GB2382045A

GB2382045A - Method of machining curved contours

Info

Publication number: GB2382045A
Application number: GB0225238A
Authority: GB
Inventors: Jonathan Count
Original assignee: Alstom SA
Current assignee: Alstom SA
Priority date: 2001-11-01
Filing date: 2002-10-30
Publication date: 2003-05-21
Anticipated expiration: 2022-10-30
Also published as: GB2382045B; GB0126217D0; GB0225238D0

Abstract

The present invention provides a method of machining a curved contour in a workpiece 2 within a predetermined tolerance band 6 defined by an inner tolerance line 8 and an outer tolerance line 10. The method includes a first step of machining a plurality of adjacent arcuate concave profiles 12 in the workpiece 2 outside the inner tolerance line 8, and a second step of using an abrasive flow machining process to remove portions 14 of the workpiece 2 between the adjacent profiles 12 that lie outside of the outer tolerance line 10.

Description

METHOD OF MACHINING CURVED CONTOURS Field of the Invention The present invention relates to a machining method which can be used to achieve finely toleranced contours on workpieces.

Background of the Invention Abrasive flow machining is a well-known machining process in which an abrasive putty is extruded past or though a workpiece to effect abrasive working of the workpiece. The process is commonly used to de-burr and polish difficult to reach surfaces. It is particularly useful for de-burring very hard materials that cannot be deburred using conventional filing, grinding, lapping or honing processes.

To carry out the abrasive flow machining process the workpiece is secured in a workpiece chamber positioned between two directly opposed cylinders. The abrasive putty consists of a visco-elastic material containing abrasive grit. One of the cylinders is filled with this abrasive putty and the other cylinder is empty. The abrasive putty is then extruded from the filled cylinder, through the workpiece chamber and into the other cylinder. As the abrasive putty flows through the workpiece chamber it flows past the workpiece to abrade its external surfaces and any bores or passages provided in the workpiece in a smooth and uniform fashion. The abrasive putty continues to be extruded back and forth between the cylinders until the workpiece is sufficiently abraded.

When manufacturing components for aero-engines and gas turbines it is often necessary to machine very tiny curved contours. Such components are usually made of very hard materials and are expensive to produce. In some cases, conventional machining techniques such as milling, or other types of machining processes using multi-toothed cutting tools, or even single-point tools, cannot be used to machine these curves to within the extremely small tolerances that are required. Even when conventional machining techniques can be used the whole process is often inefficient and there is always the chance that the component may have to be rejected if the curve

is even slightly inaccurate. The aim of the present invention is therefore to provide a new method of machining curved contours that overcomes these problems.

Summary of the Invention The present invention provides a method of machining a curved contour in a workpiece within a predetermined tolerance band defined by an inner tolerance line and an outer tolerance line, said method comprising the steps of : (a) machining into said workpiece a plurality of adjacent concave profiles, such that all portions of said profiles lie outside said inner tolerance line and at least some portions of said profiles lie outside said outer tolerance line; and (b) using an abrasive flow machining process to remove from said workpiece at least said profile portions which lie outside said outer tolerance line, thereby leaving on said workpiece only profile portions which lie within said predetermined tolerance band.

The machining process of step (a) above may be a milling process, such as end milling, which typically produces an array of adjacent concavely curved profiles, in which neighbouring profiles intersect each other at cusp points.

The combination of the two processes, each known per se, provides a method of machining a highly accurate curved contour that lies within the predetermined tolerance band. The initial step of machining the workpiece is only constrained by the inner tolerance line and it does not matter if portions of the adjacent profiles extend outside the outer tolerance line. The portions of the adjacent profiles that lie outside the outer tolerance line are then removed using an abrasive flow machining process that is quick and can be carefully controlled.

The method may further comprise: (c) using an abrasive flow machining process to remove further portions of said adjacent profiles that lie within said predetermined tolerance band, so producing a substantially smooth curved contour.

Description of the Drawing Figure 1 illustrates the various steps of machining a curved contour in a workpiece using the method according to the present invention.

Figure 1 shows a workpiece 2 into which a curved contour 4 is to be machined. A contour line 4 is tightly toleranced within a tolerance band 6 defined by an inner tolerance line 8 and an outer tolerance line 10. Although the contour line 4 illustrates the path of the curved contour that is to be machined in the workpiece, it will be readily appreciated from the following description that the curved contour actually produced by the method is not completely smooth and is only an approximation to the contour line 4 within the tolerance limits determined by the tolerance band 6.

The first step of the method involves machining the workpiece 2, e. g. , with an end mill or other multiple tooth cutter, or even a single-point tool, as appropriate, to achieve an approximation to the desired curved profile. We prefer end milling the workpiece 2 using conventional machining techniques, to produce a plurality of adjacent arcuate concavely curved profiles 12. These intersect their neighbouring profiles at cusp-like projections 14 as shown. The profiles 12 lie completely outside the inner tolerance band 8 and generally follow the contour line 4, but some of the projections 14 extend outside the outer tolerance line 10. The part of the workpiece 2a shown ghosted is separated from the rest of the workpiece 2 by the end milling process and can be scrapped.

The second step of the method involves abrading the projections 14 using an abrasive flow machining process as previously described. Depending on the nature of the workpiece the abrasive flow machining process can be used to remove the projections 14 completely to produce a substantially smooth curved contour. Alternatively, the abrasive flow machining process can be used to remove just enough of the projections 14 to bring the curved contour completely inside the outer tolerance line 10.

Claims

1. A method of machining a curved contour in a workpiece within a predetermined tolerance band defined by an inner tolerance line and an outer tolerance line, said method comprising the steps of : (a) machining into said workpiece a plurality of adjacent concave profiles, such that all portions of said profiles lie outside said inner tolerance line and at least some portions of said profiles lie outside said outer tolerance line; and (b) using an abrasive flow machining process to remove from said workpiece at least said profile portions which lie outside said outer tolerance line, thereby leaving on said workpiece only profile portions which lie within said predetermined tolerance band.

2. A method according to claim 1, further comprising: (c) using an abrasive flow machining process to remove further portions of said adjacent profiles that lie within said predetermined tolerance band.