Tap

Abstract: A cutting tap (1) comprises a threaded portion (3) including a lead (3a) whose thread diameter gradually decreases toward a front end of the lead (3a), a complete thread portion (3b) continued from the lead (3a), and four flutes (3c) arranged at equal intervals in a circumferential direction. In the cutting tap (1), a first cutting edge portion (4) for processing a prepared hole is formed at a front end surface of the lead (3a), and a second cutting edge portion (5) for finishing a minor diameter of an internal thread is formed in a part of an outer periphery of the complete thread portion (3b), the second cutting edge portion (5) being one or more pitches long. The cutting tap (1) can perform processing of a prepared hole, tapping, and finishing of a minor diameter by a single step, and can also perform tapping even if a prepared hole for an internal thread is a blind hole or the like and does not have enough space.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tap according to the preamble of claim 1 suitable for tapping of a cored hole or the like (see for example US-2,898,612). 2. Description of the Related Art Formation of an internal thread has so far been performed in succession to drilling. This has posed the inconvenience of requiring facilities and time for replacement of tools. Thus, a proposal has recently been made, as in Japanese Utility Model Publication No. 25782/93, for a tap for a cored hole which has a guide portion connected to a front end of a tapping portion, and a cutting edge capable of face cutting which is provided at a front end of the guide portion so that the processing of a prepared hole and tapping can be performed by a single step. With the above-mentioned conventional tap for a cored hole, an incomplete thread portion of the tap increases by the amount of the guide portion. Thus, if the prepared hole for an internal thread is a through hole or has enough space, no problem is posed. However, if the prepared hole for an internal thread is a blind hole or the like and does not have enough space, tapping cannot be performed. Even if tapping is possible because the prepared hole for an internal thread is a through hole or has enough space, an inner peripheral surface of the prepared hole worked by the tapping portion becomes, as is, a crest surface of an internal thread. For this reason or else, sufficiently satisfactory finished surface roughness or accuracy is not always obtained depending on the machining conditions. US-2,898,612 from which the precharacterising first part of claim 1 starts out, discloses a combined drilling and tapping tool having a drill portion and a tap portion arranged end to end along the same axis. Cutting edges of the drill portion extend into the tap portion for deburring the thread formed by the tap portion. This arrangement has the same disadvantage as the above-discussed prior art in that it does not allow for cutting of a thread into a blind hole of limited depth. The same document discloses a modified arrangement without the drill portion but with cutting edges in the tap portion for deburring the thread just as in the unmodified arrangement. The modified arrangement cannot cope with eccentricity between the tool and a prepared hole in the work. SUMMARY OF THE INVENTION The present invention has been accomplished in view of the above-described circumstances. It is an object of the invention to provide a tap; which can performprocessing of a prepared hole, tapping, and finishing of a minor diameter by a single step, and which can carry out tapping even if a prepared hole for an internal thread is a blind hole or the like. This object is solved by a tap as set forth in claim 1. The subclaims are directed to preferred embodiments of the invention. An embodiment of the invention for attaining the above object is a tap for performing processing of a prepared hole and tapping by a single step, the tap having a threaded portion for performing tapping by chipping, the threaded portion including a lead whose thread diameter gradually decreases toward a front end of the lead, a complete thread portion continued from the lead, and a plurality of flutes arranged in a circumferential direction; a first cutting edge portion formed at a front end surface of the lead for processing a prepared hole; and a second cutting edge portion formed in a part of an outer periphery of the complete thread portion for finishing a minor diameter of an internal thread, the second cutting edge portion being one or more pitches long. Accordingly, processing of a prepared hole, tapping, and finishing of a minor diameter can be performed by a single step without replacement of tools, the processing man-hours and the processing time can be markedly decreased, and productivity can be increased. Furthermore, the first cutting edge portion is directly provided at the front end surface of the lead of the threaded portion, without a guide portion provided there, and the second cutting edge portion is provided in the threaded portion. Thus, the entire length of the tap needs not to be increased, and tapping can be performed even if a prepared hole for an internal thread is a blind hole or the like and does not have much space. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: Fig. 1 is an overall constitution drawing of a cutting tap according to a first embodiment of the present invention; Fig. 2 (a) is a view taken on line I-I of Fig. 1, Fig. 2 (b) is an end face view taken on line II-II of Fig. 1, Fig. 2(c) is a sectional view taken on line III-III of Fig. 1, and Fig. 2 (d) is a sectional view taken on line IV-IV of Fig. 1; Fig. 3 (a) is a view taken on line I-I of Fig. 1 showing a modification of the tap, and Fig. 3(b) is an end face view taken on line II-II of Fig. 1 showing the modification of the tap; and Figs. 4 (a) and 4 (b) are explanation drawings for actions of the tap. PREFERRED EMBODIMENTS OF THE INVENTION A tap according to the present invention will now be described in detail by way of the following Embodiments with reference to the accompanying drawings, but it should be understood that the invention is not restricted thereby. Fig. 1 is an overall constitution drawing of a cutting tap according to a first embodiment of the present invention. Fig. 2(a) is a view taken on line I-I of Fig. 1, Fig. 2(b) is an end face view taken on line II-II of Fig. 1, Fig. 2(c) is a sectional view taken on line III-III of Fig. 1, and Fig. 2 (d) is a sectional view taken on line IV-IV of Fig. 1. Fig. 3(a) is a view taken on line I-I of Fig. 1 showing a modification of the tap, and Fig. 3(b) is an end face view taken on line II-II of Fig. 1 showing the modification of the tap. Figs. 4 (a) and 4(b) are explanation drawings for actions of the tap. As illustrated in Figs. 1 and 2 (a) to 2 (d), a cutting tap 1 comprises a shank portion 2 having a nearly tetragonal cross sectional shape at a rear end site 2a so as to be chucked by a tapping machine (not shown), and a threaded portion 3 for thread cutting a cored hole W1 of a workpiece W (to be described later on), a material to be cut. The threaded portion 3 has a lead 3a whose thread diameter gradually decreases toward a front end of the lead 3a, a complete thread portion 3b continued from the lead 3a, and four straight flutes (may be spiral flutes) 3c arranged at equal intervals in a circumferential direction. In the threaded portion 3, a first cutting edge portion (an end mill blade) 4 for processing a prepared hole is formed which is situated at a front end surface of the lead 3a, and which comprises four cutting edges 4a and four flanks 4b (see Figs. 2(a) and 2(b). In the threaded portion 3, a second cutting edge portion (a reamer blade) 5 for finishing a minor diameter is also formed at a pair of opposed lands situated on a front end side of the complete thread portion 3b (at a site where the lead 3a ends). The second cutting edge portion 5 comprises cutting edges 5a and flanks 5b. (See Fig. 2(c).) The cutting edge 4a and the cutting edge 5a are formed on a forward side in a direction of rotation during normal rotation shown by an arrow in Fig. 2(c). The cutting edge 5a is formed over a length of several pitches. In the drawings, the numeral 6 denotes a center hole/oil hole (i.e., a center hole concurrently serving as an oil hole). As shown in Figs. 3 (a) and 3 (b), a spot facing 7 may be formed at a peripheral edge of an opening of the center hole/oil hole 6 so that a cutting resistance will be decreased during bottom face processing of a blind hole. For tapping, the cutting tap 1 constituted as described above is screwed into the cored hole W1 of the workpiece W. First of all, the cored hole W1 is cut, along its wall, with the four cutting edges 4a of the first cutting edge portion 4 at the front end of the threaded portion 3, as shown in Fig. 4. Then, the lead 3a of the threaded portion 3 bites into the cut wall, and the complete thread portion 3b forms an internal thread. At this time, the cutting edges 5a of the second cutting edge portion 5 having a larger outside diameter than the minor diameter of the threaded portion 3 finish the minor diameter of the internal thread to fall within the tolerance. In Fig. 4(b), the left half shows a processing example in which the minor diameter of the internal thread has been finished by the second cutting edge portion 5 to fall within the tolerance, while the right half shows a processing example in which the cutting tap 1 does not have the second cutting edge portion 5. The four cutting edges 4a of the first cutting edge portion 4 also cut a flash present at the bottom of the cored hole W1. According to the present embodiment, as described above, processing of a prepared hole, tapping, and finishing of a minor diameter are performed by a single step without tool replacement. Thus, the processing man-hours and the processing time can be markedly decreased, and productivity can be increased. Furthermore, the first cutting edge portion 4 is directly provided at the front end of the threaded portion 3, without a guide portion provided there, and the second cutting edge portion 5 is provided midway in the threaded portion 3. Thus, the entire length of the tap need not be increased, and tapping can be performed even if.a prepared hole for an internal thread is a blind hole or the like and does not have enough space. . Besides, the first cutting edge portion 4 for processing of a prepared hole is provided at the front end of the threaded portion 3. Hence, even if the axis of the cored hole W1 of the workpiece W is eccentric, tapping can be performed, with the eccentricity being corrected with the first cutting edge portion 4. This invention being thus described, it will be obvious that the same may be varied in many ways, including changes in the numbers of the flutes, the first cutting edge portion, and the second cutting edge portion. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.

Claims: A tap for performing processing of a prepared hole and tapping in a single step, comprising a threaded portion (3) for tapping by chipping, including a lead (3a) whose thread diameter decreases gradually towards its front end, a complete thread portion (3b) continued from the lead (3a), a plurality of flutes (3c) at different circumferential positions, and a cutting edge (4, 5) including a first cutting edge portion (4, 4a) on a front face of the tap for processing said prepared hole and a second cutting edge portion (5, 5a) for finishing a minor diameter of an internal thread, characterised in that the first cutting edge portion (4, 4a) is formed directly at said front end of the lead (3a) without a guide portion provided there and the second cutting edge portion (5, 5a) is formed only over several pitches of the tap in the outer periphery at the front end of the complete thread portion (3b). The tap of claim 1, wherein the prepared hole is a blind hole. The tap of claim 1, wherein cutting edges (4a, 5a) of the first and second cutting edge portions (4, 5) are formed on a forward side of said cutting edge portions in a direction of rotation during normal operation. The tap of claim 1, wherein a centre hole or oil hole (6) is formed in the threaded portion (3), and a spot facing (7) is formed at a peripheral edge of an opening at the front end of the centre hole or oil hole (6).

Inventors: SAWABE TERUO, SAWABE, TERUO, Sawabe, Teruo, c/o Tanoi Mfg. Co., Ltd.
Applicants: TANOI MFG CO LTD, TANOI MFG. CO., LTD.
Application year: 1999
Main classification: B23G5
IPC codes: B23G5, B23B51