GB2295109A

GB2295109A - Centre drill insert for a trepanning tool

Info

Publication number: GB2295109A
Application number: GB9522584A
Authority: GB
Inventors: Roland Lay; Guenter Wahl; Otto Glarner
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1994-11-19
Filing date: 1995-11-03
Publication date: 1996-05-22
Anticipated expiration: 2015-11-03
Also published as: GB9522584D0; ITMI952335A1; GB2295109B; DE4441275A1; ITMI952335A0; IT1276128B1; FR2727043A1; CH690568A5; FR2727043B1; DE4441275B4

Description

Centre drill insert for a trelpannina tool 229.5109 The invention starts

from a centre drill insert for a trepanning tool according to the precharacterizing clause of Claim 1 or from a trepanning tool according to the pre-characterizing clause of Claim 9. A centre drill insert is already known (DE-GU 93 02 487), which has a plurality of holding arms projecting radially outwards and able to be inserted into slots, open at the end face, in the trepanning tool. In order to prevent the centre drill insert from falling out of the trepanning tool, it was proposed to make the slots in the trepanning tool oblique, so that the centre drill insert can tilt in the slots. This measure accordingly presupposes a particular configuration of the annular bit. in the case of conventional trepanning tools provided with an axial longitudinal slot the centre drill insert in not prevented from falling out.

In addition, a centre drill insert in known on the market, which has radially projecting, flexible plastics tabs which are intended to prevent the centre drill insert from falling out. The..plastics tabs extend in the space between two neighbouring holding arms, so that on the insertion of the centre drill insert they can yield into the trepanning tool in the axial direction of the centre drill. Because of the relatively low resistance to bending of the plastics material, the pressure force which the plastics tabs can exert on the inner wall of the trepanning tool is relatively slight, so that secure mounting of the centre drill insert in the trepanning tool is not ensured in every case. In some circum tances rock dust in deposited between the tabs and the wall, whereby the holding action is further reduced.

Advantages of the invention In contrast thereto the centre drill insert according to the invention, which has the characterizing features of Claim 1, has the advantage that the centre drill insert is held securely in the trepanning tool. The centre drill insert can be inserted in a simple manner, and no wear can be f ound on the centre drill mounting even after frequent insertion and removal of the centre drill insert.

Through the measures specified in the dependent claims advantageous developments and improvements of the centre drill Insert indicated in Claim 1 are possible.

It in particularly advantageous for the clamp wings to be formed by sheet metal strips having the necessary resistance to bending. If the sheet metal strips are made sharp- edged at their ends, their ability to grip firmly on the inside wall of the trepanning tool in improved. If the sheet metal strips have sharp- edged corners, this effect in further intensified, because in particular the corners of the sheet metal strips bear against the curved inner wall.

Through the arrangement of the clamp wings along the holding arms on the side rem te f rom the centre drill bit, the centre drill insert can be inserted particularly well into the trepianning tool, since then the spring travel of the f ree ends in limited by the holding arms. Moreover, the cutting edges do not then interfere in the insertion or extraction of the centre drill insert.

if the f ree ends of the clamp wings are bent over in the direction of the centre drill bit, the insertion of the centre drill insert can be additionally facilitated. Through the provision of two bending points having opposite curvatures. the axial length of the centre drill insert can be kept short.

Drawings One exemplary embodiment of the invention is illustrated in the drawings and explained in more detail in the following description. Figure 1 shows a side view of a trepanning tool, Figure 2 a cross-section through the trepanning tool with the centre drill insert inserted in it, and Figure 3 shown a plan view of a centre drill insert having a configuration according to the invention.

Description of the exemplary embodiment

A trepanning tool designated 10 in Figure 1 is in the form of an annular bit. The trepanning tool 10 has a bowl-like shape with an approximately cylindrical bowl bottom 11 and a hollow cylindrical bowl wall 12 adjoining the latter. on the end face of the bowl wall 12, at the end remote from, the bowl bottom 11, are provided cutting segments 13 serving to remove material. In addition, a plurality of longitudinal slots 14 are provided for cooling purposes in the bowl wall 12. On the side of the bowl bottom, 11 rem te from the cutting segments in situated a central extension 15, in which a threaded bore 16 is provided for the connection of the trepanning tool to a drill spindle of a power tool, particularly a handheld drill. On the outer circumference of the extension 15 are formed parallel spannerapplication surfaces 17 serving for the application of an open-end spanner. The bowl bottom 11 and the bowl wall 12 are f or example of two-part construction, but may by all means also be made in one piece, for example by deep-drawing or machining. on the end face there are a total of four axially extend- ing slots 18, which are open at the edge and of which only one can be seen in Figure 1. The slots 18 may also be made obliquely to a longitudinal axis 35 of the trepanning tool 10.

in Figure 2 the end-face part of the bowl wall 12 is shown in section. A centre drill insert 20, which is used to give a reliable start to the drilling of the rock which in to be drilled, is inserted into the interior of the trepanning tool 10. The centre drill insert comprises a centre drill bit 21, which in arranged centrally and projects axially beyond the cutting segments 13 of the trepanning tool 10. On the shank end 23 of the centre drill bit 21 are provided holding arms 22, which project radially outwards and which are offset by 9011 relative to each other. The holding arms 22 together form a centring cross 24, which in shown in detail in Figure 3. The centring cross 24 in fastened, for example soldered or welded, to the centre drill bit 10 21 for rotation therewith. On a side facing the cutting segments 13 the centring cross 24 in supported by a disc 25. On the other side of the centring cross 24 is arranged a holding device 26, by which the centre drill insert 20 is held inside the trepanning tool 10. is The holding device 26 consists of two, clamp wings 27, which project radially outwards and are dia- trically offset relative to one another. The two clamp wings 27 of the holding device 26 are formed in the case of the example by a holding metal shoot 28 which Is fastened 20 centrally to the shank end 23 of the centre drill bit 21 by means of an upset 29. The free ends 30 of the clamp wings 27 are provided with sharp edges 31, which under prestress firmly grip against an inner wall 32 of the trepanning tool 10. The shape of the clamp wings 27 is characterized by a first bending point 33 which lies radially further inwards and has a small radius of curvature, with the curvature facing the centre drill bit 21, and by a second bending point 34 which lies further outwards and has a 30 bending radius larger than the first-mentioned radius, with a curvature facing away from the centre drill bit 21. The radial length of the clamp wings 27, measured from the longitudinal axle 35 of the trepanning tool 10, is greater than the inside radius of the bowl wall 12. On 35 the insertion of the centre drill insert 20, the clamp wings 27 bend around the bending points 33, 34. Because of this deformation, after the insertion of the centre drill insert 20 the clamp wings 27 lie against the inside wall 32 under prestress. In consequence of the preformed bending points 33, 34, the free ends 30 then enclose a relatively sharp angle a preferably smaller than 45 degrees, with the inside wall 32 or the longitudinal axis 35.

In Figure 3 the centre drill insert 20 in shown in plan view. The centrally arranged centre drill bit 21 with its cutting edges 40, the disc 25 and the centring cross 24 can be seen. The centring cross 24 is formed by a sheet-like body whose edges are flanged for stiffening purposes. Beneath two holding arms 22 diametrically offset relative to one another is situated the holding device 26, which in formed by the sheet metal strip 28, preferably of spring steel. The clamp wings 27 of the holding device 26 are provided at their free ends 30 with is the sha rp edges 31, which, together with side faces 41 of the clamp wings 27, form sharp corners 42.

CLATES 1. Centre drill insert for a trepanning tool, particularly for an annular drill, having a centre drill bit (21) which is provided with a plurality of holding armw (22) which project radially outwards and are able to be inserted into slots (18), open at the end face, in the trepanning tool (10), the centre drill insert (20) having at least two clamp wings (27) which extend radially outwards and which at one end are joined to the centre drill insert (20), while their radial length in greater than an inside radius of the trepanning tool (10), so that when the centre drill insert (20) is inserted they are braced at their free end (30) against an inside wall (32) of the trepanning tool (10), characterized in that the clamp wings (27) are made of resiliently stiff material and at their ends are bent into a preformed shape towards the centre drill bit (21).

Claims

2. Centre drill insert according to Claim 1, characterized in that the

clamp wings (27) are formed by sheet metal strips.

3. Centre drill insert according to Claim 2, characterized in that the clamp wings (27) have sharp edges at their free end (30).

4. Centre drill insert according to one of the preceding claims, characterized in that the clamp wings (27) extend along a side of the holding arms (22) which is remote from the centre drill bit (21).

5. Centre drill insert according to one of the preceding claims, characterized in that the clamp wings (27) each have, in the direction of their free end (30), a first bending point (33), which bends the clamp wing (27) away from the centre drill bit (21), and a second bending point (34) which lies further outwards and has a curvature facing away from the centre drill bit (21) and having a larger radius of curvature than the first bending point (33).

6. Centre drill insert according to one of the preceding claimw, characterized in that in the region of their free ends (30) the clamp wings (27) extend at an angle a smaller than 45 degrees to a longitudinal axis (35) of the trep-nning tool (10).

7. Centre drill insert according to one of the preceding claims, characterized in that a total of four holding armw (22) arranged in the form of a cross and two clamp wings (27) lying diametrically opposite one another are provided.

8. Centre drill insert according to Claim 7, characterized in that the two clamp wings (27) are formed on a radially continuous sheet metal strip which at the centre is joined to the centre drill insert (20).

9. Trepanning tool, particularly an aninular drill bit for power tools, characterized in that in the front of the trepanning tool (10) a centre drill insert (20) according to one of Claims 1 to 8 is clamped fast.

10. A centre drill insert substantially as herein described with reference to the accompanying drawings. 11.

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