GB2216050A - Method of surface profiling honeycomb material - Google Patents

Abstract

A burr-free and tear-free method of three-dimensional surface profiling workpieces of synthetic honeycomb material having soft and flexible cells uses a programme-controlled cutting tool 10 in the shape of a bell, with convergent external and internal cutting surfaces terminating in a cutting rim, 22. The interior surface provides the control surface and the exterior surface provides the cutting edge. <IMAGE>

Description

METHOD OF SURFACE PROFILING HONEYCOMB MATERIAL This invention concerns a method of surface profiling honeycomb material and although it is not so restricted, it will hereafter be particularly described with reference to its use in the aerospace industry to profile cut in three dimensions synthetic honeycomb materials that provide high strength, low weight fillers and supports for structures such as aircraft wings, tailplanes and control surfaces.

Aircraft manufacturers are making increasing use of honeycomb structural materials in aluminium, plastics and the new aromatic polyamides such as that marketed under the Trade Mark 'Nomex' in order to reduce structural weight and increase strength. These materials are used as fillers glued between metal, laminated carbon fibre or homogeneous plastic sheets, and have to be fitted very accurately to the often complex shape of the components between which they are located. It is obviously important that the adhesive bond be sound and uniform. This in turn requires that there are no burrs at the interface between the material and its 'skin'. Also, typical honeycomb materials such as the polyamide ones mentioned are soft and flexible, except in compression along the axes of the cells. Conventional machine cutting tools tear honeycomb materials to an unacceptable degree.

In one known technique standard honeycomb panels are first cut as closely as possible to their final dimensions, using horizontal and vertical saw-blades having negative-incidence teeth in an attempt to reduce ripping out the edges of the cells.

Then, the form of the component is finalized, using special 'gentle' techniques to avoid burring. A concave circular saw of some 60 cm diameter with extremely fine teeth is rotated at 18,000-20,000 revs/min. As this technique is very difficult to use for complex shapes, moulding or freezing processes are sometimes employed to stabilize the honeycomb material, followed by machining with conventional milling tools. This approach gives good results, but at high cost.

Another known technique employs complex purpose-built machines involving five-axis numerically controlled machines with spindle speeds of the order of 20,000 r.p.m.

A further factor with regard cutting tools is that they are increasingly used in conjunction with computer numerically controlled (CNC) or numerically controlled (NC) machine programs.

It is therefore of great importance that the periodical sharpening of the cutting tool should not affect the program, i.e. the parameters set by the program should remain unaffected by changes in tool dimensions consequent upon tool sharpening or regrinding. This requirement may be met, to a certain extent, by the use of carbide tools which, given careful use, will give longer cutting life. Also, in a production run back-up tools of the same size as the tool in use would be provided.

The present invention seeks to overcome the drawbacks of expense and complicatedness of the prior art described above and to provide a cutting tool which can cut honeycomb materials in a substantially burr-free manner. A preferred embodiment has the added advantage that successive sharpening of the tool does not affect NC machine programs because the tool diameter is always constant.

According to one aspect of the present invention, there is provided a method of surface profiling a workpiece made of a flexible and/or synthetic honeycomb material, comprising using a rotary cutting tool a hollow body having convergent internal and external surfaces terminating a cutting rim, the diameter of the internal surface defining the control diameter of cutting and the external surface is the cutting surface.

The tool may be made from sintered carbide fused to a steel body or as inserted units with special clamping arrangements. A carbide tool- would be appropriate for production of large aircraft skins where a larger diameter tool would be desirable.

Carbide would also be desirable for cutting metal honeycomb material. Note, however, that carbide tools cannot be honed to a razor edge because of their basic composition, i.e. powder bonded in a matrix. A high-speed steel tool can, on the other hand, be honed to a true knife edge.

Preferably, the external surface is tapered at a small angle, typically 100, to the internal surface which is plain cylindrical. One would not expect good results with cutter angles in excess of 300, and whilst 200 angles might be functional it would be desirable to run tests before use to show whether rupture or bruising of honeycomb cells was likely in use.

The hollow annular body may be of a variety of shapes, e.g.

a reverse taper-cone shape, provided that a 'knife' edge was established and maintained at the intersection of internal and external surfaces. The tool preferably has an internal 'bell' shape to help contain swarf and prevent snatch and tear-off.

The tool is preferably provided with a spindle for engagement in a chuck of a drive unit, preferably an air motor.

The use of an air motor has the advantages of being inexpensive, readily fittable to the spindle nose of a CNC machine, and requiring a supply of air which is readily available in most works where cutting is performed.

A special adaptor is preferably provided to mount the air motor on a CNC machine to permit location in both 'X' and 'Y' planes, and have a rotational capability through 900 from vertical 'Z', to horizontal.

An axis pivot may be located in the 'Z' plane face, to jigged holes bored at 900, 450 and 300, being the desired programmed angles for tool attack.

The invention is illustrated, by way of example only, with reference to the accompanying drawing, wherein: Figure 1 is a cross-section of a cutting tool for use with the method according to the invention, Figure 2 is a perspective view of an adaptor mounting for the tool of Figure 1 on a CNC cutting machine, Figure 3 is a cross-section of part of an alternative cutting tool for use with the method according to the invention; Figure 4 is a perspective view of a workpiece made of a synthetic honeycomb material, juxtaposed parts of which have been machined by a prior art method and by the method according to the present invention.

Referring to Figure 1, there is shown a cutting tool 10 of, essentially, bell shape. The tool 10 has a spindle 12 and a cutter portion 14. The cutter portion 14 is a hollow annular body having internal and external surfaces 16, 18, respectively.

The internal surface 16 is a plain cylindrical annulus closed by a radial surface 17 so as to form a generally bowl shape which facilitates swarf removal without tear-off and snatch. The external surface 18 is a cutting surface. It, too, is for the most part plain cylindrical but its end portion 20 tapers, e.g. at an angle 21 of 100, toward the internal surface 16 so as to meet in a circular line edge 22.

The edge 22 is in effect coincident with the internal surface 16 and represents the control diameter for cutting by the external surface 18. This is the reverse of conventional practice.

In use, the tool 10 maintains a constant diameter. If the cuting edge requires sharpening, this may be effected manually while still mounted on the cutting machine, or elsewhere, but in either case the cutting diameter is unchanged. Thus no alteration of the program of a NC or CNC machine is required.

To produce effective and economic operation, the cutting tool 10 requires to be driven at relatively high spindle speeds, typically 8000 r.p.m. It was found that the best combination of economy, convenience and noise is afforded by an air motor fitted with silencers, not shown.

To mount such an air motor on a three-dimensional CNC machine, a special spindle nose adaptor assembly is required and this is shown in Figure 2. The adaptor assembly is generally designated by 23 and has the facility of permitting location in both the X and Y planes as well as permitting rotation through 900 in the Z plane from the vertical to the horizontal.

More particularly, the assembly includes a spindle nose mounting collar 23a to which a vertical support block 24 is secured. The block 24 carries an angularly adjustable pivot plate 26 representing the Z plane. The plate 26 is provided with a plurality of holes 28 bored at various angular intervals, e.g.

900, 450 and 300, being the desired angles programmed in the CNC program for tool attack. The pivot axis is shown at 30.

The plate 26 is unitary with an orthogonally extending clamp plate 32 having a slotted aperture 34 for receiving and clamping the non-illustrated air motor.

The 900 vertical position being used for routing, end milling etc., and waterline roughing preparation work, where surface finish is not required to be 'burr' free. The 300 angle location was used when the tool according to the invention is in operation. This gives a cutter to workpiece clearance of 230 and 280.

A cutter portion 34 of an alternative tool design is shown in Figure 3. The shape of this tool is described as a reverse taper cone. The cutter portion is again a hollow annular steel body with internal and external surfaces 36 and 38, respectively.

However, in this case the internal and external surfaces 36 and 38 are generally conical and convergent terminating in an annular, honed carbide cutting edge 40 fused to the steel body.

The cutting edge taper is again of the order of 100. This tool, whilst allowing wider scope for tool approach than the tool of Figure 1, does not have the latter's constant cutting edge diameter advantage. However, the generally bowl-shaped internal surface 36 allows for swarf removal without tear-off and snatch, as before.

In use of the cutting tool illustrated in Figures 1 and 2 or Figure 3 it was found that the requirements of producing complex surfaces substantially without burr and without appreciably distorting the cells of flexible honeycomb materials such as 'NOMEX' have been achieved.

One example of the method according to the invention will now be described, by reference to the three-dimensional contouring of synthetic honeycomb material, such as NOMEX (Trade Mark), to make an aerofoil-section member, such as an aircraft wing.

A block of honeycomb material is placed on a flat-bed and, overhead, the air driven, rotary cutting tool according to Figures 1 and 2 is mounted at an angle relative to the block.

Adjacent to the tool, a suction pipe is disposed to remove honeycomb swarf. To machine the top surface of an aerofoil crosssectioned wing the tool is caused to make successive chord-wise spaced span-wise passes along the block rising and falling to the required contour under computer control. To machine the undersurface, the block is then placed in a female mould of the top-surface, inverted, located and clamped with respect to a datum on the bed, with the initially planar lower surface of the block exposed and uppermost, and then subjected to further, appropriately programmed chord-wise spaced, span-wise passes of the tool.

The cutting proceeds with the plane containing the circular cutting edge transverse to the direction of travel of the tool.

Because of the high rate of cutting, heat generated in the cutting area is confined to the removed swarf and is not transferred to the tool or the remainder of the workpiece. Hence no coaling lubricant is required.

Stiffening ribs or spars may be inserted in the finished aerofoil by machining out slots, lining the slots with glassreinforced plastic liners and gluing the ribs and spars in the lined slots.

Figure 4 shows honeycomb material roughly machined in conventional cutter (left-hand portion as viewed) and, in contrast, profiled by the method according to the invention using the cylindrical cutting tool (right-hand portion, as viewed) that in the latter there is a practically complete absence of burrs, tears and deformations of the material; while such burrs, tears and deformations are very clearly evident in the left-hand portion. An outer skin could be glued to the right-hand section, but not to the left-hand section, with complete confidence that every honeycomb cell would be in contact and load-bearing.

Claims (8)

CLPAS

1. A method of surface profiling a workpiece made of a flexible and/or synthetic honeycomb material, comprising using a rotary cutting tool a hollow body having convergent internal and external surfaces terminating a cutting rim, the diameter of the internal surface defining the control diameter of cutting and the external surface is the cutting surface.

2. A method accordng to claim 1, wherein the cutting tool is programme-controlled to follow the described contour of the said workpiece.

3. A method according to claim 2, wherein the cutting is performed by moving said tool relatively to said workpiece with the plane containing the said cutting rim extending transversely to the direction of relative movement of the tool.

4. A method according to any preceding claim, wherein said workpiece is an aerofoil-shaped member.

5. A method according to claim 4, wherein the said tool is passed repeatedly in a chord-wise spaced apart span-wise directon along the workpiece.

6. A method according to claim 4, wherein the said tool is passed repeatedly in a chord-wise spaced apart span-wise direction along the top of the workpiece, then the workpiece is placed in a female mould of the top surface, inverted, secured in position, and then the said tool is passed repeatedly in a chordwise spaced apart span-wise direction along the workpiece.

7. A method according to any preceding claim, wherein the said tool is air-driven.

8. A method according to claim 1, substantially as herein described with reference to and as shown in the accompanying drawings.