US2141155A - Method for producing abrasive articles - Google Patents

Description

Dec. 20, 1938.

E. s. MERR|AM METHOD FOR PRODUCING ABRASIVE ARTICLES Filed April 28, 1938 Patented Dec. 20, 1938 UNITED STATES PATENT OFFICE Edmund S. Merriam, Marietta, Ohio Application April 28, 1938, Serial No. 204,849

5 Claims.

This invention relates to a method for producing abrasive articles and more particularly to the production of abrasive grinding wheels.

The method of the present invention is concerned with the production of abrasive wheels in which the abrasive grains are bonded together by a vitreous bond. I

In my prior United States Patent No. 2,050,112, I have disclosed an advantageous method of producing abrasive wheels of this type. The present invention is concerned with certain improve ments upon the process disclosed in my said issued patent.

It is the purpose and object of the present invention to provide a method for controlling the conditions under which the abrasive wheels are fired or burned so as to insure high efficiency in the production of wheels of uniform quality and with low losses due to rupture.

In accordance with the teachings of ny patent above referred to, the abrasive wheels'are formed by first molding a mixture of suitable abrasive.

grains, a vitrifiable bond, a temporary bonding agent, and combustible fuel effective when burned 25 to re-fuse the vit rifiable bond into the form of a green wheel. /A series of these wheels is then stacked in a retort, and the combustible material "therein is ignited and burned to fuse the contained vitreous bond.

30 Combustion is effected by embedding the molded and previously dried wheels in sand or grain which also contains fuel. The upper part of the stack is then ignited and combustion of the fuel effected by drawing a current of a combustion 3 supporting gas through a series of wheels untilall of the fuel has been burned. The combustion of the fuel generates heat within the wheels, raising the temperature to a point where the bond softens and binds the grains of the wheel to- 40 gether. After the burning operation has been completed, the air stream is allowed to continue to flow through the stack and thereby effect a gradual cooling of the wheels.

In this operation it has been found that, as the 45 burning of the fuel in the stack of wheels in the retort progresses, that portion of the stack from which the fuel has already been burned is immediately subjected to the cooling effect of the combustion supporting gas which is being drawn 50 into the stack to support combustion of the fuel at the more advanced zone of burning. This has a tendency to effect a premature cooling of the first burned wheels, frequently causing strain suflicient to crack them.

In accordance with the present invention, in-

equalities in the cooling of the different wheels are avoided, and wheels of true uniform character are produced throughout the stack..

In accordance with the present process, I preferably burn a secondary fuel in the retort in such 5 relationship to the wheels that no objectionable premature cooling occurs. Additionally, I conductthe final cooling operation after the wheels have been burned under controlled temperature conditions such' as to insure the production of superior and uniform wheels.

In the foregoing discussion, I have particularly referred to the progressive burning of wheels in a stack. It will be appreciated that in the production of exceedingly large wheels, the burning of the contained combustible material for fusing the bond will occur progressively through different portions of the wheel. The principles of the present invention are applicable as well to such operations as they are to the burning ofv successive wheels in a stack.

Coming now to a detailed description. of one application to the present invention, reference is made to the accompanying drawing illustrating a suitable retort for carrying out the process of the invention, in which like reference characters designate like parts. 7

When the invention is applied to that type of operation wherein a stack of preformed green abrasive wheels containing combustible material. is burned, there is employed a retort I provided with sui. able trunnions 2 to permit of tilting or purposes of emptying the charge therefrom. The retort I is advantageously provided at the bottom with a suction line 3 leading-to a suitable 5 suction pump, not shown. Interiorly of the retort I, there is supported a perforated circular plate 4 which is positioned several inches above the bottom of the retort and supported by standards 5. The circular disc 4 is surrounded by a short cylinder 6 which functions to form beneath the disc 4 a circular chamber having an area co-extensive with that of he disc. Supported upon the disc4 is a second cylinder I which forms the inner wall of the retrt. The space between the uter wall of the retort I and the cylinder -1 is packed with a suitable insulating material, such as infusorial earth, which is held in place by an annular cover member 8. The top of the retort is adapted to be covered by a removable funnel-shaped top 9 open to the atmosphere as shown at Ill and carrying an annular rose II in the form of a burner adapted to function either as a gas-injecting device or as a burner.' The rose II is provided with a suitable connection l2 adapted to lead to an appropriate source of combustible gas. There is additionally shown a separate pipe-like cylinder l3 which may be employed for a purpose hereinafter described.

There is preferably placed upon the disc 4 a fine screen of nichrome wire to prevent sifting of the packing sand or abrasive grains down into the cylindrical chamber below the disc 4. d

When the retort is to be charged, a layer of abrasive grains or sand having a depth of several inches is first placed upon the disc 4. The pre-' viously dried green wheels are then piledggrto the retort one on top of the other. Each wheelpreferably has molded therewith a thin layer of packing grains on one side which is free from any permanent bonding material and functions to facilitate separation of the individual wheels after burning. After the wheels are placed in the retort, there is packed in the central hole of the column and around the periphery thereof a mixture of sand or abrasive grains and fuel. This packing material holds the stack of wheels in position and compels the current of combustion supporting gas to pass through the column of wheels. The top of the stack of wheels is also covered over with a layer of packing grains carrying incorporated therein a supply of combustible fuel.

Ignition of the combustible fuel may be effected in numerous waysz- In one method of operation, the burner ll inside of the funnel-shaped cover 9 is supplied with gas and ignited. It is preferable to dust over the top of the packing grains in the retort with a layer of charcoal. When the pump is started and suction applied on the suction line 3 at the bottom of the retort, air is drawn into the open end H) of the funnel-shaped cover 9 and passes down through the column. When the burner is lighted, the flames play upon the top charcoal dusted surface and ignite the.

entire surface in the course of a few minutes.

After ignition, in the above manner, the gas fiame may be extinguished and air alone may be drawn into the retort for some time, say for approximately one-half hour, by which time the combustion of the contained combustible material at the top of the column will have reached a depth of around two inches. The burning of the contained combustible material has at this time begun its progressive descent through the retort and the upper surface will have cooled below a red heat. Gas, which is preferably natural gas, although other gases and vapors may be used, is again admitted in controlled amount through the burner II, but the gas is preferably not lighted at the burner. The gas mixes with the incoming air, and the air-gas mixture is drawn down into the column. The gas oxidizes when it reaches material having a temperature of 1350 F. and burns at this point. The funnel-shaped cover 9 serves as a mixing chamber for the air and gas. There is never any danger of explosion in the operation for two primary considerations: first, the gas-air mixture is so lean that it is below the explosive limit for the gas'employed, and, second, the temperature of the topmost layer of packing sand or grains will have fallen below 350 F. before the gas is admitted, and this low temperature layer of granular material prevents the passage of flame into the air-gas mixture just as the screen in afsafety lamp acts) to prevent back-firing of such a mixture.

The burning of the combustible fuel in the wheels and the surrounding packing grain continues to progress downwardly through the stack of wheels and to successively melt the bond contained in the wheels. The oxidation of the gas in the upper portion of the column acts to preheat all of the air which passes down to support combustion in the lower zones of the column.

about twenty per cent of the oxygen content of the incoming air. The ignition temperature of CH4 in air is about 1350 F. The gas will not oxidize until it reaches a point in the mass of hot grains where a temperature of 1350 F. prevails. The gas then oxidizes completely at this point, and a zone is formed where the temperature is maintained at a fairly constant point. All of the air employed to sustain combustion in the lower regions of the retort must, of course, pass through this zone. The location of the zone of burning of the gas is under close control and can be varied by merely varying the ratio of the gasair mixture.

Due to the close proximity of the peripheral portions of the wheels to the retort walls, there is some tendency for the peripheral zone to burn at a somewhat lower temperature than the central portion of the wheels. With adequate insulation, this temperature gradient from center to rim may be kept quite small. I have. found, however, that substantially constant temperature conditions throughout the cross-sectional area of the stack can be maintained by employing the cylindrical baflle or shield l3 in the relationship shown in the drawing. The placing of this cylindrical shield or bailie I3, the proper size for which may be determined by trial, enables the central parts of the retort to be supplied with an air-gas mixture in which the'proportion of air will be greater than in the air-gas mixture supplied to the peripheral portions of the retort. In this manner, more of the heat of oxidation of the gas will be developed around the periphery of the column and thus tend to offset lateral loss of heat.

It will be appreciated that the supply of the auxiliary fuel in the form of a gas in the man- 'ner hereinbefore described, in addition to affordinga method for effecting a precision control over the temperature conditions prevailing in the various portions of the retort, likewise increases the efllciency of the operation by supplementing the heat made available through the'burning of the combustible material incorporated in theretort charge.

It is possible to so control the conditions of operation that the final condition, after the last wheel in the stack has been burned, may be one in which the temperature from top to bottom of the stack is practically uniform. This insures that the wheels will all be standard and-conform to uniform specifications.

It is also possible to obtain good results by employing the rose H as a gas burner throughout the operation. In such operation, no attempt oxidized at the burner II and by suitably regulating the quantity of gas supplied, it is possible to maintain the temperature of the incoming air or combustion supporting gas within any desired range. Since the oxygen required for oxidizing the gas is but a fraction of the oxygen content of the air, ample excess oxygen is present for supporting combustion of the fuel incorporated in the retort charge.

Regardless of whether the gas is ignited at the rose burner H or whether it is oxidized in the top layer of grains above the topmost wheel, it is preferable to keep the temperature of the combustion supporting gas which reaches and passes through the zone occupied by the upper wheels in the retort at a temperature at least.

approximating the upper limit of the annealing range for the particular bond employed. It will be appreciated that the annealing range varies depending upon the precise vitreous bonding agent which has been incorporated in the green wheels.

In either operation, that is, where the gas is ignitedat the rose burner H or where it is oxidized within the top layer of grains, the operation is effective to preclude premature or shock cooling of the first burned wheels and to permit maintaining of controlled temperatures throughout the stack of wheels during the burning operation.

Thus far I have described the burning operation as conducted in accordance with my improved method. When all of the wheels in the stack have been burned, that is, after the combustible material in the retort charge has been wholly consumed, the cooling operation is instituted. When the gas is being oxidized in the retort charge, this is accomplished by temporarily shutting off the supply of gas through the burner l l to quench the oxidation of the gas in the retort charge, whereupon gas is again supplied and ignited at the burner. Proper control over the temperature of the cooling medium may then be effected by regulating the amount of air mixed with the products of combustion resulting from the burning of the gas at the burner H. The products of combustion together with any desired amount of air are drawn down through the charge to effect cooling of the wheels. In this manner it is possible to send through the wheels a current of air of any desired temperature. By regulating the ratio of air to combustion gases,

it is possible to employ any suitable temperature throughout the cross-sectional area of the retort charge and prevent diiferential cooling of different portions of the wheels. The factors of size of the cylindrical shield l3 and the proper velocity of air and gas can be determined for any given set of conditions by trial, during which thermocouples can be located inside the central hole in the stack of wheels in any given charge and also in the packing grains surrounding the periphery of the wheels. Once appropriate conditions of operation are established for a given type of charge, i. e., wheels of a given size, close adherence to the determined conditions will insure the attaining of a uniform product for each operation without the necessity for close observation of instruments during the operation.

In this way it is possible to effect the burning as well as the cooling of the wheels under precision control and in such manner that all the 7 wheels of each stack burned will be of absolutely uniform quality, and difliculties due to differential cooling of different portions of the wheels and of different wheels in the stack are obviated.

What I claim is:-

1. A method for burning a stack of preformed green abrasive wheels containing combustible material effective on burning to fuse the contained vitrifiable bond in said wheels, comprising embedding said stack of green wheels in a bed of inert granular material in a retort, initially igniting the combustible material contained in the retort charge at one end thereof, and, as the zone of burning of the combustible material in the retort charge progresses through the stack of Wheels, eifecting oxidation of a gaseous fuel to the rear of the advancing zone of burning of combustible material in the retort charge to preclude premature coo-ling of the first burned wheels in the stack.

2. A method for burning a stack of preformed green abrasive wheels containing combustible material comprising embedding the wheels of the stack in a bed of inert granular material in a retort, igniting the combustible material contained in the retort charge at one end of the stack, effecting a draft of a combustion supporting gas through the retort charge from the end of the retort where the combustible material is first ignited toward the opposite end of the stack, and

concurrently oxidizing gas in said retort charge in a zone so arranged with respect to said stack of wheels as to preclude any premature cooling of the first burned wheels in said stack.

3. A method for producing abrasive articles comprising molding the article in the desired shape from a mixture comprising abrasive grains, a vitriflable bond, and combustible material effective when burned to fuse said bond, placing the article in a retort, igniting said combustible material at one face of the article and burning it progressively through the article, and concurrently oxidizing a second fuel in the retort at a point to the rear of the zone where the burning of said combustible material is occurring at any given time to thereby avoid premature cooling of said article.

4. A process for producing abrasive wheels comprising mixing suitable abrasive grains, a vitreous bonding material in finely divided form, a temporary binder, and a combustible material sufficient in amount to generate, when burned in situ, adequate heat to fuse said vitreous bonding material, molding a plurality of green wheels, placing said wheels in a retort, igniting said combustible material at one end of the retort, effecting the burning thereof progressively therethrough, during burning of said combustible material, flowing a combustion supporting gas through said retort, and maintaining said combustion supporting gas at a controlled temperature by oxidizing in the presence thereof a combustible gas in an amount inadequate to materially deplete the oxygen content of said combustion supporting gas.

5. A process for producing abrasive wheels comprising mixing suitable abrasive grains, a previously fused vitreous bonding material in finely divided form, a temporary binder, and a substantially non-residue producing combustible material suflicient in amount to generate, when burned in situ, adequate heat to again fuse said vitreous bonding material, molding a plurality of green wheels, placing said wheels in a retort, igniting and burning said combustible material, effecting the burning of said combustible mate- EDMUND S. MERRIAM.

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