US 20040238154 A1
A corrosion resistant and durable forstner bit is formed from stainless steel using an investment casting process.
1. A method of manufacturing a forstner bit comprising:
making a wax pattern of a forstner bit;
dipping said wax pattern in a refractory slurry to coat the wax pattern;
rolling the coated wax pattern in sand;
drying the coated and rolled wax pattern;
repeating said steps of dipping, rolling and drying until a prescribed coating thickness is achieved;
placing the entire coated pattern in a heated environment to melt away the wax, thereby to produce a mold ready for receiving molten material;
pouring molten stainless steel into said mold;
cooling the stainless steel; and
breaking away the casing to reveal the molded forstner bit.
2. A method of casting as in
3. A method of casting as in
4. A method of casting as in
5. A method as in
6. A stainless steel forstner bit formed by the process of
7. A method of fabricating a forstner bit comprising:
fabricating a wax model of a forstner bit body, said wax model substantially representing a periphery of the bit body;
forming a mold from said wax model by applying a refractory mold material to external surfaces of said model and hardening said refractory mold material;
removing said wax model from said mold;
pouring molten stainless steel into at least one cavity of the resultant mold to form a bit body substantially corresponding in configuration and size to said wax model.
8. A method as in
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12. A stainless steel forstner bit formed by the process of
 The present invention relates to an improved forstner bit and, more specifically to a forstner bit investment cast from stainless steel.
 Conventionally, forstner bits have been manufactured from high carbon steel. Several methods have been used to form such conventional forstner bits.
 The first manufacturing method is forging. Forging is a process by which metal is heated and shaped by plastic deformation by suitably applying compressive force. Usually the compressive force is in the form of hammer blows using a power hammer or a press.
 According to such a method, a metal ingot is forged into a shape close to that of the bit and then machined to its final form. This manufacturing method is expensive because the intermediate form that is to be machined is usually harder material which wears the machining tools rapidly. Additionally, the initial tooling is very expensive and wears fast. Since, this process requires expensive tooling that wears rapidly and requires a lot of maintenance, the bits formed by this process are expensive to produce. Accordingly, the bits must be priced high for sale and are therefore hard to market.
 Forstner bits may also be machined from solid stock. According to such a process, a solid bar is machined into the bit configuration using, e.g., a CNC turning center. Machining from solid stock requires expensive tooling/machines and produces large amounts of wasted stock, but the waste material can be recycled for only a fraction of the cost of the solid stock. Therefore, while a great deal of waste is produced by this process, that waste may be reused to reduce the cost of the overall process.
 More recently, carbon steel forstner bits have been made using investment casting. The investment casting process requires that the mold be made each time and is thus labor intensive. The tooling however, is minimal in that the wax pattern's dye tooling lasts a long time due to little to no wear. Also the material used to make the mold is reusable. Although this process is time consuming, the tooling is not expensive and the finished product requires little to no secondary machining so that ultimately the finished product can cost less than those produced using other manufacturing processes.
 The inventors recognized that forming forstner bits using the investment casting process from high carbon steel produces an inexpensive bit but have also recognized that the thus produced forstner bit is disadvantageously subject to surface deterioration due to corrosion when exposed to moist and/or corrosive environments as may be encountered in a commercial or home hobbyist's workshop. Significantly, the inventors have recognized that forming a forstner from stainless steel using investment casting would produce a superior but relatively low cost bit which will be durable and will maintain effective cutting edges in spite of adverse ambient conditions, thereby making the bit highly attractive to the consumer. Accordingly, the inventors have adapted a generally conventional investment casting process to produce a forstner bit from stainless steel.
 Accordingly, the invention is embodied in a method of manufacturing a forstner bit comprising: making a wax pattern of a forstner bit; dipping the wax pattern in a refractory slurry to coat the wax pattern; rolling the coated wax pattern in sand; drying the coated and rolled wax pattern; repeating the steps of dipping, rolling and drying until a prescribed coating thickness is achieved; placing the entire coated pattern in a heated environment to melt away the wax, thereby to produce a mold ready for receiving molten material; pouring molten stainless steel into the mold; cooling the stainless steel; and breaking away the casing to reveal the molded forstner bit.
 The invention may also be embodied in a method of fabricating a forstner bit comprising: fabricating a wax model of a forstner bit body, the wax model substantially representing a periphery of the bit body; forming a mold from the wax model by applying a refractory mold material to external surfaces of the model and hardening the refractory mold material; removing the wax model from the mold; pouring molten stainless steel into at least one cavity of the resultant mold to form a bit body substantially corresponding in configuration and size to the wax.
 The invention is further embodied in a stainless steel bit formed using an investment casting process as summarized above.
 These and other objects and advantages of this invention, will be more completely understood and appreciated by careful study of the following more detailed description of the presently preferred exemplary embodiments of the invention taken in conjunction with the accompanying drawing, in which:
FIG. 1 is a perspective view of a forstner bit formed from investment cast stainless steel in accordance with the invention.
 Investment casting is also known as the lost wax process. This process allows intricate shapes to be made with high accuracy. The types of materials that can be cast using investment casting include Aluminum alloys, Bronzes, tool steels, stainless steels, Stellite, Hastelloys, and precious metals. Metals that are hard to machine or fabricate are particularly good candidates for this process. It may also be used to make parts that cannot be produced by forging or machining, such as turbine blades having complex shapes or airplane parts that have to withstand high temperatures.
 In accordance with the investment casting process, the forstner bit mold is made by making a forstner bit pattern using wax that can be melted away. As used herein, a “pattern” is a full-scale representation or model of the forstner bit that is either machine or hand made. The wax pattern is made by pouring molten wax into a two half machined steel mold, then separating the mold and removing the wax pattern when the wax is solidified.
 This wax pattern is dipped in refractory slurry, which coats the wax pattern and forms a skin, and is then rolled in sand. This is dried and the process of dipping in the slurry, rolling and drying is repeated until a robust thickness is achieved. The target thickness will vary with the size of the forstner bit. As presently proposed, the thickness would be at least about ¼ to ½ inch thick. The materials used for the slurry are a mixture of plaster of Paris, a binder and powdered silica, a refractory, for low temperature melts. For higher temperature melts, sillimanite, an alumina-silicate, is used as a refractory, and silica is used as a binder. Depending on the fineness of the finish desired additional coatings of sillimanite and ethyl silicate may be applied. In an exemplary embodiment of the invention, due to the high temperatures of the molten metal, the material used for the slurry is silliamanite.
 The entire pattern is then placed in a hot environment and the wax is melted away. A hot water bath is generally the easiest method to remove the wax and recapture it for reuse. In addition or in the alternative, a firing process is employed according to which the mold is heated to a firing temperature, e.g., to about 1000° C. (1832° F.) for firing the mold, to insure the strength of the mold will accommodate molten steel.
 Thus the aforementioned steps produce a mold that can be filled with the molten metal. The mold thus produced can be used directly for light castings, or can be reinforced by placing it in a larger container and reinforcing it with more slurry. In the presently proposed embodiment, the layers of sand and slurry create a wall thickness adequate for making the forstners such that additional reinforcement is generally not required.
 Just before the pour, the mold is pre-heated to about 1000° C. (1832° F.) to remove any residues of wax, and harden the binder. The pre-heating step may comprise the firing step. Pouring in the pre-heated mold also ensures that the mold will fill completely. Pouring can be done using gravity, pressure, or vacuum conditions. Attention must be paid to mold permeability when using pressure, however, to allow the air to escape as the pour is performed. In an exemplary embodiment, gravity is used for pouring the molten metal into the cast to produce the forstner bit of the invention.
 After the molten metal sets and cools the casting is broken away to reveal the desired part. The bit is then machined as necessary or desirable, to its final form. As noted above, parts made with investment castings often do not require any further machining, because of the close tolerances that can be achieved.
 Because the mold is formed around a one-piece pattern, which does not have to be pulled out from the mold as in a traditional sand casting process, very intricate parts and undercuts can be made. Tolerances of 0.5% of length are routinely possible, and as low as 0.15% is possible for small dimensions. Castings can weigh from a few grams to 35 kg (0.1 oz to 80 lb), although the normal size ranges from 200 g to about 8 kg (7 oz to 15 lb). Normal minimum wall thicknesses are about 1 mm to about 0.5 mm (0.040-0.020 in) for alloys that can be cast easily. The forstner bits provided in accordance with exemplary embodiments of the invention range from 0.5 ox. to 2 lbs and have wall thicknesses from 0.25 in. to 0.825 in. depending on the size.
 As mentioned above, investment casting is used to produce relatively inexpensive high carbon steel forstner bits, but those bits lack the corrosion resistance and thus the durability desired by the consumer.
 In accordance with the invention, a forstner bit 10, as illustrated in FIG. 1, is formed from stainless steel using an investment casting process as summarized above. Four hundred series stainless steel was selected as a preferred material for the forstner bits because the thus produced bit may be heat treated for strength and wear resistance. In an exemplary and presently most preferred embodiment of the invention, the composition for 416 stainless steel was selected for investment casting. Type 416 is a chromium grade of stainless steel modified by the addition of phosphorus and sulfur to produce a free-machining steel. As a readily machinable stainless steel, the bit produced according to the invention using the investing casting process may be machined after molding if deemed necessary or desirable.
 The composition of (416) stainless steel adopted in an exemplary and presently preferred embodiment of the invention is as follows: C=max 0.15, Mn=max 1.25, P=max 0.060, S=0.15/.40, Si=max 1.00, Cr=12.00/13.50, Ni=max 0.75, Mo=max 0.60, Cu=max 0.50. In the alternative, the bits may be made out of 440 stainless, which has a higher Cr and C level. Actually, the bits can be fabricated out of any of the stainless steels series which respond to any- heat treat process giving hardenability ranges above RC 35. However, as mentioned above, the preferred material is 416 SS because of its availability, competitive cost, and machining ease.
 Stainless steel is most readily available in stock form and therefore forging or machining stainless steel would be the manufacturing options of choice to the skilled artisan. However, forging and machining have the deficiencies noted above. By forming a forstner bit from stainless steel using an investment casting method, a forstner bit can be formed inexpensively and the forstner bit produced is advantageous in its corrosion resistance and hardness, and thus durability.
 While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not- to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.