DRILL BITS AND HOLDERS THEREFOR

Abstract: A drill bit comprising a shank (2) of nominal 10mm diameter for engagement within a tool holder (20), said shank comprising diametrically-opposed driving keyways (4, 6) opening onto an end face of the shank and at least one locking groove (12) spaced from the end of the shank, the two driving keyways being of different cross section.

Description: DRILL BITS AND HOLDERS THEREFOR The present invention relates to drill bits and more particularly to a drill bit for use in a hammer drill for drilling holes into concrete or other masonry. The invention also relates to a tool holder for such a drill bit. In the building construction industry, substantial use is made of hammer drills to drill holes into masonry. Several systems currently available for use in the building industry utilise a tool holder mounted on the drill and which is sized to receive a specific diameter of drill bit, with the tool holder having at least one key way and at least one spring-loaded ball or similar which engage in correspondingly shaped formations in the shank of the drill bit to drivingly couple the bit to the tool holder while, also, permitting quick insertion of the drill bit into, and quick release of the drill bit from, the tool holder. Currently the most commonly used drilling system for use in building construction utilises drill bits with a nominal shank diameter of 10mm and holders sized to receive such drills. A drilling system which operates with a nominal drill shank diameter of 10mm can drill holes of up to 30mm in diameter which is sufficiently large for most building applications. However as the drill bit diameter increases, the torque loading on the system increases and with existing designs this has resulted in rapid wear of the driving keys within the tool holder. Although an increase in size of the driving keys can result in reduced wear rates, the increase in driving key size will result in a commensurate increase in the size of the key ways within the shank of the drill bit which can result in a significant reduction in the torsional strength of the shank which is contrary to requirements in a situation where the system may be under an increased torque loading. We have, however, determined that a critical relationship can be established between the size of the driving keys in the tool holder and driving keyways in the 10mm diameter shank of the drill bit which will result in improved wear rates of the driving keys whilst providing good torsional strength of the drill bit shank which will ensure good resistance to high torque loadings as may be encountered in some drilling situations. According to the present invention, there is provided a drill bit comprising a shank of nominal 10mm diameter for engagement within a tool holder, said shank comprising diametrically-opposed driving keyways opening onto an end face of the shank and at least one locking groove spaced from the end face of the shank, the two driving keyways being of different cross-section. Advantageously, there are two such locking grooves diametrically-opposed. Advantageously the two driving keyways each have a cross-section which is symmetrical about a respective radial plane of the shank and have planar driving faces preferably lying on a common diametral plane. Preferably, the two driving keyways have different widths as considered in a circumferential direction and either the same or different radial depths. Preferably, the circumferential width of the smaller of the two driving keyways as determined by the included angle between its driving and non-driving faces is at least 40° but not greater than 42° and the circumferential width of the larger of the two driving keyways considered in the same manner is at least 50° but not more than 90°. Preferably, for each of the driving keyways the radial distance between the axis of the shank and base of the key way is not less than 2.25mm and not greater than 3.25mm and is preferably within the range 2.5mm to 2.75mm. Preferably, the locking grooves are symmetrical about a common diametral plane which is inclined by approximately 70° in advance, as considered in the direction of rotation, to the diametral plane containing the driving faces of the keyways. Slight variation of one or two degrees can be tolerated from this nominal offset. Optionally, the shank of the drill bit may include a third locking groove associated with the smaller of the two driving keyways. According to another aspect of the invention, there is provided a tool holder having diametrically-opposed driving keys and diametrically-opposed locking members, the driving keys and locking members being sized and positioned to engage within the driving keyways and locking grooves of a drill bit as defined above, the driving faces of the driving keys being substantially planar and being located on a common diametral plane. The configuration defined above can provide an increase in bearing surface areas between the driving keys and keyways of at least about 50% in comparison with prior arrangements. Further increase may be obtained by increasing the axial length of the driving keys to a length in excess of about 13.5mm. According to another aspect of the invention, there is provided a tool holder having diametrically-opposed driving keys and diametrically -opposed locking members, wherein the axial length of each driving key is from about 20 to 27mm. In a preferred embodiment the driving keys are sized to fit closely within the driving keyways of the drill bit but without providing an interference fit which would cause difficulty in the insertion of the shank. Preferably, the locking members of the tool holder are substantially circular in cross-section (for example, spherical) and are substantially identical, the locking members being symmetrically arranged on a common diametral plane which is approximately 70° in advance, as considered in the direction of rotation, from the diametral plane containing the driving faces of the driving keys. An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 a schematic cross-section through the shank of a drill bit in accordance with a preferred of the invention; and Figure 2 is a schematic cross-section through a tool holder in accordance with a preferred embodiment of the invention. With reference to Figure 1 of the drawings there is shown a shank 2 of the, drill bit, the shank 2 being of nominal 10mm diameter and comprising two driving keyways 4,6 which are substantially diametrically-opposed. As will be apparent, both driving keyways 4,6 open onto the end face of the driving shank to permit location of the driving keys of the tool holder into these keyways when the shank is pushed into the tool holder. Each key way 4,6 is symmetrical about a respective radial plane 8a, 8b. Preferably, the driving faces 4a, 6a of the keyways 4,6 are substantially planar and lie on common diametral plane 10. The cross- sectional profiles of the two driving keyways 4,6 differ at least in the width (circumferential) direction and possibly also in the depth (radial) dimension. We have determined that a shank having capacity for high torque transmission whilst achieving low wear rates of the matching driving keys of the tool holder can be constructed within the general parameters defined above by forming the keyways to a depth wherein the radial distance from the base of each keyway to the axis of the shank along the radial plane of symmetry is not less than 2.25mm and not greater than 3.25mm and it is particularly advantageous for this distance to be between 2.55mm and 2.75mm, with the said distance either being the same for each of the two keyways or different. The width of the two keyways as defined by the included angle between its two faces (driving and non-driving) is at least 40° but not more than 42° for the smaller of the two keyways (as shown, the keyway 6 with included angle o^), and at least 50° and not more than 90° for the larger of the two keyways (as shown, the keyway 4 with included angle ^. In one particularly preferred embodiment t is 42° and α 2 is 56°. The shank 2 also includes diametrically-opposed locking grooves 12 of part-cylindrical form for receiving spring-loaded locking members within the tool holder in order to retain the shank 2 within the tool holder against accidental release. In order to provide the retention effect, the ends of the locking grooves 12 are displaced axially forwardly of the end face of the shank. The two locking grooves 12 are symmetrical about a common diametral plane 14, and preferably this plane is inclined by an angle β of about 70° in advance (as considered in the direction of rotation) of the diametral plane 10 containing the driving faces 4a, 6a of the two driving keyways 4,6. Optionally, a third locking groove 16 of substantially the same profile as the locking grooves 12, at least partially intersects the smaller of the two driving keyways (the keyway 6, as shown), the radial plane of symmetry 18 of the third locking groove 16 being displaced by an angle γ of about 75° in advance of the diametral plane of symmetry 14 of the two 5 diametrically-opposed locking grooves 12, and the third locking groove 16 preferably being so related to the associated driving keyway 6 that the driving face 6a of that keyway is substantially tangential to the locking groove 16. The tool holder 20 (Figure 2) comprises diametrically-opposed driving keys 22,24 10 sized for engagement within the driving keyways 4,6 of the shank 2 when the shank 2 is inserted into the tool holder 20, and diametrically-opposed spring-loaded locking members 26 of spherical form for engagement within the diametrically-opposed locking grooves 12 to retain the shank 2 within the tool holder; alternatively, the locking members 26 could be of cylindrical form or of other form which provides a circular cross-section. The driving faces 15 22a,24a of the two driving keys 22,24 lie on a common diametral plane 28. The use of the two driving keyways and corresponding driving keys of different circumferential width ensures that the drill bit can only be inserted into the tool holder in one orientation commensurate with the asymmetry between the locking members and driving 0 keys. The configuration defined above permits the driving keys 22,24 to be of a size which enables high torque transmission to be achieved, particularly when using larger diameter drill bits but without resulting in relatively rapid wear of the driving keys to a point at which the 5 tool holder requires frequent replacement. This is achieved not only by appropriate selection of the depth of the driving keyways and correspondingly of the driving keys which results in greater available load-transmitting area but also as a result of the circumferential width of each driving keyway and correspondingly of the driving key and particularly of the larger width key which can remain effective even after substantial wear has occurred even to the 30 extent that the smaller width key has worn completely and is no longer effective in transmitting drive. On the other hand, the defined configuration of the driving keyways in the shank enables the shank to carry the high torque loading without undue weakening of the shank. In particular, the circumferential size range of the larger keyway and larger driving key optimises the strength and wear resistance of that key whilst ensuring that the shank can still carry the required torque loading. Although the shank described herein is primarily designed for use with a tool holder as described herein, the shank configuration as described also permits the usage with some existing tool holders even although in those circumstances there may not necessarily be an optimal relationship between the shank and tool holder. It is for this reason that the third locking groove is present in order to co-operate with a locking member present in that position in an existing tool holder. It is also to be noted that the tangential configuration of the third locking groove to the driving face of the associated driving keyway maximises the retention effect of the locking member without reducing the drive transmitting area of the driving face as would arise if the third locking groove were to intersect that face. The particular circumferential size range of the associated driving keyway also ensures an adequate retention effect of the locking member. In the configuration particularly described, the depth range of the driving keyways and corresponding driving keys permits an increase of at least about 50% in the bearing surface area between the driving faces of the keys and keyways, in comparison with prior configurations. This has the effect of significantly increasing the working life of the tool holder and drill bit. In particular, the increase in bearing surface area increases the available volume of material of the driving key available to resist the effects of wear and also leads to a reduction in contact pressure at the interface of the key and keyway, this resulting in a reduction in the frictional force which, in turn, leads to a further reduction in wear rate. The increase in bearing surface area resulting from increase in radial depth of the driving keys provides a far more significant increase than would arise if the length of the driving key were to be increased. However it is within the scope of the present invention to provide an increase in driving key length in order to achieve a reduction in contact pressure and an increase in working life. In current systems the notional length of the driving keys is about 13.5mm and an increase of about 6.5mm in key length would equate to the 50% increase in bearing surface area discussed above as provided by the increased radial depth. Of course, any increase in length when combined with increase in radial depth will further increase the bearing surface area and from that standpoint is advantageous although from a manufacturing perspective may not be as cost effective as the improvements obtained by increase in radial depth. However there is advantage in not only increasing the radial depth but also increasing the axial length of the driving keys beyond 13.5mm. In an alternative construction in which no increase in radial depth is provided, the axial length of the driving keys should be increased to a length of between about 20 and 27mm thereby providing an increase in bearing surface area of between about 50 and 100% over existing designs. Throughout this specification and claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers or steps but not the exclusion of any other integer or group of integers. The embodiment has been described by way of example only and modifications are possible within the scope of the invention.

Claims: CLAIMS :- 1. A drill bit comprising a shank of nominal 10mm diameter for engagement within a tool holder, said shank comprising diametrically-opposed driving keyways opening onto an end face of the shank and at least one locking groove spaced from the end face of the shank, the two driving keyways being of different cross-section. 2. A drill bit according to claim 1, wherein there are two such locking grooves diametrically-opposed. 3. A drill bit according to claim 1 or claim 2, wherein the two driving keyways have a cross-section which is symmetrical about a respective radial plane of the shank and each have planar driving faces. 4. A drill bit according to claim 3, wherein the driving faces lie on a common diametral plane. 5. A drill bit according to any one of claims 1 to 4, wherein the two driving keyways have different widths as considered in a circumferential direction. 6. A drill bit according to claim 5, wherein the driving keyways have the same radial depth. 7. A drill bit according to claim 5, wherein the driving keyways are of different radial depth. 8. A drill bit according to any one of claims 5 to 7, wherein the circumferential width of the smaller of the two driving keyways as determined by the included angle between its driving and non-driving faces is at least 40° but not greater than 42° and the circumferential width of the larger of the two driving keyways considered in the same manner is at least 50° but not greater than 90°. 9. A drill bit according to any one of claims 1 to 8, wherein for each of the driving keyways the radial distance between the axis of the shank and the base of the keyway is not less than 2.25 mm and not greater than 3.25 mm. 10. A drill bit according to claim 9, wherein the said radial distance is within the range 2.5 mm to 2.75 mm. 11. A drill bit according to claim 2 and claim 4 or any claim dependent thereon, wherein the locking grooves are symmetrical about a common diametral plane which is inclined by approximately 70° in advance, as considered in the direction of rotation, to the diametral plane containing the driving faces of the keyways. 12. A drill bit according to claim 2 or any claim dependent on claim 2, wherein the shank includes a third locking groove associated with the smaller of the two driving keyways. 13. A tool holder having diametrically -opposed driving keys and a locking member, the driving keys and locking member being sized and positioned to engage within the driving keyways and locking groove of a drill bit as claimed in claim 1. 14. A tool holder having diametrically -opposed driving keys and diametrically-opposed locking members, the driving keys and locking members being sized and positioned to engage within the driving keyways and locking grooves of a drill bit as claimed in claims 2 and 4 or any claim dependent thereon, the driving faces of the driving keys being substantially planar and being located on a common diametral plane. 15. A tool holder according to claim 13 or claim 14 wherein the axial length of the driving keys is in excess of about 13.5 mm. 16. A tool holder for a drill bit comprising a shank of nominal 10 mm diameter, the tool holder having diametrically-opposed keyways and diametrically-opposed locking members for engagement respectively in driving keyways and locking grooves of the drill bit, the axial length of each driving key being from about 20 to 27 mm.

Inventors: HOWARD DOUGLAS JAMES, HOWARD, DOUGLAS, JAMES
Applicants: RAMSET FASTENERS AUST PTY LTD, RAMSET FASTENERS (AUST.) PTY. LIMITED
Application year: 1998
Main classification: B25D17
IPC codes: B25D17