US 3183632 A
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Description (OCR text may contain errors)
3 y 18, 1965 H. w. FERCHLAND 3,183,632
61 GRINDING TOOL Filed July 9, 1962 INVENTOR.
Unitcd States Patent 3,183,632 GRINDING TOOL Harold W. Ferchland, Birmingham, Mich., assignor to General Motors Corporation, Detroit, Mich, a corporation of Delaware Filed July 9, 1962, Ser. No. 208,304 4 Claims. (Cl. 51-406) The subject matter of this invention is an improved grinding tool and method for making same.
In recent years there have been increased requirements for fuel injection nozzles and other articles of manufacture having extremely close tolerance small diameter cylindrical passages. Also, it has been recognized in recent years that such passages must be accurately cylindrical throughout their entire length in order to perform their intended function most efficiently. A slightly oval sectioned bore can, for example, have a detrimental effect on fuel passage and atomization. Hence, need has arisen for improved methods and tools for accomplishing close tolerance cylindrical bores in metal parts. The present invention provides a greatly improved and yet inexpensive grinding tool which fulfills this need.
The commonly used method for forming an elongated, small diametered bore in a metal part consists of first drilling the bore and then grinding and polishing its surface to the desired smoothness. In practice, the latter step is performed by reciprocating longitudinally through the bore a grinding wheel having a slightly smaller diameter than that of the bore while simultaneously rotating the grinding wheel and the metal part. At the present state of the art the grinding wheels used for such purpose consist of a ceramic annulus working portion cemented to a metal shank. Since the outer diameter of the ceramic annulus itself must be small it will be obvious that the diameter of the supporting metal shank must be even smaller, thereby resulting in a shank which is comparatively flexible. This shank flexibility makes extremely difiicult the attainment of an accurate, truly cylindrical bore. As an attempt to circumvent this problem the metal shank can be made of a diameter almost as great as that of the ceramic annulus. However, this leads to a serious problem in that the cement which binds the ceramic to the metal shank becomes exposed at the outer surface of the ceramic, and the presence of such cement at the working surface is disadvantageous. Such cements are of an organic material and tend to soak into the inherently somewhat porous, ceramic material during tool manufacture. The grinding tool of the present invention, which will now be described by reference to the accompanying drawings, solves these problems.
In the drawings:
FIGURE 1 shows a perspective view of a grinding tool made in accordance with the invention; and FIGURE 2 shows a sectional view of the tool shown in FIGURE 1.
In accordance with the invention and as illustrated in the drawings, both the working portion 1 and the shank portion 2 of the grinding tool are of ceramic and form a monolithic or unitary structure thereby dispensing entirely with any requirement for a cement. The shank portion of the tool can therefore be of only slightly less diameter than the working portion. Also, the fact that ceramic inherently has a very high modulus of elasticity provides a shaft rigidity which is much greater than that attainable with the conventional metal shank type tool structure. Shafts with a modulus of elasticity in excess of 45 are simply attainable in accordance with the invention. As will be noted from the more specific description of preferred embodiments hereinafter set forth, the preferred ceramic for the grinding tools is a high alumina base material containing upwards of 85% by weight aluminum oxide. It is desirable that the working portion of the tool be of a relatively coarse grain porous structure and that the shank portion be of a more dense, nonporous fine grain structure so as to have high strength and a high modulus. This can be accomplished by first forming and firing the shank portion from an alumina raw batch which will provide optimum shank characteristics, inserting one end of the shank into a formed but yet unfired annulus formed of a relatively coarse grain alumina raw batch and then firing the assembly. During this firing operation the relatively coarse grain annulus sinters to its final desired structure and while doing so forms a strong sintered or fused bond with the end of the shaft. The result is a unitary ceramic tool, both the shaft and working portions of which have the optimum desired physical characteristics.
The preferred ceramic for the shank is one containing upwards of by weight aluminum oxide together with suitable grain growth inhibitors or flux ingredients such as silica and alkaline earth metal oxides. Such ceramics, after firing to sintering temperature, consist of a dense mass of alumina crystals bonded to each other either directly or through a thin interstitial layer of glass formed in situ during firing, and have outstanding strength and other desirable physical characteristics. Typical are the compositions disclosed by United State Patent 2,760,875 issued to Karl Schwartzwalder and Helen Barlett. Substantially alumina oxide ceramics or those containing only a small addition of a mineralizer or grain growth inhibitor such as MgO are equally good shank materials.
The ceramic composition for the relatively porous working portion of the tool may be selected from any of those conventionally used for grinding purposes. For example, it can be of substantially 100% sintered aluminum oxide or a glass-bonded aluminum oxide, the grain size of the aluminum oxide being selected to provide the exact abrading characteristics desired. Generally a relatively coarse grain porous structure is preferred. For manufacture of a glass-bonded aluminum oxide working portion the raw batch can consist of granular aluminum oxide such as Borolon or Tabular Corundum, and a small amount of powdered glass. Alternatively, the batch can consist of aluminum oxide together with small amounts of flux ingredients such that the glass is formed in situ during firing. It will be understood, of course, that ceramic abrasive other than aluminum oxide may be used if desired. When the tools are made by the preferred method of manufacture wherein the working annulus is fired onto a prefired shank, it is important that the firing temperature for the working annulus be less than that which would cause softening of the shank and, hence, the composition of the working annulus should be selected taking this factor into account.
The methods used for forming the shank and working portions of the tools can be any of those conventionally used in the ceramic art. For example, the shank portion can be formed by isostatic or other molding or by the extrusion technique wherein the ceramic batch is extruded to form a rod which is then cut to the desired size pieces and fired. The working portion can be cold-pressed in a steel die or can be made by the aforementioned so-called isostatic molding technique well-known in the art. With the latter, a small amount of organic binder such as wax is included in the raw batch and the batch is then isostatically molded in rubber molds to form blanks which are then fired to burn out the wax and sinter the ceramic. Of course, in accordance with the preferred method of manufacture of the tools a tired shank would be inserted into the wax-bonded annulus-shaped blank prior to the final firing operation. The following specific example will serve to illustrate.
A. raw batch consisting of about 99.8 aluminum oxide (Tabular Corundum) and about .2% magnesium oxide was ball milled to form a uniform admixture, the final grain size of the alumina being less than about 10 microns and 90% being less than microns. A small quantity of paraffin wax binder, about 3% by weight, was then uniformly mixed into the batch, this by adding a wax-water emulsion and then drying. The wax-containing batch was then isostatically molded in rubber molds to form small rods. The dimensions of these rods, both the length and diameter, were slightly greater than those desired in the final piece in order to compensate for shrinkage during subsequent firing. After molding, the rods were fired to about 3000" F. on a 24-hour firing schedule to burn out the wax binder and cause the ceramic to sinter to a dense, extremely hard piece. The sintered rods had a modulus of elasticity of about 50x10? To form the Working portion of the tool a raw batch was made by thoroughly mixing 34 grams of 100 mesh aluminum oxide (Borolon grain), 6 grams of powdered borosilicate glass having a grain size of 325 mesh (glass COl'DpOSltlOHI Slog, B203, A1203, Na O), and 4 grams of naphthalene dissolved in cc. chlorethane. After mixing, the chlorethane was allowed to evaporate and 2.4 grams carboxy methyl cellulose binder dissolved in water were mixed into the batch and the water then allowed to evaporate. The purpose of the naphthalene was to provide a filler which would burn out during subsequent firing and thereby leave a porous structure.
A portion of this batch was then poured into a cylindrical steel die opening in which the end of a ceramic shaft, prepared as above, was concentrically positioned. Pressure of about 10,000 poundsper square inch was then applied by means of an annular punch, which fitted around the protruding ceramic shaft, so as to form the batch into an organic bonded, self-sustaining, uniformly pressed annulus tightly fitted to the end of the shaft. The assembly was then removed from the steel die and fired to a temperature of 1650" F. (this temperature being reached in 30 minutes) to burn out the naphthalene and carboxymethyl cellulose binder and to cause the annulus portion to bond to a hard tough porous condition. During firing the annulus bonded to the end of the shaft to form a unitary ceramic structure.
The amount of pressure used in the, pressing operation will, of course, depend on the porosity desired in the working annulus. Pressures as low as 5,000 pounds per square inch are satisfactory. To assure a more continuous durable bond between the shank and the working annulus, particularly where lower pressures are used in the annulus pressing operation, it is desirable to glaze the end of the shank to which the annulus is to be bonded with a thin layer of glass, preferably of the same composition as that used in the annulus. The firing temperatures and schedules used to obtain a strong fired structure will de pend on the exact composition of the piece being fired as is well known in the ceramics art.
The final dimensions of the fired tool manufactured as described were as follows: Over-all length 2.5, length 5 of exposed portion of shaft 2", diameter of working portion .275, diameter of shaft .200". The tool had a full useful life until the working portion wore to a diameter only slightly greater than that of the shaft.
It will be understood that while the grinding tool and method of the invention have been described specifically with reference to a preferred embodiment thereof various modifications may be used all within the full and intended scope of the following claims.
1. A grinding tool for making a close tolerance cylindrical bore in metal parts and adapted for high speed ro tation and longitudinal movement within said bore, said grinding tool comprising a cylindrical shaped ceramic working portion of relatively large diameter havinga relatively coarse porous grinding structure and an elongated ceramic shaft portion of relatively small cross-sectional area having a fine grain non-porous structure with a high modulus of elasticity extending from said working portion.
2. A grinding tool as-defined in claim 1 wherein the ceramic of said working portion and the ceramic of said shaft portion are both at least 85% by weight aluminum oxide.
3. A grinding tool as defined in claim 1 wherein the Working portion and the shaft portion are bonded together by glass. 7
4. A grinding tool as defined in claim 1 wherein the working portion is of glass bonded relatively coarse grain aluminum oxide and the shaft portion is of fine grain nonporous sintered highalumina ceramic.
Pharmacy International, volume 3, Issue No. 12, pages 12 and 13, December 1949.
LESTER M. SWINGLE, Primary Examiner. FRANK I-LBRONAUGH, Examiner.