GB2492465A - Trepan drill bit with abrasive tip - Google Patents

Abstract

A drill bit (1) includes a mount (2) to be mounted on a power tool; and a body portion (4A) having a hollow cylindrical shape with center axis (X), a cylindrical surface, a base end, and a tip end (4B). The body portion being connected with the mount at the base end to rotate coaxially with the mount portion and cut a workpiece. A flat tip-end portion is provided at the tip end extending in a direction perpendicular to the axial direction. An outer circumferential portion (42) is provided adjacent to the tip end of the body portion; and a connecting face portion (43) is provided between the outer circumferential portion and the tip-end portion, all of which are covered with abrasive grains to form an abrasive grain layer (5A). The connecting face may taper towards the tip-end portion. The thickness of abrasive grain layer on the connecting face may be thicker than the layer on the tip-end portion.

Description

Drill bit The invention relates to a drill bit and, more specifically, to a drill bit that is capable of boring concrete and rigid tile.

Japanese Patent Application Publication No. 2011-121323 discloses a drill bit having a body section of substantially a cylindrical shape and having diamond abrasive grains provided at a tip end of the body section. Tn the drill bit, diamond abrasive grains are provided at a tip-end section located at a tip-end surface of the cylindrical shape and at an outer circumferential surface extending from the tip-end pianar section toward a base-end side. With the diamond abrasive grains, the drill bit grinds and bores a workpiece such as concrete and rigid tile.

In the above-described drill bit, cutting is performed by diamond abrasive grains.

Hence, the cuffing capability decreases if diamond abrasive grains are lost due to falling off, abrasion, etc. In view of the foregoing, it is an object of the invention to provide a drill bit that suppresses a decrease in the boring capability.

The present invention features a drill bit comprising: a mount portion configured to be mounted on a power tool; and a body portion having a hollow cylindrical shape having a center axis extending in an axial direction, the body portion having a cylindrical surface, a base end, and a tip end, the body portion being connected with the mount portion at the base end so as to rotate coaxially together with the mount portion and cut a workpiece with the tip end, the body portion further comprising a flat tip-end portion provided at the tip end and extending in a direction perpendicular to the axial direction; an outer circumferential portion provided adjacent to the tip end of the body portion on the cylindrical surface; and a connecting face portion provided between the outer circumferential portion and the tip-end portion, wherein the outer circumferential portion, the connecting face portion, and the tip-end portion are covered with an abrasive grain by bonding material to form an abrasive grain layer, an abrasive grain having a diameter, the connecting surface portion in a cross-section including the center axis has a length which is twice to triple of the diameter of the abrasive grain.

Preferably, at least two particles of the abrasive grains are arranged on the length of the connecting surface portion.

Preferably, the connecting face portion has a tapered shape toward the tip end and the length is a generatrix.

Preferably, a radial length of the tip-end portion in the direction perpendicular to the axial direction is ito 1.6 times as large as the particle diameter of the abrasive grain.

Preferably, the abrasive grain layer has a thickness which is 1.5 to 2 times as large as the particle diameter of the abrasive grain on the connecting surface.

Preferably, wherein the abrasive grain layer has a thicker thickness on the connecting face portion than the thickness on the tip-end portion.

Preferably, the abrasive grain comprises a diamond abrasive having #30 to #200 mesh.

According to a drill bit of the present invention, a decrease in the boring capability can be suppressed.

Embodiments in accordance with the invention will be described in detail with reference to the following figures wherein: Fig. 1 is a side view of a drill bit according to an embodiment of the invention; Fig. 2 is a partial cross-sectional view showing a state in which a tip-end portion of the drill bit according to the embodiment is inserted in a forming jig; Fig. 3 is a side view showing a state in which the drill bit according to the embodiment is mounted on a power tool; Fig. 4 is a graph showing relationships between chamfer dimensions and boring ratios; Fig. 5 is a cross-sectional view showing a state in which the tip-end portion of the drill bit according to the embodiment is inserted in the forming jig; Fig. 6 is a view showing a state in which the power tool is driven in a state where the drill bit according to the embodiment is mounted on the power tool; Fig. 7 is a view showing relationships of forces acting on the drill bit according to the embodiment; and Fig. 8 is a partial cross-sectional view showing a state in which a tip-end portion of a drill bit according to a modification is inserted in a forming jig.

is A drill bit according to some aspects of the invention will be described while referring to Figs. 1 through 7. A drill bit 1 shown in Fig. 1 mainly includes a mount section 2 to be mounted on an impact driver 1A (Fig. 3) serving as a power tool, a body section 4 having a hollow cylindrical shape having an axis X and formed integrally with the mount section 2 so as to rotate coaxially together with the mount section 2, and a flange section 3 located between the mount section 2 and the body section 4.

The mount section 2 is a so-called hexagonal shaft having a hexagonal shape in cross-section. As shown in Fig. 3, the mount section 2 enables the drill bit 1 to be mounted on a chuck of the impact driver 1A. The size of the hexagonal shape is specified in accordance with a predetermined industrial standard such as Japanese Industrial Standards (JIS). A recessed section 2a is formed continuously in the circumferential direction, approximately at the center of the mount section 2 in the axial direction. Balls of a so-called single-touch (quick-operation) attachment mechanism (not shown) of an impact driver etc. can fit into the recessed section 2a.

The flange section 3 has a disc shape of which outer diameter is larger than the outer diameter of the body section 4. The flange section 3 is formed integrally with the mount section 2 and the body section 4 so as to be coaxial with the mount section 2. The flange section 3 serves to reduce dust that is generated during a boring operation and that directly flies toward the operator side, and also serves to define a maximum boring depth of the drill bit I. The body section 4 is connected at one end side of the mount section 2 in the axial direction with the flange section 3 interposed therebetween. The body section 4 has a hollow cylindrical shape of which the center axis is coaxial with the rotational axis of the mount section 2. In the following descriptions, a lip-end direction is defined as an axial direction fiom the mount section 2 toward the body section 4, a base-end direction is the direction opposite the tip-end direction, and a radial direction is a direction perpendicular to the axial direction. The body section 4 has an outer diameter of approximately 14.5 mm, which is identical to an outer diameter of a cuffing blade 5 described later. The body section 4 includes a main body portion 4A located at the base-end side and a tip end portion 4B located at the tip-end side of the main body portion 4A. Each of the main body portion 4A and the tip end portion 4B has a hollow cylindrical shape.

The main body portion 4A has a wall thickness (thickness in the radial direction) of approximately 1.5 mm so as to reduce cutting resistance and to maintain its strength.

The base-end side of the main body portion 4A is connected with the flange section 3.

A helical groove 4b is formed on the outer circumferential surface of the main body portion 4A. The axis of the helical groove 4b (the axis of the helix) is the rotational axis of the body section 4. The helical groove 4b can lead dust, which is generated when a workpiece W (Fig. 3) such as concrete is cut with the cutting blade 5, to outside the bore and can discharge the dust. A core discharge hole 4a having an oval shape extending toward the tip-end side is formed in the main body portion 4A at a position adjacent to the connection with the flange section 3.

The tip end portion 4B has the same inner diameter and outer diameter as those of the main body portion 4A. The tip end portion 4B is located at the tip-end side of the main body portion 4A. A notch 4c is formed at the same position as the position of the core discharge hole 4a in the circumferential direction and extends from the tip edge of the tip end portion 4B toward the base-end side. The cutting blade S is provided adjacent to and on the surface of the tip end portion 4B. As shown in Fig. 2, the cutting blade 5 includes an abrasive gain layer 5A including diamond abrasive grains SB of #35 mesh (mesh opening 500 jim) with brazing material portion 5C that retains the diamond abrasive grains SB and integrates the diamond abrasive grains SB with the tip end portion 4B.

The body portion 4A further includes a tip-end portion 41, an outer circumferential portion 42, and a connecting face portion 43. The tip-end portion 41 is provided at the tip end of the tip end portion 4B. The tip-end portion 41 extends in a planar shape in a direction perpendicular to the axial direction. The outer circumferential section 42 is provided at the circumference of the tip end portion 4B. A cross-section of the outer circumferential section 42 perpendicular to the axial direction has a ring shape. The outer circumferential section 42 has a radial length in the direction perpendicular to the axial direction.

The connecting face section 43 is provided between the tip-end portion 41 and the outer circumferential portion 42 so as to serve as a connecting surface between the sections 41 and 42. The connecting face portion 43 has a tapered shape to the tip-end portion 41.

In another embodiment, the connecting face portion 43 may have a frustoconical shape or a truncated cone shape. The connecting face portion 43 may have a tapered face to the tip-end portion 41. The connecting face portion 43 may have a slope shape connecting the tip-end portion 41 and the outer circumferential portion 42.

The connecting face portion 43 may be a chamfered surface having a chamfer dimension 0.7. As described above, because the wall thickness (radial length) of the tip end portion 4B has the same value as a wall thickness T of the main body portion 4A, a radial length Ll (length in the radial direction) of the tip-end portion 41 is 0.8 mm. This value is included in a range of 1 to 1.6 times as large as the particle size (particle diameter) of the diamond abrasive grain SB. Further, a distance from the connection of the connecting face portion 43 and the tip-end portion 41 to the connection of the connecting face portion 43 and the outer circumferential portion 42, that is, a length L2 of the connecting face portion 43 in a cross-section including the center axis of the body section 4 is 1.0 mm. The length L2 of the connecting face portion 43 is so-called a generatrix of a tapered shape. This value is greater than or equal to 2 times as large as the particle size of the diamond abrasive grain SB.

And, the tip-end portion 41 has such a size that at least one particle of the diamond abrasive grain SB is able to be arranged in the radial direction. Thus, one particle of the diamond abrasive grain SB is arranged at the tip-end portion 41 in Fig. 2, The connecting face portion 43 has such a size that at least two particles of the diamond abrasive grain SB are arranged within the length in the cross-section including the center axis of the body section 4. Thus, two or more particles of the diamond abrasive grain 5B are arranged within the length of the connecting face portion 43.

In forming the cutting blade 5, after the brazing material SC consisting mainly of nickel metallic powder is applied to the tip end portion 4B, the diamond abrasive grain SB is sprinkled on the brazing material SC. The tip end portion 4B is then inserted in a concave section lOa of a forming jig 10 shown in Fig. 5. Then, by heating the tip end portion 4B, the diamond abrasive grains SB are held at the tip end portion 4B with the brazing material SC in a state where the diamond abrasive grains SB are arranged in the forming jig 10. As shown in Fig. 2, the shape of the concave section lOa of the forming jig 10 is so configured that a distance L3 between the tip-end portion 41 and a bottom surface of the concave section 1 Oa (the distance L3 is also a distance between the outer circumferential portion 42 and a side surface of the concave section lOa) is approximately 0.7 mm, and that a portion of the concave section lOa confronting the connecting face portion 43 has a rounded shape. In this state, the abrasive gain layer SA is formed such that a maximum distance L4 between the connecting face portion 43 and the rounded-shape portion of the concave section lOa is 1.5 to 2 times as large as the particle diameter of the diamond abrasive grain SB. With this configuration, the diamond abrasive grains SB in the abrasive gain layer 5A arc supported on the surface of the connecting face portion 43, and the diamond abrasive grain SB is further stacked on the diamond abrasive grain SB which has already been placed on the connecting face portion 43 in the abrasive gain layer SA. That is, one or more particle of the diamond abrasive grain SB can be arranged in a direction normal (perpendicular) to the connecting face portion 43 shown in Fig. 2. With this configuration, even if particles of the diamond abrasive grain SB located at the outermost position fall off or separate, grinding can still be performed with the abrasive grain attached to the connecting face portion 43.

Further, as described above, the distance L4 between the connecting face portion 43 and the rounded-shape portion of the concave section lOa is larger than the distance L3 between the tip-end portion 41 and the bottom surface of the concave section IDa.

Hence, the thicker brazing material 5C can be applied to the connecting face portion 43 than to the tip-end portion 41, and a larger number of particles of the diamond abrasive grain 5B can be attached to the connecting face portion 43 than to the tip-end portion 41.

This improves retaining performance of the diamond abrasive grain SB at the connecting face portion 43 and increases bonding strength between the diamond abrasive grain 5B and the brazing material 5C. This also suppresses falling off and separation of the diamond abrasive grain 5B and improves retaining performance of the diamond abrasive grain 5B at the connecting face portion 43.

As a result of boring operations with the above-described drill bit 1, it has been found that the largest load acts on the connecting face portion 43, not on the tip-end portion 41, during a boring operation. The following reasons can be conceived.

First, the center axis of the outer circumferential portion of the blade tip shifts from the center axis of the mount section 2 due to a tolerance in mounting of the drill bit I onto the impact driver 1 A, dimension errors of the impact driver 1 A and the drill bit 1, or tolerance and circularity of the blade tip of the drill bit 1. Further, as shown in Fig. 6, the drill bit 1 rotates while the tip end makes a sweeping motion about the center axis, due to centrifugal force. Hence, when boring is performed in this state, as shown in Fig. 7, with respect to downward force S 1 in a direction in which the drill bit 1 progresses, radial force 82 and oblique downward force S3 are generated. The radial force S2 is centrifugal force which acts in a radial direction. The oblique downward force S3 is the resultant force of the downward force 81 and the radial force 52. It can be understood, from the arrow lengths of the forces, that the oblique downward force 83 is the largest.

Based on the above, it has been confirmed from experiments that, even if some diamond abrasive grains 5B of the tip-end portion 41 are lost due to falling off or the like, boring can still be performed as long as the diamond abrasive grain SB at the outer circumferential portion 42 remains.

Based on the above experimental result, in order to find the optimal value (dimension) of the connecting face portion 43, boring-ratio tests have been performed with four drill bits.

The four drill bits have slope sections of chamfer dimensions 0.4, 0.45, 0.6, and 0.7, respectively, and the remaining portions are identical. Here, the boring-ratio tests are tests for examining how many holes with a predetermined depth can be formed for a fixed time period. The test results are shown in the graph of Fig. 4, in which the boring ratios are shown in percentage in the vertical axis. According to the test results, the boring ratio increases only by 30% when the chamfer dimension changes from 0.4 to 0.6, but the boring ratio increases by as much as 4S% when the chamfer dimension changes from 0.6 to 0.7. Thus, it is preferable that the chamfer dimension be larger than or equal to 0.7W Further, when the chamfer dimension is 0.7, the length L2 of the connecting face portion 43 is 1.0 mm, which is a length in which two or more particles of the diamond abrasive grain SB can be arranged, as described above. Thus, the chamfer dimension is 0.7 in the present embodiment.

If the chamfer dimension is increased, the radial length Li of the tip-end portion 41 decreases, and the radial length Li becomes a length in which one or more particle of the diamond abrasive grain SB cannot be arranged. Hence, it is not preferable to increase the chamfer dimension excessively. Specifically, if the chamfer dimension is larger than or equal to 1.0, the radial length Li becomes smaller than or equal to 0,5 mm, and one or more particle of the diamond abrasive grain SB cannot be arranged at the tip-end portion 41. Thus, it is preferable that the chamfer dimension be 0.7 to 1.0. With this range, the radial length Li is 1 to 1.6 times as large as the particle size of the diamond abrasive grain SB, and the length L2 of the connecting face portion 43 is 2 to 3 times as large as the particle size of the diamond abrasive grain SB.

While the invention has been described in detail with reference to the above aspects thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the claims. For example, although the diamond abrasive grain SB of #35 mesh is used in the above-described embodiment, the disclosure may not be limited to the specific embodiment thereof. With diamond abrasive grain of #30 to #200 mesh, concrete and rigid tile can be ground appropriately. Further, although an impact driver has been described as an example of a power tool, the disclosure may not be limited to the specific embodiment thereof For example, a drill bit can be mounted on a drill etc. that outputs rotational force for performing operations.

As described above, retaining performance of the diamond abrasive grain SB at the connecting face portion 43 improves due to the thickness difference of the brazing material 5C at the connecting face portion 43 and at the tip-end portion 41. This improvement in the retaining performance can be obtained, irrespective of the relationships between the lengths of the connecting face portion 43 and the tip-end portion 41 and the particle diameter of the diamond abrasive grain 5B.

Although the connecting face portion 43 has a planar shape in the above-described embodiment, the disclosure may not be limited to the specific embodiment thereof As shown in Fig. 8, for example, a slope section 143 may have a concave shape. This configuration can increase the area of the slope section 143 and hence increase the area in which the brazing material SC adheres, thereby increasing the quantity of the brazing material SC applied to the slope section 143. By increasing the quantity of the brazing material SC, a larger number of particles of the diamond abrasive grain SB can be retained, and thus the cutting performance at the slope section 143 can be ensured more reliably.

Claims (1)

1. <claim-text>Claims l.A drill bit comprising: a mount portion configured to be mounted on a power tool; and a body portion having a hollow cylindrical shape having a center axis extending in an axial direction, the body portion having a cylindrical surface, a base end, and a tip end, the body portion being connected with the mount portion at the base end so as to rotate coaxially together with the mount portion and cut a workpiece with the tip end, the body portion further comprising a flat tip-end portion provided at the tip end and extending in a direction perpendicular to the axial direction; an outer circumferential portion provided adjacent to the tip end of the body portion on the cylindrical surface; and a connecting face portion provided between the outer circumferential portion and the tip-end portion, wherein the outer circumfcrential portion, thc connecting face portion, and thc tip-end portion are covercd with an abrasive grain by bonding material to form an abrasivc grain layer, an abrasivc grain having a diameter, the connecting face portion in a cross-section including the center axis has a length which is twice to triple of the diameter of the abrasive grain.</claim-text> <claim-text>2. The drill bit according to claim 1, wherein at least two particles of the abrasive grains are ananged on the length of the connecting face portion.</claim-text> <claim-text>3. The drill bit according to claim 1 or 2, wherein the connecting face has a tapered shape toward the tip end and the length is a generatrix.</claim-text> <claim-text>4. The drill bit according to claim 1 or 2, wherein a radial length of the tip-end portion in the direction perpendicular to the axial direction is I to 1.6 times as large as the particle diameter of the abrasive grain.</claim-text> <claim-text>5. The drill bit according to claims 1, wherein the abrasive grain layer has a thickness which is 1.5 to 2 times as large as the particle diameter of the abrasive grain on the connecting face.</claim-text> <claim-text>6. The drill bit according to claims 1 or 5, wherein the abrasive grain layer has a thicker thickness on the connecting face portion than the thickness on the tip-end portion.</claim-text> <claim-text>7. The drill bit according to claim 1, wherein the abrasive grain comprises a diamond abrasive having #30 to #200 mesh.</claim-text> <claim-text>8. A drill bit substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings.</claim-text> <claim-text>9. A drill bit substantially as hereinbefore described with reference to Figures 1 to 7 of the accompanying drawings as modified by Figure 8 of the accompanying drawings.</claim-text>