GB1081901A - Electrolytic machining - Google Patents

Abstract

An electrolytic machining operation is so controlled that the end gap, formed between the tool and workpiece as a result of relative infeed movement of the tool and workpiece towards one another, is continuously reduced at every point during at least the major part of the progress of the operation. The electrolytic machining is either by a pure forming method, i.e. where the finished form of the workpiece is determined by the shape of the tool in stationary position relative to the workpiece at the end of the machining, or by a combined forming and generating method in which, in addition to the infeed movement, the relative movement between the tool and workpiece has a component perpendicular to the infeed movement. The reduction in gap may be achieved by continuously reducing the voltage drop between the tool and workpiece as they approach one another. The infeed rate may also be progressively reduced during the operation, the rate of fall of voltage drop then being arranged to ensure that the gap continues to diminish; alternatively a constant slow infeed rate may be used. The conductivity <PICT:1081901/C6-C7/1> <PICT:1081901/C6-C7/2> of the electrolyte is kept substantially constant. The electrolyte flow may be as produced by a pressure of 100 lbs per square inch upwards. Jerkiness infeed is avoided by using an electric feed motor controlled by a known electronic comparison circuit. Fig. 1 (not shown) illustrates electrolytic machining using a pyramidal cathode (37), the gap being reduced by reducing the voltage. Over a small final feed distance the voltage may be kept constant. Fig. 2 shows apparatus for forming grooves in a workpiece 32 using cathodic templates 29. In a modification, jets of electrolyte are supplied parallel to the defining edges of the template 29. More than one set of grooves may be similarly formed using further sets of templates. The grooves may be parallel or may cross one another or may be radial or spiral or in concentric circles thus permitting relative rotation between the tool and workpiece during electrolytic machining. The voltage can be reduced manually or automatically and may be regulated according to tool position, modifying the regulation according to electrolyte pressure or flow and/or current. Alternatively the reducing voltage may be obtained by using a constant voltage supply with a resistance in series with the gap. As the gap closes the resistance between the tool and workpiece falls so that a smaller proportion of the voltage drop is across the gap. The resistance may be such that it will increase with rising current. The feed rate may be reduced by using a hydraulic feed system or an electric servomotor system controlled according to tool position and also according to electrolyte pressure or flow and/or current flow. The forming method may be used in shaping turbine blades, forming dies and coining dies. Fig. 3 (not shown) illustrates a combined forming and generating method using a tool having two conductive lands (25, 26). Tools of this form can be used in producing internal or external rotary shapes where the relative movement is a combination of rotary and axial. Fig 4 (not shown) illustrates a spark detecting system (see Division H2) which can switch off the electrolyzing power and stop the feed of the tool. Specified electrolytes are (i) aqueous HCl at a concentration of 1/2 to 6% with a corrosion inhibitor such as an aliphatic or aromatic amine, e.g. 0.04% pyridine, hexamine, quinoline or Galvene (Trade Mark) and (ii) an electrolyte containing up to 0.4% HCl and 0.5% gelatine (iii) NaNO2 solution (iv) solutions of chlorinated aliphatic acids. Electrolyte temperature in an electrolyte reservoir 43, Fig. 5, is controlled by a thermostat 49 which controls heating and cooling means 51, 52 to maintain the temperature within suitable limits. The thermostat is also arranged to switch on a conductivity comparator 53 when the temperature is at the correct value and this controls the addition of concentrated electrolyte from a tank 46. During circulation of electrolyte to and from the location of machining 56 it passes through a filter 55.