The manufacture of gas turbine engine airfoil members, such as fan and axial compressor blades, typically entails alternately heating and forging metallic billets through a series of progressive dies until the airfoil meets the engineering design requirements as to surface contour, thickness, twist, etc. An engineering master drawing typically defines the airfoil contour requirements at a plurality of radially spaced longitudinal slices or sections through the airfoil. To be acceptable, the actual airfoil surface must fall between maximum and minimum contours at each radial location. Further the airfoil surfaces must smoothly transition from each section to the next. Due to the inherent complexity of the airfoil contour as well as the variability in the forging process, reasonable tolerances are applied. After the airfoil has been formed, subsequent machining steps are required to remove excess material and extruded flash from the leading and trailing edges, and correctly machine attachment areas such as dovetails. The smooth blending of the surface contours from forged to machined areas is critical to meeting the design requirements. Any steps or other surface irregularities give rise to undesirable stress concentrations in the airfoil which limit blade life. Further, these discontinuities adversely affect the airflow over the blades causing localized turbulence and a consequent reduction in pumping efficiency.
Fan and compressor systems for large turbofan engines often incorporate midspan shrouds. These aerodynamically shaped shrouds extend generally circumferentially from both the pressure and suction faces of each airfoil and abut adjacent shrouds, serving to stabilize the airfoil members thereby preventing excessive detrimental vibratory response during engine operation. Due to the complexity of the airfoil configuration and difficulty in forging the shrouds to correct size and precise location, excess material is incorporated to form oversized protrusions from which the desired shroud contour is machined. Conventionally, the majority of the excess material is removed from the protrusion by a numerically controlled (NC) milling machine using the previously machined dovetail as a reference datum. Since the as-forged airfoil surface is complexly contoured and has a relatively large dimensional tolerance, a standoff dimension, for example 0.050 inch, is incorporated in the NC part program which prevents the milling cutter from disturbing the excess material proximate the dimensionally acceptable airfoil surface. This remaining material must be subsequently removed via tedious, time and labor intensive manual benching or grinding to smoothly blend the two areas. This results in blades which are costly to produce and which lack consistent dimensional uniformity in the blend area, where additional requirements exist as to fillet radius to maintain low stress.
A known method of accommodating the automatic machining of parts having variable geometry on NC machines utilizes a mechanical probe to measure the location of surface features prior to machining. An example of such a method is described in U.S. Pat. No. 4,382,215 System and Method of Precision Machining issued to Barlow et al and assigned to the same assignee as the present invention. Such methods, however, are limited to shifting or translating an NC part program in total along one of the X, Y or Z axes of the machine tool, depending on the offset from nominal calculated from the probe hit data. While effective for machining a uniform surface such as an outer diameter of a cylindrical component, such a technique would not be of benefit in the blending requirement at issue due to the continuously contoured airfoil surface. Total deviation from nominal of the part surface for a conventional blade having a radial airfoil length of approximately eight inches and a nominal chord length of two inches due to contour, thickness and twist in the worst case stackup can easily exceed 0.030 inch. For a smooth blend requirement of .+-.0.0015 inch, it is clear that offsetting the NC program, which is drawn to the nominal airfoil surface would be insufficient. Allowable contour variation alone at a given section can exceed 0.010 inch. Similarly, attempts to skew the toolpath by the offset, for example from a maximum value at the leading edge to zero at the trailing edge, or by adding half the offset at the leading edge and subtracting half at the trailing edge do not meet the requirement, as ultimately, they rely on simply rotating the nominal tool path and do not match the actual airfoil contour.
To properly blend the shroud to the airfoil, the airfoil surface proximate the area to be machined should be measured and the appropriate NC part program generated to match the specific contour of each airfoil. Initial attempts applied to machining airfoil squealer tips utilize fixed gaging, specific to each blade design, to measure individual blades. Measurement data is uploaded to a computer aided design (CAD) system on a host computer where the part surface is regenerated and a unique tool path NC part program is defined for each blade. The unique program is then downloaded and the part is machined. While achieving acceptable final results, this technique utilizes costly fixed gaging, entails two way communication between the host and machine tool computers and requires up to four hours of host computing time to regenerate the surface and the unique tool path part program to machine that single blade only. Application of this technique to the more complex contoured airfoil surface problem at hand in a high volume production environment is clearly unacceptable.