The present invention relates to a casting having a cast portion which is made of casting material and a cap nut which is enveloped in the cast portion, and also to a method of making such casting.
When a component is assembled to a large or heavy casting, a fasting bolt is often used and inserted into a cap nut which is enveloped in cast material of the casting. For enveloping the cap nut in the cast material, casting material or molten metal is poured with a bolt inserted into a threaded hole of the cap nut with the head of the bolt buried in the sand mold. In the above envelopment casting, however, the cap nut expands and contracts under the influence of heat of the casting material and cooling after casting. Additionally, part of the casting material may enter between the cap nut and the bolt thereby causing seizure therebetween. The expansion and contraction of the cap nut or the seizure between the nut and the bolt cause the threaded hole to be damaged or deformed, so that the threaded hole of the cap nut needs to be formed or threaded after casting. Thus, additional work for forming the threaded hole of the cap nut has caused a decrease in productivity and an increase in manufacturing cost.
There has been recently proposed an art for solving such problems. For example, FIG. 8 illustrates a casting method for enveloping a cap nut 50 in the casting by a full mold process. The cap nut 50 has formed therein a stepped hole 51 which includes a small-diameter threaded portion 51a on the bottom side and a large-diameter threaded portion 51b on the opening side. The cap nut 50 also has formed therearound adjacent to the large-diameter threaded portion 51b an annular recess or a circumferential cut 52 having a V-shaped cross-section. With this annular recess 52 as the boundary, the cap nut 50 has a nut body 50a on the bottom side thereof and a cylindrical extension 50b on the opposite side.
This casting method will now be described. The cap nut 50 is initially inserted into a support hole 54 which is formed in an evaporative pattern 53 made of, for example, expanded polystyrene. The cap nut 50 is inserted into the support hole 54 to such a depth that the annular recess 52 is positioned flush with the surface of the evaporative pattern 53. After the cap nut 50 is fixed relative to the evaporative pattern 53, a bolt 55 is screwed into the large-diameter threaded portion 51b of the cap nut 50. Thereafter, a sand mold 56 is made so that the mold 56 surrounds the evaporative pattern 53 and also that the bolt 55 and the cap nut 50 are partially buried in the sand mold 56. As shown in FIG. 8A, the nut body 50a is supported by the evaporative pattern 53, while the cylindrical extension 50b is buried in the sand mold 56.
The casting material is then poured into the sand mold 56. The casting material runs in the cavity of the sand mold 56 and replaces the evaporative pattern 53, so that the cap nut 50, which is fixed to the sand mold 56 by the bolt 55, is enveloped in the casting. Because the hole 51 of the cap nut 50 is then closed by the bolt 55, the casting material does not enter into the hole 51.
As shown in FIG. 8B, the resulting casting 58, which is taken out of the sand mold 56 after cooling, includes a cast portion 57 made of the casting material and the cap nut 50 enveloped in the cast portion 57, and the bolt 55 is still inserted in the cap nut 50. As shown in FIG. 8C, the bolt 55 is sheared at the annular recess 52. As a result, the bolt 55 and the cylindrical extension 50b of the cap nut 50 are removed, and the body 50a of the cap nut 50 remains in the cast portion 57. The sheared surface adjacent to the hole 51 of the nut body 50a is positioned flush with the surface of the cast portion 57. A component may be fixed to the casting 58 by using a fastening bolt (not shown) inserted into the small-diameter threaded portion 51a of the cap nut 50 which is enveloped in the cast portion 57.
Thus, the above-described casting method contributes greatly to productivity improvement and reduction of manufacturing cost, and the cap nut 50 having the annular recess 52 on the outer peripheral surface thereof is appropriate for performing the casting method which is disclosed in pages 2 through 4 and FIGS. 1 and 2 of Unexamined Japanese Patent Publication No. 2002-192326.
However, the conventional casting method or the casting does not accomplish sufficiently high productivity of castings or sufficiently low manufacturing cost yet. According to the conventional casting method, the cap nut needs to be machined previously to have the annular recess with V-shaped cross-section in the outer peripheral surface thereof, and this machining process impedes the productivity improvement and reduction of the manufacturing cost of castings.
After the casting, the bolt inserted in the cap nut is struck to shear the cap nut at the annular recess. Thus, the conventional method requires a shearing process, as well as a process for smoothening the irregularly-shaped surface which remains on the sheared surface, which further impedes higher productivity and lower manufacturing cost.
Furthermore, since the conventional cap nut requires the shearing after casting, the outer diameter of the cap nut and the inner diameter of the hole suitable for the shearing should be determined previously, which inhibits the freedom of establishing the outer diameter of the cap nut and the inner diameter of the hole into which the bolt is inserted. Therefore, there is a need for providing a casting and a casting method therefor which do not require a forming work for the hole of the cap nut, while making possible higher productivity and lower manufacturing cost of castings, and additionally higher degree of freedom for designing the cap nut.