JP5152022B2 - Drilling tools - Google Patents

Description

  The present invention connects a drilling bit for excavating the ground and a drilling rod for transmitting a striking force and a rotational force from a drilling device to the drilling bit, or a coupling for connecting the drilling rod and the drilling rod to each other. The present invention relates to a drilling tool having a screw shaft in which a male screw portion is formed and a screw hole in which a female screw portion is formed.

  For example, in Patent Documents 1 and 2, a screw hole having a female screw portion on the inner periphery is formed on the rear end surface of a drilling bit in which a tip for excavating the ground is implanted at the tip, and this In the screw hole, there is described one in which a screw shaft having a male thread portion formed on the outer periphery of a drilling rod tip that transmits a rotational force, a striking force, and a thrust during excavation to the drilling tool main body is screwed.

  In addition, in order to excavate a predetermined depth, the excavation rod is formed with a screw hole similar to that of the excavation tool main body at the rear end surface thereof, and a screw shaft at the tip of the next excavation rod is screwed or screwed at both ends. In the excavation rod in which the shaft is formed, these screw shafts are screwed into a coupling in which both inner peripheral portions of the cylinder are screw holes, and are sequentially added.

International Publication No. 03/042493 Pamphlet JP 2007-162220 A

  By the way, in such an excavation tool connected by screwing between the screw shaft and the male and female screw portions of the screw hole, the screw is loosened in the drilling hole particularly when the impact force as described above is applied, for example, the tip of the excavation bit. Depending on the direction of the external force applied to the drill bit, the center axis of the drill bit and the drill rod, that is, the center line of the screw hole and the center line of the screw shaft do not coincide with each other. This causes a phenomenon that the center line of the screw is greatly inclined and the rear end portion of the screw shaft contacts the opening of the screw hole. When such a phenomenon occurs, breakage of the excavation rod, breakage of the excavation bit, etc. occur at this contact portion at an early stage, and the life of the excavation tool is significantly impaired.

  Here, in the said patent document 1, providing the cylindrical part which has a clearance in the rear-end part rather than the external thread part in a screw shaft and the opening part side rather than the internal thread part of a screw hole is proposed. However, in such a configuration, although the effect of correcting the mismatch of the central axes can be expected, the diameter of the cylindrical portion must be larger than the outer diameter of the male screw portion, The outer diameter of the hole part is relative to the outer diameter of the tip of the excavation bit from the viewpoint of securing a space for discharging excavation debris (slime) generated during excavation between the inner periphery of the drilling bit and the like. Therefore, it is difficult to reliably prevent the occurrence of breakage since the thickness of the excavation bit becomes thin in the cylindrical portion.

  The present invention has been made under such a background. While reducing the mismatch between the screw hole and the center line of the screw shaft as described above, the present invention provides a split in the screw hole opening of the drill bit, drill rod, and coupling. An object of the present invention is to provide a drilling tool capable of preventing damage.

  In order to solve the above-described problems and achieve such an object, the present invention provides a screw shaft in which a male screw portion for connecting a drill bit and a drill rod, drill rods, or a drill rod and a coupling is formed. And a screw hole in which a female screw portion is formed, and a cylindrical bearing portion that is fitted in sliding contact with at least the tip end portion of the screw shaft and the hole bottom portion of the screw hole. Are formed around the center lines of these screw shafts and screw holes, respectively.

  In this way, in the excavation tool in which the cylindrical surface bearing portions that are slidably engaged with each other are formed at the tip end portion of the screw shaft and the hole bottom portion of the screw hole, the center portion of the screw shaft and the screw hole is formed by the bearing portion. Of course, since the diameter of the bearing portion is smaller than the inner diameter of the female screw portion of the screw hole, a large thickness is formed between the outer diameter of the drilling bit, the drilling rod, or the coupling. Can be secured. For this reason, for example, even if the bearing portion is formed in the portion of the excavation space for the excavation waste in the excavation bit as described above, high strength can be given to the outer peripheral side of the bearing portion, and the excavation bit is screwed It is possible to prevent a situation where the hole breaks or breaks. This is the same even when the excavation rods are directly connected to each other or when connected via a coupling.

  In the present invention, in addition to forming the bearing portion at the tip end portion of the screw shaft and the hole bottom portion of the screw hole, the rear end portion of the screw shaft and the hole opening portion of the screw hole are also in sliding contact with each other. The cylindrical bearing portions that fit together may be formed around the screw shaft and the center line of the screw hole, respectively. In this case, since the screw shaft is supported at both ends of the hole bottom and the opening of the screw hole, the screw shaft and the screw hole are more accurately compared to, for example, the excavation tool described in Patent Document 1. The center line of the screw shaft can be matched, and the length of the cylindrical surface of the bearing portion of the screw hole opening can be shortened. Even if the portion contacts the opening of the screw hole, it can be prevented that they are greatly deformed and damaged.

  Here, even when the bearing portion is formed only at the tip end portion of the screw shaft and the hole bottom portion of the screw hole, or when it is also formed at the screw shaft rear end portion and the screw hole opening portion, these are slidably fitted to each other. It is desirable that the clearance between the screw shafts and the screw hole bearing portions forming a cylindrical surface to be combined be smaller than the clearance between the male screw portion and the female screw portion. As a result, even if the screw shaft and the center line of the screw hole are inclined due to the looseness of the male and female screw portions, the screw shaft and the center line of the screw hole do not coincide with each other more than the clearance of the female and male screw portions.

  More specifically, the clearance between the bearing portions is desirably 0.1 mm or less in the radial direction. If the clearance is larger than this, even if the clearance is larger than this, the front and rear ends of the screw shaft, the bottom of the screw hole, and the opening Even if the bearing part is formed on the part, the center line of the screw shaft is largely inclined with respect to the screw hole center line due to the looseness of the male and female screw parts, and the damage to the screw shaft and the screw hole opening is surely prevented. There is a risk that it will not be possible. On the other hand, based on the inclination of the center line, the clearances of the bearings are sized such that the inclination of the center line of the screw shaft with respect to the center line of the screw hole is 0.2 ° or less. Is desirable.

  As described above, according to the present invention, the cylindrical shaft-shaped bearing portions that are slidably engaged with each other maintain the screw shaft of the excavation tool and the center line of the screw hole so as to coincide with each other. In addition, it is possible to suppress the inclination of the center line due to the looseness of the screw, and such a bearing portion is formed and fitted to at least the screw shaft tip portion and the screw hole hole bottom portion. The thickness of the tool can be ensured and damage can be prevented from occurring.

It is sectional drawing which shows the excavation tool which connected the excavation bit and excavation rod which are the 1st Embodiment of this invention. It is sectional drawing which shows the detail of the screw shaft and screw hole in the 1st Embodiment of this invention. It is sectional drawing which shows the detail of the screw shaft and screw hole in the modification of the 1st Embodiment of this invention. It is sectional drawing which shows the excavation tool which connected the excavation bits which are the 2nd Embodiment of this invention directly. It is sectional drawing which shows the excavation tool which connected the excavation bits which are the 3rd Embodiment of this invention via the coupling.

  In the first embodiment shown in FIG. 1, the excavation tool is configured by connecting the tip end portion (right side portion in FIG. 1) of the excavation rod 2 to the rear end portion (left side portion in FIG. 1) of the excavation bit 1. . The excavation bit 1 has a substantially bottomed cylindrical shape centered on the center line A with a steel material or the like, and this bottomed portion is used as a front end portion so that the outer diameter is expanded by one step from the rear end portion of the substantially cylindrical shape. A large number of chips made of a hard material such as cemented carbide are implanted on the tip surface. The inner peripheral portion of the rear end portion, which is one step smaller in diameter than the tip end portion, is a screw hole 11, and a female screw portion 12 centering on the center line A is formed on the inner peripheral surface. Yes.

  On the other hand, the excavation rod 2 is also formed of a steel material or the like and has a multi-stage shaft shape centered on the center line B. In this embodiment, the tip portion is a screw shaft 21 and the outer peripheral surface has the above-described surface. A male screw portion 22 that is screwed into the female screw portion 12 is formed around the center line B. The portion on the rear end side of the screw shaft 21 has a hexagonal column shape centered on the center line B, for example, having a smaller diameter than the outer diameter of the tip portion formed so as to increase the diameter of the excavation bit 1 by one step. It is said that. Further, a drilling device (not shown) that applies a rotational force around the center line B and a striking force or thrust force in the direction of the center line B to the drill rod 2 at the rear end portion of the drill rod 2 will be described later if necessary. 2. Connected after another drilling rod is added as in the third embodiment.

  Further, in the excavation rod 2, a supply hole 3 is formed so as to penetrate the excavation rod 2 along the center line B, and a fluid such as air for discharging excavation waste generated during excavation is provided. Supplied from the excavator. On the other hand, the excavation bit 1 is formed with a discharge hole 4 so as to open on the center line A of the bottom surface of the screw hole 11 and communicates with the supply hole 3 when connected to the excavation rod 2. At the same time, the fluid is ejected by opening the tip surface. Further, the drilling waste swept from the hole bottom of the drilling hole by the fluid ejected in this way is sent to the rear end side, and the drilling rod 2 and the drilling hole from the space between the rear end of the drilling bit 1 and the inner periphery of the drilling hole. It is discharged from the hole through the inner circumference.

  And in this 1st Embodiment, it mutually slide-contacts at least to the front-end | tip part rather than the external thread part 22 of the said screw shaft 21, and the hole bottom part rather than the internal thread part 12 of the said screw hole 11 corresponding to this. Bearing portions 23 and 13 having cylindrical inner and outer peripheral surfaces to be fitted are formed around the center lines B and A of the screw shaft 21 and the screw hole 11, respectively. That is, in the screw shaft 21, the most distal portion in the screwing direction to be screwed into the screw hole 11 is the bearing portion 23 whose outer peripheral surface forms a cylindrical surface centered on the center line B, and the rear end of the bearing portion 23. A male screw portion 22 is formed following the side. On the other hand, in the screw hole 11, the most advanced hole bottom portion in the screwing direction is the bearing portion 13 whose inner peripheral surface forms a cylindrical surface centered on the center line A, and the rear end side of the bearing portion 13 Subsequently, a female screw portion 12 is formed.

  Furthermore, in the present embodiment, the cylindrical surface bearing portions 24 and 14 that are also slidably fitted to each other at the rear end portion of the screw shaft 21 and the hole opening portion of the screw hole 11 are provided at the center. They are formed around the lines B and A, respectively. That is, the screw shaft 21 of the present embodiment is formed with the bearing portion 24 whose outer peripheral surface forms a cylindrical surface centering on the center line B on the rear end side after the male screw portion 22 and the screw shaft 21. The hole 11 is formed with a bearing portion 14 having an inner peripheral surface forming a cylindrical surface centered on the center line A following the rear end side of the female screw portion 12, and the bearing portion 14 is formed at the rear end of the excavation bit 1. To be opened.

  Here, the screw shaft 21 having the bearing portion 23 formed at least at the tip portion is screwed into the screw hole 11 having the bearing portion 13 formed at the hole bottom so that the bearing portions 13 and 23 are fitted. The fitting diameter d1 of the bearing portions 13 and 23 is the minimum diameter of the female and male screw portions 12 and 22, that is, the thread diameter of the female screw portion 12, and the screw diameter of the male screw portion 22. Smaller than the diameter of the valley.

  On the other hand, while screwing the male and female screw parts 12 and 22 while fitting the bearing parts 13 and 23 in this way, the bearing part 24 at the rear end of the screw shaft 21 is further fitted into the bearing part 14 at the opening of the screw hole 11. The fitting diameter d2 of the bearing portions 14 and 24 is the maximum diameter of the female and male screw portions 12 and 22, that is, the diameter of the valley of the screw in the female screw portion 12, and the screw in the male screw portion 22. Be larger than the diameter of the mountain. Therefore, the fitting diameters d1 and d2 are d1 <d2.

  However, since these bearing portions 13 and 23 and the bearing portions 14 and 24 are inserted by inserting the bearing portions 23 and 24 of the screw shaft 21 into the bearing portions 13 and 14 of the screw hole 11, Each has a very small clearance. This is the same between the male and female screw portions 12 and 22 in which the male screw portion 22 is screwed into the female screw portion 12. The clearances between the bearing parts 13 and 23 and between the bearing parts 14 and 24 are made smaller than the clearances between the male and female screw parts 12 and 22, respectively. Is set to 0.1 mm or less in the radial direction with respect to the center lines A and B in a state in which they are matched.

  Further, since the clearances are formed in this way, for example, the rear end of the excavation rod 2 in the radial direction with respect to the center line A with respect to the excavation bit 1 which is imperfectly fixed by loosening of screws generated during excavation, for example. When the side is pressed, the center lines A and B become inconsistent so that the center line B of the excavation rod 2 is inclined with respect to the center line A. Here, in particular, in the present embodiment, as described above, the clearances of the bearing portions 13 and 23 and the bearing portions 14 and 24 are made smaller than the clearances of the male and female screw portions 12 and 22. If the rear end side of the excavation rod 2 is pressed downward, the tip upper portion P of the bearing portion 23 in FIG. 2 contacts the bearing portion 13 or the rear end lower portion Q of the bearing portion 24 in FIG. The center line B of the screw shaft 21 inclines upward in FIG. 2 as it goes to the front end side with respect to the center line A of the screw hole 11 until it contacts 14 or both.

  In this embodiment, the inclination (inclination angle) θ of the center line B of the screw shaft 21 with respect to the center line A of the screw hole 11 when contacted in this way is set to 0.2 ° or less. That is, the clearance is set so that the inclination θ does not increase even when the excavation rod 2 is pressed.

  The hole bottom surface 15 of the screw hole 11 and the tip end surface 25 of the screw shaft 21 are formed in a planar shape perpendicular to the center lines A and B, respectively, and the male and female screw portions 12 and 22 are screwed to the screw shaft. By screwing 21 into the screw hole 11, the excavation bit 1 and the excavation rod 2 are connected while the hole bottom surface 15 and the distal end surface 25 are pressed against each other, and the supply hole 3 and the discharge hole 3 are discharged. The hole 4 is communicated.

  In the excavation tool configured as described above, the excavation rod 2 is attached to the bearing portion 13 formed at the bottom of the screw hole 11 having the cylindrical inner peripheral surface with the center line A of the excavation bit 1 as the center. A bearing portion 23 formed at the tip end portion of the screw shaft 21 having a cylindrical outer peripheral surface with the center line B as the center is fitted so as to be in sliding contact with each other, and the center lines A and B coincide with each other. Thus, the excavation bit 1 and the excavation rod 2 are joined. Therefore, the excavation bit 1 can be accurately rotated around the center line A by the rotational force transmitted from the excavator, and efficient excavation can be performed by the striking force and thrust applied in the direction of the center line A.

  And in the said excavation tool, these bearing parts 13 and 23 are formed in the hole bottom part of the screw hole 11, and the front-end | tip part of the screw shaft 21, and the fitting diameter d1 is made into the opening part of the screw hole 11, for example. And the fitting diameter d2 of the bearing portions 14 and 24 formed at the rear end portion of the screw shaft 21 can be reduced as described above. Therefore, in the excavation bit 1, the thickness from the bearing portion 13 to the outer peripheral surface can be ensured to maintain the strength, and the excavation bit 1 tip portion is provided for the space for discharging excavation waste as described above. Even if this bearing portion 13 is formed at the rear end portion reduced in diameter by one step, it is possible to prevent damage such as breakage from the bearing portion 13 to the excavation bit 1, thereby reducing the tool life. Can be extended.

  In addition to this, in the present embodiment, bearing portions 14 and 24 are also formed at the opening portion of the screw hole 11 and the rear end portion of the screw shaft 21, and the bearing portions 13 and 23 at the bottom and tip portions of the hole. Since the screw hole 11 and the screw shaft 21 can be fitted and screwed at both ends of the center lines A and B, the center lines A and B can be aligned more accurately.

  Moreover, the bearing portions 14 and 24 formed in this way have a depth of the screw hole 11 and a female width compared to the drilling tool described in Patent Document 1, for example, in which a cylindrical portion is formed only at the screw hole opening and the screw shaft rear end. When the length of the screw portion 12 is the same, that is, when an attempt is made to secure a fitting length equal to the direction of the center line A, the bearing portion of this opening is equivalent to the amount of the bearing portion 13 formed at the bottom of the hole. The length of 14 can be shortened. Therefore, even if the outer diameter of the excavation bit 1 on the opening side is a small diameter for excavating waste, the portion where the wall thickness is reduced between the bearing portion 14 can be made as small as possible. According to this embodiment, it is possible to prevent the excavation bit 1 from being damaged on the opening side of the screw hole 11.

  The clearance between the bearing portions 14 and 24 and the clearance between the bearing portions 13 and 23 are also smaller than the clearance between the screw hole 11 and the male and female screw portions 12 and 22 of the screw shaft 21 as in this embodiment. It is desirable to be done. If the clearance between the bearing portions 13 and 23 and the clearance between the bearing portions 14 and 24 is larger than the clearance between the male and female screw portions 12 and 22, depending on the length of the bearing portions 13, 14, 23, and 24, the male and female screws The clearance between the parts 12 and 22 determines the mismatch or inclination θ due to the looseness of the screw that occurs during the drilling of the center lines A and B of the screw hole 11 and the screw shaft 21, and these center lines A and B exactly match. There is a risk that it cannot be made to occur.

  More specifically, the clearance between the bearing portions 13 and 23 and between the bearing portions 14 and 24 is desirably 0.1 mm or less in the radial direction as in the present embodiment. It is possible to more reliably prevent the lines A and B from becoming largely inconsistent and the inclination θ becoming larger. Even if the center lines A and B do not coincide with each other and the inclination θ is generated as long as the inclination θ is a very small range of 0.2 ° or less as in the present embodiment, the excavation bit 1 is accurate. Therefore, the excavation bit 1 is not damaged even if the bearing portion 14 is formed on the opening side of the screw hole 11.

  In the present embodiment, the bearing portion 14 is also formed on the opening side of the screw hole 11 of the excavation bit 1 in this way, and the bearing portion 24 at the rear end portion of the screw shaft 21 of the excavation rod 2 is in sliding contact. Although it can be fitted, the opening of these screw holes 11 and the rear end portion of the screw shaft 21 are not provided with such bearing portions 14 and 24 to be fitted, for example, a modification shown in FIG. As described above, the female screw portion 12 of the screw hole 11 may be opened at the rear end of the excavation bit 1 as it is. In the modification shown in FIG. 3 and the second and third embodiments described later, the same reference numerals are assigned to the same elements as those in the first embodiment, and the description thereof is omitted.

  Also in the excavation tool of such a modification, the center line A of the excavation bit 1 and the center line B of the excavation rod 2 are formed by fitting the bottom of the screw hole 11 and the bearing portions 13 and 23 at the tip of the screw shaft 21. It is possible to make them coincide with each other accurately and to prevent the excavation bit 1 from being damaged at the positions of the bearing portions 13 and 23. In addition, in this modified example, since the thinned portion does not occur at the opening of the screw hole 11 of the excavation bit 1, it is possible to prevent damage to the excavation bit 1 more reliably and further improve the tool life. Can be planned.

  Next, FIGS. 4 and 5 show the second and second excavation tools in which the present invention is applied to the connection between the excavation rods 2 instead of the connection between the excavation bit 1 and the excavation rod 2 as in the first embodiment. 3 shows a third embodiment. Among them, in the second embodiment shown in FIG. 4, the excavation rods 2 are directly connected to each other, whereas in the third embodiment shown in FIG. 5, the excavation rods 2 are connected to each other through the coupling 3. ing. However, in these second and third embodiments, the excavation rod 2 has a different rear end portion.

  That is, the rear end portion of the excavation rod 2 in the second embodiment has a hexagonal column shape, which is larger than the portion slightly larger in diameter than the screw shaft 21, for example, an excavation bit. The outer diameter is about the same as the rear end portion of the one-stage reduced diameter, and the rear end portion of the excavation rod 2 is the same as that formed at the rear end portion of the excavation bit 1 of the first embodiment. The screw hole 11 is formed. The supply hole 3 extends along the center line B from the bottom surface 15 of the screw hole 11.

  Accordingly, in such a second embodiment, when the excavation rod 2 connected to the excavation bit 1 as in the first embodiment is inserted into the drilling hole to a predetermined depth by the progress of excavation, The screw shaft 21 at the tip of the next excavation rod 2 is screwed into the screw hole 11 at the rear end of the excavation rod 2, and the excavation tool of the second embodiment is configured by these excavation rods 2. The screw hole 11 and the screw shaft 21 have the same configuration as that of the first embodiment. Therefore, the rear end of the excavation rod 2 on the front end side is prevented from being damaged and the tool life is improved. Hole bending that occurs as a result of the accumulation of the inclination θ generated in the plurality of screw portions can be prevented by making the center lines B coincide with each other, and accurate and efficient excavation can be promoted.

  On the other hand, in the third embodiment, the screw shaft 21 similar to the tip portion is formed at the rear end portion of the excavation rod 2, provided that the screw shafts 21 at both ends of the excavation rod 2 are the male screw portions. The 22 twists are in the same direction. On the other hand, the coupling 5 for connecting the excavation rod 2 is formed of a steel material or the like and has a substantially cylindrical shape centered on the center line C, and the outer diameter is also the hexagonal column shape of the excavation rod 2. The diameter is larger than that of the portion, for example, about the same as the rear end portion of the drill bit 1.

  And in the inner peripheral part of this coupling 5, a pair of the screw holes 11 of the said structure are each in order of the bearing part 14, the internal thread part 12, and the bearing part 13 toward the center part from the both ends of the coupling 5, respectively. The bearing portion 13 is formed in a single cylindrical surface continuous with each other at the center portion of the coupling 5 in the center line C direction. Note that the twist directions of the female screw portions 12 are the same even in the pair of screw holes 11.

  Also in the third embodiment, when the excavation rod 2 on the distal end side (excavation bit 1 side) is inserted into the drilling hole, the screw shaft 21 at the rear end of the excavation rod 2 is connected to one screw hole. The coupling 5 is attached so as to be screwed into 11, and the drilling rod 2 is added by screwing the screw shaft 21 of the next drilling rod 3 into the other screw hole 11 facing the rear end side of the coupling 5. Thus, the pair of excavation rods 2 can be connected.

  In the present embodiment, the screw shafts 21 of the pair of excavation rods 2 are screwed so that the distal end surfaces 25 abut each other, and the supply holes 3 communicate with each other. The excavation tool of the third embodiment is configured by at least one of the pair of excavation rods 2 and the coupling 5.

  Accordingly, also in the excavation tool of the third embodiment, the pair of excavation rods 2 are connected to the coupling 5 so that the center lines B coincide with the center line C, and the center lines B are connected to each other. Can be connected so as to be coaxial with each other, and accurate and efficient excavation can be promoted, and the bottom of each screw hole 11, that is, the inner periphery of the coupling 5 in which the pair of screw holes 11 are formed. Since the bearing part 13 is formed in the center part in the center line C direction and the bearing part 23 in the screw shaft 21 of the excavation rod 2 is fitted, the breakage of the coupling 5 from this part can be prevented, The tool life can be extended.

  In the second and third embodiments as well, as in the modification of the first embodiment, the bearing portion 14 on the opening side of the screw hole 11 and the rear end portion of the screw shaft 21 fitted therein. The bearing portion 24 may not be provided. The shape of the male and female screw portions 12 and 22 may be a trapezoidal screw or a triangular screw in addition to the round screw shape shown in FIG.

DESCRIPTION OF SYMBOLS 1 Drilling bit 2 Drilling rod 5 Coupling 11 Screw hole 12 Female screw part 13, 14 Bearing part of screw hole 11 21 Screw shaft 22 Male screw part 23, 24 Bearing part of screw shaft 21 A Center line of drilling bit 1 B Drilling Center line of rod 2 C Center line of coupling 5 θ Inclination of center lines A and B

Claims (5)

  A drilling tool having a screw shaft formed with a male screw part and a screw hole formed with a female screw part for connecting a drilling bit and a drilling rod, drilling rods, or a drilling rod and a coupling, at least the above-mentioned Cylindrical bearings that are fitted in sliding contact with each other are formed around the center line of the screw shaft and the screw hole, respectively, at the tip of the screw shaft and the hole bottom of the screw hole. Features a drilling tool.   Cylindrical bearings that are slidably engaged with each other are formed on the rear end portion of the screw shaft and the hole opening portion of the screw hole, respectively, with the center line of the screw shaft and the screw hole as a center. The excavation tool according to claim 1, wherein:   The excavation tool according to claim 1 or 2, wherein a clearance between the screw shaft and a bearing portion of the screw hole is smaller than a clearance between the male screw portion and the female screw portion.   The excavation tool according to any one of claims 1 to 3, wherein a clearance between the bearing portions is 0.1 mm or less in a radial direction.   The excavation tool according to any one of claims 1 to 4, wherein an inclination of a center line of the screw shaft with respect to a center line of the screw hole is 0.2 ° or less.

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