US3093464A - Magnetic abrasive wheels, method of making them, and machine tools for using them - Google Patents

Description

June 11, 1963 COES, JR 3,093,464

MAGNETIC ABRASIVE WHEELS, METHOD OF MAKING THEM, AND MACHINE TOOLS FOR usmc THEM Filed Jan. 16, 1959 AMI.

if M 8 INVENTOR /6 L OE/NG C055 JE.

United States Patent M 3,693,464 MAGNETIC ABRASIVE WHEELS, METHUD 0F MAKING THEM, AND MACHHJE TGOLS FOR USING TIEM Luring Coes, Jr., Brookfield, Mass, assignor to Norton Company, Worcester, Mass, a corporation of Massachusetts Filed Jan. 16, 1959, Ser. No. 787,216 4 Claims. (Cl. 51-295) The invention relates to magnetic abrasive wheels, method of making them and machine tools for using them.

One object of the invention is to provide a grinding wheel or lapping wheel of good vitrified abrasive characteristics which :has a good magnetic permeability, i.e. will affect a magnetic field. Another object of the invention is to provide a grinding or lapping wheel of high magnetic susceptibility which also has good vitrified abrasive characteristics. Another object of the invention is to provide a simple process for the manufacture of such wheels and other abrasive products. Another object is to provide an abrasive wheel for use in precision grinders and lapping machines of magnetic properties to act as a component in controlling the operation of the machine. Another object of the invention is to provide dense wheels and wheels of fine abrasive size with the characteristics indicated. Another object is to provide organic bonded grinding wheels and other abrasive products which are magnetic. Any organic bond can be used such as phenolic resin, shellac, natural and synthetic rubber. Another object is to provide magnetic grinding wheels or other abrasive products bonded with inorganic bonds which mature at a low temperature. Examples are magnesium oxyc-hloride and silicate bonds.

Another object is to provide machine tools and electronic components with a magnetically susceptible abrasive wheel for controlling grinding operations. Another object is to provide a combination of instrumentalities including a magnetically susceptible abrasive product for controlling an abrasive operation with precision.

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

In the accompanying drawings,

FIGURE 1 is a magnified sectional view of an abrasive wheel according to the invention,

FIGURE 2 is a block and wiring diagram of a thread grinding machine to illustrate one application of the abrasive wheel of the invention,

FIGURE 3 is a cross sectional view of a furnace which can be used to produce the abrasive wheel of the invention.

Although abrasive grains are bonded with many different kinds of bond to produce grinding wheels and lapping wheels, for use in machine tools for precision grinding and lapping, the vitrified bond has for many years been preferred. Abrasive grains are bonded with vitrified bond by mixing them with any of a combination of clays, fluxes such as feldspar, frits (crushed glass of known composition according to formula for the production of bond of specific composition). Vitrified bond is a combination of oxides, with usually a large proportion of silica so that most vitrified bonds are silicates. They are analyzed as a combination of oxides of which, besides silica SiO alumina A1 0 is usually prominent along with MgO, CaO, Na O, K 0, B 0 and frequently some TiO Iron oxide is usually kept at a low figure because of its excessive fluxing properties and because it discolors the wheels irregularly. It also gives a bond somewhat unpredictable characteristics in firing.

The invention provides a thoroughly practical vitrified bonded abrasive wheel, a grinding wheel or a lapping wheel or according to any other designation, of free cutting precision quality which has at the same time 3,093,464 Patented June 11, 1963 2 magnetic properties, that is it is permeable and will affect a magnetic field. It is easy to manufacture as the following description will show. The magnetic material does not deleteriously atfect the grinding, lapping or other abrasive properties.

According to the invention the magnetic material may be magnetite, Fe O This is sometimes said to be a combination of one mol of FeO plus one mol of Fe O but is usually written Fe O This oxide is magnetic.

I reject iron because it too readily oxidizes to a nonmagnetic oxide and furthermore its presence in the .pores of a grinding wheel would cause smearing of the work piece being ground which would be undesirable. The magnetic metals cobalt and nickel, singly or in combination, can however be used as they do not have bad smearing properties and furthermore dont readily oxidize to non-magnetic oxides.

Instead of magnetite Fe O I can use cobaltous cobaltic cobalt oxide, C0 0 or cobalt ferrite, CoO-Fe O or nickel ferrite NiO-Fe O Nickel doesnt have any magnetic oxide of its own. Mixtures of the twometals with the three oxides in any combinations can be both produced and used.

To produce a grinding wheel according to the invention or to produce any other type of abrasive wheel according to the invention, I first procure a vitrified grinding wheel manufactured in accordance with any known technique. Nearly all vitrefied abrasive wheels are porous to some extent, enough for the present purposes.

As the greater part of precision grinding and lapping is done with vitrefied wheels having fused alumina abrasive which means that the abrasive is alumina A1 0 of the alpha variety which has been fused in an electric furnace, I will first describe the invention in connection with such a wheel.

EXAMPLE I I procured a grinding wheel which was made of white aluminum oxide abrasive grains of a purity of better than 98%, half of it being 280 grit size and the other half (by weight) being 320 grit size, and having a volume percentage of abrasive of 47.3%, a volume percentage of bond of 15.0%, and a volume percentage of pores of 37.7%. These pores were interconnected. The bond formula is given in the following table.

This wheel was 12" in diameter, /1" thick and had a 3" central hole. It was made in the usual way, by coating the abrasive grains with a water solution of dextrine and then mixing therewith the bond in dry powdered form, molding, firing.

I placed this wheel in a controlled atmosphere furnace heated to 400 C. A stream of nitrogen gas saturated with iron carbonyl vapor was passed through the furnace at the rate of two litres per minute for two hours. Then the furnace was flushed out with nitrogen and steam was passed through it at the rate of two litres per minute for two more hours, the temperature still being held at 400 C. The furnace was then allowed to cool and when the aceaaee wheel was cool it was complete. Naturally more than one wheel will be made at one time.

FIGURE 1 shows the condition of this wheel. The fused aluminum oxide abrasive grains of 280 and 320 grit size are indicated by the numeral 1. The bond is indicated by the numeral 2, the pores by the numeral 3. The coating of mangetite Pe o, on the pore walls is indicated by the numeral 4 and the wheel as a whole is indicated by the numeral 5. This wheel is sufliciently magnetically permeable for the purposes of this invention.

The furnace used in making the wheel of Example I is shown in FIGURE 3. It consisted of a chamber comprising a bottom 6 and a top 7 connected by a sand seal 8 with an inlet pipe 9 an outlet pipe 10 and a bar 11 to hold the wheels 5. The inside diameter of the bottom 6 was 24" and the overall height of the furnace was 44" and the other dimensions were about in proportion thereto as shown in FIGURE 3. All parts except the sand were made of stainless steel. The furnace was cylindrical.

Wheels according to the invention are useful for controlling grinding operations as illustrated in the diagram of FIGURE 2. In FIGURE 2 the Work piece 12 is being ground by a grinding wheel 5 made in accordance with the invention. In Example I the manufacture of a straight wheel was described, but this can be trued to a shape like that shown in FIGURE 2 and this is the way to make such a shape. The wheel 5 is mounted on a spindle 13 which is rotated as by means of belts 14. As FIGURE 2 is simply a diagram, a feed nut 15 is represented which is connected to a slide 16 to move the spindle 13 forward and back in the direction of the work, the spindle 13 being journalled on the slide 16. The nut 15 is moved by a screw shaft 17 driven by an electric motor 18. Referring now to the lower right of FIGURE 2 and working to the left and up, the armature of the motor 18 is energized by power lines 20 through a double relay switch 21, another double relay switch 22, lines 23, a slide controlled switch 24 and lines 25. The slide controlled switch 24 has an insulated element attached to the slide itself which, when it withdraws to a certain point, opens the circuit as diagrammatically indicated. In this description the abrasive product is moved relative to the work piece as a result of the action of the magnetic sensing device, but since it is only the relative motion between the abrasive and the work that is necessary, it is obviously possible operatively to position the sensing device to move the work piece relative to the abrasive, or to move both, in order to achieve the desired result.

Referring now to the upper right hand part of FIG- URE 2 and working to the left, the stopping and starting of the motor 18 is controlled by power lines 28 to an ofl? and on control 29 energized by an amplifier 30 which receives its signal from a bridge 31 energized by an oscillator 32. The bridge 31 is connected by wires 34 and 35 to a magnetic sensing device 36. This magnetic sensing device carries more current with a given E.M.F. when the wheel 5 comes nearer to it on account of the magnetic permeability of the wheel 5. When the device 36 carries more current, the bridge 31 has a greater output. The input to the bridge 31 is from the oscillator 32. The circuit of the bridge 31 is from the wire 34 to the device 36 to the wire 35 through a variable resistance 40 through a resistance 41 and a variable condenser 42 through a variable inductance 43 and back to the wire 34. As controlled by the setting of the bridge 31, when the wheel 5 has moved a certain distance into the work piece 12, the output from the bridge 31 is amplified by the amplifier 30 to a high enough value to cause the off and on control 29 to actuate the relay solenoid 44 which opens the switch 22 and stops the infeed. But when the wheel 5 has been worn away some thus becoming of less diameter, the Spindle 13 has to advance farther to actuate the ofi and on control 29. The face of the wheel 5 will be at the same spot. In thread grinding machines, the grinding wheels are trued at frequent intervals, but this mechanism in combination with a magnetically susceptible wheel lbrings the wheel every time to a position to cut the thread in the work piece 12 to the same depth. The magnetic sensing device 36 is one or more coils or the equivalent.

The arrangement shown in FIGURE 2 is particularly useful for thread grinding of all kinds and can also be used for surface grinding and cylindrical traverse grinding. Since this invention is in a grinding wheel structure and a method of producing it I dont need completely to describe the machine, since the above description is merely to show the utility of the wheel and such machines actually exist.

However the machine should have a circuit to cause the slide 24 to withdraw at the end of a grinding operation. Referring to the lower right of FIGURE 2, a push button switch 45 is connected by lines 46 to the lower lines 20 and when closed, through lines 47 energizes a relay solenoid 48 which closes a double relay switch 49 and opens the double relay switch 21. The double relay switch 49 is connected by lines 50 to the power lines 20 and by lines 51 to the lines 23, and it will be seen that the direction of the current is reversed through the double relay switch 49 as compared with through the double relay switch 21. The motor 18 is a reversible motor. When the push button switch 45 is closed the armature circuit is reversed so that the slide 16 retreats, and when it gets to a certain position the switch 24 opens the circuit and the motor 18 stops thus stopping the withdrawal of the slide. Referring to the bottom middle left of FIG- URE 2, to start the machine up again the operator momentarily presses a push button switch 55 which connects lines 56 to lines 57, the former being connected to lines 23 and the latter to lines 25. Later the circuit is reestablished through the slide switch 24. Similarly, my combination of a magnetic abrasive product and a magnetic sensing device operating relatively to feed the abrasive member and work piece toward and away from each other may be readily applied to abrasive operations involving segments, discs, sticks and other shapes in surface grinding and other types of abrading operations as well as cylindrical grinding.

EXAMPLE II 1 coated pores of the same kind of a grinding wheel as specified in Example I with magnetite, Fe O in the following manner:

The wheel, the same size and composition and volume percentage of abrasive bond and pores 'as in the case of Example I, was placed in a tightly fitting can, open at the top, and was covered with a 30% water solution of ferric nitrate, Fe(NO although other water soluble salts of iron could be used. This can was then placed in a closed container having an atmosphere of ammonia and the can with the wheel in it immersed in the ferric nitrate solution was kept in this atmosphere of ammonia for three days. The ammonia NH reacted with the water to produce ammonium hydroxide NH OH, which reacted with the ferric nitrate to produce ammonium nitrate NH NO and iron hydroxide Fe(OH) This caused the precipitation of the iron hydroxide in the pores of the wheel leaving a water solution of ammonium nitrate also in the pores with very little else. This wheel was then dried at C. for twenty-four hours which precipitated the ammonium nitrate into the pores.

The wheel was then fired in a muffle furnace at 550 C. for four hours during which time an atmosphere of steam was maintained in the furnace. The steam removed the ammonium nitrate and converted the ferric hydroxide to magnetite Fe O This wheel weighed about 6% more than it did originally and had a magnetic susceptibility of 40x110- c.g.s. units. This wheel is also fully illustrated in FIGURE 1 and is usable as already described for the wheel of FIGURE 1. The furnace of FIGURE 3 is a mufile furnace which is used to fire this wheel of Example II. For purposes of complying with the patent statute, this example is selected as representing the best mode for vitrified bonded wheels. I

EXAMPLE III Vitrified grinding wheels have their pores coated with cobalt ferrite Coo-R by proceeding as in Example II substituting a solution containing cobaltic nitrate and ferric nitrate for the 30% ferric nitrate of Example II. All other steps are identical.

EXAMPLE IV In order to coat the pores of la vitrified wheel with nickel ferrite, NiO-Fe o I proceed as in Example II substituting a water solution containing 10% of nickel nitrate and 20% :of ferric nitrate.

EXAMPLE V In order to coat the pores of [a vitrified wheel with cobaltous cobaltic oxide, C0 0 I proceed as in Example 11 but use a 30% water solution of cobaltic nitrate, am- 3h- All of these wheels are magnetic, but I have only measured the magnetic susceptibility of the wheel made according to Example II. This had a magnetic susceptibility of 40x10" c.g.s. units. For use in accordance with this invention a magnetic susceptibility of at least 10- c.g.s. units may be satisfactory for operating some equipment of this type. While there is no upper limit, it would be extremely diificult to coat the pores of a grinding wheel to give it a magnetic susceptibility of more than 600x 10- c.g.s. units. The examples herein produce wheels having a susceptibility of more than l0 l0- c.g.s. units which is desirable.

EXAMPLE VI For the manufacture of a grinding wheel having in the pores thereof cobalt metal, I may proceed in accordance with Example I using cobalt carbonyl vapor instead of iron carbonyl vapor, stopping the process after passing the cobalt carbonyl vapor through the furnace in the stream of nitrogen gas. This causes the deposit of cobalt in the pores of the wheel.

EXAMPLE VII For the manufacture of a grinding wheel having in the pores thereof nickel metal, I may proceed in accordance with Example I using nickel carbonyl vapor instead of iron carbonyl vapor, stopping the process after passing the nickel carbonyl vapor through the furnace in the stream of nitrogen gas. This causes the deposit of nickel in the pores of the wheel.

EXAMPLES VIII AND IX In order to coat the pores of a wheel with cobalt, I may proceed 'as in Example II using a 30% water solution of cobalt nitrate (instead of ferric nitrate) firing the wheel in the mufiie furnace at 550 C. for four hours in an atmosphere of nitrogen without any steam. The cobalt hydroxide is converted to cobalt metal and the ammonium nitrate is driven off. In the same way the pores of a wheel can be coated with nickel, substituting nickel nitrate for the ferric nitrate of Example LI and proceeding as in Example II through the firing in the muffle furnace in an atmosphere of nitrogen without any steam.

FURTHER EXAMPLES Mixtures and complexes of the oxides and the metals can be deposited on the surfaces of the pores of a grinding wheel by proceeding in accordance with the principles of Example II and using a single nitrate mentioned herein or mixtures of these nitrates and firing in the muffle furnace for a short time in steam, the rest of the time in an atmosphere of nitrogen. Mixtures of metals can be deposited using mixtures of nitrates and [firing in nitrogen without steam.

I have given 400 C. as the temperature for heating the abrasive wheel in carbonyl vapor. This temperature must be above about 150 C., which is the approximate decomposition point of the carbonyl vapor to form metal. The upper limit for use in my process is indefinite because other factors come into consideration such as the initial temperature at which the wheel was fired, the ease of apparatus operation, etc. ordinarily to avoid premature decomposition of the carbonyl, the temperature should not be much above 500 C. However the grinding wheel should not be deformed by the heat. The non-oxidizing atmosphere mentioned throughout is nitrogen gas. However any other non-oxidizing atmosphere can be used including the inert gases, argon, helium, etc. The oxidizing atmosphere specified herein is steam. However other oxidizing atmospheres such as oxygen itself can be used. Ordinary air can be used but more time would be needed to effect the conversion.

For coating the pores of vitrified wheels using nitrate solutions, I have specified 550 C. for firing the wheels in a muffie furnace to convert the hydroxide to magnetic oxide or metal or combination. Ordinarily, a temperature above 400 C. would be used to decompose the hydroxide and temperatures above about 1000 C. are undesirable because of possible loss of magnetic properties through the formation of ferrous oxides.

It has recently been found that for precision grinding if high wheel speeds are used as measured in surface feet per minute (s.f.p.m.), organic bonded grinding wheels give superior performance in many cases, such as in the thread grinding described. Organic bonded grinding Wheels, especially phenolic resin bonded grinding wheels, can be operated at higher speeds measured in s.f.p.m. than vitrified bonded wheels because the former are stronger. This more than overcomes the superior cutting rate of vitrified bonded wheels at a given s.f.p.m. This has caused the development of new grinders capable of operating at higher wheel speeds, but these are now available in some quantities.

So far as the machine tool combination of the present invention is concerned, or other abrasive member, the grinding wheel can be made with various bonds. Of the organic bonds, phenolic resin is usually preferred for grinding operations of the type herein disclosed. This wheel is made by incorporating into the wheel mixture any one or combination of the magnetically susceptible materials previously listed in sufiicnient quantities to give the wheel a magnetic susceptibility of at least 10- c.g.s. units.

EXAMPLE X A wheel 8" in diameter, /2" thick with a /2" central hole was made in the following manner. The ingredients were:

Weight/VOL, 2.92.

The fused alumina of mesh grit size), was first wet with 20 cc. of resinper pound of bond, then phenolic resin which was in powder form and of the brand BR2417 together with the magnetite and the calcium oxide (to take up water during curing) were added and thoroughly mixed whereupon 10 cc. of 'anthracene oil per pound of bond was added to settle the dust. This mixture was cold molded in a closed mold and baked for twenty-four hours at C. The wheel had a magnetic susceptibility of about 60x10 c.g.s.

For purposes of complying with the patent statute, this Example X is selected as the best mode for organic bonded wheels.

Other magnetic material besides those already mentioned can be used in this phase of the invention. Among these are various magnetic ferrites, iron and its alloys, aluminum-nickel-cobalt alloys, copper-manganese alloys and rare earth garnets can be used. They are preferably provided in finely powdered form to smooth out mixing procedures and avoid difiiculties of poor distribution. The reason why many other magnetically susceptible materials can be used is that the temperature of baking is very low in comparison with the temperature of vitrifying. Since wheels are usable in the invention if they have the mum magnetic susceptibility of at least 10 c.g.s. units, it is unnecessary to define a complete group of the magnetic materials which do not come under any recognized classification with respect to reactivity properties.

Other commonly used organic bonds can be employed to make grinding wheels or other abrasive products as components of the apparatus of the invention. Such bonds include shellac and natural or synthetic rubber. Also such inorganic bonds for grinding wheels as magnesium oxychloride and silicate, by which is meant bonds based on the use of alkali silicate as a primary bonding ingredient in the mixture, may be employed, and the wheels are made magnetic by the same procedure used for organic wheels. These wheels are made by methods known to the art with the addition of the magnetically susceptible material as a filler. Metal bonded wheels are readily made magnetic by using magnetic metals as a constituent of the bond.

It will thus be seen that there has been provided by this invention magnetic abrasive wheels and other abrasive products, methods of making them and machine tools for using them 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. A porous vitrified grinding wheel having on the surfaces of the pores thereof magnetically susceptible material seleoted from the group consisting of magnetite Fe O cobaltous cobaltic oxide C0 0 cobalt ferrite CoO-Fe O nickel ferrite NiO'Fe O cobalt and nickel and mixtures thereof, and the wheel having a magnetic susceptibility of at least 10- c.g.s. units.

2. A porous vitrified grinding wheel according to claim 1 in which the material is magnetite.

3. A porous vitrified grinding wheel according to claim 1 in which the material is cobalt.

4. A porous vitrified grinding wheel according to claim 1 in which the material is nickel.

References Cited in the file of this patent UNITED STATES PATENTS 1,187,431 King June 13, 1916 2,038,727 Geyer Apr. 28, 1936 2,052,194 Sandorff Aug. 25, 1936 2,114,160 Whitacre Apr. 12, 1938 2,114,166 Leeuw Apr. 12, 1938 2,125,782 Heald Aug. 2, 1938 2,164,476 Scutt July 4, 1939 2,377,995 Coes June 12, 1945 2,426,139 Bishop et al Aug. 19, 1947 2,657,505 Price Nov. 3, 1953

Claims (1)

1. A POROUS VITRIFIED GRINDING WHEEL HAVING ON THE SURFACES OF THE PORES THEREOF MAGNETICALLY SUSCEPTIBLE MATERIAL SELECTED FROM THE GROUP CONSISTING OF MAGNETITE FE3O4, COBALTOUS COBALTIC OXIDE CO3O4, COBALT FERRITE COO.FE2O3, NICKEL FERRITE NIO.FE2O3, COBALT AND NICKEL AND MIXTURES THEREOF, AND THE WHEEL HAVING A MAGNETIC SUSCEPTIBILITY OF AT LEAST 10**6 C.G.S. UNITS.

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