WO1994027773A1

WO1994027773A1 - Trepanning tool for drilling machines

Info

Publication number: WO1994027773A1
Application number: PCT/FI1994/000197
Authority: WO
Inventors: Eero Joona
Original assignee: Joona, Anna-Liisa; Joona, Erkki; Joona, Aaro
Priority date: 1993-05-21
Filing date: 1994-05-17
Publication date: 1994-12-08
Also published as: FI867U1; AU6652294A; FIU930297U0

Abstract

According to the invention, at least two cutting blades (3a, 3b) are attached to a body (4) on the opposite sides of a centre bit (2) at substantially an equal distance from the centre bit (2). The cutting point (6a) of the first blade (3a) is on the outer edge of the blade (3a), and the cutting point (6b) of the second cutting blade (3b) is on the inner edge of the cutting blade (3b) whereby, when a hole is drilled, a wide cut is formed in the object drilled by the action of the cutting blades (3a, 3b).

Description

TREPANNING TOOL FOR DRILLING MACHINES

The invention relates to a box drill bit comprising a fastening rod for securing the box drill bit to the chuck of the drilling machine, a centre bit on the same longitudinal axis as the fastening rod, a blade at a distance from the centre bit, said blade cutting substantially in the direction of the longitudinal axis of the centre bit, and a body portion connecting the fastening rod, the cutting blade and the centre bit.

In a conventional box drill bit a planar portion is mounted on the rod of a normal drill bit, and an annular saw blade is attached to the planar portion at a distance of a certain radius from the centre line of the drill bit for cutting a hole for a box. When a hole is drilled, the normal drill bit, which serves as a guide, is at first pressed against the object to be drilled. When the annular saw blade has travelled such a distance that it makes contact with the object to be drilled, it presses against the surface of the object and penetrates into the object at a certain radius from the guiding bit. A narrow cut is thus formed in the object. When the bit drills through a board, for instance, an annular disc, in the middle of which there is the guiding hole made by the normal drill bit, loosens from the board. Also, some known box drill bits are provided with an adjustable cutting point which is attached to the rod of the drill bit. The cutting point which is attached to the drill bit rod by means of a slide, whereby it moves substantially perpendicularly to the longitudinal axis of the bit. In this case cutting is only one-sided, wherefore when the blade rotates, the hole becomes easily asymmetric and the cut is usually not clean. An annular cutting blade is easily jammed in its cut and does not make a clean cut. For instance, when the annular blade has penetrated through a board, it may be stuck between the disc and the board, in which case it is difficult to detach the blade from the hole. In addition, the manufacture of conventional box drill bits is usually very complicated, and the time and special materials required for manufacturing them make them expensive. It is also rather difficult to maintain and sharpen them.

The object of the present invention is to provide a box drill bit by which the above-described problems can be avoided.

The invention is characterized in that at least two cutting blades are attached to the body on the opposite sides of the centre bit at substantially an equal distance from the centre bit, that the cutting point of the first blade is on the outer edge of the blade, and that the cutting point of the second cutting blade is on the inner edge of the cutting blade whereby, when a hole is drilled, a wide cut is formed in the object drilled by the action of the cutting blades. An essential feature of the invention is that when a hole for a box, e.g. a branch box, is drilled in a board or the like there is formed in the board, in addition to a centering hole, a cut as large as the circumference of the box and a cut slightly smaller than the circumference of the box, whereby the blades in conjunction with each other chip a wide cut in the wall board. A further essential feature is that the blades for cutting a box hole are shaped in such a manner that the blade making the outer cut supplies the cuttings towards the centre bit while the other cutting blade of the box drill supplies its cuttings away from the centre bit so that the centre of the box drill is not clogged by the cuttings. As the blades chip the board from the opposite edges of the cut, the force exerted on the blades is even and helps to center the box drill bit. Yet another advantageous feature is that the centre bit, which centers the box drill bit, is both conical and angular; when pressed against the object, it retards the blade penetration rate and keeps the box drill bit accurately centered.

An essential advantage of the invention is that when a box hole is being drilled, the centre bit keeps the box drill bit accurately centered. The box hole is cut in the board in such a manner that one of the blades cuts the board and the other one cuts the disc to be detached from the board; this prevents the bit from being clogged and jammed. According to a preferred embodiment, the blades narrow slightly from the cutting points towards the rod in the lateral direction of the cut, whereby the upper parts of the blades are not in contact with the edges of the cut, and the blades are thus not easily jammed. Furthermore, owing to the advantageous shape of the center bit, the penetration rate of the box drill bit is suitable, which thus allows the blades to chip evenly.

The invention will be described in greater detail in the accompanying drawings, in which

Figure 1 is a side view of the box drill bit according to the invention, Figure 2 is another side view of the box drill bit according to the invention, and

Figure 3 is an end view of the box drill bit according to the invention.

Figure 1 illustrates a box drill bit 1 comprising a center bit 2, blades 3a and 3b for cutting a box hole, a body portion 4 for securing the cutting blades 3a and 3b to the center bit 2, and a fastening rod 5 for securing the drill bit to the chuck of a drilling machine. The box drill bit 1 is most preferably made from a preformed steel blank which is machined to provide the necessary blade surfaces. After machining, the box drill bit 1 can be heated and hardened to improve its durability. The center bit 2 of the box drill bit 1 is most preferably shaped so as to have a rectangular cross-section as shown in Figure 3, whereafter its sides are finished so that it becomes conical when seen from the side, widening from the point towards the body portion 4, and penetrates into the object to be drilled. The conical shape of the center bit 2 also retards the penetration rate of the box drill bit 1. The cutting blades 3a and 3b are shaped in such a manner that the cutting point 6a of blade 3a is further away from the center bit 2 than the cutting point 6b of cutting blade 3b. The cutting is also facilitated by cutting bevels 8a and 8b, which are bevelled from the points 6a and 6b of the cutting blades 3a and 3b and which shear the bottom of the cut formed in the drilled object. Blade 3a is bevelled so that the cuttings move towards the centre bit 2, and blade 3b is bevelled so that the cuttings move away from the center bit 2. During drilling, the actual cuttings remain thus in the cut formed by the cutting blades 3a and 3b and are not allowed to enter the space between the centre bit 2 and cutting blades 3a and 3b of the box drill bit 1. Thus the cuttings do not clog the box drill bit 1. When a box hole is made to the object drilled, the box drill bit is at first aligned by means of the centre bit 2, whereafter the blades 3a and 3b make a wide cut to the object while the box drill bit 1 is slowly pressed into it. When the box drill bit 1 has penetrated so deeply into the object that the cutting blades 3a and 3b have reached the other surface of the object, the disc that has been formed can be easily detached from the board or the like, as the cut made by the blades 3a and 3b is wide and the box drill bit 1 is therefore not wedged between the edges of the formed cut.

Figure 2 illustrates the box drill bit seen from another side. The numbers used in Figure 2 correspond to those used in Figure 1. The figure shows particularly well the shape of the cutting point 6a with respect to the direction of rotation of the box drill bit 1.

Figure 3 is an end view of the box drill bit 1. The numbers used in Figure 3 correspond to those used in Figures 1 and 2. The centre bit 2 shown in Figure 3 has a rectangular cross-section, wherefore it penetrates into the object while rotating, centering the box drill bit 1 and also retarding its penetration rate. Figure 3 also illustrates the positions of the cutting points 6a and 6b: point 6a cuts a circle that has a greater radius than the circle cut by point 6b. Thus a relatively wide cut is made to the object to be drilled to facilitate, for example, treatment of the cuttings and detachment of the disc from the hole formed. Figure 3 also shows guiding surfaces 7a and 7b, which make the cuttings remain in the cut.

The drawings and the description relating thereto are intended merely to illustrate the inventive concept. In its details the box drill bit according to the invention may vary within the scope of the appended claims. Thus, for example, the shape of the centre bit 2 can be freely chosen as long as the centre bit continuously centers the box drill bit. The centre bit may be, for example, drill bit-like either along its entire length or, most preferably, only at its point, in which case the centre bit, at about the level of the cutting blade points, is a uniform, smooth, round pin of even thickness so that no cutting forces are exerted on the walls of the centre bit hole. Also, the cutting blades 3a and 3b may vary to a great extent, depending on the use and the object to be drilled.

Claims

1. A box drill bit comprising a fastening rod (5) for securing the box drill bit (1) to the chuck of the drilling machine, a centre bit (2) on the same longitudinal axis as the fastening rod (5), a blade (3a) at a distance from the centre bit (2), said blade cutting substantially in the direction of the longitudinal axis of the centre bit (2 ) , and a body portion (4) connecting the fastening rod (5), the cutting blade (3a) and the centre bit (2), c h a r a c t e r i z e d in that at least two cutting blades (3a, 3b) are attached to the body (4) on the opposite sides of the centre bit (2) at substantially an equal distance from the centre bit (2), that the cutting point (6a) of the first blade (3a) is on the outer edge of the blade (3a), and that the cutting point (6b) of the second cutting blade (3b) is on the inner edge of the cutting blade (3b) whereby, when a hole is drilled, a wide cut is formed in the object drilled by the action of the cutting blades (3a, 3b).

2. A box drill bit according to claim 1, c h a r a c t e r i z e d in that the centre bit (2) is an angular bit which widens from the point towards the body portion (4).

3. A box drill bit according to claim 1, c h a r a c t e r i z e d in that the centre bit (2) is drill bit-like at its point along part of its length.

4. A box drill bit according to any one of claims 1 to 3, c h a r a c t e r i z e d in that the cutting bevels (8a, 8b) of the cutting blades (3a, 3b) are bevelled in the same direction.