Chemistry: electrical and wave energy – Processes and products – Electrostatic field or electrical discharge
Patent
1987-07-13
1988-09-06
Valentine, Donald R.
Chemistry: electrical and wave energy
Processes and products
Electrostatic field or electrical discharge
204224M, 204225, B23H 304, B23H 914
Patent
active
047691182
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for the formation of curvilinear holes in engineering components.
Some engineering components require the formation therein of very fine bore holes. Examples of such components include gas turbine engine blades and nozzle guide vanes. Such holes, when they are linear, may be formed by various methods such as, for example, by casting-in by means of removable cores, drilling with a conventional rotatable drill bit, laser or electron beam drilling, electro-discharge machining (EDM) and elecro-chemical machining (ECM). One such ECM method is described in GB No. 1 516 014 which describes an apparatus for forming a plurality of straight holes, the holes having non-parallel axes. The apparatus includes a guide member having the required number of holes therein, the holes in the guide member being relatively long and having their axes coaxial with the holes to be drilled or formed in the workpiece. It is a relatively simple matter to produce a long straight hole in a guide member. Straight glass capilliary tubes for ECM drilling are also freely available.
The necessity arises in some component designs to produce holes having curvilinear paths. The trailing edges of gas turbine blade aerofoils which have high curvature, for example, may have such holes to assist cooling. Some of the newer aerospace materials of which oxide dispersion strengthened (ODS) alloy is one example, are not amenable to casting as a production route. Therefore, the inclusion of holes by means of cast-in, linear or otherwise, cores is not possible. In some cases drilling of holes is the preferred route even though casting-in may be technically feasible. Such cases may include those for reasons of economy or metallurgical reasons as in the case of single crystal components.
GB No. 946,448 describes a process for producing curved recesses in a metallic workpiece by EDM. The electrodes used are large and rigid and do not require additional guidance means.
Curved fine glass, capilliary electrodes would be unable to be self-supporting for drilling small diameter holes in such components whilst the type of electrode guidance shown in GB No. 1 516 014 of a long curved guide member would be impractical since it is extremely difficult to form both a long curved guide member and curved glass capilliary of exactly compatible curvature reliably. Problems arise due to friction between the guide member and the capilliary resulting in excessive stress and consequent breakage of the capilliary. Where a plurality of holes are being drilled simultaneously slight misalignments of the glass capilliaries in the manifold holding them accentuate the strains and hence stresses involved.
One method which has apparently been employed to produce curvilinear holes is to flatten the aerofoil in the region where the hole is required, drill a straight hole and then bend the drilled region back into the required curve. Clearly, such techniques must in some circumstances have detrimental effects on the metallurgical properties of the alloy in a highly stressed part.
It has now been discovered that fine curvilinear holes may be formed in workpieces with curved glass capilliary electrodes without the need for a guidance member the aperture of which is coaxial with the hole to be drilled.
According to the present invention a process for the production of a curvilinear hole in a workpiece comprises the steps of producing relative motion along a curvilinear path between a workpiece in which it is desired to form the curvilinear hole and a drill bit characterised in that the drill bit comprises a curved delivery tube of an ECM apparatus and which curved tube passes through an aperture in a guidance member which is positioned adjacent the workpiece, the aperture length being less than the diameter of the tube.
In one embodiment the tube is a glass capilliary tube.
Capilliary electrodes in ECM apparatus are normally secured to a manifold by a curable adhesive resin. During curing of the resin stresses are generated which may result in misal
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AE PLC
Hinds William R.
Valentine Donald R.
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