US2798846A - Method and apparatus for the electrolytic erosion of work pieces - Google Patents

Description

July 9, 1957 G. E. coMsTocK 3D 2798,846

METHOD AND APPARATUS FOR THE ELECTROLYTIC saosxou OF WORK moss Filed April 28, 1953 2 Sheets-Sheet l A 77oz NEY G. E. COMSTOCK 30 ,7 46 METHOD AND APPA TUS FOR THE ELECTROLYTIC EROSION OF WORK PIECES Filed April 28, 1953 July 9, 1957 v 2 Sheets-Sheet 2 ATTORNEY Unitedstates PatentO METHGD AND APPARATUS FOR THE ELECTRG- LYTlC ERQSKGN (OF WORK PIECES George E. Cornstock 3d, Holden, Mass, assignor to Norton Company, Worcester, Mass, a corporation of Massachusetts Application April 28, 1953, Serial No. 351,722 9 Claims. (Cl. 204-143) The invention relates to a machine, wheel and method for the electrolytic erosion of work pieces.

One object of the invention is to provide a machine and method for the electrolytic erosion of work pieces so that an abrasive wheel will not have to be used. Another object of the invention is to provide a wheel for the electrolytic erosion of work pieces which itself will not be eroded and which will therefore have a long life. Another object of the invention is to erode work pieces without abrasion. Another object is to eliminate or to reduce detrimental arcing in the art of the electrolytic erosion of work pieces.

Other objects will be in part obvious or in part pointed out hereinafter. i

In the accompanying drawings illustrating one embodiment of the machine tool and two embodiments of'the eroding Wheel,

Figure 1 is a view partly in side elevation and partly in vertical section of an electrolytic eroding machine and a wheel therefore according to the invention,

Figure 2 is an enlarged elevation of the wheel spindle showing various parts thereon in cross section,'

Figure 3 is a diametral sectional view on an enlarged scale of the eroding wheel shown in Figure 1, the section being taken along the line 3-3 of Figure 1,

Figure 4 is an axial sectional view on the same scale as Figure 3 of a modified form of wheel,

Figure 5 is a wiring diagram.

Referring first to Figure. 1, in an illustrative embodiment of the invention, the machine tool may be of the type disclosed in U. S. Patent No. 2,375,619 to W. M. Bura with the special features which are appurtenant to the present invention. The base supports a bed plate 11 which supports a tool holder 12 which holds the workpiece 13 which illustratively is itself a tool, for example a tool bit for use in a lathe. The tips 14 of such tool bits are usually made of cemented hard carbide which is diflicult to grind but which can be ground with a diamond grinding wheel and also can be shaped by electrolytic erosin.

Referring to Figures 1 and 3, the wheel 15may consist of a metal back plate 16 having radial grooves 17 and a metal front plate 18 with a fiat rear face so that, When the plates 16 and 18 are secured together as by means of bolts 20 the grooves 17 became channels. The front plate 18 has an annular projecting portion 21 with a front face which is in a plane perpendicular to the axis of the Wheel 15 and in this portion 21' are many holes 22 extending from the front face to the grooves 17. There are a plurality of holes 22 for each groove 17 preferably the same number of holes 22 for each groove 17, for example four for each as shown, and each hole 22 may be parallel to the axis of the wheel 15 as shown although holes at angles to the axis might be preferred in some cases.

Referring now to Figures .1 and 2, I provide a steel spindle 25 having a straight axial bore 26 extending from its rear end nearly but not quite to the front end 27 which is threaded. The spindle 25 has an enlarged diameter portion 28 having square shoulders at its ends. Against this portion 28 the wheel 15 is held by means ofa nut 31 on the threaded portion 27. A bronze bearing sleeve 32 abuts the portion 28 and has a shoulder 33 which abuts a corresponding shoulder in an insulating sleeve 35. The sleeve 35 has a flange 36 which contacts the front face of a wheel head 4% which has aligned bores for the reception of the sleeve 35 and a similar insulating sleeve 45.

The sleeve 45 has a flange 46 which is located in a recess in the rear face of the wheel head 40. In the sleeve 45 is a bronze bearing sleeve 47 similar to the sleeve 32 and which has a shoulder 48 abutting a shouldered portion of the sleeve 45. The spindle 25 is journalled in the bearing sleeves 32 and 47 and turns therein.

Secured to the spindle 25 by means of a nut 50 on a threaded end 51 of the spindle 25 and by means of a key 52 is a pulley 53 connected by belts 54 to a pulley 55 of a motor 57 secured by means of screws 58 to the outside of the machine base 10.

In the bore 26 of the spindle 25 is a tube 60 having enlarged ends which are carefully ground and fit the bore 26 closely. The left hand end of the tube 60 is threaded on the outside and is connected to a pipe elbow 65 which is connected to a pipe 66 which is supported by an insulating bracket 67 which is supported by a metal bracket 68 secured to the wheel head 41 The lower end of the pipe 66 is connected to a valve fitting 7 0 having a valve wheel 71 by the angular adjustment of which the flow of liquid through the valve fitting 76 can be regulated. Connected to the latter on the under side is a rubber hose 73.which is connected to a dome accumulator 74 which is mounted by means of a bracket 75 on an insulating plate 76.

The dome accumulator 74 is connected by a pipe 77 to a diaphragm pump 78 which is connected by a rubber pipe 79 to the bottom of a rubber tank 80 located between Walls 81 and 82 of the base 10 in the inside thereof. The tank 80 can be made out of hard rubber such as the variety which is used to make the shells of electric storage batteries. The pipe 79 can also be made of the same kind of hard rubber while the hose 73 can more conveniently be made of soft rubber.

The diaphragm pump is operated by an electric motor 84 which is attached to the insulating plate 76. The diaphragm pump includes a box-like portion 85 containing the mechanical elements of the pump including an eccentric 86 and a follower 87 and this box-like portion 85 is likewise fastened to the insulating plate 7 6. I have not deemed it necessary further to illustrate the diaphragm pump in detail since such pumps are well known and are readily available on the market. A characteristic of such pumps is that detritus and dirt entrained in the liquid being pumped does not rapidly wear 'out the pump because it has no piston sliding in a cylinder.

The rubber tank 80 contains liquid 90 which in accordance with this invention is an electrolyte. Ordinary salt water can be used, for example clean ocean water can be used although in most places it will probably be more convenient to use a solution of sodium chloride in water. However it is preferable to inhibit rusting of the machine and of steel parts of the work piece so far as possible. In most grinding operations the coolant has been water and it has been general practice to introduce a rust inhibitor into the Water. Most of these will serve as the electrolyte. Particular salts which can be used in water solution to make the electrolyte with rust inhibiting characteristics are sodium nitrite, sodium chromate, potassium nitrite, potassium chromate', the amines of sodium nitrate, sodium and potassium dichromate, sodium and 2 8, x a T potassium chlorate and sodium and potassium chlorite. Two good formulae are disclosed in British Patent No. 491,023 as follows:

Formula N0. 1 A 1% concentration in water of:

Percent Potassium chromate 22 Potassium dichromate 78 Formula N0. 2

A 1% concentration in water of:

Percent Potassium chromate ;a 12 Potassium dichromate 86 Sodium chlorate 1 Sodium chlorite 1 The above formulae for electrolytic eroding concentrations. t

Referring now to Figures 2 and 3, a series of arcuate grooves 95 are milled in the spindle 25, near the front end 27. At the bottom of these grooves 95 are holes 96 extending to the bore 26. At the same position longitudinally as these holes 96 is a single hole 97 through the tube 60. It will now be seen that liquid 90, pumped by the pump 78 from the tank 80, must pass through the tube 60 and enter the grooves 17 and flow out of the holes 22 and furthermore because there is only one hole 97 through the tube 60, the pressure and the flow will be greatest at somewhere between one and two oclock (using the artillery terminology) of the wheel 15 which is where the tip 14 of the workpiece 13 being eroded is located. The pressure of the liquid keeps the tip 14 from actual contact with the front plate 18 of the wheel 15 thus to prevent wearing of the latter.

For electrolytic erosion of a workpiece, the workpiece should be positive and the electrode as represented by the revolving wheel 15 should be negative; Figure illustrates a simple circuit for achieving this result. A direct current source of electricity such as a storage battery 100 has its positive terminal connected by a wire 101 to a switch 102 which is connected by a wire 103'to the bed plate 11 which of course is electrically connected to the tip 14, and the diagram so indicates. It will be readily appreciated that the bed plate 11 is electrically connected to the entire machine because it is supported by the base and so therefore in Figure 5 there is represented a. path 105 to ground 106, but this is purely diagrammatic and in actual practice a real wire 103 connects the battery to the bed plate 11 because it is desirable to avoid any variation of conductivity involved in a film of oil between the bed plate 11 and the base 10 are rust inhibiting formulae and I prefer, in general, about 5%] upon which it is slidable, the bed plate 11 being movable forwardly and rearwardly by means as set forth in the Bura patent.

Referring now to Figure 1, I provide an insulating holder 111 secured to the front of the wheel head 40 and containing brushes 112 pressed by springs 113 against the enlarged portion 28 of the spindle 25. The brushes 112 are connected, as shown in Figure 5, by wires 115 (represented by a single line in Figure 5) to a variable resistance 116 which is connected by a wire 117 and a wire 118 to the negative side of the battery 100. Thus the workpiece is positive (when the switch 102 is closed) and the wheel is negative, and the amount of current flow can be adjusted by adjustment of the variable resistance 116. As there is an electrolyte between the workpiece tip 14 and the wheel 15, the workpiece will be eroded and the action is quite analogous to grinding because the flat face of the front plate 18 and the revolutions of the wheel 15 cause an even erosion of the workpiece to produce, in this case, a plane surface thereon.

In an optional embodiment of the invention I may, as

of a hand wheel 110 indicated in Figure 1, provide a wheel eroding electrode 120 which can be a piece of cemented tungstein carbide secured to a holder 121 having a spherical portion 122 supported by a hollow spherical end 123 of a spring steel bar 124 secured by bolts 125 to an insulating bracket 126 secured by means of a bolt 127 to the machine base 10. The spring steel bar 124 urges the electrode 120 against the face of the wheel 15 with light pressure. Referring now to Figure 5, this electrode 120 is shown as connected by means of a wire 128 to a variable resistance 129 which is connected by a wire 130 to the wire 118.

Thus while the wheel is cathode to the workpiece tip 14, it is anode to the electrode 120 which is a cathode. At the same time by adjustment of the resistances 116 and 129 as much of the current can be sent through the electrode 120 as desired up to a certain maximum or all of the current can be sent through the brushes 112 by removing the electrode 120. This electrode 120 if connected and energized as shown in Figure 5, serves to true the front face of the wheel 15 to keep it in a plane. However for most operations this electrode 120 will not be needed.

Referring now to Figure 4, the wheel eroding electrode 120 is perhaps more useful in connection with this embodiment of the invention. The wheel 15a of Figure 4 comprises a metal back plate having grooves 136 and a rim 137. A front plate 138 is secured to the back plate 135 by means of bolts 139 and thus the grooves 136 are channels for conducting the electrolyte. The front plate 138 has a rim 140 and between the rims 137 and 140 is an annular piece 141 of recrystallized silicon carbide having an interlocking projection 142 located behind a shoulder 143 of the front plate 138. p

This wheel of Figure 4 is mountable upon the spindle 25 and the erosion takes place by electrolytic action as already described. However the recrystallized piece of silicon carbide 141 is capable of doing some real abrading by conventional abrading action. In the case of cutting cemented carbide pieces, such as the tool bit 14, I have. observed that the electrolytic erosion removes the metal bond, usually cobalt, but probably does not erode the tungsten carbide (or tantalum carbide or titanium carbide etc.) as fast as the metal is eroded. It is therefore a help to have a true abrasive action which needs only to clear away the carbide from the surface of the workpiece. Thus the combination of electrolytic erosion and light abrading has proved to be able to remove stock practically as fast as a diamond wheel can remove the stock but of course a piece of recrystallized silicon carbide is far cheaper than diamond abrasive and furthermore the former doesnt appear to wear out any faster than the latter.

Recrystallized silicon carbide was described by Francis A. Fitzgerald in U. S. Letters Patent No. 650,234 patented May 22, 1900. It is made by heating crystals of silicon carbide in proximity to each other to the temperature of recrystallization which means the temperature of further crystal growth. As a practical matter some adhesive has to be used and Fitzgerald suggests a dilute solution of glue which is mixed with grains (crushed crystals) of silicon carbide, the mixture being then pressed to the desired shape. Fitzgerald says that the mass so molded is then heated in an electric furnace to or about the temperature originally used to produce the silicon carbide and that when so treated the silicon carbide will recrystallize and produce a very hard dense mass without substantial loss of material by vaporization except the volatilizing of the material which may have been employed as a temporary bond. Later on it was found more practical to use sodium silicate as the temporary bond for this, in a reducing atmosphere, is also converted to silicon carbide as the sodium component is lost by volatilization and the silicon unites with the carbon of the reducing gas,carbon monoxide, to form additional silicon carbide. Any oxygen present is of course removed in the reducing atmosphere which is created by the crucible or tube or other container made of carbon or of graphite.

For the manufacture of wheels according to the invention, I may proceed as follows: I make a mixture of silicon carbide grains and sodium silicate solution. For example, 45% by weight of No. 20 grit size silicon carbide and 55% by weight of No. 100 grit size silicon carbide are thoroughly mixed. Then for each pound of this mixture of silicon carbide grain, I provide 14 cubic centimeters of 3 to 1 sodium silicate solution, that is to say 3 parts of water to 1 part of sodium silicate by weight. Making a thorough mixture of the silicon carbide grain and the sodium silicate solution, I mold the mixture into the desired shape under pressure, for example in an hydraulic press, dry the pressed wheel for the removal of all but about 2% of the water, and then fire the pressed and dried wheel to a temperature to recrystallize the silicon carbide. A good recrystallizing temperature is 2500 C. and I can use a graphite tube furnace or an induction furnace having a graphite crucible to hold the wheel or wheels being recrystallized. A satisfactory furnace is illustrated in Raymond R. Ridgways U. S. Letters Patent No. 2,125,588. This procedure produces recrystallized silicon carbide wheels having a crss-bending modulus of rupture in the range of 5,000 to 7,000 pounds per square inch and a room temperature resistivity that may vary from as high as to 50 ohms centimeter if carefully selected crystals of green silicon carbide were used, to a value several orders of magnitude smaller if black or gray grain were used. It may be noted at this point that the unit ohm centimeter is exemplified in a cubic centimeter since resistivity increases proportionally to the length and decreases proportionally to the area of cross-section. This same unit used to be called ohm per cubic centimeter. For making a porous piece of recrystallized silicon carbide, I mold with only a small amount of pressure, for example of the order of 200 pounds per square inch.

The electrolyte of course comes out through the porous piece 141 and covers the face of the workpiece 14. In this embodiment of the invention, however, the pressure between the tool tip 14 and the wheel 15 can be sufiicient to cause real contact to assist in removing stock, whereas where a plane metal front plate 18 is used the pressure can be less so that the pressure of the fluid will hold the tip 14 and wheel 15 out of actual contact, thus to prolong the life of the front plate 18. However, since the latter is not expensive, greater pressures can be used in the first embodiment of the invention so that actual contact is made and the electrode 120 can be relied upon for truing the face of the front plate 18. In this illustrative embodiment of the invention, the pressure between workpiece and wheel is hand pressure exerted by the operator but in many other embodiments of the invention the machine tool will have a power feed, either positive or pressure.

Advantages of the invention are that a diamond free wheel can be used, in fact the front plate 18 can be an ordinary piece of cast iron. The back plate 16 is, of

course, good for an indefinite length of time.

Diamonds for industrial use are scarce and high in price and furthermore the known supply is not inexhaustable. On the contrary, at the present rate of consumption present stock piles of diamonds would be exhausted in a few years, and the present rate of consumption is far greater than the rate of finding of the diamond stones. Alluvial stones are the chief source of supply and little can be done to accelerate the discovery of stones. Consequently any development for shaping hard carbides and other hard materials without using diamonds at all is greatly to be desired.

The electrode 120 will having a metal front plate 18, to therefore to true the working face.

function, in the case of a wheel remove metal and Here again the machine functions without a diamond as truing is achieved without the use of a diamond tool, whereas it has been standard practice for many years to true grinding wheels with individually mounted diamonds. In the case of the annular piece of silicon carbide 141, the electrode will not true the abrasive but it will remove any metal load that may gather thereon. When eroding workpieces with relatively strong currents, that is to say 250 amperes per square inch of workpiece face, the front face of the annular piece of recrystallized silicon carbide 141 will stay true for a very long time. Eventually it can be dressed with a diamond tool but this will be infrequently necessary.

One advantage of using the recrystallized silicon carbide is that the resistivity is high so arcing due to local concentration of current is largely avoided. However equally in the case of the metal front plate 18, arcing is largely avoided because the pressure of the water flowing through the holes 22 eliminates direct metal to metal contact and thus the resistivity will never be so low at any particular place as to cause detrimental arcing. This therefore is an important feature of the present invention, involving hydraulic pressure keeping tool and workpiece spaced apart by a minute distance over the entire working area. Of course if an edge of the workpiece is presented to the wheel the water pressure may be insufficient to prevent direct contact but what will happen is what is wanted, namely quick removal of stock to produce a plane surface on the workpiece which can be converted into a round surface by swinging the workpiece. The machine may have the usual stop plate secured to the-bed plate 11, engageable by a template 146 secured to the tool holder 12.

Thus the machine, wheel and method provide protec tion against short circuits and elimination of detrimental arcing while nevertheless permitting a high rate of cut, and when there is an urge for fast cutting the erosion can be rapid whereas when a fine finished surface is wanted the erosion can be controlled. Thus in a sense the new erosion system is self-regulating.

It will be noted that the spindle 25 is insulated from the machine tool by the insulating sleeves 35 and 45. The belts 54 are made of rubber or some insulating material so no electric current can pass through them. The pipe 66 is insulated by the bracket 67 while the hose 73 is made of rubber. The accumulator 74, the pump '78 and the motor 84 and associated parts are insulated by the plate 76 while the pipe 79 is made of rubber and the tank 80 is also made of rubber. Thus while the electrolyte is a conductor no current will be lost through it via the inside of the tube 60. With regard to the dripping of electrolyte from the negative wheel 15 or 15a on to the grounded bed plate 11 actual experience has shown that the electric current loss through dripping electrolyte is so small as to be inconsequential, probably because of the discontinuity thereof.

It will thus be seen that there has been provided by this invention a machine, an eroding wheel and a method in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As many possible embodiments may be made of the above invention and as many changes might be made in the embodiments above set forth, it is to be understood that all matter hereinbefore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. The method of electricflly eroding electrically conductive work pieces which comprises providing a rotating electrically conductive wheel, connecting the rotating wheel and the work piece in :an electric circuit so that the work piece is positive and the wheel is negative, flooding the wheel with liquid electrolyte, holding the work piece in the air up to the wheel where it is flooded with electrolyte to close the electric circuit and cause electric current to flow therein through the electrolyte with the work piece positive and the wheel negative, and repelling the work piece from the wheel without opening the circuit through the electrolyte by forcing the electrolyte through the wheel under hydraulic pressure to emerge from a face thereof against the work piece.

2. A machine tool comprising a wheel head, bearings in said wheel head to journal a spindle, a spindle rotatably mounted in said bearings, said spindle having a passage lengthwise thereof, means to pump electrolytic liquid to and through said passage in said spindle, an electrically conductive wheel secured to said spindle, said wheel having radial passages leading to openings in the wheel located in a circular area, electric connections to make the wheel negative, an electrically conductive support fora work piece located to hold a work piece very closely adjacent to the wheel at the circular area thereof having the openings, electric connections to make the support positive, and means to rotate the spindle and therefore to rotate the wheel, whereby to erode work pieces electrically and at the same time to restrain the work pieces from actual contact with the wheel by hydraulic pressure,

3. A machine tool as claimed in claim 2 having an insulated tank for the electrolytic liquid, the means to pump being likewise insulated and said machine tool having insulated connections from the tank to the pump and from the pump to the spindle to convey said electrolytic liquid from the tank to the Wheel, whereby the machine as a whole can be at ground potential with the wheel negatively charged avoiding any substantial current loss through the electrolytic liquid that extends from the tank through the pump to the Wheel.

4. A machine tool comprising a Wheel head, hearings in said wheel head to journal a spindle, a spindle rotatably mounted in said bearings, said spindle having a passage lengthwise thereof, an electrically conductive wheel secured to said spindle, said wheel having radial passages leading to openings in the wheel located in a circular area, an electrically conductive support for a work piece located to hold a work piece very closely adjacent to the wheel at the circular area thereof having the openings, means to pump electrolytic liquid into said spindle, said means being electrically insulated from the machine base, and electrical connections to the wheel to charge the wheel with negative electricity.

5. A machine tool comprising a wheel head, a spindle journalled in said wheel head, an electrically conductive wheel on said spindle having a circular area with openings therein, a pump and connections from the pump through the spindle to the openings in the circular area to pump electrolytic liquid through said openings, means to charge said'wheel negatively electrically, a support for a work piece locatedtohold a work piece very closely adjacent to the wheel at the circular area thereof having theopenings, and means to charge said support positively electrically relative to said wheel.

6. A machine tool as claimed in claim 5 having an insulated tank for the electrolytic liquid, the pump being likewise insulated, and having insulated connections from the tank to the pump and from the pump to the openings, whereby the machine as a whole can be at ground potential with the wheel negatively charged avoiding any substantial current loss through the electrolytic liquid that extends from the tank through the pump to the wheel.

7. A machine tool as claimed in claim 5 having in the circular area of said wheel a porous annulus the pores of which constitute said openings whereby the electrolytic liquid is pumped through the porous annulus which is part of the conductive wheel and is inset therein, said porous annulus being itself electrically conductive.

8. A machine tool according to claim 7 in which the porous annulus besides being electrically conductive is also abrasive.

9. A machine tool according to claim 8 in which the porous annulus is made of recrystallized silicon carbide.

, References Cited in the file of this patent UNITED STATES PATENTS 387,467 Webster Aug. 7, 1888 1,095,893 Landreth May 5, 1914 2,142,512 Hartel Jan. 3, 1939 2,385,198 Engle Sept. 18, 1945 2,433,018 Ronay Dec. 23, 1947 2,526,423 Rudortf Oct. 17, 1950 2,532,907 Hangosky Dec. 5, 1950 2,718,581 Thomas Sept. 20, 1955 FOREIGN PATENTS 16,475 Great Britain of 1912 18,643 Great Britain of 1899 OTHER REFERENCES Report #MAB-18M, National Research Council, Washington 25, D. C., Jan. 18, 1952. Appendix VI cited.

Claims (1)

1. THE METHOD OF ELECTRICALLY ERODING ELECTRICALLY CONDUCTIVE WORK PIECES WHICH COMPRISES PROVIDING A ROTATING ELECTRICALLY CONDUCTIVE WHEEL, CONNECTING THE ROTATING WHEEL AND THE WORK PIECE IN AN ELECTRIC CIRCUIT SO THAT THE WORK PIECE IS POSITIVE AND THE WHEEL IS NEGATIVE, FLOODING THE WHEEL WITH LIQUID ELECTROLYTE, HOLDING THE WORK PIECE IN THE AIR UP TO THE WHEEL WHERE IT IS FLOODED WITH ELECTROLYTE TO CLOSE THE ELECTRIC CIRCUIT AND CAUSE ELECTRIC CURRENT TO FLOW THEREIN THROUGH THE ELECTROLYTE WITH THE WORK PIECE POSITIVE AND THE WHEEL NEGATIVE, AND REPELLING THE WORK PIECE FROM THE WHEEL WITHOUT OPENING THE CIRCUIT THROUGH THE ELECTROLYTE BY FORCING THE ELECTROLYTE THROUGH THE WHEEL UNDER HYDRAULIC PRESSURE TO EMERGE FROM A FACE THEREOF AGAINST THE WORK PIECE.

Images