Abstract
A fixture for locating a straight edge of a curved surface of a part and holding the part in a fixed position during performance of a machining operation on the straight edge is described. The fixture includes: one or more supports for seating a convex curved surface of the part, one or more detachable end stops arranged to extend in a plane orthogonal to that in which the supports sit and against which, in use, the straight edge can be aligned, and a clamp arranged, in use, to push the convex curved surface against the supports such that the straight edge contacts the supports without obstruction of the straight edge.
Claims
1. A fixture for locating a straight edge of a curved surface of a part and holding the part in a fixed position during performance of a machining operation on a concave side surface adjacent the straight edge, the fixture comprising; two or more elongate and spaced apart supports for seating a convex curved surface of the part each elongate support extending straightly and in parallel with a chord of the convex curved surface, one or more detachable end stops arranged to extend in a plane orthogonal to that in which the elongate supports extend and against which, in use, the straight edge can be aligned, a clamp arranged, in use, to push the convex curved surface against the elongate supports such that the straight edge contacts the support without obstruction of the concave side surface adjacent the straight edge.
2. A fixture as claimed in claim 1 wherein the supports comprise two or more spaced ribs.
3. A fixture as claimed in claim 2 wherein the ribs are in the form of two or more rods in parallel alignment.
4. A fixture as claimed in claim 2 wherein the supports comprises a base onto which are integrally formed the two or more protruding ribs.
5. A fixture as claimed in claim 2 wherein the ribs have a rounded cross section.
6. A fixture as claimed in claim 1 wherein the detachable end stop comprises two or more spaced apart rods.
7. A fixture as claimed in claim 6 wherein the rods have a rounded cross section.
8. A fixture as claimed in claim 1 wherein the clamp comprises a pivoted clamp.
9. A fixture as claimed in claim 8 wherein the clamp is centrally pivoted.
10. A fixture as claimed in claim 1 further comprising a convex surface counter-support for countering any undesirable load from the clamp which may cause slippage and/or tilting of the part after it has been positioned.
11. A fixture as claimed in claim 10 wherein the counter support comprises a wedge or a bar whose position is slidably adjustable to accommodate various degrees of curvature.
12. A fixture as claimed in claim 10 wherein the counter-support comprises a pivoted support.
13. A fixture as claimed in claim 1 further comprising one or more additional stops arranged in a plane orthogonal to both the plane in which the support sits and the plane in which the detachable end stops extend.
14. A fixture as claimed in any of claim 1 further comprising a pair of compliant gripping fixtures configured to receive opposing ends of the part in a manner which does not permit the part to rotate relative to the compliant gripping fixtures.
15. Use of a fixture according to claim 1 to align and secure an aerofoil with its trailing edge accessible to a machine tool and performing a machining operation with the machine tool to remove material from the trailing edge on a concave surface side of the aerofoil.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Embodiments of the invention will now be further described with reference to the accompanying Figures in which;
[0019] FIG. 1 shows a cast turbine blade having a curved surface and a straight trailing edge of the surface;
[0020] FIG. 2 illustrates how the turbine blade of FIG. 1 might be machined to provide a thinner trailing edge for a more aerodynamic performance;
[0021] FIG. 3 shows a view from a leading edge of the turbine blade of FIG. 1 arranged in tooling in accordance with a first embodiment of the invention;
[0022] FIG. 4 shows the arrangement of FIG. 3 with the blade clamped in the desired position and orientation;
[0023] FIG. 5 shows points at which the concave and convex surfaces of the blade of FIG. 4 are supported;
[0024] FIG. 6 shows material being removed from the trailing edge of a blade aligned and held using tooling in accordance with an embodiment of the invention.
[0025] FIG. 7 shows the loading applied to the blade by the clamp of the tooling of FIG. 8;
[0026] FIG. 8 illustrates a second embodiment of tooling in accordance with the invention applied to a blade which is the same as the blade of FIG. 1;
[0027] FIG. 9 illustrates a second embodiment of the tooling with the clamp shown in FIGS. 7 and 8 removed
DETAILED DESCRIPTION OF DRAWINGS AND SOME EMBODIMENTS
[0028] As can be seen the blade of FIG. 1 has a concave surface 1 on an opposite face to a convex surface 2. The two surfaces 1, 2 meet at a trailing edge 3. The blade has a tip platform 4 and a root platform 5. FIG. 2 shows the blade of FIG. 1 in the process of having a surface of the trailing edge 3a machined by a tool 6. For example, the machine tool 6 may be of a milling or grinding machine.
[0029] FIG. 3 shows the blade of FIG. 1 arranged in a fixture according to an embodiment of the invention. As can be seen, the fixture comprises a pair of support bars 10a, 10b onto which the convex surface 2 of the blade is seated. Extending in an orthogonal plane is a pair of detachable end stops in the form of rods 11. The trailing edge 3 of the blade is aligned against the end stops 11. Desirably, the convex surface 2 adjacent the trailing edge 3 is positioned flat against the supports 10a, 10b. This may be facilitated by means of a pivoted convex surface support 12. The tooling may further include a stop 13 in a third orthogonal direction against which the tip platform is aligned. In alternatives, the position of this stop could be on the opposite face of the tip platform 4, or on either face of the root platform 5. The stop 13 may be fixedly connected to a frame which also comprises the support bars 10a, 10b.
[0030] As can be seen in FIG. 4, once the trailing edge 3 is aligned with the detachable rods 11, the blade is secured in position by a pivoted clamp 14. The pivoted convex surface support 12 serves to counter any load from the clamp 14 which might push the trailing edge out of alignment with the rods 10a and 10b. This is better illustrated with the schematic cross-sectional view of the blade in FIG. 5.
[0031] In the fixture shown in FIGS. 4 and 5, the centrally pivoted clamp 14, is located approximately ⅓ of the way along the chordal length of the blade from the trailing edge 3 to the leading edge. This clamp 14 acts on the concave surface to force the part firmly against the supports 10a, 10b and the centrally pivoted counter-support 12, but without excessive force so as to cause distortion of the trailing edge. The blade is thus firmly gripped as a result of the reactionary forces from the clamp 14, the supports 10a, 10b and the centrally pivoted convex support 12. The trailing edge end stops 11 can now be retracted (e.g. vertically) away from the remainder of the tooling to allow access for the cutting tool 6 as shown in FIGS. 2 and 6. It is important to note that the detail of the tooling design will depend on the specific geometry of the part to be machined, and in particular the relative distances and angles between the clamp 14 and the counter-support 12 and the trailing edge supports 10a and 10b, to ensure that the part is held firmly in position during the machining operation. Following clamping of the part and retraction of the trailing edge stops 11, the trailing edge 3 can be machined along the concave surface side of the trailing edge to form the desired thinner trailing edge profile. To ensure an accurate trailing edge profile is generated, a measurement process, such as a scanning probe or a laser system, may be used to measure the trailing edge thickness from the tip platform 4 to the root platform 5, or visa-versa, to make allowances for any part distortion, such as trailing edge bow, during the machining operation.
[0032] In an alternative embodiment as shown in FIG. 8, the tip and root platforms 4, 5 of the blade are supported in compliant gripping fixtures 15, 16. The compliant gripping fixtures may be rotated and locked into position once the convex surface adjacent the trailing edge is in contact with the support bars 10a, 10b and the detachable end stops 11. Before the gripping fixtures 15, 16 are locked in position the clamp 14 (which in this embodiment is positioned closer to the trailing edge 3) pushes the convex surface intimately against the supports 10a, 10b. With the blade properly positioned for subsequent machining, the compliant gripping fixtures 15, 16 are locked to maintain the position and the clamp 14 can be removed providing access to the concave side trailing edge for machining.
[0033] In this embodiment, the clamp 14 can be dispensed with once compliant gripping fixtures 15, 16 have been locked in position. For example, in FIG. 7, the clamp 14 is arranged such that there is negligible force in a plane parallel to the support bars 10a, 10b. Thus, the position of the blade can be maintained whilst compliant gripping fixtures 15, 16 are locked in position.
[0034] As can be seen in FIG. 9, once the blade has been positioned and secured into position, the detachable rods 11 are removed allowing a machine tool 6 to remove material 17 from the trailing edge on the concave surface side.
[0035] In the tooling arrangement shown in FIGS. 7 and 8 the centrally pivoted clamp 14 is positioned much closer to the apex of the trailing edge 3. This allows the trailing edge to be located more intimately against the supports 10a, 10b, thereby improving the accuracy of the setting and thus the final profile of the trailing edge. With this arrangement, the blade is clamped in position after it has been appropriately positioned and oriented. As an alternative to the counter support 12 of the previously described embodiment, the compliant gripping fixtures 15, 16 can be arranged to hold a platform 4, 5 of the blade in a given orientation. Once the blade and compliant gripping fixtures 15, 16 are in the desired position and orientation, the ends of the fixtures 15, 16 are firmly clamped. The end stops 11 and clamp 14 are retracted away from the trailing edge 3 to allow access for the cutting tool 6 to machine the trailing edge 3. The material removal 17 is carried out relative to the datum system of the tool, in particular the supports 10a, 10b and the position of the trailing edge end stops 11 (prior to their removal).
