GB2399781A

GB2399781A - Tool and method for multi-axis back spot facing

Info

Publication number: GB2399781A
Application number: GB0219767A
Authority: GB
Inventors: Michael Adrian Thompson
Original assignee: Alstom SA
Current assignee: Alstom SA
Priority date: 2002-08-24
Filing date: 2002-08-24
Publication date: 2004-09-29
Also published as: GB0219767D0

Abstract

A method for machining a flat area in a surface surrounding an aperture through a workpiece, where the surface is located at the rear of the workpiece, includes a machine tool comprising a shaft 7 adapted to be mounted to a mechanism for rotating the tool about a longitudinal axis 11 and a tip 9 having a cutting edge 10 extending outwardly from the axis, inclined to the axis by an acute angle, and facing towards the rear face of the workpiece when the tool is inserted through the aperture. The method comprises the steps of: inserting the tool through the hole; rotating the tool about its longitudinal axis; tilting the longitudinal axis of the tool through the acute angle so as to engage the cutting edge with the rear face of the workpiece; precessing the longitudinal axis of the tool around the hole so that the longitudinal axis describes the surface of a cone, whereby the cutting edge cuts a flat region in the rear face of the workpiece around the periphery of the hole; returning the axis of the tool to substantially normal to the said flat region; stopping rotation of the tool about its longitudinal axis; and extracting the tool from the workpiece.

Description

239978 1

TOOL AND METHOD FOR MULTI-AXIS BACK SPOT FACING

Field of the Invention

The present Invention relates to the machining of a flat region around the edge of a hole through a workpiece where the rear face of the workpiece is not readily ac cessible. Such machining is generally known as back spot facing.

Background to the Invention

In certain machine operations on workpieces, for example casings or other parts for turbines, it is often necessary to form a flat region on a face of the workpiece surrounding a through hole. This flat region may be provided to receive a bolt head, nut or washer, for example, used to assemble the apparatus of which the workpiece forms part. The provision of this flat area is particularly difficult when it has to be pro vided on a face of the workpiece that cannot be reached easily from the "front". Un der such circumstances, a tool for machining the flat has to be inserted into the work piece so as to effect the machining operation from the "rear". It is appreciated that terms such as "front" and "rear" are relative but it is apparent to the average skilled worker in the field that certain faces of a workpiece can be regarded as accessible more easily from one face than another. This is especially true where the workpiece is a cas ing part and there is literally no other aperture available through which the tool can be inserted in such a way as to reach all of the sites for machining a flat region.

Hitherto, there have been two alternative techniques for machining flats in workplaces from the rear face. One technique involves the use of a special tool that folds or flips from an inoperative position, enabling it to be inserted through a suitable front aperture, and an operative position, where it can be deployed from the rear face to access the site of machining. In the other technique, a tool is inserted in pieces through a front aperture and assembled/deployed by hand.

Neither of these techniques is entirely satisfactory. The tools used in this type of machining are compact. The first technique is subject to failure from distortion as the cutting force applies a large bending moment to a small joint. The second technique is poor for automation because it requires hand assembly and disassembly of the tool.

Furthermore, failing to disassemble the tool before its withdrawal can cause massive damage. Failure of the tool tip is the most common reason for failure of the whole ma Speaficaton ALSOlGBP1038.GBA- 2002-08-22 doc 2 chining operation. The problems associated with back spot facing is particularly marked when there is a large ratio between the outside diameter to the inside diameter since the "reach" of the tool tip in the radial sense relative to the diameter of the shaft of the tip is such that excessive forces are exerted on the tip and shaft.

There is thus a need for a technique for back spot facing holes that is more reli able and effective and gives more satisfactory results than afforded by the prior art.

Summary of the Invention

The invention provides a tool for machining a flat area in a surface surrounding an aperture through a workpiece, the surface being located at the rear of the work piece and the tool being adapted to be inserted into the aperture from a front face of the workpiece, the tool comprising a shaft adapted to be mounted to a mechanism for rotating the tool about a longitudinal axis and a tip having a cutting edge extending out wardly from the axis and facing towards the rear face when the tool is inserted through the aperture.

The cutting edge of the tool faces back towards a mounting portion of the tool and is inclined to the longitudinal axis of the tool by an acute angle, preferably 60 de grees, measured from the distal end of the tool axis.

The cutting edge Is preferable laterally offset relative to a radius extending from the said axis. The cutting edge is also preferably inclined, for example by 9 degrees, from said radius. The cutting edge can thereby be brought into engagement with the rear face of the workpiece by tilting the longitudinal axis of the tool through said acute angle, once the tool has been inserted through the hole in the workpiece.

The outermost edge of the cutting tip preferably extends at 90 degrees to the cutting edge so that the back spot face cut by the tool has a side wall that extends nor many to the cut back spot face.

The invention also provides a method for machining a flat area in a surface sur rounding an aperture through a workpiece, the surface being located at the rear of the workpiece, the tool comprising a shaft adapted to be mounted to a mechanism for ro tating the tool about a longitudinal axis and a tip having a cutting edge extending out wardly from the axis and facing towards the rear face when the tool is inserted through the aperture, the method comprising the steps of (1) inserting the tool through the Speaficabon ALS01GB P1038.GBA 2002-08-22.doc - 3 hole; (2) rotating the tool about its longitudinal axis; (3) tilting the longitudinal axis of the tool through the said acute angle so as to engage the cutting edge with the rear face of the workpiece; (4) processing the longitudinal axis of the tool around the hole so that the longitudinal axis describes the surface of a cone, whereby the cutting edge cuts a flat region in the rear face of the workpiece around the periphery of the hole; (5) returning the axis of the tool to substantially normal to the said flat region; (6) stopping rotation of the tool about its longitudinal axis; and (7) extracting the tool from the workpiece.

The method is performed by moving the workpiece on a controllably movable table; moving the rotatable tool over the workpiece which is mounted on a stationary table; or a combination thereof. The operation is preferably computer-controlled.

Brief Description of the Drawings

The invention will now be described with reference to the following drawings, in which: Figure 1 is a diagrammatic representation of part of a workpiece with which the present invention is effective; Figure 2 is an end elevation of a tool for use in the invention; Figure 3 is a side elevation of the tool of Figure 2; Figure 4 is a top plan view of the tool; and figure 5 (a) to (h) illustrate steps involved in performing the invention.

Detailed Description of the Illustrated Embodiments Referring first to Figure 1, a section of a part of a workpiece 1 is shown. The workpiece is hollow and may be substantially enclosed. It may be part of a casing or housing, for example for part of a gas turbine. A through hole 2 is provided in a wall of the workpiece for passage of a bolt or other similar fastening means (not shown). In the workpiece illustrated, the inner wall 3 is curved in the neighbourhood of the hole 2 to such an extent that there is no flat surface to accommodate the head of the bolt or perhaps a nut or washer for the bolt. A flat or back spot face 4 has therefore to be provided.

Figures 2 to 4 illustrate a tool 5 that may be used in accordance with the nven tion in order to create the back spot face 4 in the workpiece 1. As shown, the tool 5 includes a mounting portion 6, by which it may be attached to a mechanism for rotating

Specification ALSO l GB P103 8.GBA 2002-08-22.doc - 4

and traversing the tool (as will be described later). The portion 6 may be provided with a slot 7 by which the tool is attached to the mechanism and which may be used for rota tional timing The operative end of the tool has a narrower neck portion 8 tapering towards the cutting tip 9. The tip Includes a cutting edge 10, as best seen in Figure 4, that Is in clined at an angle to the axis 11 of the tool. That angle may be typically 60 degrees, measured from the distal end of the axis, but can be any suitable angle. The cutting edge 10 is also offset relative to a radius taken from the axis 11 and is also inclined rela bve to a radius so that the cutting edge performs a slicing action. The cutting edge may make an angle of 9 degrees, for example, relative to a radius taken from the axis 11.

This angle is known as the "top rake" angle.

The outer edge 12 of the cutting tip 9 is also angled relative to the axis 11. In the present example, that angle is 30 degrees. The edge 12 and the cutting edge 10 therefore make a right angle with one another. This is so that the recess 3 cut by the tip has sides that are at right angles to the base, as will become apparent.

The technique employed in the present invention is illustrated schematically in Figures 5(a) to 5(h). It is important to recognise that, in this description, reference is made to rotation of the tool and to translation and orientation of the tool in various di rections and at various angles relative to the workpiece. This is intended to encompass the option of the tool being moved relative to a stationary workpiece, or the workpiece being moved relative to a rotating but not translating tool, or to a combination of the two. What is important is that there is relative movement between the tool and the workpiece that achieves the required relative movement necessary to create the back spot.

In the first position (Fig 5(a)), the tool is stationary relative to the workpiece and is aligned with the hole 2. The still non-rotating tool is then passed through the hole 2 so that the cutting edge 10 is behind the rear face of the workpiece (Fig 5(b)) and is then positioned, by lateral movement, until the cutting edge 10 is behind the sur face of the workpiece surrounding the edge of the hole where the back spot is to be machined, as shown in Fig 5(c).

Speaficaton ALS01GB P1038 GBA 2002-08-22.doc - 5 Fig 5(d) shows the tool rotating about its axis. Whilst the tool Is thus rotating, the axis 11 is inclined to the normal axis 13 through the hole 2, as represented by the arrows, so that the tool cutting edge 10 Is engaged with the rear face and the outer edge 12 is substantially parallel to the axis 13 so as to cut out a back spot face with sides normal to the base.

Fig 5(e) shows movement of the tool about the normal axis 13 so as to create the back spot face 4. The axis 11 of the tool effectively describes a surface of a cone as the tool tip rotates about the axis 11. This is indicated schematically by the arrows and the broken lines describing the conical surface. When the back spot face has been ma chined to the required depth, the tool is tilted back towards the normal axis 13 (Fig 5(f)) to move the cutting edge 10 away from the workpiece. The tool is still rotated about its own axis 11 during this time.

finally, rotation of the tool is stopped and the tool is laterally moved to a pos'- tion (Fig 5(g)) in which the tip 9 is aligned with the hole 2 and the tool can then be withdrawn (Fig 5(h)) from the workpiece, ready to move on to the next hole that re- quires to be machined.

As already mentioned, the above description has been presented as though the workpiece is stationary and the tool is moved (translated and oriented) relative to the workpiece. In practice, what is more likely is that the tool is mounted on a relatively stationary head and the workpiece is supported on a movable work table. The hole where the machining operation is being performed is not at the centre of rotation of the workpiece. Therefore, when the tool and workpiece are driven relative to one an- other in order to define the machine surface of the back spot face, it is necessary for the work table to be rotated, translated and oriented and for the tool to be translated over the front face of the workpiece in such a way that their combined relative move- ment describes the required back spot face. Often, the path taken by the tool relative to the workpiece is elliptical.

The operation is likely to be effected by a computer-controlled machine, requir- ing a program to be designed to implement the necessary relative movements in all the necessary planes and axes.

Speaficaton ALS01 GB-P1038.GBA 2002-08-22.doc - 6

Claims

1. A machine tool for machining a flat area in a surface surrounding an ap- erture through a workpiece, the surface being located at the rear of the workpiece and the tool being adapted to be inserted into the aperture from a front face of the work- piece, the tool comprising a shaft adapted to be mounted to a mechanism for rotating the tool about a longitudinal axis and a tip having a cutting edge extending outwardly from the axis and facing towards the rear face when the tool is inserted through the aperture.

2. A machine tool as claimed in Claim 1, wherein the cutting edge of the tool faces back towards a mounting portion of the tool and is inclined to the longitudinal axis of the tool by an acute angle measured from the distal end of the tool axis, whereby the cutting edge can be brought into engagement with the rear face of the workpiece by tilting the longitudinal axis of the tool through said acute angle, once the tool has been inserted through the hole in the workpiece.

3. A machine tool as claimed in Claim 2, wherein the acute angle is substan tially 60 degrees.

4. A machine tool as claimed in any of the preceding claims, wherein the cutting edge is laterally offset relative to a radius extending from the said axis.

5. A machine tool as claimed in Claim 4, wherein the cutting edge is n clined to said radius.

6. A machine tool as claimed in Claim 5, wherein the angle of inclination is substantially 9 degrees.

7. A machine tool as claimed in any of the preceding claims, wherein the radially outermost edge of the cutting tip extends at 90 degrees to the cutting edge so that the back spot face cut by the tool has a side wall that extends normally to the back spot face.

8. A method for machining a flat area in a surface surrounding an aperture through a workpiece, the surface being located at the rear of the workpiece, the tool comprising a shaft adapted to be mounted to a mechanism for rotating the tool about a longitudinal axis and a tip having a cutting edge extending outwardly from the axis and Speaficaton ALS01GBP1038.GBA2002-08-22 doc - 7 facing towards the rear face when the tool is inserted through the aperture, the method comprising the steps of: inserting the tool through the hole; rotating the tool about its longitudinal axis; tilting the longitudinal axis of the tool through the said acute angle so as to en gage the cutting edge with the rear face of the workpiece; processing the longitudinal axis of the tool around the hole so that the longtudi nal axis describes the surface of a cone, whereby the cutting edge cuts a flat region in the rear face of the workpece around the periphery of the hole; returning the axis of the tool to substantially normal to the said flat region; stopping rotation of the tool about its longitudinal axis; and extracting the tool from the workpiece.

9. A method according to Claim 8, wherein the workpiece is moved rela tive to the tool on a controllably movable table. On which the workpiece is mounted.

10. A method according to Claim 8, wherein the tool is moved relative to the workpiece, which is mounted on a stationary table.

11. A method according to Claim 8, wherein the tool and the workpiece are both moved during the machining operation.

12. A method according to Claim 8, wherein the machining operation is computer-controlled.

Specification ALS01GB P1038.GBA 2002-08-22.doc