GB2222977A

GB2222977A - Electrode

Info

Publication number: GB2222977A
Application number: GB8822222A
Authority: GB
Inventors: Frederick John Rolinson; Paul Daren Winfield
Original assignee: ERODEX
Current assignee: ERODEX
Priority date: 1988-09-21
Filing date: 1988-09-21
Publication date: 1990-03-28
Also published as: GB8822222D0

Abstract

An electrode for electrochemical drilling or machining a workpiece comprises a metal or alloy tube 1 of uniform outer diameter coated over an operating length L by a non-conducting insulating coating 2 of uniform thickness selected from the group consisting of alkyd phenols, fluorinated ethylene and propylene polymers and copolymers, polyvinyl fluorines and shellacs. The tube is ground at 1b to expose the tube and form a frusto conical working end. <IMAGE>

Description

ELECTRODE This invention relates to electrodes and in particular to an electrode for electrochemical drilling or machining of a workpiece.

Electrochemical drilling or machining of workpieces is a process in which the workpiece and an appropriately shaped electrode are immersed in a suitable electrolyte and metal is removed from the workpiece by applying a potential between an anode constituted by the workpiece and a cathode constituted by the electrode. The process enables ultra-hard metal workpieces to be drilled or machined with a high degree of accuracy by careful control of the type, quality and geometry of the electrode.

One particular application of the process is to drill very small diameter holes, typically of 0.025 inch diameter, through long sections of aero engine components, 4 inches being not uncommon, to provide cooling passages through the high temperature zones of jet engine components.

For such application, the electrode consists of a straight tube insulated over an operating length by a non-conducting coating.

It is essential that the coated tube is of uniform outer diameter over the operating length and the coating must adhere strongly to the tube under the operating conditions and be totally free of microporosity. Typically, the tube and coating must resist 30% nitric acid in aqueous solution.

According to the present invention we provide an electrode for electrochemical drilling or machining a workpiece comprising a metallic tube or rod insulated over an operating length by a non-conducting coating selected from the group consisting of alkyd phenols, fluorinated ethylene and propylene polymers and copolymers, polyvinyl fluorines and shellacs.

The tube or rod is preferably straight and of uniform outer diameter over the operating length.

Titanium, stainless steel and molybdenum are preferred materials for the tube or rod but any other metal or alloy to which the coating adheres strongly may be used.

The coating is preferably of uniform thickness over the operating length and may be applied by various techniques, for example dipping or spraying. The coating is preferably air or heat dried depending on the material selected for the coating.

The coating may be formed by applying a single layer of the required thickness but more preferably the coating is formed by applying several thin layers in succession to build-up gradually to the required thickness of the coating with each layer being dried before application of the next layer.

Applying the coating in several thin layers improves uniformity of the coating and the resultant coating adheres strongly to the tube or rod and is free of micro-porosity.

The uncoated tube or rod is preferably cleaned and/or sensitised prior to application of the coating to improve adhesion, for example the tube or rod may be electrochemically etched or ultrasonically cleaned.

The coated tube or rod is preferably of uniform outer diameter over the operating length and may be finished, for example by grinding the coating, to set accurately the outer diameter within close tolerances.

The invention will now be described in more detail with reference to the accompanying drawing in which the single Figure is a side view of an electrode according to the present invention.

The electrode shown in the accompanying drawing is for electrochemical machining a hole in a workpiece such as a jet engine component and comprises a straight titanium tube 1 having a concentric non-conducting coating 2 of an alkyd phenol insulating resin over an operating length 'L'. The tube 1 is of uniform outer diameter and the coating 2 is of uniform thickness.

The tail end la of the tube 1 is left uncoated to provide an electrical contact for the electrochemical machining process and the tip end ib is ground to a conical configuration to expose the tube and improve tne accuracy of the hole formed in the workpiece during the electrochemical machining process. In this embodiment, the taper angle 'A' is 70t 10 but this angle may be altered as desired.

The coating 2 is applied in several layers by dipping the cleaned and sensitised tube 1 in a xylene solution of the alkyd phenol insulating resin (for example "SLIMVAR 5037" manufactured by Glasurit Beck Limited of Horsham, Sussex) to build up gradually to the required coating thickness with each layer being dried before application of the next layer.

The coated tube is finished by grinding the coating 2 to ensure a uniform outer diameter over the operating length 'L' corresponding to the internal diameter of the. hole to be machined in the workpiece.

In use, the workpiece and electrode are immersed in a suitable electrolyte and the hole is machined in the workpiece by applying a potential between the workpiece which constitutes an anode and the electrode which constitutes a cathode whereby metal is removed from the workpiece and the electrode is advanced into the hole as it is formed.

In this embodiment the tube 1 has an outer diameter of 0.02 inches, the coating 2 is 0.0015inches thick and the operating length 'L' is 11.75 inches with 2.25 inches of the tube 1 being left uncoated to form the electrical contact.

It will be understood by those skilled in the art that such dimensions are not limiting and that the outer diameter of the tube 1, thickness of the coating 2 and operating length 'L' of the coated tube may be selected depending on the size of hole to be formed in the workpiece. In general, however, it is preferred that the coating thickness should not be less than 0.001 inches for best results.

Claims

Claims:

1. An electrode for electrochemical drilling or machining a workpiece comprising a metallic tube or rod insulated over an operating length by a non-conducting coating selected from the group consisting of alkyd phenols, fluorinated ethylene and propylene polymers and copolymers, polyvinyl fluorines and shellacs.

2. An electrode according to Claim 1 wherein the tube or rod is straight and of uniform outer diameter over the operating length.

3. An electrode according to Claim 1 or Claim 2 wherein the tube or rod is made of a metal or alloy selected from the group consisting of titanium, stainless steel and molybdenum.

4. An electrode according to any one of the preceding Claims wherein the coating is of uniform thickness over the operating length.

5. An electrode according to Claim 4 wherein the coating thickness is at least 0.001 inches.

6. An electrode according to any one of the preceding Claims wherein the tip end is of conical configuration.

7. An electrode according to Claim 6 wherein the tail end of the electrode is uncoated.

8. An electrode according to any one of the preceding Claims wherein the coating comprises an alkyd phenol insulating resin.

9. An electrode according to Claim 8 wherein the coating is applied to the tube or rod from a xylene solution of the alkyd phenol insulating resin.

10. An electrode for electrochemical drilling or machining a workpiece substantially as hereinbefore described with reference to the accompanying drawing.