Percussion or hammer drill

[0001] This invention concerns a percussion drill or hammer drill.

[0002] During drilling, a percussion or hammer drill, also known as a masonry, concrete or stone drill, performs a percussive movement along the drilling axis and a rotary movement about the drilling axis. Both components of its movement contribute to the removal of material in the drill hole. The axial movement shatters the material in the drill hole. The rotary movement, by abrasion, causes a reduction of the material into drilling dust and carries the drilling dust away out of the drill hole.

[0003] Known percussion or hammer drills consist of a drill shaft with a hardened metal plate, the cutting plate, set in it. Spiral grooves run along the shaft to evacuate the drilling dust from the drill hole. The cutting plate extends across the diameter of the drill shaft. At its exposed front surface, chip surfaces and free surfaces arranged in a wedge shape form cutting edges. These consist of two linear main edges offset parallel to a plane in which the drilling axis lies, and a transverse edge linking the two main edges through the drilling axis. In order to achieve satisfactory centering when starting to drill, there is an angle not exceeding 130° between the two main cutting edges.

[0004] A task of this invention was to optimize removal of material in the drill hole by suitable design of the cutting plate. This task is fulfilled by a percussion or hammer drill in accordance with claim 1 or 15.

[0005] Such a percussion or hammer drill, also designated by the generic term “masonry drill”, consists of the standard layout of a drill shaft with a cutting plate set in its head end. This cutting plate extends right across the diameter of the drill shaft and displays an exposed front surface. In this front surface—in a first embodiment—chip surfaces and free surfaces arranged in a wedge shape form linear main cutting edges diametrically opposite to each other. In this arrangement, a plane running through the drilling axis forms a central plane of both main cutting edges. With this arrangement of the main cutting edges, the percussive energy is transmitted into the material more advantageously than with two main cutting edges offset parallel to a plane in which the drilling axis lies. Thus the percussive energy is applied more effectively for demolishing material in the drill hole.

[0006] Further optimization of material removal in the drill hole is achieved by the fact that the two main cutting edges meet at an apex angle that is greater than 130° and lies preferably in the region of 150° to 170°, for example 155° to 165°. The increase in apex angle similarly yields more effective application of the percussive energy for removal of material in the drill hole.

[0007] In order to achieve satisfactory centering when starting to drill with a wide apex angle, it is advantageous if a centering point is provided between the two diametrically opposite main cutting edges. The apex angle of this central point is then smaller than the apex angle between the two main cutting edges. It may lie, for example, in the range of 80° to 130°.

[0008] Between the centering point and the chip surfaces or the free surfaces, as the case may be, it is advantageous to provide rounded transition zones, in order to prevent stress concentrations between the centering point and the main cutting edges.

[0009] The centering point may be formed in plane symmetry with two planes perpendicular to each other, both of which pass through the drilling axis and one of which also constitutes the central plane of the two main cutting edges. Such plane symmetry makes it possible to design a centering point that contributes to high material removal performance, high stability and outstanding resistance to wear of the cutting plate. As an alternative, the centering point can be designed with rotational symmetry.

[0010] According to another embodiment of the invention, removal of material from the drill hole is optimized through corresponding shaping of the cutting plate. If the apex angle between the main cutting edges increase radially from the inside towards the outside, the main cutting edges are better matched to the prevailing loads and removal of material is optimized. The apex angle is then increased steadily in an outward direction along a main cutting edge by 20° to 40°. In this arrangement, the smallest apex angle then advantageously lies in the range between 70° and 90°, and the greatest apex angle lies advantageously in the range between 90° and 130°.

[0011] In this connection it should be mentioned that in this embodiment, the two main cutting edges may, but need not, lie diametrically opposite to each other.

[0012] Furthermore, the wear resistance of the cutting plate can be improved if the angle between the angle bisector of the cutting wedge and the mid-plane of the two main cutting edges increases in an outward direction along the main cutting edges. In this respect, it has been shown to be advantageous if this angle is increased from about 5° to 25°.

[0013] The stability of the cutting plate is further enhanced by a cutting wedge that is rounded off at the tip, the radius of this rounding being greater in the outer zone than in the inner zone.

[0014] In addition, it has proved advantageous, with the tip angles of 150° to 170° used here, to increase the stability of the cutting edge by a protective chamfer of the outer edge.

[0026] The percussion or hammer drill 12 shown in FIG. 1 in a drill hole 10 consists of a drill shaft 14 in the head end 16 of which is set a hard metal plate 18, the drill plate or cutting plate as it is also known. This extends right across the diameter of the head end 16. Spiral grooves 20, 20′ run along the drill shaft 14 to carry away the drilling dust out of the drill hole 10. The drilling axis is designated by the reference number 21. The percussion drill 12 performs a percussive movement along the direction of the drilling axis 21 and a rotary movement (see arrow 22) about the drilling axis 21. Both of these components of its motion contribute to the destruction of material in the drill hole 10. The axial movement shatters the material in the drill hole 10. The rotary movement causes reduction of the material into drilling dust by abrasion and carries away the drilling dust out of the drill hole 10.

[0027] A first embodiment of a cutting plate 18 for a percussion or hammer drill 12 according to the invention will be described with reference to FIGS. 2 to 6. Such a cutting plate 18 comprises a fixing shaft 24, to all intents and purposes prismatic in shape, which is welded into a corresponding slit in the head end 16 of the drill shaft 14 (see also FIG. 3, in which the outlines of the head end 16 are indicated by a broken line). The fixing shaft 24 is provided with narrow sides 26 and 26′, each formed of a cylindrical face 28, 28′ and a flat face 30, 30′. The cylindrical face 28, 28′ in each case precedes the flat face 30, 30′ in its direction of rotation and ensures that the drill 12 follows the drill hole 10. The flat face 30, 30′ is set slightly back with respect to the diameter of the drill hole 10, thus reducing the friction of the narrow sides 26 and 26′ in the drill hole.

[0028] The end of the cutting plate 18, which protrudes axially from the head end 16 of the drill shaft 14, displays a profiled front face 32, the profile of which is described in greater detail below. In this exposed front face 32, the chip surfaces 34, 34′ in combination with the free surfaces 36, 36′ respectively are arranged together in a wedge shape so as to form the main cutting edges 38, 38′. As can be seen most clearly in FIG. 3, the cutting plate 18 is provided with two linear main cutting edges 38, 38′, arranged diametrically opposite to each other so that the plane 40 through the drilling axis 21 constitutes a central plane of the two main cutting edges 38, 38′. In the cutting plate 18 shown, this central plane 40 forms an angle 44 of approximately 8° with the central plane 42 of the fixing shaft 24. In this arrangement, the central plane 42 intersects each of the two narrow sides 26, 26′ just behind the transition point between the flat faces 30, 30′ and the cylindrical faces 28, 28′.

[0029] FIG. 2 shows how the two main cutting edges 38, 38′ slope down from the inside towards the outside. In the central plane 40, they form what may be termed an apex angle 46, of 162°, for example, in the cutting plate 18 shown (in previous hammer drills this apex angle was no greater than 130°). The effect of the very blunt apex angle 46, together with the common central plane 40 of the two main cutting edges 38, 38′, is that the percussive energy during drilling is strongly concentrated in the material being drilled, while friction is low. These two features thus contribute to a significant optimization of shattering of the material in the drill hole 10.

[0030] As can best be seen in FIGS. 2 and 5, the two main cutting edges 38, 38′ are separated by a centering point 48 which is centered on the drilling axis 21. This centering point 48 is in plane symmetry with respect to the two planes 40, 70, which are perpendicular to each other. The first plane 40 is the central plane described in greater detail above. The second plane 70 also includes the drilling axis 21 and is perpendicular to the central plane 40. As can be seen in FIG. 3, the centering point 48 is oval in cross-section, the longer axis of the oval lying in the central plane 40 and the shorter axis in the plane 70. As can be seen in FIG. 5, the centering point 48 displays more or less the shape of a cutter such as is commonly used in mining. However, it is significantly more blunt in shape in the direction of the central plane 40. It should also be noted that the centering point 48 contributes to higher boring performance, greater stability and outstanding resistance to wear of the cutting plate 18. The rounded transition surfaces 52, 52′ are designed to prevent phenomena of stress concentration between the centering point 48 and the main cutting edges 38, 38′, which could generate stress peaks leading to fracture during drilling. Another point to note is that the transition surfaces 52, 52′ in the transition zones between the centering point and the chip surfaces can have a different radius of curvature from those between the centering point and the free surfaces.

[0031] The cutting wedge constituted by the chip surface 34 and the free surface 36 will now be described in greater detail with reference to FIG. 6. This cutting wedge will be defined, for each point on a main cutting edge 38, 38′, by a tangent 54 to the free surface 36 and a tangent 56 to the free surface 36, in a sectional plane which is perpendicular to the central plane 40 and parallel to the axis of rotation 21. As the projections of the chip surfaces 34, 34′ and the free surfaces 36, 36′ in the cutting plate 18 of FIG. 6 in the sectional plane are largely flat, the tangents 54, 56 effectively constitute the lines of intersection between the chip surface 34, 34′, resp. free surface 36, 36′ and the sectional plane.

[0032] It should be noted that the point of the cutting wedge is rounded, or to put it another way, each of the main cutting edges 38, 38′ is rounded off. A large edge radius here favors the stability of the cutting plate. A smaller edge radius, on the other hand, favors drilling performance. In the cutting plate 18, the edge radius of the main cutting edges 38, 38′, as can best be seen in FIG. 3, is fairly constant in the inner zone, but becomes significantly greater in proximity to the narrow sides 26, 26′. By this means, the main cutting edges 38, 38′ are strengthened in a particularly critical outer zone, but in the inner zone display a relatively small edge radius, which favors drilling performance.

[0033] Returning to FIG. 6, it may also be noted that the angle of the cutting wedge (referred to from now on as the wedge angle 57) along the main cutting edges 38, 38′ is not constant, but increases from the inside to the outside. In the cutting plate 18 in FIG. 6, for example, the wedge angle 57 shows a linear increase with the radius, from about 80° at the two transition surfaces 52, 52′ to about 110° at the two narrow sides 26, 26′. It can also be observed that the orientation of the cutting wedge along the main cutting edges 38, 38′ is not constant either. This orientation is measured as the angle 58 between the angle bisector 60 of the cutting wedge and the central plane 40. In the cutting plate 18 in FIG. 6, this angle 58 increases with the radius, from about 5° at the two transition surfaces 52, 52′ to about 25° at the two narrow sides 26, 26′. Both the radially varying orientation of the cutting wedge and the radially varying wedge angle 57 of the cutting wedge give improved stability to the cutting plate 18. This therefore becomes significantly stronger in its radially outer zone, that is to say where its tangential velocity is highest, and nevertheless displays outstanding drilling performance. It should also be noted that a larger wedge angle 57 in the outer zone results in a larger amount of material, reducing wear on the corners of the cutting plate, something that has to be kept in view during percussion or hammer drilling. Such wear on the corners leads, among other effects, to a reduction in the diameter of the drill hole 10, so that slowing down wear on the corners extends the life of the drill 12.

[0034] In addition, with the very blunt apex angle 46 used here, it has proved advantageous to provide the outer edge of cutting edge, chip face and free face with a protective chamfer 54, 54′ as a means of further increasing the stability of the cutting plate. The shape illustrated for the protective chamfer 54, 54′ is only one of a variety of possible embodiments.

[0035] The cutting plate 18 in FIGS. 7 to 10 differs from the cutting plate 18 shown in FIGS. 2 to 6 primarily in the design of the centering point 48. This no longer displays plane symmetry with two planes perpendicular to each other, but displays rotational symmetry instead. In this arrangement, the centering point 48 is given an apex angle 50 which is significantly smaller than the apex angle 46 between the two main cutting edges 38, 38′, in order to enable satisfactory centering of the drill when starting a hole. In the cutting plate 18 illustrated, the apex angle 50 shown as an example for the centering point 48 is 90°, that is to say 72° less than the apex angle 46 of the two main cutting edges 38, 38′. The centering point 48 of the cutting plate 18 is made to all intents and purposes rotationally symmetrical, with transition surfaces 52, 52′ enabling a rounded transition towards the chip surfaces 34, 34′ and the free surfaces 36, 36′.

[0036]

Key to references
	10	Drill hole
	12	Percussion or hammer drill
	14	Drill shaft
	16	Head end
	18	Hard metal plate
	20, 20′	Spiral grooves
	21	Drilling axis
	22	Direction of rotation
	24	Fixing shaft
	26, 26′	Two narrow sides
	28, 28′	Cylindrical face
	30, 30′	Flat face
	32	Front face
	34, 34′	Chip surface
	36, 36′	Free surface
	38, 38′	Main cutting edges
	40	Central plane
	42	Central plane
	44	Angle
	46	Apex angle
	48	Centring point
	50	Apex angle
	52, 52′	Transition surfaces
	54, 54′	Protective chamfer
	57	Wedge angle
	58	Angle
	60	Angle bisector