GB2114479A - Drilling tool - Google Patents

Abstract

The tool comprises at its head (13) at least three main cutting edges (14, 15, 16) arranged at approximately equal peripheral angle intervals from one another and respectively associated centre cutting edges (24, 25, 26, 34, 35, 36) which reach to the central axis (27). Each centre cutting edge consists of an inner cutting edge (34, 35, 36) and an outer cutting edge (24, 25, 26) which meet in an axially protruding cutting edge point (28, 29, 30) all of which have equal radial distances from the central axis (27) and are arranged at the same axial height. Helical flutes (17, 18, 19) issue from the head (13). At least one radially extending part of each centre cutting edge and/or each main cutting edge is arranged, in the direction of rotation of the tool, in advance of the centre of the head, which in Figure 2 shown, coincides with the edge to the rear of each cutting edge. <IMAGE>

Description

SPECIFICATION Drilling tool The invention relates to a drilling tool of the kind having a cutting head which is formed with at least three main cutting edges arranged at approximately equal peripheral angle intervals from one another, respectively associated centre cutting edges reaching to the central axis of the drill, each centre cutting edge being formed by an inner cutting edge extending at an obtuse angle to the central axis and an outer cutting edge extending at an acute angle to the central axis, which edges in each case converge to axial, protruding cutting edge points which are all arranged at equal radial distances from the central axis and which all extend in the axial direction at equal by the same amount, and flutes emanating from the head in the shape of helical grooves.

In one known drilling tool of this kind (Fed. Ger. Ut.

Mod. 77 10 873) each main cutting edge and associated centre cutting edge possesses a 0 rake angle. In this case each main cutting edge with associated centre cutting edge proceeds within a diametrical line coinciding with the tool centre. The flutes are formed as rectilinear channels which run parallel with the central axis to the shank of the drilling tool. Such a drilling tool is suitable without exception for the working of short-chip materials, that is cast metal materials. In the case of other materials of greater toughness the tool breaks because of defective chip formation and chip removal.Admittedly with such a tool it has proved advantageous that by reason of the cutting edge points drilling is possible directly into the solid material without special aids or centre drilling, and this without danger of shifting and/or cutting edge fracture, for in the feed movement of the drilling tool on to a workpiece all cutting edge points come into contact with the workpiece at the same time, so that as early as the initial phase in drilling there is a symmetrical cutting edge loading in the radial direction, because each cutting edge point engaging in the workpiece forms a support for the other cutting edge point.In the initial phase of the drilling operation an annular groove of approximately Vshaped cross-section, corresponding to the cutting edge geometry in the region of the cutting edge points and extending concentrically with the centre axis, is cut into the workpiece by means of all the cutting edge points. Only when the engagement of the cutting edge points into the workpiece progresses will the annular groove become ever wider until it reaches the nominal diametrical dimensions. Since the centre cutting edges are set back somewhat from the main cutting edges, a chip division occurs in this region, so that excessively wide drilling chips do not occur.As stated, such a drilling tool with the described advantages, especially great accuracy of truth of rotation and precision as regards correct dimensions, form and position of the bore to be produced, is without exception suitable for shortchip materials. It has appeared that on the other hand machining of tougher materials, especially steel, leads to fracture of the drilling tool and fails.

An object of the invention is to produce a drilling tool of the initially stated kind which, while retaining the described advantages, opens up a substantially widerfield of use and with equal advantages, including great durability, is suitable not only for short-chip materials but also other, even up to long-chip materials.

According to the invention there is provided a drilling tool having a cutting head which if formed with at least three main cutting edges arranged at approximately equal peripheral angle intervals from one another, respectively associated centre cutting edges reaching to the central axis of the drill, each centre cutting edge being formed by an inner cutting edge extending at an obtuse angle to the central axis and an outer cutting edge extending at an acute angle to the central axis, which edges in each case converge to axial, protruding cutting edge points which are all arranged at equal radial distances from the central axis and which all extend in the axial direction at equal by the same amount, and flutes emanating from the head in the shape of helical grooves at least one part of the centre cutting and the main cutting edge, extending at least substantially in a radial direction, being arranged in the cutting direction in advance of the centre of the head.

Helical grooves as flutes are known per se, for example in twist drills. It is known that unfavourable conditions arise in twist drills at the transverse cutting edge. The cutting angle is here greater than 90 , so that here one cannot speak of cutting, but rather of crushing or the like. As soon as the transverse cutting edge is no longer pressing against the material, in the emergence of the drill from the drill hole, the spindle holding the drill springs back into its original position. The feed motion chages suddenly. The drill can jam and break as a result of overstressing. Even when, in the case of tough materials, the transverse cutting edge of the twist drill is shortened by pointing, these problems are not eliminated. As is known these relationships are the reason why bores of high quality cannot be produced by means of twist drills.Centring is necessary in every case. Moreover as from specific drill hole diameters a preliminary centring by means of smaller bores is necessary.

The drilling tool in accordance with the invention combines the advantages of a tool self-centred by means of the centre cutting edges, especially their cutting edge points, with those of a twist drill, as regards the improved chip guidance and chip removal by reason of flutes formed as helical grooves.

Moreover thanks to the geometry of the main cutting edges and associated centre cutting edges a special chip formation and chip form are achieved. Thus a chip of specific geometry and form is produced in the region of each centre cutting edge. Moreover in the region of each main cutting edge which outwardly adjoins the associated centre cutting edge a separate, likewise correspondingly shaped chip occurs, and this chip is subjected to deformation with compression, whereby the chip becomes smaller than the flute and thus good chip flow and removal result. By reason of this special chip formation and removal the drilling tool, retaining all the other advantages set forth at the outset, is suitable for cutting even tough materials and steel, that is those materials which not only form short and medium chips but also are long-chip.

Further details and advantages appear from the following description.

The invention is explained in greater detail below by reference to examples of embodiment which are shown in the drawing, wherein Figure 1 shows a diagrammatic lateral elevation of the lower part of a drilling tool with three cutting edges, in which for the sake of simplification two cutting edges are represented as turned into the plane of the drawing, Figure 2 shows an end view of the head of the drilling tool, at the lower end Figure 3 shows a diagrammatic view, somewhat similarto that in Figure 2, of only one cutting edge part of a drilling tool, in a modified form of embodiment.

The drilling tool 10 according to the first example of embodiment in Figures 1 and 2 in the usual way comprises a shank 11, which is smoothly cylindrical and serves for chucking, and a drill body 12, of which the lower end is here designated as head 13. The head 13 comprises three radially outwardly seated main cutting edges 14, 15 and 16, arranged at approximately equal peripheral angle intervals of 1200 from one another, and respective associated centre cutting edges 24, 25 and 26, which reach to the central axis 27.

Each centre cutting edge 24 - 26 if formed from an inner cutting edge 34, 35 and 36 respectively, extending at an obtuse angle to the central axis 27 in lateral elevation (Figure 2), and of an outer cutting edge 44,45 and 46 respectively extending at an acute angle to the central axis 27, these converging in each case into axial downwardly protruding cutting edge points 28, 29 and 30 respectively. The cutting edge points 28, 29 and 30 in the view from beneath according to Figure 2 appear each as a section of an inner circular line, by reason of the material cross-section and the clearance grinding.

All cutting edge points 28, 29 and 30 have the same radial distance from the central axis 27 and are arranged at the same height in the axial direction, so that when set upon a surface of a workpiece all the cutting edge points 28, 29, 30 are in contact at the same time.

Three flutes 17,18 and 19 issue from the head 13.

Each flute 17, 18 and 19 is formed in a special manner as a helical groove, as known fundamentally from twist drills. The twist angle in the usual manner lies between about 250 and 35 , but depends on the material of the workpiece in each case.

At least one part, extending in the radial direction, of each centre cutting edge 24, 25 and 26 and/or of each main cutting edge 14, 15 and 16 is arranged, considered in the cutting direction according to the arrow 20, in advance of the centre of the head 13. In the illustration according to Figure 2 this centre by chance coincides with the edge which, to the rear of the main cutting edges and centre cutting edges, there defines the clearance. Thus this edge can serve here as reference line.

In the example of embodiment as shown in Figures 1 and 2 fjirstly each main cutting edge 14,15 and 16 is arranged in advance of the centre, the distance from a diametrical plane passing through the centre lying approximately in the order of magnitude between 0.4 and 1.2 mm. In Figure 2 the mean distance is entered only for the main cutting edge 14, asd, Furthermore in the first example of embodiment, as regards the centre cutting edges 24, 25 and 26, the arrangement is made such that in these the inner cutting edge 34to 36 in each case and the respectively associated outer cutting edge 44 to 46 extend on a radial line and are likewise arranged in advance of the centre.Each centre cutting edge 24,25 and 26 includes, with a diameter passing through the centre, an angle of the order of magnitude of about 270 to 320. This angle is entered in Figure 2, as regards the centre cutting edge 24, as a. It amounts for example to 280. As may be seen each centre cutting edge 24, 25, 26 passes over at the radially outer end of its outer cutting edge 44, 45, 46 in somewhat arcuate form into the associated, radially further outwardly situated main cutting edge 14,15 and 16 respectively.

By reason of this particular configuration the drilling tool 10 can be used for drilling into practically all materials, that is not only short-chip materials, for example cast iron, but also long-chip materials, thus even steel. The drilling tool is suitable, thanks to the centring cutting edge points 28, 29 and 30, for drilling operations into the solid. Preliminary drilling is not necessary. In the cutting operation the cutting edge points 28, 29 and 30 serve for the self-centring of the drilling tool. An extraordinarily great accuracy of form and dimensions of the bore results, that is calibre precision, high accuracy regarding rounding and rectilinearity of the bore and also good surface quality of the drill hole wall.The specially configured main cutting edges 14to 16 and centre cutting edges 24 to 26, to which here different functions are allocated, have an essential share in this. In the cutting operation the centre cutting edges 24to 26 produce a chip each with corresponding form and width. On the other hand the respectively associated main cutting edges 14,15 and 16 each generate their own chip of different form and width, the respective chip form beng determined by the geometry of the cutting edges. In the region of the main cutting edges 14, 15 and 16 a chip deformation, namely a chip compression, occurs for the chip produced there. Thus the chips produced there become smallet than corresponds to the cross-section of the respective flute 17, 18 and 19. This has the advantage that the chip occurring there is removed without problem by way of the flute 17, 18 and 19.

Not lastly, the special manner of formation of different chips and the achieved good chip removal contribute to responsibility for the fact that, with the aid of the configuration of the centre cutting edges 24,25 and 26 with the cutting edge points 28. 29 and 30, by means of this drilling tool 10 extraordinarily great accuracies are achievable, that is accuracies of truth of rotation on the machine side and high accuracies as regards the produced bores, namely as regards form and accuracy of dimensions. At the same time this drilling tool 10 guarantees great durability and useful life, without danger of damage or even destruction.

In the example of embodiment as shown in Figure 3, two different variants are contained in one illustration. Thus in chain lines a variant is shown in which the outer cutting edge 144 does not proceed in radial extension of the inner cutting edge 134, but rather likewise before the diametrical plane passing through the centre. The inner cutting edge 134 and the outer cutting edge 144 form a point where they meet. This then lies for example on the radius passing through the cutting edge point. The main cutting edge 144, as in the first example of embodiment, externally adjoins the outer cutting edge 144 somewhat arcuately, forming a hollow.

In the variant shown in solid lines in Figure 3 the centre cutting edge 224 is of the same nature exactly as in the first example of embodiment in Figure 1, that is to say the outer cutting edge extends in radial extension of the inner cutting edge, and at the angle a before the centre. On the other hand the main cutting edge 214 outwardly adjoins the centre cutting edge 224 and extends without curve, rectilinearly and approximately parallel to the diameter passing through the centre.

Claims (7)

1. A drilling tool having a cutting head which is formed with at least three main cutting edges arranged at approximately equal peripheral angle intervals from one another, respectively associated centre cutting edges reaching to the central axis of the drill, each centre cutting edge being formed by an inner cutting edge extending at an obtuse angle to the central axis and an outer cutting edge extending at an acute angle to the central axis, which edges in each case converge to axial, protruding cutting edge points which are all arranged at equal radial distances from the central axis and which all extend in the axial direction at equal by the same amount, and flutes emanating from the head in the shape of helical grooves at least one part of the centre cutting edge and of the main cutting edge, extending at least substantially in a radial direction, being arranged in the cutting direction in advance of the centre of the head.

2. A drilling tool according to Claim 1, wherein at least the inner cutting edge of each centre cutting edge is arranged in advance of the centre.

3. A drilling tool according to Claim 1 or 2, wherein the inner cutting edges and associated outer cutting edges extend on a radius and are arranged in advance of the centre.

4. A drilling tool according to one of Claim 1,2 or 3, wherein the main cutting edges are also arranged in advance of the centre and have a mean spacing of approximately the order of magnitude of 0.4 to 1.2mm. from a diametrical plane passing through the centre.

5. A drilling tool according to Claim 1,2,3 or 4 wherein the part arranged in advance of the centre, of each centre cutting edge includes, with a diameter passing through the centre, an angle approximately of the order of magnitude of 270 to 320.

6. A drilling tool according to Claim 5, wherein the angle amounts to 280.

7. A drilling tool substantially as herein described with reference to the accompanying drawings.