Drill Bit

Abstract

There is provided a drill bit for cutting tiles comprising a hollow body portion bearing an annular cutting surface at one end and a wax plug located within the hollow body portion, wherein an internal wall of the hollow body portion has at least one region of reduced thickness so as to provide one or more channels along which melted wax can flow to reach cutting surface. The channels can be provided by an offset central passage within body portion, or as elongate grooves around the internal wall of the hollow body portion. The channels can be helical grooves.

Claims

1. A drill bit for cutting tiles, comprising: a hollow body portion bearing an annular cutting surface at one end and a wax plug located within the hollow body portion, wherein an internal wall of the hollow body portion has at least one region of reduced thickness and the hollow body portion is formed by a passage offset from the cross-sectional center of the hollow body portion.

2. The drill bit according to claim 1, wherein the hollow body portion is cylindrical.

3. The drill bit according to claim 1, wherein the region of reduced wall thickness extends along the hollow body portion at least as far as the wax plug.

4. The drill bit according to claim 1, wherein the passage is offset from the cross-sectional center of the hollow body portion by 50% or less of the width of the hollow body portion.

5. The drill bit according to claim 1, wherein the passage is offset from the cross-sectional center of the hollow body portion so that the region of reduced wall thickness has a minimum wall thickness of 0.5 mm.

6. The drill bit according to claim 1, wherein at least one channel is formed within an internal wall of the hollow body portion.

7. The drill bit according to claim 6, wherein a plurality of channels is provided.

8. The drill bit according to claim 7, wherein the plurality of channels is circumferentially equispaced grooves around the internal wall of the hollow body portion.

9. The drill bit according to claim 6, wherein the at least one channel is a helical groove.

10. The drill bit according to claim 9, wherein the helical groove is provided as a reverse spiral thread.

11. The drill bit according to claim 1, wherein the hollow body portion has an external diameter in the range 3 to 16 mm.

12. The drill bit according to claim 1, wherein the hollow body portion has an external diameter in the range 3 to 7 mm.

13. The drill bit according to claim 1 when used with a drilling tool operating between 500 to 30,000 rpm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will now be described by way of example with reference to the accompanying drawings in which:

[0014] FIG. 1 is a perspective view of a first embodiment of a drill bit in accordance with the invention;

[0015] FIG. 2 is an illustrative diagram showing an internal rotational path of the first embodiment;

[0016] FIG. 3 is a perspective view of a second embodiment;

[0017] FIGS. 4(a) and (b) are end and horizontal cross-sectional views respectively of the second embodiment;

[0018] FIG. 5 is a perspective view of a third embodiment; and

[0019] FIGS. 6(a) and (b) are end and horizontal cross-sectional views respectively of the third embodiment.

DETAILED DESCRIPTION OF THE INVENTION

[0020] FIG. 1 shows drill bit, or hole saw, 10 comprising a shank 12 for attachment to a manually held drill or angle grinder and an elongate thin-walled cylindrical tube 14 having an annular cutting surface 16 for engaging with a workpiece such as a tile or work surface. Typically, the external diameter of tube 14 will be in the range 3 mm to 16 mm. Cutting surface 16 has abrasive cutting material 18 attached to it, typically in the form of adhered particles of granular diamond or tungsten carbide. Rotation of drill bit 10 by a drill or angle grinder connected to shank 12 rotates abrasive cutting surface 16 so removing circular discs of material from a workpiece.

[0021] Internal passage 20 through tube 14 is circular in cross-section and is offset by at most 50% from a centre axis 22 about which drill bit 10 rotates. Passage 20 is thus typically displaced between 0.1 to 1 mm from centre axis 22 so that the wall thickness of tube 14 varies, with a minimum wall thickness of around 0.5 mm at the narrowest region 24. Wax plug 26 is located within passage 20 close to cutting surface 16. Wax plug 26 gradually melts from heat generated by friction as cutting takes place, with liquid wax acting as a coolant on cutting surface 16 and passing between the granular cutting material to achieve cooling.

[0022] Rotation of drill bit 10 results in an elliptical rotation path 28 of offset passage 20 as shown in FIG. 2. Where a slug 30 forms due to waste material mixing with liquid wax, slug 30 will be circular in cross-section and of reduced diameter compared to passage 20, leaving gap 32 unblocked and forming an internal channel within tube 14 that reaches wax plug 26. Thus, during rotation there exists an internal channel extending within tube 14 along which wax can flow to reach cutting surface 16.

[0023] The creation of a channel that cannot be blocked by slug formation maintains wax cooling at abrasive cutting surface 16, ensuring cutting material 18 is not damaged through excess heat. This increases the working lifetime of drill bit 10 as can be seen in Table 1 below where the number of holes drilled through 8 mm thick porcelain tiles at 1100 rpm was compared for prior art hole saws with wax plugs and hole saws according to the invention with an offset internal passage and wax plug. Drill bits of external diameters 5 mm and 6.5 mm were compared, with the offset passage or offset hole version increasing the number of holes drilled by over 130%. This is a significant improvement on existing hole saws.

TABLE-US-00001 TABLE 1 Comparison of holes drilled Diameter 3/16″ or 5.0 mm ¼″ or 6.5 mm without with without with offset offset offset offset Modification hole hole hole hole Test 1 18 100 26 65 Test 2 27 48 20 93 Test 3 23 46 19 69 Test 4 31 44 19 56 Test 5 57 17 56 Test 6 32 Test 7 28 Average Life 24.8 59.0 23.0 67.8 Difference ref. 138% ref. 195%

[0024] A second embodiment shown in FIGS. 3 and 4 comprises hollow tube 34 having an internal passage 36 centred on rotational axis 22 and which is formed with a plurality of internal semi-circular channels 40 extending along tube 34 towards shank 12, with the four channels 40 being circumferentially equispaced and providing multiple pathways for melted wax from plug 26 to reach a workpiece. Typically channels 40 extend into wall 42 to produce a minimum wall thickness 24 of 0.5mm, with the wall thickness in regions without a channel being around 1 mm. Any slug formed from waste material combined with melted wax will sit within passage 36 and will not obstruct flow of liquid wax along channels 40.

[0025] In a third embodiment shown in FIGS. 5 and 6, one or more internal helical channels 50 are provided within tube 52, such that the channels extend along and around the internal wall. The helical channel can be provided as a single thread, or as two, three or four multiple threads so as to provide multiple exit points adjacent cutting surface 16. The threads can be provided as reverse spiral threads, such that the handedness of the helical groove left-handed and opposite to the sense of rotation of the drill bit during use.

[0026] Increasing the number of helical threads or helical channels provides increased flow of coolant/lubricant, delivering more cooling.

[0027] While the present invention has been illustrated by description of various embodiments and while those embodiments have been described in considerable detail, it is not the intention of Applicants to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications will readily appear to those skilled in the art. The present invention in its broader aspects is therefore not limited to the specific details and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of Applicants' invention.