Also in the storage means are data relating to the machine tool zero or home position. Further, each time the probe 152 touches a point on the workpiece (e.g., airfoil 10) or on the gauge block 116, data indicative of the machine tool position at that instant are placed in the storage means. During operation the computer 204 accesses the machine program and selected data in the storage means 206, and performs certain calculations on the stored data to generate a polynomial equation which represents the feature of the workpiece to be machined, in this case the trailing edge of the airfoil, within the accuracy required by the engineering drawing. This equation is then processed by another computer program which converts the generated equation into the instructions necessary for the machine control 202 to operate the machine tool 200 such that the workpiece will be correctly machined.

Prior to actual machining of the airfoil the machine program instructs the machine tool 200 to move the probe and fixture such that the probe contacts the gauge block 116 on several surfaces, such as the surfaces a, b, c and d. (Figure 3 shows the probe 152, in phantom, about to contact the gauge block.)

The data put into the storage means 206 as a result of those probe contacts are accessed and used by the computer 204 to calculate the length of the probe, the size of the stylus tip 156, and deviations of the position and orientation of the machine tool components from the "home" position. These deviations or "machine offsets" are stored in the storage means 206 for use in the subsequent step of airfoil measurement and analysis.

In this embodiment, the airfoil 10 is defined on engineering drawings by a series of airfoil sections which are plane sections through the airfoil perpendicular to the stacking line 24 at specified locations along the length of the airfoil. The phantom line in Figure 2 shows one such section. The distance between the stacking point 24 and the center point 28' of the trailing edge is a given nominal dimension D* for each of the several sections used to define the airfoil The dimensions D' for these airfoil sections are input into the storage means 206 (Figure 9) and are the nominal engineering design dimensions referred to above.

After determining the machine offsets, the airfoil surface adjacent to the unmachined trailing edge is probed (per instructions from the machine program) at locations corresponding to the engineering drawing sections which define the airfoil. In Figure 4, each of these sections is represented by a pair of horizontally spaced apart cross marks 160 which are superimposed upon the drawing for purposes of illustration. Each pair of points 160 lies in a plane perpendicular to the stacking line 24. There are seventeen such pairs of points 160 for the airfoil in this example, which represent the seventeen airfoil sections used to define the engineered design of the finished airfoil For the particular airfoil of this example, which is about 17 inches long and 5 inches wide, the points 160 closest to the unmachined trailing edge are approximately 1/10 inch in from that edge; and each pair of points 160 are about 1/10 of an inch apart. These distances are somewhat exaggerated in the drawing for clarity. The probe is also programmed to touch a point 162 located 1/10 inch above (i.e., in the Y direction) each of the points 160 closest to the unmachined trailing edge (see Figure 8) . (The points 162 could equally as well be below the points 160. The machine program is written according to where the programmer desires to have the probe contact the part.) Thus, for each airfoil section, three points are probed (one is actually above the section plane) . As the probe touches each point the position of that point, with appropriate machine offsets applied, is stored in the storage means 206.

According to the method of the present invention, the probe is programmed to touch (in some preselected efficient order) all the points 160, 162 along the length of the trailing edge, thereby placing into the storage means 206 data for the position of every one of those points. Note that the order of contacting the points is not critical, except the machine program must be written to access the correct point information when doing its calculations.

In this example, there is no rotation of the airfoil during probing. The fixture moves only in the X direction to allow the probe to contact each of the pair of points 160 at each section. The probe itself moves only parallel to the Y axis as it moves from section to section and between 160, 162 at each location. No rotation of the airfoil 10 is necessary since the airfoil does not have a large amount of curvature at its trailing edge. Some airfoils may have very large amounts of twist about their stacking line resulting in a highly curved trailing edge. For airfoils such as these, it would be required that the airfoil be rotated about the stacking line at each new airfoil section being probed so that the surface of the airfoil near the trailing edge was always approximately perpendicular to the spindle axis 125.

Returning to the present example, after the airfoil has been probed, the probe 152 is removed from the spindle 124 (such as by a robot arm or by hand) and is replaced by a cutting tool or cutter which rotates about the spindle axis 125. Figure 7 shows a full-form cutter 154 in position in the spindle 124 and in the process of cutting the workpiece at the section shown. The cutter teeth form circular arcs which are bisected by the plane 156 which is perpendicular to the spindle axis. In this example, the pressure and suction surfaces of the airfoil 10 are parallel as the surfaces approach the trailing edge. In order for the cutter 155 to properly cut a radius into the trailing edge at each point along the length of the trailing edge, the airfoil surfaces adjacent to the trailing edge must be perpendicular to the axis 125 of the cutter at that point. Thus, if the lead line 158 is tangent to the airfoil pressure surface adjacent to the trailing edge in the plane of the airfoil section containing the spindle or cutter axis, the proper machining of the airfoil of this example requires that the angle A be 90° (or very close to it) at all times. Similarly, a line bisecting the trailing edge should be perpendicular to the axis 125 and in the plane 156.

Accomplishing the foregoing with the airfoil 10, which has a curved trailing edge, requires that, as the cutter 154 moves in the Y direction, the airfoil be continuously reoriented relative to the cutter to maintain the appropriate angular orientation between the cutter and the trailing edge. Simultaneously, the airfoil must be moved in the X direction, such that the distance D at each airfoil section is maintained to the nominal engineering dimension D¹ (Figure 2). It is apparent that the equation generated by the machine program using the data gathered by probing determines how much the airfoil must be rotated about the Y axis in order to orient the tangent to the airfoil section mean chordline 163 at the trailing edge perpendicular to the spindle axis 125 and in the plane 156. The phantom lines in Figure 8 show the cutter 155 in operation when the spindle axis 125 is aligned with the point 160 adjacent the edge 14 (Figure 6) . The calculated machine instructions have caused the airfoil to be rotated (from it's probed position shown in full) an appropriate amount about the C axis such that the tangent to the airfoil section mean chordline 163 is perpendicular to the spindle axis. The machine instructions also adjust the location of the spindle in the Y direction to compensate for the change in the Y coordinate of the point 160 as a result of the rotation of the airfoil about the C axis.

Referring to Figures 7 and 8, during cutting, the points 160, 162 are a known distance G from a reference plane 164 which is perpendicular to the spindle axis. The nominal thickness of the trailing edge in this example happens to be constant along the length of the airfoil. This nominal thickness dimension was previously input into storage means 206. When cutting, the computer continuously positions the cutter such that its bisecting plane 156 is a distance from the reference plane 164 which is equivalent to the dimension G plus one half the nominal thickness of the trailing edge.

When formulating the machine control instructions, the computer program compares a predicted cutter path with the acceptable tolerance range of the workpiece. If, because of sharp contours in the workpiece, the predicted cutter path will not remain within the tolerance band, a new predicted cutter path is selected which covers a shorter distance of the workpiece. Thus, one of the unique features of this invention, compared to prior art techniques, is the ability of the computer control to interpolate between measured data points in order to improve the accuracy of the machining operation. The machine control 202 sets the appropriate distance between the spindle axis and the stacking point 24 to result in the dimension D being equal to D' at any required point along the feature being machined. The machine program automatically compensates for Z and X direction movement of the points 160 resulting from rotations about the Y axis which were required to orient the tangent to the airfoil section mean chordline 163 perpendicular to the spindle axis.

As set forth above nominal engineering dimensions for the finished airfoil are input into and stored in the storage means 206 for the 17 airfoil sections corresponding to the location of the pairs of points 160 shown in Figure 4. Using the curve-fit equation based on the measured data points, the computer generates machine instructions which position the cutting tool 155 correctly at any point along the airfoil trailing edge. The cutting tool is programmed to travel along the airfoil trailing edge at a constant rate of speed in the Y direction until the machine control instructs the machine tool to reorient the workpiece relative to the cutter. The machine program causes all linear movements of the airfoil and cutting tool in the X, Y and Z directions and the rotations if any of the airfoil about the C and Y axes to be at appropriate constant rates of speed between adjacent points on the workpiece such that the cutting tool and airfoil simultaneously arrive at the next section appropriately positioned. By that technique the cutting of the trailing edge smoothly transitions from one section to the next. In this example, the cutter 154 is a full-form cutter.

In the machining of certain parts, it may be preferable or necessary to use a half-form cutter; however, that would require each of two cutters to make a pass along the length of the airfoil trailing edge, each cutter forming half the trailing edge shape. Half-form cutters may be particularly useful when, for example, the trailing edge is highly curved, or when the thickness of the trailing edge is variable either by design or due to significant manufacturing tolerances which cannot be ignored. In the case of a variable thickness edge, it may be required to probe points on both the pressure and suction surfaces of the airfoil. The machine program would be designed to use that information to calculate a point equidistant from both surfaces at each section, and thereby determine the center point of the trailing edge (e.g., corresponding to point 28' in Figure 7), so as to enable the machine control to correctly position the cutter as it moves along the edge.

Although in this example a five axis machining system is described, it should be apparent that some parts may be machined using only a three or four axis machining center. For example, if the unmachined trailing edge of the airfoil 10 of the example described above were straight and parallel to the stacking line within certain acceptable tolerances then there would be no need to make the angular adjustment about the C axis as shown and described with respect to Figure 8. Therefore, a machining system without the capability of rotating about an axis corresponding to the C axis could be used. Similarly, for certain airfoil designs, it might also be unnecessary to rotate the airfoil about its stacking line during the cutting operation.

Although this exemplary embodiment was directed to the machining of the trailing edge of an airfoil, it is equally as applicable to the machining of the leading edge of an airfoil, the edge of a rotor blade platform, or even the outermost tip of an airfoil, such as the tip of a compressor or turbine rotor blade. Actually, the method of this invention is readily adaptable and useful for machining any feature of a workpiece, the location and orientation of which cannot be accurately predicted when the workpiece is initially fixtured for the machining operation. For example, in the machining of the inner surface of the metal skins which form the airfoil surfaces of hollow airfoils, measurements of the outer surfaces of these metal skins can be used to establish the cutter paths required to machine the pockets and supporting rib structures on the inner surface, where the requirement for a very thin, but uniform, airfoil surface requires that each component piece be machined relative to it's individual unique configuration. Use of the technique of this invention eliminates the need for strict uniformity of shape of the starting material for each skin. A further example is the machining of mating parts wherein the acceptable tolerance of the mated assembly is significantly less than the normal capability of the machining system to produce the close-tolerance. By measuring a first part, and creating a machining program which will machine a mating part which corresponds to the dimensions of the first part, the tolerance requirements of the assembly can be met.

Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.

Claims

We claim: 1. A method for machining a workpiece utilizing a numerically controlled machining system, comprising the steps of: (a) positioning the workpiece in a fixture means on a coordinate measurement system; (b) contacting a plurality of points on the workpiece in the region to be machined, generating by such contacts data indicative of the position of such contacted points relative to the fixture means; (c) storing in a memory of a computer such contacted point data; (d) storing in the memory of the computer preselected data indicative of desired after machining dimensions of the workpiece; (e) generating a polynomial equation representative of the region of the workpiece to be machined using the contacted point position data and the preselected data; (f) generating machine tool instructions based on the generated equation; (g) positioning the workpiece in the fixture means on a numerically controlled machining system; and (h) machining the workpiece under the direction of the machine tool instructions, cutting the workpiece to preselected dimensions as the cutting tool travels relative thereto, the fixture means and holder being reoriented relative to each other as the tool moves along the workpiece to maintain the cutting tool in appropriate angular positioned relation to the workpiece over the region to be machined. 2. A method for machining a workpiece utilizing a numerically controlled machining system, comprising the steps of: (a) positioning the workpiece in fixture means of the system; (b) positioning a probe in a holder which is part of the system and moveable relative to the fixture means; (c) moving, under the direction of a machine program, the holder and fixture means relative to each other to cause the probe to contact a plurality of points on the workpiece in the region to be machined, and generating by such contacts data indicative of the position of such contacted points relative to the fixture means; (d) storing in a memory of the system such contacted point position data; (e) storing in the memory of the system preselected data indicative of desired after machining dimensions of the workpiece; (f) removing the probe from and positioning a cutting tool in the holder; and (g) machining the workpiece under the direction of a machine program which accesses the stored data and causes the cutting tool to follow the region to be machined on the workpiece, cutting the -workpiece to preselected dimensions as the tool travels relative thereto, the fixture means and holder being reoriented relative to each other as the tool moves along the workpiece to maintain the tool in appropriate angular and positioned relation to the workpiece over the region to be machined. 3. A method for machining a leading or trailing edge of an airfoil workpiece using a numerically controlled machining system, the workpiece having an airfoil stacking line, said system comprising: (a) machining apparatus including a work table, fixture means for holding an airfoil workpiece, said fixture means being secured to said work table, spindle means of holding, alternatively, a cutting tool and a probe, said spindle means having a spindle axis, and being moveable relative to said fixture means; (b) programmable machine control means adapted to control relative movement between the fixture means and spindle means and including data storage means and computer means; said method comprising the steps of: (1) fixing an airfoil workpiece within the fixture means such that the location and orientation of the stacking line of the workpiece relative to the fixture means is known within predetermined tolerances, and the location, relative to the fixture means, of at least one point on the workpiece not on the stacking line is known within predetermined tolerances; (2) entering into the data storage means desired engineering design dimensions to which the workpiece is to be machined; (3) placing the probe in the tool holding means and probing therewith at least one preselected point on the surface of the airfoil workpiece at or near each of a preselected number of spaced apart locations along the length of the workpiece, adjacent the workpiece edge to be machined, the step of probing causing data to be stored in the data storage means indicative of the actual position of each point probed relative to the fixture means; (4) removing the probe from the spindle means and replacing it with the cutting tool which rotates about the spindle axis; (5) machining the edge of the airfoil workpiece by machine control under the direction of a computer program which accesses the data in said storage means, generates a polynomial equation which represents the measured data points and the desired engineering design dimensions, and uses that equation to generate instructions to the machine control which, in turn causes the cutting tool to be brought into position at an initial one of said probed locations along the airfoil workpiece edge and to accurately follow along that edge, cutting the edge to a desired shape as it travels relative thereto, the workpiece and the cutting tool being reoriented relative to each other as the cutting tool moves along the edge to maintain the spindle axis in appropriate angular and positional relation to the workpiece over the length of the edge.