Tap

Abstract: A tap having a main body (11,21) having a crest, a heel, and an outer circumference is disclosed. The tap main body has a male screw part at the crest side. At this male screw part, a plurality of grooves (13,23) opening to the crest and the outer circumference of tap main body is formed. A tap main body has a cutting edge (14,24) at each ridge part defined by a wall of said plurality of grooves and an outer circumference of a male screw part. These cutting edges, each have a crest part and a heel part. The crest part of at least one cutting edge has an axial rake angle (ϑ n ) which of differs from an axial rake angle (ϑ m ) of the crest part of another cutting edge.

Description: The present invention relates to a tap according to the precharacterising part of claim 1 which is used when a female screw is manufactured in a workpiece. Heretofore, among taps used for manufacturing female screws in workpieces, for example, those shown in Figures 7 and 8, there are known taps designed for, in particular, decreasing the cutting resistance and for improving chip discharge quality. The taps shown in these figures are formed by a male screw part 2 on the crest side of the tap main body 1 forming a shaft, and are formed by a plurality of grooves 3 spiraling around the axis of tap main body 1 in this male screw part 2, and are further formed by a cutting edge 4 on the ridge part of the wall of these grooves 3 and on the outer circumference of male screw part 2; due to the spiraling cutting edge along the entire length, this type of tap is calted a "spiral tap". As indicated in Figure 9, in the spiral tap 5 in the illustration, all the circumferential pitches P on the crest of cutting edge 4 are set to be equal. In addition, the helical angles ϑ of cutting edges 4 are all set to be equal. In the spiral tap 5, as the axial rake angle (= ϑ ) of cutting edge 4 increases in the positive angle direction, the cutting resistance decreases, and since the chips discharged in grooves 3 are forced out toward the heel side of tap main body 1 by the walls of grooves 3, the efficiency of chip discharge improves. Moreover, there are other known taps having improved chip discharge efficiency (in addition to the spiral tap 5) for example, those shown in Figures 10 and 11. The taps shown in these figures have in common certain features shown in Figures 7 and 8 in that a plurality of groove parts 6 is formed by male screw parts 2 of tap main body 1 and in that cutting edge 7 is formed by the ridge part of these groove parts 6 and the outer circumference of male screw part 2; and they differ from tap 5 of Figures 7 and 8 in that the crest side walls of groove parts 6 are formed by a twisting surface spiraling in the direction opposite to that of the spiral tap 5, towards the axis of tap main body 1; spiral part 8 is formed by the crest side of cutting edge 7, and chips produced by said spiral part 8 are discharged to the crest side of the tap; this type of tap is called a "gun tap". As indicated in Figure 12, the circumferential pitches P of the tap crest of the cutting edge 7 are set to be mutually equal in gun tap 9. Furthermore, the axial rake angles ϑ of spiral part 8 are also set to be equal. In gun tap 9, instead of the spiral part 8, a straight notch crossing the axis of tap main body 1 at a skewed angle is cut in the crest side of groove parts 6, and therefore an inclined part at the crest side of cutting edge 7 is provided, and the axial rake angles of the crest parts of cutting edge 7 are all set to be equal; such a construction is disclosed for instance in DE-A 2 331 927. In the above-mentioned taps 5 and 9 of the prior art, the thread formed in the workpiece by cutting edges 4 and 7, and the above male screw part 2 of taps 5 and 9, make contact and advance during processing; however, at times the cutting resistance load on cutting edges 4 and 7 changes and produces an irregular radial deflection in male screw part 2. Since the thread formed by cutting edges 4 and 7 and male screw part 2 do not come into perfect contact when such a deflection occurs, the shape of the resulting thread differs from the intended shape, production accuracy deteriorates, and the thread is defective. Recently, due to greater control of the rotation frequency and feed rate of the tap main body 1 for which numerically controlled machine tools are used, automation and increased speed of female screw production is widely practiced. The production of female screws by such mechanical means is much faster when compared to the cutting speed attained when female screws are manufactured by manual means. Consequently, the vibration of the tap due to irregular changes of the cutting load put on cutting edges 4 and 7 easily exceeds manageable limits, manufacturing accuracy deteriorates extremely, and it is possible that damage to the tap will result. It is an object of the present invention to prevent deterioration of manufacturing accuracy due to radial irregular deflections of the tap main body, and moreover, to provide a tap which can control the potential increased vibration during the manufacturing process. In order to attain the above-mentioned object, the tap of the present invention provides a tap having a male screw part formed at the crest side of a tap main body; in the male screw part of the tap main body, a plurality of groove parts open to the crest side and to the outer circumference of the tap main body; the tap having cutting edges formed between the walls of the groove parts and the outer circumference of the male screw part on the ridge parts thereof; the tap characterized in that among the cutting edges, there is at least one cutting edge having an axial rake angle value which differs from the axial rake angle values of other cutting edges on the crest side. In the tap of the above configuration, since the component in the radial direction of the cutting load of the tap main body changes in correspondence with the change in the axial rake angle, the balance of the cutting load acting on the cutting edge of the tap main body is disrupted, a part in the circumference of the male screw part is constantly pressed harder on the inner wall of the workpiece compared to other parts of the male screw during processing. Accordingly, if the engagement of the tap main body with the thread of the workpiece is freed of irregular deflections, the engagement becomes more uniform. Moreover, as a certain part of the tap main body is firmly pressed against the thread of the workpiece, the burnishing effect due to the abrasion of the male screw part on the inner wall of the workpiece becomes large. Accordingly, the surface roughness of the thread of the workpiece is improved and the manufacturing accuracy is advanced in comparison to the prior art. Since the axial rake angle of the cutting edge changes, the circumferential pitches of the plurality of corresponding cutting edges also change. Therefore, when the tap main body is rotated at a set speed, the distance at which each cutting edge cuts into the workpiece changes irregularly and the cutting load transmitted by each cutting edge to the tap main body also varies irregularly. Accordingly, the generation of a periodic vibration of the tap main body is disrupted, the vibration transmitted from each cutting edge is preferably countered, and an increment of vibration is suppressed. In this way, manufacturing accuracy is still further improved and such problems as damage to the tap are avoided. In the Drawings: Figure 1 shows a side view of a tap in the first embodiment of the present invention. Figure 2 shows the form of the tap in Figure 1 when viewed in the direction of arrow II. Figure 3 shows an outer radial development of the male screw part of the tap in Figure 1. Figure 4 shows a side view of a tap in a second embodiment of the present invention. Figure 5 shows the form of the tap in Figure 4 when viewed in the direction of arrow V. Figure 6 shows an outer radial development of the male screw part of the tap in Figure 4, Figure 7 shows a side view of a tap of the prior art. Figure 8 shows the form of the tap in Figure 7 when viewed in the direction of arrow VII. Figure 9 shows an outer radial development of the male screw part of the tap in Figure 7. Figure 10 shows a side view of an example tap of the prior art. Figure 11 shows the form of the tap in Figure 10 when viewed in the direction of arrow XI. Figure 12 shows an outer radial development of the male screw part of the tap in Figure 10. The first embodiment of the present invention will be explained with reference to Figures 1 to 3. The corresponding part 10 in Figures 1 and 2 is a tap of the preferred embodiment. This tap 10 has a tap main body 11 forming a shaft, and male screw part 12 is formed on the crest side of tap main body 11. On male screw part 12, a plurality (three in this figure) of groove parts 13a to 13c is formed in such a way that the groove parts spiral around the axis X of tap main body 11. Groove parts 13a to 13c open to the crest and the outer circumference of the tap main body 11, and cutting edges 14a to 14c are formed on the ridge part consisting of the wall of groove parts 13a to 13c and the outer circumference of male screw part 12. Furthermore, cutting edges 14a to 14c follow the outer circumference of tap main body 11, each reaching from the crest to the heel, and are formed as spiral cutting edges spiraling in a spiral form. As shown in detail in Figure 3, the axial rake angles ϑ₁ to ϑ₃ of each cutting edge 14a to 14c are all set to different values. Furthermore, the relative values of the axial rake angles ϑ₁ to ϑ₃ are ϑ₁ < ϑ₂ < ϑ₃ . The difference in the axial rake angles ϑ₁ to ϑ₃ of each cutting edge 14a to 14c is suitably set in correspondence with the screw diameter of the female screw formed in the workpiece and the cutting speed during manufacturing. However, if possible, the difference of the maximum value ϑ₃ and minimum value of ϑ₁ of the axial rake angles ( ϑ₃ - ϑ₁ ) should most preferably be determined so that it lies in the range of 1° to 15°. In the case where the difference in axial rake angles ( ϑ₃ - ϑ₁ ) is less than 1°, there is some risk that the positive improvement in manufacturing speed and suppression of vibration obtained by changing the axial rake angles ϑ₁ to ϑ₃ is not sufficient. Moreover, when the difference of axial rake angles ( ϑ₃ - ϑ₁ ) does exceed 15°, considerable difference occurs in the radial components of the cutting load of tap main body 11, and when cutting edges 14a to 14c cut into a workpiece, there is a risk that male screw part 12 will vibrate considerably, and manufacturing accuracy would therefore deteriorate. As shown in Figure 3, cutting edges 14a to 14c are at equal distance intervals P in the circumferential direction of tap main body 11 in a plane S orthogonal to the axis X of tap main body 11. Plane S may be at any position along the entire length L of cutting edges 14a to 14c; however, plane S″ is arranged such that its position only lies at a fixed distance l = L/3 to 2L/3 from the crest to the heel of the tap main body 11. By positioning plane S between either end of cutting edges 14a to 14c, and by making axial rake angles ϑ₁ to ϑ₃ of cutting edges 14a to 14c different from each other, the values of the circumferential pitches of P₁ to P₃ of tap main body 11 in the crest of cutting edges 14a to 14c become different from one another. Then, the magnitude of the relationships of these distances P₁ to P₃ become P₁ < P₂ < P₃ . The reason why plane S in the example shown in Figure 3 is separated from the crest of tap main body 11 at a distance of L/3 to 2L/3 is that if plane S is at the crest or the heel of cutting edges 14a to 14c, the width of the groove parts 13a to 13c at the opposite side of the cutting edges create a wide difference, and therefore there is a risk that the chip discharge quality at groove parts 13a to 13c greatly decreases, especially when the width is small. In contrast, if the plane S is at any position between both ends of cutting edges 14a to 14c, the magnitude relationships of the groove widths on the crest side with respect to plane S are reversed when compared to the magnitude relationships in the heel part. However, the position of plane S does not necessarily have to be provided between either edge of cutting edges 14a to 14c. The positioning of plane S can be modified according to the dimensions of tap 10, for example, it can be positioned at the crest or heel part if there is a tap which is so short that L/D = 1 (D = outer diameter). If plane S is not provided, it is acceptable that the circumferential pitch of each cutting edge 14a to 14c on the tap main body 11, which extends from the crest part of cutting edges 14a to 14c to the heel, be nonuniform. According to tap 10 obtained by the above construction, since the axial rake angles ϑ₁ to ϑ₃ of the cutting edges 14a to 14c are set to have mutually differing values, the radial components of tap main body 11 of the cutting load imposed on cutting edges 14a to 14c are all different. Due to this, the balance of the cutting load in the radial direction of tap main body 11 is disrupted during the manufacturing process, and one part of male screw part 12 is more firmly pressed against the wall of the workpiece than other parts thereof. Therefore, in this condition, the thread and male screw part 12 are in contact when cutting edges 14a to 14c are screwed in and are advancing. When the radial components of the cutting load imposed on the cutting edges 14a to 14c are balanced, it is logical that the force pressing male screw part 12 to the wall of the workpiece is not acting. However, since the cutting load actually acting on cutting edges 14a to 14c is in practice always acting, the balance of the radial components of the cutting load on tap main body 11 is constantly disturbed, and this creates an irregular vibration in male screw part 12. This irregular vibration influences the engagement between male screw part 12 and the thread of the workpiece and deteriorates the accuracy of the thread. However, by deliberately causing the unbalanced condition of the cutting load in the preferred embodiment, since a set part of male screw part 12 is normally pressed against the wall of the workpiece, a small variation of the cutting load of cutting edges 14a to 14c does not give rise to an irregular vibration in male screw part 12, and moreover, male screw part 12 and the thread formed in the workpiece are engageable in a set relationship. Moreover, by firmly pressing a set part of male screw part 12 against the wall of the workpiece, the burnishing effect arising from abrasion increases. Consequently, the degree of surface roughness of the thread formed in the workpiece is improved, and as a result of this, the manufacturing accuracy of the female screw increases when compared with that of the prior art. In particular, when tap 10 of the preferred embodiment in the machine tool is rotated at a high speed, since the circumferential distances of cutting edges 14a to 14c differ except in plane S, the distances cutting edges 14a to 14c cut into the workpiece change irregularly and the cutting load transferred from cutting edges 14a to 14c to tap main body 11 also change irregularly. Accordingly, the occurrence of cyclic vibration of tap main body 11 is prevented, and rather than counteracting the vibration transferred from cutting edges 14a to 14c, the increase in the vibration is controlled. Therefore, risks such as a deterioration in manufacturing accuracy and damage to the tap are avoided. In the above-described preferred embodiment, all cutting edges 14a to 14c were explained with reference to an example of a spiral tap in which the cutting edges spiral along the entire length thereof; however, the present invention is not so limited. For example, in Figures 4 to 6, examples are shown in which the present invention is applied to gun taps. In tap 20, as shown in these figures, the inner walls of groove parts 23a to 23c formed in the male screw part 22 of tap main body 21 form a part in the area a fixed distance A from the crest of tap main body 21 in such a manner that the spiraling surface spirals in the opposite direction of spiral tap 10, and the crest part of cutting edges 24a to 24c formed on the male screw part 22 are formed as spiral cutting edges. The part containing the spiral cutting edge is formed as a straight cutting edge extending over to the axis X of tap main body 21. The axial rake angles ϑ₁ to ϑ₃ of the spiral cutting edge of cutting edges 24a to 24c are set to be mutually different, and the magnitude relationships of these axial rake angles ϑ₁ to ϑ₃ are set so that ϑ₁ < ϑ₂ < ϑ₃ . The cutting edges 24a to 24c are arranged at a circumferential distance to tap main body 21 in a plane S orthogonal to the axis X of tap main body 21, and this is positioned at a distance A/2 from the crest of tap main body 21 so that the total length of the spiral edge of cutting edges 24a to 24c is defined as A. In this way, the circumferential distances P₁ to P₃ of cutting edges 24a to 24c on tap main body 21 fulfill the magnitude relationship P₃ < P₁ < P₂ Also, in a tap such as tap 20, the variation of axial rake angles ϑ₁ to ϑ₃ of cutting edges 24a to 24c, as in the preferred embodiment illustrated in the above-explained Figures 1 to 3, the unbalanced condition of the cutting load in the radial direction of tap main body 21 is actively produced. Accordingly, the counteracting action which prevents irregular vibration of male screw part 22 functions, manufacturing accuracy increases; the suppression of vibration is accomplished by the inconsistency of vibration frequencies and therefore increased vibration is controlled. In the tap 20, instead of forming the wall of the crest part of groove parts 23a to 23c in a spiral manner, it is possible to simply produce a linear notch slanting relative to the direction of the axis X of tap main body 21; a crest part of cutting edges 24a to 24c can also be formed as a slanting edge by slanting counter to the axis X of tap main body 21. In this case, by changing the angle of notching of the groove parts 23a to 23c, and by changing the axial rake angle of the crest part of cutting edges 24a to 24c, a result similar to the one of the aforementioned example can be obtained. In the aforementioned preferred embodiment, a cutting edge having 3 parts is referred to, and it was also decided that the value of the axial rake angles of the crest part of the cutting edges are all different, whereas this is no limitation on the present invention since a tap consisting of 4 or more cutting edges may be similarly applied. Moreover, even when not all the axial rake angles of the cutting edges are set to differ from one another and only one of the axial rake angles of the crest part of the cutting edges is set to vary from the others, the above-described effects may also be expected. Moreover, it does not matter whether the screw of the cutting edges spirals to the left or to the right. In Figures 1 to 6 illustrating the present preferred embodiment, in the cutting edges, the right-spiral or left-spiral is formed at the crest part of all the cutting edges; however, this is no limitation on the present invention since the twist may also be possible on, for example, one crest part of the cutting edges, whereas the other cutting edges may have an axial rake angle of 0° so that they are parallel to the axis X. Next, effects of the present invention will be clear from the results of a chip discharge experiment. The spiral taps shown in Figures 1 to 3 and the gun taps shown in Figures 4 to 6, having varying angles ϑ₁ to ϑ₃ in the screw diameter and the axial directions were produced, and using these taps, the torques produced during tap processing were measured. The results using the spiral taps are shown in Table 1, and the results using the gun tap are shown in Table 2. In addition, in each table, experimental characteristics of the various prepared taps are summarized. In each table, the form code indicates the tap form: ST Indicates the axial rake angles of the cutting edges are equal as in the tap of the prior art; EU and EE indicate the difference between the plane position where the distance of the circumferences of the spiral cutting edges are equal. In the case of a spiral tap, EU shows that the above plane is positioned along 1/2 of the length of the entire cutting edge; and EE shows that the above-mentioned plane is positioned at the crest of the cutting edge. In the case of a gun tap, EU shows that the plane is positioned at the heel of the spiral cutting edge, and EE shows that the plane is positioned at the crest of the spiral cutting edge. In typical processing using a tap, in order to discharge chips, counter-rotation of the tap is necessary; this is called "stepback processing", in the above experiment, the screwing distance is measured without stepback processing as long as the tap is turned in only one direction. As a result, in the ST type, stepback processing is necessary after approximately 10-15 mm of screwing distance; whereas in the EE type and EU type, stepback processing is not necessary for 17-20 mm. Therefore, the present invention is improved in that the allowable screwing distance before stepback processing is required is reduced by 5 to 7 mm. According to the above experimental results, the required torque decrease for a large diameter tap and a tap having a large screw pitch appears to be larger than that of a small-diameter tap or a tap having a small screw pitch, and these effects vary according to the angle. The resistance encountered by the drill may also be reduced in accordance with the type of material being processed, such as titanium alloys and stainless steel, and other extremely hard materials which are difficult to drill. Although only one spiral tap was tested, similar reduced resistances can also be obtained at various screw angles. The clogging of grooves with chips is evident from the data in the above table since a longer screw distance is possible in the present invention without stepback processing. If stepback processing is necessary, since the number of counter revolutions is reduced, the time required for tapping can be reduced and the costs of processing may also be reduced.

Claims: A tap having a male screw part (12,22) formed at the crest side of a tap main body ; in said male screw part of said tap main body, a plurality of groove parts (13a-13c ; 23a-23c) open to the crest side and to the outer circumference of said tap main body ; said tap having cutting edges (14a-14c ; 24a-24c) formed between the walls of said groove parts and said outer circumference of said male screw part on the ridge parts thereof, characterized in that among said cutting edges, there is at least one cutting edge having an axial rake angle value which differs from the axial rake angle values of other said cutting edges on the crest side. A tap according to claim 1, characterized in that the value of the difference between at least 2 of said axial rake angle values (ϑ₃-ϑ₁) of said cutting edges at said crest side is between 1° to 15°, inclusive. A tap according to claim 1 or 2, characterized in that said cutting edges are formed on spiral cutting edges which spiral along said outer circumference of said tap main body from the respective crest part to the representative heel part. A tap according to claim 1 or 2, characterized in that all said cutting edges have spiral cutting edges which spiral along said outer circumference of said tap main body at said crest side ; and said cutting edges have straight cutting edges which extend parallel to the axis of said tap main body at the heel part. A tap according to claim 3 or 4, characterized in that all the axial rake angles of said spiral cutting edges of said cutting edges differ from one another. A tap according to claim 3 or 4, characterized in that said spiral cutting edges are circumferentially equidistant, and said circumferentially equidistant spiral cutting edges are at a plane (S) orthogonal to the longitudinal axis of said tap main body. A tap according to claim 6, characterized in that said plane is at one of the crest and the heel of said spiral cutting edges. A tap according to claim 6, characterized in that said plane is between the crest and the heel of said spiral cutting edges. A tap according to claim 6, characterized in that said plane is at a position 1/3 to 2/3, preferably about 1/2, of the entire length of the spiral cutting edges from the crest. A tap according to claim 1, characterized in that all said cutting edges have, in the crest part, slanting cutting edges which slant relative to the longitudinal axis of said tap main body, and all said cutting edges have, in the heel part, straight cutting edges parallel to the longitudinal axis of said tap main body.

Inventors: HIYAMA NOBUO C O IZUMO INDUSTR, KATO YOSHIHARU C O IZUMO INDUS, HIYAMA, NOBUO, C/O IZUMO INDUSTRIAL CO., LTD., KATO, YOSHIHARU, C/O IZUMO INDUSTRIAL CO., LTD.
Applicants: IZUMO IND CO LTD, IZUMO INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: B23G5
IPC codes: B23G5