GB2101157A - Electro-chemical machining type face characters from solid metal - Google Patents

Abstract

A print band, daisy wheel or other component carrying type face or embossed detail is made of electrochemically machining the type face or embossed detail from solid metal. The rate of feed of the cathode in relation to the workpiece is controlled in accordance with the cathode-workpiece gap, which is maintained less than 0.010 in (0.25 mm). The gap is measured by metering the volume flow of electrolyte, and the pressure of electrolyte flowing in the gap can be used to clamp the workpiece on a base and maintain good electrical contact therebetween. In print bond manufacture elongate strip 11 is fed in steps to an electrochemical machining area where characters are machined on it, after which it is stepped through the area and a further length of strip is treated. The cathode 15 can slide towards the workpiece in a housing 14 in which it is sealed by O-ring 24 and the leading and trailing ends of the exposed surface of the length of strip being machined are sealed by end clamps 27. <IMAGE>

Description

SPECIFICATION Electro-chemical machining This invention relates to electro-chemical machining (hereinafter termed E.C.M.), a process in which a workpiece is used as an anode and a contoured tool as a cathode with an electrolyte flowing in the space between the anode and cathode through which low voltage direct current is passed. During the process, the anodic reactions yield products, often in the form of a precipitate, as the workpiece is machined away in a pattern corresponding to the contoured shape of the tool.

According to one aspect of the present invention, E.C.M., is used to machine type characters or the like from solid metal, for example, in the manufacture of a print band, a metal daisy print wheel, or any other component carrying type face or embossed detail.

In the past, a type face has been cast or moulded from plastics material, or cold formed relying on the material to flow under large applied forces or mechanically engraved or etched using photo-resist method, which latter methods are difficult and require several steps.

E.C.M. can be performed in a single operation but the machining of such intricate shapes as type characters requires special and quite new conditions of E.C.M.

In accordance with a second aspect of the invention, in a method of electro-chemical machining there is a gap of less than about .010 inches between the work and the cathode tool. It is possible to provide an indication of the gap obtaining as a measure of the volume of the electrolyte flowing through the gap, and an electrolyte volume meter can be calibrated in terms of the gap width.

For machining with such small gaps it is desirable to feed the workpiece towards the cathode tool at a rate corresponding to the rate of machining, so called equilibrium machining, and the rate of feed may be controlled in accordance with the gap measured as described above by measuring the volume of the electrolyte.

It is also important to control the rate of volume flow of electrolyte to cool the workpiece and to remove the precipitate and prevent rapid changes in the electrolyte conductivity and for that purpose, a pressure control valve is fitted prior to connection to the tool.

It is also important to maintain a high pressure within the machining gap and for this purpose a controllable pressure valve is located in the electro lyte route after the tool.

High rates of machining will enable the character to be produced reasonably quickly, but that requires higher electrical power between the anode and cathode, and a high rate of production of precipitate, and also of gas at the electrode surfaces. It is important to keep the gas so generated in solution in the electrolyte, and it may be necessary to use electrolyte pressures as high as 650 p.s.i. With such high pressures, special sealing arrangements are necessary around the electrolyte flow passage, but it is possible to use the high pressure of the electrolyte to clamp the workpiece in position on a conducting base particularly if the back of the workpiece and the surface of the base are perfectly smooth, and in intimate contact.

In order to prevent machining away of the sides of the unmachined characters, the inner surface of the contoured recesses in the tool defining the characters may carry an insulating layer.

Highly passivating electrolytes including nitrates and chlorates also assist in the process of obtaining highly defined characters by virtue of the discriminating voltage properties arising from their anode film formation thereby producing rapid transition between machining and non-machining areas.

The invention may be carried into practice in various ways, and one embodiment will now be described by way of example, with reference to the accompanying drawings; in which Figure 1 is a sketch showing how a stainless steel band can be mounted for E.C.M., Figure 2 is a sectional view, looking in the direction of the arrow II in Figure 1 at a position part way along the length of the strip, Figure 3 is a detail of Figure 2; and Figure 4 is a sketch showing a detail of the machining method.

A stainless steel print band is formed by beam welding the ends of an elongate strip 11 of stainless steel strip. The band may consist of one long strip or say four shorter strips welded end to end to form a complete print band after printing characters have been formed proud of the surface by E.C.M.

Each part length is machined separately on an individual length of continuous strip fed in steps through the forming apparatus. The length being formed is shown at 11 in Figures 1 and 3, sitting on the upper surface of a base 12 of brass, copper titanium or other conducting material which may or may not utilise a further metallic layer to impart additional conductive and protective properties.

Both the upper surface of the base and the lower surface of the strip are maintained scrupulously clean so that there will be intimate contact between them, and substantially no loss of electrolyte around the edges of the strip 11.

The strip is fed in steps longitudinally through the machine from a feed roll 10, to take-up roll, or altenatively a programmed guillotine to cut individual lengths.

The base 12 and the strip 11 act as an anode in the E.C.M. process. The cathode is constituted by a copper or other tool 15 which contains in its lower face, recesses shaped to correspond with the shape of the print characters to be machined on the upper surface of the strip 11.

The length of strip in its passage through the tool lengthwise is guided laterally on each side by an insulating block 16 of polycarbonate silicon nitride or other insulating material, which is seated on the base 12 which supports the edge of the strip and contains a venting slot 31.

The blocks 16, which extend along the full length of the part of the strip being machined, cm operate with upper blocks 14to define inlet and outlet slots 17 and 18 for a transversely flowing electrolyte supplied under high pressure at 21 and leaving at atmospheric pressure at 22.

The cathode 15 is lowered in a guide 23 defined in the blocks 14, and is sealed in the guide against escape of electrolyte by virtue of an O-ring seal 24 extending in a groove all round the external side surface of the cathode.

As the cathode approaches the start position, a point is reached at 0.030 inches above the strip 11 when lower shoulders 25 at the ends of the cathode make contact with top surfaces 26 of a pair of end clamps 27, and urge them down to clamp the strip onto the base 12 and seal against the upper faces of the strip by sealing at the undersurface of each clamp.

The flow of electrolyte is established with a pressure of perhaps 400 or 650 pounds per square inch at 17 in Figure 2. It is important to maintain the width of the gap between the electrodes, but it is difficult to measure it accurately, and a method has been devised whereby the volume of the electrolyte passing through said gap is indicated and the indication is calibrated in terms of the width of the gap obtaining by an empirical method.

Current flow is distributed across the gap of perhaps between 3000 and 6000 amps at between 3 and 8 volts for a strip 14 inches wide and 15 inches long.

During E.C.M., gas is developed at the anode 11 and cathode 15, and it is important to keep that gas dissolved in the electrolyte because local gas bubbles produce irregularities in the machined surface and cause electrical conductivity changes to the electrolyte thereby affecting the metal removal rate and therefore the machining gap.

The higher electrolyte pressure at 31 may be needed to keep the gases dissolved.

During machining ferric hydroxide representing the machined away product when the anode is of steel is carried away by the electrolyte.

It is possible with electrolyte pressure as high as 650 p.s.i. to use the pressure of the electrolyte to hold the strip 11 clamped to the base 12 by virtue of a pair of longitudinal slots 31 in the base, one under each edge of the strip, and at atmospheric pressure.

As machining progresses, it is necessary to feed the tool 15 towards the base and strip at a corresponding rate, and that is made easier if an indication of the volume of the electrolyte as a measure of the gap obtaining can be used as referred to above.

Electrolyte leaving at 18 can be fed through an adjustable back pressure valve to provide a control of the electrolyte pressure and flow rate within the machining gap. It is important that the volume rate of flow of the electrolyte is controlled in order that the heating effect arising from the flow of electric current does not induce boiling of the electrolyte or otherwise change its physical properties since, for example, a decrease in electrical conductivity results in lower metal removal, a smaller gap and quite different machining conditions. Effective operation can be achived by control of the back pressure valve.

This will effect pressure changes within the gap and cause changes in the volume rate of electrolyte flow. Such changes in back pressure and consequential volume changes clearly require a re-calibration of the gap indication equipment.

A typical rate of machining steel is .005 inches per minute. In the example being described the strip before machining is .015 inches thick, and it is desired to machine away .010 inches from those areas not occupied by the type characters, thus leaving these standing proud of the machined surface.

This operation would take about 2 minutes.

Figure 4 shows a typical cross section of a recess 32 in the tool 15, where a character 33 is to be proud of the machined surface of the strip 11. The sides of the recess could be perpendicular to the under surface of the tool 15, but in conventional practice the recesses can be machined with an included angle of about 10' or 20% For effective machining of characters, it may be necessary to control the rate of side attack on the characters. A means of achieving this may be to apply a lining of insulating or partially insulating material 37.

The lining 37 might be an organic paint or lacquer, or a glassy silica film, or an aluminium, or other inorganic oxide protected by a film.

The gap between tool and workpiece might be maintained at about 0.004 or even 0.002 inches.

The electrolyte might be a sodium or potassium chlorate solution or possibly a sodium nitrate solution, or possibly a mixture of both.

Although a method has been described, in which individual lengths of strip are treated in turn between a base and a tool which do not move longitudinally, it is also possible to feed a long length of strip between a base and the cylindrical surface of a tool which rotates about an axis perpendicular to the length of the strip and parallel to, and spaced from, the surface of the strip. The individual characters are formed around that cylindrical surface which has as great a radius as is necessary for that purpose.

The strip is fed forward at a rate corresponding to the rate of rotation of the tool.

Claims (10)

1. The use of electro-chemical machining to machine type face characters or the like from solid metal in the manufacture of a print band, metal daisy print wheel or other component.

2. A method of electro-chemical machining in which there is a gap of iess than about .010 inches between the work and the tool constituting the two electrodes.

3. A method as claimed in Claim 2 in which the rate of feed of the tool in relation to the workpiece is controlled in accordance with the gaps obtaining between those two components.

4. A method as claimed in Claim 3 in which the width of the gap between the two components is measured by use of a meter of the volume flow of electrolyte.

5. A method of machining as claimed in any of the preceding claims in which the pressure at which electrolyte is supplied to the gap between the tool and the workpiece is at least 200 or 400 or 600 p.s.i.

6. A method of machining as claimed in any of the preceding claims, in which the inner surface of a contoured recess in the tool carries an insulating or partially insulating layer.

7. A method as claimed in any of the preceding claims in which the pressure of the electrolyte in the gap between the workpiece and the tool is used to clamp the workpiece in position on a base.

8. A method as claimed in any of the preceding claims in which the pressure of the electrolyte in the gap between the workpiece and tool is used to secure effective electrical contact between workpiece and the base.

9. A method as claimed in any of the preceding claims, in which during print band manufacture, elongate strip is fed in steps to an E.C.M. area, and characters are machined on it after which it is stepped through the area and a further length of the same continuous strip has characters machined on it.

10. A method as claimed in any of the preceding claims in which the tool can slide towards the workpiece in a guide in a housing in which the tool is sealed, and the leading and trailing ends of the exposed surface of a length of strip being machined are sealed during electro-chemical machining.