CHAMFERING OF LAMINATE LAYERS

Abstract

A method of machining a fibrous sheet for a composite structure is described. The sheet comprises a resin matrix having a glass transition temperature, wherein the method comprises cooling the sheet substantially to maintain the temperature of the matrix below its glass transition temperature during machining.

Claims

1. An apparatus for machining a fibrous sheet for a composite structure, the apparatus comprising: a support for the sheet; a machining tool movable relative to the support; and a cooling system for cooling the sheet.

2. The apparatus of claim 1, wherein the cooling system comprises one or more cooled surfaces against which the sheet is supported before or during machining.

3. The apparatus of claim 2, comprising opposed cooled surfaces between which the sheet may be sandwiched.

4. The apparatus of claim 1, wherein the cooling system comprises a supply for supplying coolant to the sheet before or during machining.

5. The apparatus of claim 4, wherein the supply is arranged to move relative to the support in tandem with the machining tool.

6. The apparatus of claim 1, wherein the cooling system comprises an enclosure for holding the sheet within a cooled environment.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

[0018] FIG. 1 is a schematic perspective view of an apparatus for machining a prepreg or semi-preg ply, in which refrigerant is applied to the ply prior to machining;

[0019] FIG. 2 is a schematic perspective view of the apparatus of FIG. 1, in which the refrigerant is applied to the machining site during the machining process;

[0020] FIG. 3 is a schematic side view showing a machining tool being moveable towards and away from an edge of a ply along an arc-shaped path; and

[0021] FIG. 4 shows the apparatus of FIG. 1 located within a climate-controlled environment.

DETAILED DESCRIPTION

[0022] Referring to FIG. 1, a prepreg ply 10 is clamped between refrigerated steel blocks 12a, 12b, and a grinding wheel 14 is arranged to translate across a free edge 16 of the ply 10 to remove material from that edge to create a chamfer. Refrigerant 18 is applied locally to the free edge 16 of the ply 10 during the chamfering operation. The apparatus and chamfering technique are described in further detail below.

[0023] The prepreg ply 10 comprises a sheet of glass fibre fabric, which has been impregnated with a thermoset matrix, which in this example is pre-catalysed epoxy resin. The glass fibre fabric consists of two layers and is commonly referred to as triax. The first layer includes a set of unidirectional (ud) fibres, whilst the second layer is a layer of biax, which has a first set of unidirectional fibres oriented at an angle of +45? relative to the fibres in the first layer, and a second set of unidirectional fibres oriented at an angle of ?45? relative to the fibres in the first layer.

[0024] The steel blocks 12a, 12b are oblongs and include internal refrigeration channels 20. A refrigerant is pumped through the channels 20 to cool the blocks 12a, 12b to a temperature of ?50? C., and then continuously pumped through the channels 20 to maintain the temperature of the blocks 12a,12b at ?50? C. Alternatively, the blocks 12a, 12b may be placed in a refrigerator at ?50? C. for several hours prior to the chamfering operation. In this way, the refrigeration channels 20 may not be required.

[0025] The steel blocks 12a, 12b are placed one on top of the other, with the lower steel block 12b being located on an insulating foam block 22 to reduce heat transfer from a work surface 24 to the cold blocks 12a, 12b. An end portion of the prepreg ply 10 is sandwiched between the steel blocks 12a, 12b and the blocks are clamped together by a clamp (not shown) to hold the ply 10 firmly in place. The upper block 12a is set back from the lower block 12b by approximately 40 mm to define an elongate ledge 26. The free edge 16 of the ply 10 extends from between the steel blocks 12a, 12b onto this ledge 26.

[0026] The grinding wheel 14 is arranged to traverse along the ledge 26 in a direction parallel to the exposed free edge 16 of the ply 10 as indicated by the arrow 28 in FIG. 1. The grinding wheel 14 has an abrasive cylindrical outer surface 30, which rotates about an axis parallel to the free edge 16 of the ply 10, i.e., parallel to its direction of translation 28 across the ply 10. In use, the grinding wheel 14 is angled slightly with respect to the surface 32 of the ply 10 and traversed across the free edge 16 to create a chamfer of a desired gradient. A shallow chamfer gradient in the range of 1:20 to 1:10 i.e., approximately 2.8? to 6? is particularly desirable.

[0027] Prior to chamfering commencing, the free edge 16 of the ply 10 is sprayed with tetrafluoroethane refrigerant (R134a) from a spray can 34. It will of course be appreciated that other suitable refrigerants may be used for this purpose, for example liquid nitrogen or liquid carbon dioxide. Spraying the free edge 16 of the ply 10 with refrigerant cools the ply 10 to well below the glass transition temperature (T.sub.g) of the uncured epoxy resin in the prepreg. Typically the T.sub.g of the uncured epoxy is around ?2? C. Maintaining the temperature of the resin below its uncured T.sub.g during chamfering ensures that the resin remains hard during the chamfering process. This prevents the resin from becoming tacky and contaminating or clogging the abrasive surface 30 of the grinding wheel 14, which would otherwise occur if chamfering was conducted at room temperature. The cold steel blocks 12a, 12b ensure that any heat generated during the chamfering operation is channeled away from the ply 10.

[0028] Experimental tests have shown that a single application of the R134a refrigerant to the free edge 16 of the ply 10 prior to chamfering is sufficient to keep the temperature of the ply 10 below the T.sub.g of the uncured resin. However, if necessary, the refrigerant may be applied repeatedly or continuously during chamfering to keep the temperature of the ply 10 below the T.sub.g of the uncured resin. Applying the refrigerant continuously has the advantage that a flow of refrigerant will carry heat away from the worksite. In the example shown in FIG. 2, the refrigerant 18 is applied during machining and is applied locally at the machining site 35. A nozzle 36 supplying the refrigerant 18 may be arranged to move in tandem with the grinding wheel 14 as represented by the arrows 38 in FIG. 2. Applying the refrigerant 18 locally at the machining site 35 is advantageous because it concentrates the refrigerant 18 at the point where heat is generated.

[0029] In order to assist heat dissipation from the free edge 16 of the ply 10, rather than being translated across the ply 10 in a single motion, the grinding wheel 14 may be pressed against the free edge 16 of the ply 10 in a series of pressing operations across the width of the ply 10. This is represented schematically in FIG. 3, which shows the grinding wheel 14 being moveable towards and away from the free edge 16 of the ply 10, i.e., in and out of contact with the free edge 16, along an arc-shaped path 40. Refrigerant is continuously applied to the free edge 16 so that cooling continues between presses, i.e., whilst the grinding wheel 14 is moved out of contact with the free edge 16 of the ply 10.

[0030] Whilst not shown in the above figures, the humidity of the air surrounding the apparatus is controlled to prevent condensation from forming on the cold ply 10 or elsewhere on the apparatus itself.

[0031] Referring to FIG. 4, rather than applying refrigerant directly to the free edge 16 of the prepreg ply 10, the entire apparatus is located in a climate-controlled environment 42 that is sufficiently cold to maintain the epoxy below its uncured glass transition temperature during the chamfering process. Of course, it is also possible to apply refrigerant directly to the chamfering site 35 if necessary when the apparatus is located in a climate-controlled environment 42 such as this. In this example, refrigerant channels have been removed from the blocks 12a, 12b, however it will be appreciated that such channels may be used in combination with a climate-controlled environment 42.

[0032] It will be appreciated that many modifications may be made to the techniques described above without departing from the scope of the present invention as defined by the accompanying claims. For example, it will be appreciated that the prepreg ply described by way of example above may be substituted for a semi-preg ply or other fibrous ply comprising a resinous matrix material. Also, whilst triax is described by way of example, it will be appreciated that the invention is not limited to the use of triax. Indeed, the fibres in the ply may have any other orientation, for example the fibres may all be unidirectional (ud). In addition, whilst a grinding wheel has been described above, it will be appreciated that the invention may be used in connection with any other machining tool or technique.