Cutting by use of rotating axially moving tool – Tool or tool with support – With pitch-stabilizing ridge
Reexamination Certificate
1999-02-17
2001-04-17
Bishop, Steven C. (Department: 3722)
Cutting by use of rotating axially moving tool
Tool or tool with support
With pitch-stabilizing ridge
C408S220000, C470S198000, C470S204000
Reexamination Certificate
active
06217267
ABSTRACT:
TECHNICAL FIELD
The present invention relates in general to a cold forming tap for forming an internal thread in a workpiece by plastic deformation of the workpiece. More particularly, the invention is concerned with a cold forming tap which has an internal finish cutting edge and a long life of use and which is capable of efficiently forming an internal thread, even in a hole which has been formed through an aluminum alloy during the casting process.
BACKGROUND ART
There is known a cold forming tap which has an external thread portion including radially outwardly protruding portions and relief portions which are adjacent to and have a diameter smaller than the protruding portions. The protruding portions and the relief portions are alternately located as viewed in a helical direction of the cold forming tap in which a thread extends. The protruding portions are substantially equi-angularly spaced apart from each other as viewed in a circumferential direction of the cold forming tap, while the relief portions are also substantially equi-angularly spaced apart from each other as viewed in the circumferential direction. For forming an internal thread in a prepared hole formed in a workpiece, the protruding portions are forced into a surface of the prepared hole, for plastically deforming the surface so as to form the internal thread on the surface. As one type of the cold forming tap, there is proposed a cold forming tap having an internal finish cutting edge which is provided to finish a minor diameter of the formed internal thread.
This cold forming tap having the internal finish cutting edge is advantageously capable of eliminating a shape of incomplete thread ridge which may possibly appear on a crest of the internal thread, i.e., on the minor diameter of the internal thread. The cold forming tap having the internal finish cutting edge has another advantage of preventing a breakage of the tap due to an excessive torgue acting on the tap, which is caused by a large amount of surplus stock of the workpiece plastically flowing into a limited space, in a case where an internal thread is to be formed in a hole previously formed through the workpiece, which hole has a dimension smaller than a lower limit, such as a tapered hole formed in the workpiece in the casting process. Further, the cold forming tap having the internal finish cutting edge advantageously assures an improved accuracy in the minor diameter of the internal threads.
In the above-described conventional cold forming tap having the internal finish cutting edge, the highest portions of a root diameter of the external thread portion are located at the relief portions adjacent to the protruding portions (lobes) which correspond to the highest portions of a major diameter of the external thread portions, and flutes are formed at the highest portions of the root diameter so that the highest portions of the root diameter constitute the cutting edges.
In general, the external thread portion of the cold forming tap is formed, in a step referred to as a thread grinding, by a grinding wheel having a thread profile formed on an outer circumferential surface thereof such that the thread ridge and the root of the external thread portion are simultaneously formed. In the above-described conventional cold forming tap having the internal finish cutting edge, the crest and the root of the external thread portion have respective concave-convex shapes in the cross section perpendicular to an axial direction of the tap. However, the respective concave-convex shapes of the crest and the crest and a concave-convex shape defined by the root do not coincide with each other as viewed in the circumferential direction of the tap, resulting in a need of grinding the ridge and the root separately from each other. The separate grinding operations not only complicate the production process, but also make it difficult to control the dimensions of the thread ridge. For obtaining high dimensional accuracy of the thread ridge, it would be indispensable to improve the accuracy of the entirety of the external thread portion. Consequently, the conventional cold forming tap has a drawback of requiring a high level of grinding technique and a high cost for the production. The present invention has an object of providing a cold forming tap having an internal finish cutting edge and a method of producing the cold forming tap, wherein the external thread portion can be formed basically in a simple process as known in the art, with easy dimensional control of the thread ridge, at a reduced cost.
DISCLOSURE OF INVENTION
According to a first feature of the present invention, there is provided a cold forming tap with an internal finish cutting edge including an external thread portion in which an external thread having a uniform depth is formed to extend in a helical direction of the cold forming tap, the external thread portion including radially outwardly protruding portions and relief portions which are adjacent to and have a diameter smaller than the protruding portions, the protruding portions and the relief portions being alternately located in the helical direction, such that the protruding portions are angularly spaced apart from each other at a predetermined angular interval in a circumferential direction of the cold forming tap, and such that the relief portions are angularly spaced apart from each other at a predetermined angular interval in the circumferential direction, the protruding portions being forced into a surface of a prepared hole which has been formed in a workpiece, for plastically deforming the workpiece whereby an internal thread is formed on the surface, the cold forming tap being characterized in that at least one of the protruding portions which are located within each lead of the external thread portion has a root diameter larger than that of the other of the protruding portions, the at least one of the protruding portions being removed down to a root of the external thread, and a flute is formed in a portion corresponding to the root of the at least one of the protruding portions, so as to provide the internal finish cutting edge in the portion corresponding to the root.
According to this arrangement, the at least one of the protruding portions which are located within each lead of the external thread portion is removed down to the root whereby the cutting edge is formed at the portion corresponding to the root in the removed protruding portion. The cutting edge serves to remove surplus stock which has been displaced inwardly from the root diameter of the external thread portion as a result of the plastic deformation of the surface of the prepared hole caused by a bite by a thread ridge of the external thread portion into the surface, permitting easy formation of the internal thread with desired accuracy. Further, according to the construction of the cold forming tap having the internal finish cutting edge of the present invention, a maximum diameter position of the root is not located at each of the relief portions which are adjacent to the protruding portions of the external thread portion, and the concave-convex shape defined by the crest and the concave-convex shape defined by the root coincide with each other as viewed in the circumferential direction of the tap. This coincidence between the respective concave-convex shapes makes it possible to form simultaneously the ridge and root of the external thread portion with high accuracy, by employing a grinding wheel having a thread profile formed on its outer circumferential surface in a step referred to as a thread grinding, considerably facilitating the dimensional control of the thread of the external thread portion, and leading to a reduced cost for the production.
In the cold forming tap described above, the external thread portion preferably includes a tapered leading portion and a full-form thread portion which is adjacent to one of opposite ends of the tapered leading portion that is nearer to a proximal end of the cold forming tap, the tapered leading portion having an ou
Hayashi Shigeru
Minagawa Kazumitsu
Sugano Hiroto
Bishop Steven C.
Oliff & Berridg,e PLC
OSG Corporation
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