Patent Description:
There is known a pipe-tapered-thread machining tap for machining a pipe tapered internal thread in a pipe, a pipe adaptor, a fluid equipment or the like. In the pipe-tapered-thread machining tap, its complete thread portion as well as its chamfered portion is tapered for making the pipe tapered internal thread tapered, so that thread cutting is performed not only by the chamfered portion but also by the complete thread portion.

In the pipe-tapered-thread machining tap, the thread cutting performed by the complete thread portion is carried out not only by its crest but also by its root whereby larger cutting resistance is generated as compared with thread cutting performed by a chamfered portion in a straight thread machining tap. Thus, in the pipe-tapered-thread machining tap, tap breakage, chipping, wear and other problems could easily occur, resulting in problems such as insufficiencies of machining efficiency and tool durability.

On the other hand, there is proposed a pipe-tapered-thread machining tap in which a total of edge thickness angles is <NUM>°-<NUM>° for preventing brakeage and chipping of the tap and also preventing cutting chip clogging so as to provide a high efficiency and a long tool life even where the tap is used for machining a high-hardness material. An example of such a pipe-tapered-thread machining tap is disclosed in Patent Document <NUM>.

However, in the pipe-tapered-thread machining tap as described above, an amount of cut by one cutting edge of the complete thread portion is so small as about <NUM>, for example, so that the cutting edges are rubbed on an inner circumferential surface of the machined internal thread, thereby causing an inconvenience that tearing as a phenomenon of surface roughening of the inner circumferential surface of the internal thread is caused, particularly, where the work material is stainless steel or low carbon steel such as rolled steel for general structure. Moreover, cutting chips are bitten into a gap between a back edge (that is opposed to the cutting edge) and the inner circumferential surface of the machined internal thread when the direction of rotation of the tap is inverted, whereby indentation mark of the cutting edge is formed in the inner circumferential surface of the machined internal thread when the rotation of the tap is stopped, so that the sealing performance could be reduced.

The present invention was made in view of the background discussed above. It is therefore an object of the present invention to provide a spiral tap for machining a pipe tapered thread, which is capable of restraining occurrence of the tearing and formation of the mark of the cutting edge in the machined internal thread upon stop of rotation of the tap.

Various studies made by the inventors of the present invention under the above-described situation revealed a fact that the rubbing and the formation of the mark of the cutting edge upon stop of rotation of the tap can be advantageously restrained by a construction in which flutes are spiral flutes each having a certain range of helix angle and a width of each land is larger than a width of each flute in contrary to common sense in a conventional spiral tap for machining a pipe tapered thread. The present invention was made based on the revealed fact.

The above object is solved by a spiral tap defined in claim <NUM>.

An alternative spiral tap is subject-matter of claim <NUM>.

Further developments are stated in the dependent claims.

According to claim <NUM>, in (a) a spiral tap that is to be rotated about a rotation axis for machining a pipe tapered thread, the spiral tap comprising a tapered thread portion having a thread profile that becomes from an incomplete profile to a complete profile in a direction away from a distal end of a chamfered portion toward a complete thread portion, the tapered thread portion being circumferentially divided by flutes into a plurality of lands, each of the lands having a cutting edge that is defined by one of opposite ends of the each of the lands which is located on a front side of the other of the opposite ends in a direction of rotation of the spiral tap, such that the cutting edge extends along a corresponding one of the flutes, that (b) the flutes consist of a plurality of spiral flutes that are three, four or five spiral flutes each having a helix angle that is not smaller than <NUM>° and smaller than <NUM>°, and that (c) a flute width ratio AG/(AG+AL) is <NUM>-<NUM>, where AG represents a central angle which is subtended by each of the flutes and which is defined at a center corresponding to the rotation axis in a cross section that is perpendicular to the rotation axis, and AL represents a central angle which is subtended by each of the lands and which is defined at the center in the cross section. Thus, it is possible to obtain a pipe-tapered-thread machining spiral tap in which occurrence of tearing is restrained owing to presence of the spiral flutes each having the helix angle that is not smaller than <NUM>° and smaller than <NUM>°, and in which formation of mark of the cutting edge in the machined internal thread upon stop of rotation of the tap is retrained owing to a short distance between the cutting edge and a back edge.

In the alternative spiral tap of claim <NUM>, the spiral flutes consist of four spiral flutes and the lands consist of four lands defined by the four spiral flutes, a cutting edge being provided in every other of the four lands in a circumferential direction. Thus, a depth of cut by one cutting edge is made large whereby rubbing due to slip of the cutting edge is further restrained.

According to claim <NUM>, the plurality of spiral flutes are right-hand spiral flutes, so that cutting chip clogging is restrained.

According to claim <NUM>, the plurality of spiral flutes are left-hand spiral flutes, so that cutting chip clogging is restrained.

According to claim <NUM>, each of the lands has a back edge that is defined by the other of the opposite ends of each of the lands which is located on a front side of the one of the opposite ends in a direction opposite to the direction of rotation of the spiral tap, such that the back edge extends along a corresponding one of the flutes, and that a rake angle of the back edge is smaller than a rake angle of the cutting edge. Thus, formation of mark of the cutting edge upon stop of rotation of the tap is further restrained.

According to claim <NUM>, a relief angle is defined in the chamfered portion from the cutting edge to an intermediate position in a width of each of the lands, and the relief angle is zero from the intermediate position to the back edge. Thus, the back edge is retreated relative to the cutting edge by a small amount whereby formation of mark of the cutting edge upon stop of rotation of the tap is further restrained.

According to claim <NUM>, at least the chamfered portion and the complete thread portion are subjected to a surface treatment with a titanium carbonitride TiCN film. Thus, the spiral tap has an increased durability.

Preferably, cemented carbide or high-speed tool steel is preferably used as the substrate that is to be covered with the titanium carbonitride TiCN film as a hard coating. However, any one of the other kinds of tool materials may be used as the substrate.

A pipe-tapered-thread machining spiral tap as an embodiment of the present invention will be described in detail with reference to the drawings.

<FIG> is a front view showing a pipe-tapered-thread machining spiral tap <NUM> according to an embodiment of the invention. <FIG> is a view showing, in enlargement, a cross section taken along line II-II of <FIG>. As shown in <FIG> and <FIG>, the pipe-tapered-thread machining spiral tap <NUM> has a chamfered portion <NUM>, a complete thread portion <NUM> and a shank portion <NUM> that are arranged in this order as view in a direction away from its distal end, and is to be rotated about its rotation axis rotation axis C. A tapered thread portion <NUM>, which is constituted by the chamfered portion <NUM> and the complete thread portion <NUM>, has a thread profile that becomes from an incomplete profile to a complete profile in a direction away from a distal end of the chamfered portion <NUM> toward the complete thread portion <NUM>. The tapered thread portion <NUM> is circumferentially divided by a plurality of spiral flutes <NUM> into a plurality of lands <NUM>. Although each of the spiral flute <NUM> may be either a right-hand spiral flute or a left-hand spiral flute, it is the right-hand spiral flute in the present embodiment.

Thus, a cutting edge <NUM> is defined by one of opposite ends of the land <NUM>, which is located on a front side of the other of the opposite ends in a direction A of rotation of the tap <NUM>, such that the cutting edge <NUM> extends along the spiral flute <NUM>. Meanwhile, a back edge <NUM> is defined by the other of the opposite ends of the land <NUM>, which is located on a front side of the above-described one of the opposite ends of the land <NUM> in a direction opposite to the rotation direction A, such that the back edge <NUM> extends along the right-hand spiral flute <NUM>. The spiral flute <NUM> has a cross section whose shape is adapted such that a rake angle α of the cutting edge <NUM> is about four to ten times as large as a rake angle α' of the back edge <NUM>.

The right-hand spiral flutes <NUM> function as flutes for storing or evacuating cutting chips, and consist of <NUM>-<NUM> flutes that are arranged with a constant interval between each adjacent two of the flutes <NUM> in a circumferential direction. Each of the spiral flutes <NUM> has substantially the same degree of bottom slope as the pipe tapered thread of the thread portion <NUM>. In the present embodiment, the spiral flutes <NUM> consist of four flutes, so that the thread portion <NUM> in which the pipe tapered thread is provided is circumferentially divided by the four right-hand spiral flutes <NUM> whereby the four lands <NUM> are formed. A helix angle β of each spiral flute <NUM> is not smaller than <NUM>° and smaller than <NUM>°, preferably <NUM>°-<NUM>°, and more preferably a value close to <NUM>°. It is preferable that the cutting edge <NUM> is provided in every other of the four lands <NUM> in the circumferential direction.

<FIG> shows a cross section of the complete thread portion <NUM>, which is perpendicular to the rotation axis C. As shown in <FIG> in detail, the cross sectional shape of each spiral flute <NUM> is adapted for satisfying a relationship AL>AG where "AG" represents a central angle AG which is subtended by each spiral flute <NUM> and which is defined at a center corresponding to the rotation axis C while "AL" which is subtended by each land <NUM> and which is defined at the center corresponding to the rotation axis C. More in detail, a flute width ratio, which is defined by AG/(AG+AL), is not smaller than <NUM> and is not larger than <NUM>.

<FIG> shows, in enlargement, a cross section of the chamfered portion <NUM>. As shown in <FIG>, a relieved surface E is defined such that a radial distance from the rotation axis C to the relieved surface E is reduced in a direction opposite to the rotation direction in a region from the cutting edge <NUM> to an intermediate position in a width of the land <NUM> (blade thickness), namely, in a range from the cutting edge <NUM> over an angle θ, for example, <NUM>°-<NUM>°, and such that the radial distance is constant in the direction opposite to the rotation direction, with a relief angle being zero, in a region from the intermediate position to a heel (back edge <NUM>) of the land <NUM>.

The pipe-tapered-thread machining spiral tap <NUM> is subjected, at at least the chamfered portion <NUM> and the complete thread portion <NUM>, to a surface treatment, whereby a titanium carbonitride TiCN film <NUM>, for example, is formed thereon.

The present inventor and his collaborators experimentally prepared various types of pipe-tapered-thread machining spiral taps, i.e., test samples <NUM>-<NUM>, test samples <NUM>-<NUM> and conventional samples <NUM> and <NUM>, which are the same as one another in terms of the material (high-speed tool steel), the surface treatment (titanium carbonitride TiCN), the type (Rc1/<NUM>) of tapered thread to be machined and the rake angle α (<NUM>°) of the cutting edge <NUM>, and which are different from one another in terms of the number of the spiral flutes <NUM> and the central angle of the land <NUM>. Then, they conducted cutting tests of machining a pipe tapered internal thread, by using the various types of test samples and conventional samples, under a condition specified below. <FIG> shows constructions of the test samples <NUM>-<NUM>, test samples <NUM>-<NUM> and conventional samples <NUM> and <NUM>, and results of the cutting tests.

In the test samples <NUM>-<NUM>, the respective values of the helix angle β of the spiral flutes are <NUM>°, <NUM>°, <NUM>°, <NUM>° and <NUM>°, and the flute width ratio AG/(AG+AL) is <NUM> that is common to the test samples <NUM>-<NUM>. The flute width ratio AG/(AG+AL) is a value rounded to an integer. As is obvious from the results of the tests shown in <FIG>, the tap was broken at an early stage due to cutting chip clogging in the test sample <NUM>, and the cutting edges were chipped in the test sample <NUM>. On the other hand, in each of the test sample <NUM>, <NUM> and <NUM>, a satisfactory result was obtained with the machined internal thread being acceptable in an inspection made by using a thread gauge, since tearing was not caused and occurrence of formation of mark of the cutting edge upon stop of rotation of the tap was restrained. These facts revealed that the tap was broken as a result of cutting chip clogging due to absence of twist of the flutes in the test sample <NUM>, that the cutting edges were chipped due to insufficiency of the tool rigidity in spite of presence of twist of the flutes in the test sample <NUM>, and that the cutting chip clogging and the chipping of the cutting edges were not caused where the helix angle β of the spiral flutes <NUM> is not smaller than <NUM>° and smaller than <NUM>°.

The conventional sample <NUM> and the test samples <NUM>, <NUM>, <NUM>, <NUM> are the same as one another in that the number of the flutes is four and the flute helix angle β is <NUM>° and are different from one another in terms of the flute width ratio AG/(AG+AL). Further, the conventional sample <NUM> is different from the conventional sample <NUM> in that the number of the flutes is five. In according to the tests results shown in <FIG>, in the conventional sample <NUM>, the cutting chips could not be separated from the machined internal thread, and the tool was broken upon change of the direction of the tool rotation. In the test sample <NUM> and the conventional sample <NUM>, the cutting chips bunched up together and caused clogging. On the other hand, in each of the test samples <NUM>, <NUM>, <NUM>, a satisfactory result was obtained with the machined internal thread being acceptable in an inspection made by using a thread gauge, since tearing was not caused and occurrence of formation of mark of the cutting edge upon stop of rotation of the tap was restrained.

However, a noise was caused by the bitten cutting chips upon change of the direction of the tool rotation in the test sample <NUM> and slight cutting chip clogging was seen in the test sample <NUM>. These facts revealed that the tap was broken as a result of cutting chip clogging due to absence of twist of the flutes in the test sample <NUM>, that the cutting edges were chipped due to insufficiency of the tool rigidity in spite of presence of twist of the flutes in the test sample <NUM>, and that the cutting chip clogging and the chipping of the cutting edges were not caused where the helix angle β of the spiral flutes <NUM> is not smaller than <NUM>° and smaller than <NUM>°. These facts revealed that the cutting chips could not be separated from the machined internal thread and caused clogging, due to the flute width ratio AG/(AG+AL) that is larger than <NUM> in each of the conventional samples <NUM> and <NUM>, that the cutting chips bunched up together and caused clogging , due to the flute width ratio AG/(AG+AL) that is smaller than <NUM> in the test sample <NUM>, and that the tap breakage due to the cutting chip clogging and the unseparation of the cutting chips was not caused where the flute width ratio AG/(AG+AL) is not smaller than <NUM> and is not larger than <NUM>.

As described above, the pipe-tapered-thread machining spiral tap <NUM> according to the present embodiment is to be rotated about the rotation axis C for machining a pipe tapered thread, wherein the spiral tap <NUM> includes the tapered thread portion <NUM> having a thread profile that becomes from an incomplete profile to a complete profile in a direction away from a distal end of the chamfered portion <NUM> toward the complete thread portion <NUM>. The tapered thread portion <NUM> is circumferentially divided by the spiral flutes <NUM> into the plurality of lands <NUM>. The lands <NUM> has the cutting edge <NUM> that is defined by one of opposite ends of the land <NUM> which is located on a front side of the other of the opposite ends in the direction of rotation of the spiral tap <NUM>, such that the cutting edge <NUM> extends along a corresponding one of the spiral flutes <NUM>. The spiral flutes <NUM> consist of a plurality of spiral flutes <NUM> that are three, four or five spiral flutes each having a helix angle β that is not smaller than <NUM>° and smaller than <NUM>°. The flute width ratio AG/(AG+AL) is <NUM>-<NUM>, where AG represents a central angle which is subtended by each spiral flute <NUM> and which is defined at a center corresponding to the rotation axis C in a cross section that is perpendicular to the rotation axis, and AL represents a central angle which is subtended by each land <NUM> and which is defined at the center in the cross section. Thus, it is possible to obtain the pipe-tapered-thread machining spiral tap <NUM> in which occurrence of tearing is restrained owing to presence of the spiral flutes <NUM> each having the helix angle β that is not smaller than <NUM>° and smaller than <NUM>°, and in which formation of mark of the cutting edge in the machined internal thread upon stop of rotation of the tap is restrained owing to a short distance between the cutting edge <NUM> and the back edge <NUM>.

In the pipe-tapered-thread machining spiral tap <NUM> according to the present embodiment, the spiral flutes <NUM> consist of four spiral flutes <NUM> and the lands <NUM> consist of four lands <NUM> defined by the four spiral flutes <NUM>, and that the cutting edge <NUM> is provided in every other of the four lands <NUM> in a circumferential direction. Thus, a depth of cut by one cutting edge is made large whereby rubbing due to slip of the cutting edge <NUM> is further restrained.

In the pipe-tapered-thread machining spiral tap <NUM> according to the present embodiment, the plurality of spiral flutes <NUM> are right-hand spiral flutes or left-hand spiral flutes, so that cutting chip clogging is restrained whereby breakage is further restrained.

In the pipe-tapered-thread machining spiral tap <NUM> according to the present embodiment, the land <NUM> has the back edge <NUM> that is defined by the other of the opposite ends of the land <NUM> which is located on a front side of the above-described one of the opposite ends in a direction opposite to the direction A of rotation of the spiral tap <NUM>, such that the back edge <NUM> extends along a corresponding one of the spiral flutes <NUM>, and that a rake angle α' of the back edge <NUM> is smaller than the rake angle α of the cutting edge <NUM>. Thus, formation of mark of the cutting edge <NUM> upon stop of rotation of the tap is further restrained.

In the pipe-tapered-thread machining spiral tap <NUM> according to the present embodiment, the relief angle is defined in the chamfered portion <NUM> from the cutting edge <NUM> to an intermediate position in a width of the land <NUM>, namely, in a range from the cutting edge <NUM> over an angle θ, and the relief angle is zero from the intermediate position to the back edge <NUM>. Thus, the back edge <NUM> is retreated relative to the cutting edge <NUM> by a small amount whereby formation of mark of the cutting edge upon stop of rotation of the tap is further restrained.

In the pipe-tapered-thread machining spiral tap <NUM> according to the present embodiment, at least the chamfered portion <NUM> and the complete thread portion <NUM> are subjected to a surface treatment with the titanium carbonitride TiCN film <NUM>, so that the spiral tap <NUM> has an increased durability.

Claim 1:
A spiral tap (<NUM>) that is to be rotated about a rotation axis (C) for machining a pipe tapered thread, said spiral tap comprising a tapered thread portion (<NUM>) having a thread profile that becomes from an incomplete profile to a complete profile in a direction away from a distal end of a chamfered portion (<NUM>) toward a complete thread portion (<NUM>), said tapered thread portion (<NUM>) being circumferentially divided by flutes (<NUM>) into a plurality of lands (<NUM>), each of said lands (<NUM>) having a cutting edge (<NUM>) that is defined by a front side of the land (<NUM>) in a direction (A) of rotation of said spiral tap (<NUM>) such that said cutting edge (<NUM>) extends along a corresponding one of said flutes (<NUM>),
characterized in that
said flutes (<NUM>) consist of a plurality of spiral flutes (<NUM>) that are three, four or five spiral flutes (<NUM>), each of said spiral flutes (<NUM>) having a helix angle (β) that is not smaller than <NUM>° and smaller than <NUM>°, and
a flute width ratio AG/(AG+AL) is <NUM>-<NUM>, where AG represents a central angle (AG) which is subtended by each of said flutes (<NUM>) and which is defined at a center corresponding to said rotation axis (C) in a cross section of the complete thread portion (<NUM>) that is perpendicular to said rotation axis (C), and AL represents a central angle (AL) which is subtended by each of said lands (<NUM>) and which is defined at said center in said cross section.