Power tool attachment for forming spherical or partially spherical surfaces

Abstract

A power tool attachment for forming a spherical or partially spherical surface on a workpiece comprises a bit that is connectable to a rotatable drive head of a power tool. An end of the bit comprises a cutting tool. The power tool attachment further comprises a base engageable with the power tool for bearing against the workpiece. The base comprises at least one annular guide surface dimensioned such that, when the base is engaged with the power tool, the annular guide surface extends around the bit and faces toward or away from a longitudinal axis of the bit for guiding the cutting tool relative to the workpiece to form the spherical or partially spherical surface on the workpiece.

Description

POWER TOOL ATTACHMENTFOR FORMING SPHERICAL OR PARTIALLY SPHERICAL SURFACESField

[0001] The present invention relates to power tools and related attachments. More particularly, the present invention relates to a power tool attachment for forming spherical or partially spherical surfaces on workpieces made of wood or other materials.Background

[0002] Carpenters and craftspersons often need to form spherical or partially spherical surfaces on workpieces. For example, a concave partially spherical surface may need to be formed in a piece of wood to create a wooden bowl. A convex partially spherical surface may need to be formed on a piece of wood to create a sculpture or decorative feature for an item of furniture.

[0003] To create a partially spherical surface in a wooden workpiece, a rough surface is initially cut into the workpiece using a power tool such as a router or using a hand tool such as a curved scraper or chisel. The initial surface is then worked on by sanding to achieve a smooth finish. These methods are laborious and it is difficult to form a partially spherical surface that has a constant radius of curvature. Irregularities and imperfections often remain in the surface and it may take a significant amount of time and effort to achieve a satisfactory result.

[0004] The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the present application.Summary

[0005] According to the present invention, there is provided a power tool attachment for forming a spherical or partially spherical surface on a workpiece, wherein the power tool attachment comprises:a bit that is connectable to a rotatable drive head of a power tool, wherein an end of the bit comprises a cutting tool; and a base engageable with the power tool for bearing against the workpiece, wherein the base comprises at least one annular guide surface, wherein the annular guide surface is dimensioned such that, when the base is engaged with the power tool, the annular guide surface extends around the bit and faces toward or away from a longitudinal axis of the bit for guiding the cutting tool relative to the workpiece to form the spherical or partially spherical surface on the workpiece.

[0006] The annular guide surface may be rounded such that the workpiece contacts the annular guide surface tangentially when the cutting tool is being guided by the annular guide surface.

[0007] The base may comprise a pair of annular guide surfaces, wherein the annular guide surfaces are dimensioned to face toward and away from the longitudinal axis of the bit respectively for forming convex and concave surfaces on the workpiece respectively.

[0008] The pair of annular guide surfaces may, together, be partially toroidal in shape.

[0009] A position of the base relative to the cutting tool may be adjustable for adjusting a radius of curvature of the spherical or partially spherical surface to be formed on the workpiece.

[0010] The power tool attachment may further comprise a sleeve engageable with the power tool, wherein the base is attachable to the sleeve at a plurality of positions for adjusting the radius of curvature.

[0011] The base may be moveable into each of the positions relative to the sleeve by a sliding action.

[0012] The base and sleeve may comprise one or more ribs received into cooperating channels for governing the sliding action.

[0013] The ribs may be provided on an internal surface of the base and the channels may be provided on an external surface of the sleeve.

[0014] The base may comprise a rotatable collar comprising an internal screw thread that is engageable with a cooperating external screw thread on the sleeve for moving the base into each of the positions.

[0015] The base may comprise a clamp assembly for fastening the base into each of the positions.

[0016] The clamp assembly may comprise a band extending about a circumference of the base and a restraint mechanism for tightening the band around the base and the sleeve.

[0017] The band may extend at least partially around the circumference and comprise a pair of end portions, wherein the restraint mechanism is configured to pull the end portions together to tighten the band.

[0018] The restraint mechanism may comprise a pin, wherein the pin extends through a pair of apertures provided in the end portions respectively, and a fastening mechanism for causing the pin to pull the end portions together.

[0019] The fastening mechanism may comprise a lever pivotably connected to the pin by a cam, wherein the cam causes the pin to pull the end portions together when the lever is pivoted.

[0020] The power tool attachment may comprise an arm configured to bear against the power tool to prevent the power tool attachment from rotating about the longitudinal axis relative to the power tool in an uncontrolled manner in use.

[0021] The arm may be spring loaded to bias the arm against the power tool when the arm engages the power tool.

[0022] The cutting tool may comprise one or more blade members for cutting into the workpiece.

[0023] The cutting tool may comprise a pair of the blade members that are together in an annular configuration.

[0024] The cutting tool may comprise an abrasive disc for wearing away the workpiece.

[0025] The bit may be connectable to a collet of a router.

[0026] The bit may be connectable to a drive shaft of an angle grinder or drill.

[0027] The present invention also provides a power tool for forming a spherical or partially spherical surface on a workpiece, wherein the power tool comprises: a rotatable drive head with a bit connectable thereto or provided thereon, wherein an end of the bit comprises a cutting tool; and a base for bearing against the workpiece, wherein the base comprises at least one annular guide surface, wherein the annular guide surface is dimensioned such that it extends around the bit and faces toward or away from a longitudinal axis of the bit for guiding the cutting tool relative to the workpiece to form the spherical or partially spherical surface on the workpiece.Brief Description of Drawings

[0028] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which:FIG. 1 is an isometric view of a power tool attachment for forming a spherical or partially spherical surface on a workpiece according to an example embodiment of the invention;FIG. 2 is an isometric view of a power tool and a workpiece, wherein the power tool attachment is attached to the power tool;FIG. 3A is a side elevation view of the power tool attachment cutting a first concave surface in the workpiece, wherein the first concave surface has a first radius of curvature;FIG. 3B is a side elevation view of the power tool attachment cutting a second concave surface in the workpiece, wherein the second concave surface has a second radius of curvature;FIG. 30 is a side elevation view of the power tool attachment cutting a third concave surface in the workpiece, wherein the third concave surface has a third radius of curvature;FIG. 4A is a sectional side elevation view of the power tool attachment cutting a first convex surface in the workpiece, wherein the first convex surface has a first radius of curvature;FIG. 4B is a sectional side elevation view of the power tool attachment cutting a second convex surface in the workpiece, wherein the second convex surface has a second radius of curvature;FIG. 40 is a sectional side elevation view of the power tool attachment cutting a third convex surface in the workpiece, wherein the third convex surface has a third radius of curvature;FIG. 5 is an isometric view of the power tool and power tool attachment, wherein a base and sleeve of the power tool attachment are shown in a disconnected condition;FIG. 6 is an isometric view of the base and sleeve in the disconnected condition;FIG. 7 is an exploded isometric view of the power tool attachment;FIG. 8A is a plan view of a cutting tool of the power tool attachment;FIG. 8B is an isometric view of the cutting tool;FIG. 80 is a further isometric view of the cutting tool;FIG. 8D is a further isometric view of the cutting tool;FIGS. 9A and 9B are enlarged sectional side elevation views of a base of the power tool attachment, wherein the power tool attachment is cutting a convex surface in a workpiece;FIGS. 90 and 9D are enlarged sectional side elevation views of a base of a power tool attachment according to a further example embodiment of the invention, wherein the power tool attachment is cutting a convex surface in a workpiece;FIG. 10A is a perspective view of a workpiece that has a spherical convex surface formed thereon by the power tool attachment of FIG. 1A; andFIG. 10B is a perspective view of a workpiece that has a partially spherical concave surface formed thereon by the power tool attachment of FIG. 1A.Description of Embodiments

[0029] Referring to FIGS. 1 to 3, an example embodiment of the present invention provides a power tool attachment 10 for forming a spherical or partially spherical surface 12 on a workpiece 14. The power tool attachment 10 comprises a bit 16 that is connectable to a rotatable drive head of a power tool 18, wherein an end of the bit 16 comprises a cutting tool 20. The power tool attachment 10 also comprises a base 22 that is engageable with the power tool 18 for bearing against the workpiece 14, wherein the base 22 comprises an annular guide surface 24 dimensioned such that, when the base 22 is engaged with the power tool 18, the annular guide surface 24 extends around the bit 16 and faces toward or away from a longitudinal axis 26 of the bit 16 for guiding the cutting tool 20 relative to the workpiece 14 to form the spherical or partially spherical surface on the workpiece 14.

[0030] More particularly, in the example depicted the base 22 comprises a conical skirt 27 that has an annular support member 29 extending circumferentially around its lowermost end for bearing against the workpiece 14. The annular support member 29 is shaped similar to a torus. The annular guide surface 24 of the base 22 is located on the annular support member 29. The annular guide surface 24 faces generally away from the longitudinal axis 26 of the bit 16 and is used to form concave surfaces in the workpiece 14. The annular guide surface 24 is hereinafter referred to as the outward facing annular guide surface, and is labelled 25 in each of FIGS. 3A-3C.

[0031] Three different example concave surfaces that can be formed using the power tool attachment 10 are labelled 12A-12C in FIGS. 3A-3C respectively. In FIG. 3A, a portion of the outward facing annular guide surface 25 that is toward the leftmost side of the figure is labelled 25.1. A portion of the outward facing annular guide surface 25 that is toward the rightmost side of the figure is labelled 25.2.

[0032] The annular support member 29 of the base 22 also comprises an inward facing annular guide surface that is labelled 28 in each of FIGS. 4A-4C. The guide surface 28 faces generally toward the longitudinal axis 26 of the bit 16 and is used to form convex surfaces in the workpiece 14. Three different example convex surfaces are labelled 12D-12F in FIGS. 4A-4C respectively. In FIG. 4A, a portion of the inward facing annular guide surface 28 that is toward the leftmost side of the figure is labelled 28.1. A portion of the inward facing annular guide surface 28 that is toward the rightmost side of the figure is labelled 28.2.

[0033] The two guide surfaces 25, 28 of the support member 29 are both annular and, therefore, they extend concentrically around the longitudinal axis 26 of the bit 16. Each annular guide surface 25, 28 has a rounded profile such that the surfaces 25, 28 are, when viewed together, partially toroidal in shape. The rounded profile of each guide surface 25, 28 causes the workpiece 14 to contact the relevant surface 25, 28 tangentially when the cutting tool 20 is being guided by the surface 25, 28. This allows each surface 25, 28 to guide the cutting tool 20 smoothly along a spherical or partially spherical trajectory to form the relevant spherical or partially spherical surface. The two guide surfaces 25, 28 are contiguous and integral with each other such that they, together, extend in a partially toroidal configuration between inward facing and outward facing sections of the support member 29 around the lowermost end of the skirt 27. In other examples, the two guide surfaces 25, 28 may be provided on two separate components of the support member 29 that are not necessarily connected to or abutting each other.

[0034] The cutting tool 20 comprises a generally annular blade device. The center axis of the blade device is coaxial with the center axis of each guide surface 25, 28. All three center axes are coaxial with the longitudinal axis 26 of the bit 16. In the example depicted, the vertical position of the base 22 relative to the cutting tool 20 is adjustable. This feature allows the power tool attachment 10 to be switched between two different modes of operation that are used to form concave and convex surfaces respectively. This feature also allows the radius of curvature of the spherical or partially spherical surface 12 that will be formed in the workpiece 14 to be adjusted. For example, in FIG. 3A the base 22 is positioned relatively low compared to the cutting tool 20 such that onlya small portion of the cutting tool 20 is exposed and protrudes downward from the lowermost end of the base 12. In this arrangement, only the outward facing annular guide surface 25 bears against the workpiece 14 as the power tool attachment 10 moves over the surface of the workpiece 14. The position of the guide surface 25 relative to the cutting tool 20 causes a concave surface 12A to be formed in the workpiece 14 that has a relatively large radius of curvature.

[0035] In FIG. 3B, the base 22 has been moved such that it is positioned slightly higher relative to the cutting tool 20 compared to as shown in FIG. 3A. As a result of this adjustment, a slightly larger portion of the cutting tool 20 is exposed and protrudes downward from the lowermost end of the base 12. In this position, the power tool attachment 10 can be used to form a concave surface 12B that has a slightly smaller radius of curvature than the first surface 12A.

[0036] In FIG. 3C, the base 22 has been repositioned so that it is higher relative to the cutting tool 20 compared to as shown in each of FIGS. 3A and 3B. As a result of this adjustment, an even larger portion of the cutting tool 20 is exposed and protrudes downward from the lowermost end of the base 12. In this position, the power tool attachment 10 can be used to form a concave surface 12C that has a smaller radius of curvature than the first two surfaces 12A and 12B.

[0037] In each of FIGS. 4A-4C, the base 22 is positioned sufficiently low relative to the cutting tool 20 such that the cutting tool 20 is effectively recessed within the base 22. The cutting tool 20 does not, therefore, protrude at all from the lowermost end of the base 12. In each of these positions, only the inward facing annular guide surface 28 bears against the workpiece 14. The power tool attachment 10 is, therefore, used to form each of the convex surfaces 12D-F that are depicted.

[0038] Referring to FIGS. 5 and 6, in the example depicted the power tool attachment 10 comprises a sleeve 30 that is engageable with the power tool 18. The bit 16 extends longitudinally end-to-end through an internal lumen of the sleeve 30 and is rotatably fixed within the sleeve 30. The cutting tool 20 is attached to the lowermost end of the bit 16 such that the cutting tool 20 rotates with the bit 16. The base 22 is attachable to thesleeve 30 at a plurality of positions to allow the position of the base 22 relative to the cutting tool 20 to be adjusted. More particularly, the base 22 is slidably (and threadedly) attachable to the sleeve 30 at a continuous range of different operative positions. This configuration, therefore, enables spherical or partially spherical surfaces having radii falling within a continuous range of lengths to be formed using the power tool attachment 10. As shown in FIG. 7, upper and lower bearing assemblies 31 are mounted inside of the sleeve 30 that facilitate smooth rotation of the bit 16 within the sleeve 30. A circlip 33 engages around the bit 16 to prevent axial (longitudinal) movement between the bit 16 and the sleeve 30.

[0039] Referring to FIG. 6, to allow the base 22 to slide relative to the sleeve 30, the base 22 comprises an upper portion 32 that has an internal cavity that receives the sleeve 30. Both the upper portion 32 and its internal cavity are generally cylindrical in shape. A plurality of ribs 34 extend vertically along an inside surface of the upper portion 32 that protrude into the internal cavity. In the example depicted, the upper portion 32 comprises four of the ribs 34 that are arranged at regular angular intervals around the inside surface. The four ribs 34 are slidably received into, respectively, four cooperating channels 36 that extend vertically along an outer surface of the sleeve 30 at equivalent angular intervals. The cooperating ribs 34 and channels 36 mate together and govern the sliding action between the base 22 and sleeve 30. While in the example depicted the ribs 34 and channels 36 are located on the base 22 and sleeve 30 respectively, it will be appreciated that these components may be arranged the opposite way around in other examples — i.e. , cooperating ribs 34 and channels 36 may be located on the sleeve 30 and base 22 respectively.

[0040] To allow for the threaded attachment of the base 22 to the sleeve 30, a rotatable collar 38 is rotatable secured to an uppermost end of the base 22 to rotate about the uppermost end. The collar 38 comprises an internal screw thread 40 that is engageable with a cooperating external screw thread 42 provided about an outermost surface of the sleeve 30. As shown in FIGS. 5 and 7, the collar 38 is provided with a clip 44 that push-fits down onto the collar 38. The clip 44 functions to connect the collar 38 rotatably to the base 22. The clip 44 comprises a circular ring that has a pair of fingers 46 extending downward from the ring 44 at diametrically opposite sides of the ring. Thelowermost ends of the fingers 46 comprise hooks that engage in the base 22 for trapping the collar 38 in place on top of the base 22. The fingers 46 then run lengthwise in the channels 36 in the sleeve 30.

[0041] In the example depicted, the collar 38 is rotatable fixed to the base 22 and operates to pull / push the sleeve 30 toward and away from the base 22, respectively, when the collar 38 is turned. In other examples, the collar 28 may be rotatable fixed to sleeve 30 and operate to pull / push the base 22 toward and away from the sleeve 30 respectively. In other examples, the base 22 and sleeve 30 may be directly connected together by a screw thread arrangement, instead of by the rotatable collar 38.

[0042] Once the position of the base 22 relative to the sleeve 30 has been adjusted, the base 22 is provided with a clamp assembly 50 for fastening the base 22 into the relevant position. Referring to FIG. 1, the clamp assembly 50 comprises a band 52 extending about a circumference of the base 22 and a restraint mechanism 54 for tightening the band 52 around the base 22 and sleeve 30. The band 52 extends at least partially around the circumference and comprises a pair of end portions 56 arranged opposite one another. The restraint mechanism 54 is configured to pull the two end portions 56 together to tighten the band 52. The restraint mechanism 54 includes a pin 58, wherein the pin 58 extends through a pair of apertures provided in the end portions 56 respectively, and a fastening mechanism 60 for causing the pin 58 to pull the end portions 56 together. The fastening mechanism 60 comprises a lever arm pivotably connected to the pin 58 by a cam 62. The cam 62 bears against a shoulder provided on one of the end portions 56 and causes the 58 pin to pull the end portions 56 together when the lever arm 60 is pivoted.

[0043] In the example depicted, the base 22 is attachable to the sleeve 30 at a plurality of positions to allow the position of the base 22 relative to the cutting tool 20 to be adjusted. However, the power tool attachment 10 may comprise other means for adjusting the relative positioning of the base 22 and cutting tool 20. For example, the base 22 may be statically attached to the sleeve 30 (or the base 22 and sleeve 30 may be integral) and the bit 16 may be attached to the drive head of the power tool 18 by anadjustable mechanism that allows the vertical position of the cutting tool 20 to be changed.

[0044] An arm 64 is connected to the sleeve 30 for supporting the sleeve 30 in use. More particularly, as shown in FIG. 2, the arm 64 is configured to bear against a part of the power tool 18 to prevent the sleeve 30, and the attached base 22, from rotating axially about the longitudinal axis 26 in an uncontrolled manner relative to the power tool 28. The arm 64 may be configured to rest against a handle 66 of the power tool 18 or against any other suitable part of the power tool 18 to perform its function. The arm 64 may be statically connected to the sleeve 30, or the arm 64 may be pivotably connected such that the arm 64 can be pivoted vertically to engage with the power tool 18. In examples where the arm 64 is pivotably connected, the arm 64 is preferably spring loaded to bias the arm 64 against the power tool 18 when the arm 64 is vertically pivoted.

[0045] Referring to FIGS. 8A to 8D, the cutting tool 20 comprises a body 68 that has a first blade member 70 and a second blade member 72 for cutting into the workpiece 14. Each blade member 70, 72 extends in a semicircular arc around the centre of the body 68 such that the blade members 70, 72 are, together, in a generally annular configuration. The cutting tool 20 is connectable to the bit 16 such that the centre of the cutting tool 20 is axially aligned with the longitudinal axis 26 of the bit 16. The centre of the cutting tool 20, therefore, rotates about the longitudinal axis 26 in use.

[0046] A pair of cutting heads 73 are releasably attachable onto respective ends 74, 76 of the blade members 70, 72 by screws. The cutting heads 73 face generally toward the rotational direction of travel of the cutting tool 20. Referring to FIG. 8C, each cutting head 73 comprises a generally cylindrical body 79 that has a sharpened cutting edge 81 extending around a perimeter of the end of the cylindrical body 79 that faces the rotational direction of travel. In other examples, the cutting tool 20 may be a homogeneous device where the body 68, blade members 70, 72 and cutting heads 73 are integral. The cutting tool 20 is connectable to the lowermost end of the bit 16 by a washer and screw arrangement 78, as depicted in FIG. 7.

[0047] In use, the bit 16 of the power tool attachment 10 is connected to a rotatable drive head of a power tool 18. In the example depicted, the power tool 18 is an angle grinder and the bit 16 is connected to a rotatable drive shaft 80 of the angle grinder 18. The bit 16 may be connected to other types of power tools in other examples, such as to a collet of a router or to a drive shaft of a power drill. Once connected, the sleeve 30 and base 22 of the power tool attachment 10 extend down from the drive shaft 80 and the arm 64 is pivoted vertically by the user to rest against the angle grinder 18. The biasing action of the arm 64 causes the arm 64 to be held in positive engagement with the angle grinder 18.

[0048] The position of the base 22 relative to the sleeve 30 can then be adjusted by the user if required. The user can move the base 22 in order to select between cutting a concave or convex surface and / or to adjust the radius of curvature of the surface that will be formed. To adjust the position, the user pivots the lever arm 60 into an open position and then rotates collar 38 to adjust the height of the base 22. When the base 22 is in the required position, the user pivots the lever arm 60 back into a closed position to lock the base 22 in place.

[0049] The user then switches the angle grinder 18 on and offers the cutting tool 20 up against the workpiece 14. The workpiece 14 may be made of wood or other suitable material that the spherical or partially spherical surface 12 needs to be formed in, such as stone. The guide surface 24 of the base 22 bears against the workpiece 14 and guides the cutting tool 20 in a spherical or partially spherical trajectory to form the surface 12. As shown in FIGS. 3A to 3C, when the power tool attachment 10 is being used to form a concave surface 12A-12C, the outward facing annular guide surface 25 engages the workpiece 14 tangentially and guides the cutting tool 20 in a concave trajectory.

[0050] As shown in FIGS. 4A to 4C, when the power tool attachment 10 is being used to form a convex surface 12D-12F, the inward facing annular guide surface 28 engages the workpiece 14 tangentially and guides the cutting tool 20 in a convex trajectory about the workpiece 14. FIGS. 9A and 9B each provide an enlarged view of the convex surfacemember 29 of the base 22 of the power tool attachment 10 is shown raised above the surface 12F. The portion of the annular support member 29 that the inward facing annular guide surface 28 is located on is demarked by a double-headed arrow labelled 28’. The portion of the annular support member 29 that the outward facing annular guide surface 25 is located on is demarked by a double-headed arrow labelled 25’. As can be seen, the two annular guide surfaces 25, 28 are rounded and contiguous such that the annular support member 29 is partially toroidal in cross section. In FIG. 9B, the annular support member 29 is shown bearing against the surface 12F. Accordingly, the inward facing annular guide surface 28 is in contact with the surface 12F. The broken line labelled 90 corresponds to a tangent line that touches both the curved convex surface 12F and the rounded annular guide surface 28. The radius of curvature of the convex surface 12F is labelled 92.

[0051] While the two annular guide surfaces 25, 28 of the support member 29 in FIGS. 9A and 9B are rounded, each annular guide surface 25, 28 does not necessarily need to be rounded. For example, FIGS. 90 and 9D depict an example wherein each annular guide surface 25, 28 of the support member 29 is flat. In FIG. 9C, the annular support member 29 is shown raised above the convex surface 12F. In FIG. 9D, the annular support member 29 is shown bearing against the convex surface 12F and, accordingly, the flat inward facing annular guide surface 28 is in contact with the surface 12F.

[0052] The power tool attachment 10 may be used to form surfaces on a workpiece 14 that are either concave or convex and that are either partially spherical or completely spherical. For example, FIG. 10A depicts a workpiece 14 that has been worked on by the power tool attachment 10 such that it has an exterior convex surface 12 that is completely spherical. The resulting object that is produced from the workpiece 14 is a sphere that may be used for ornamental and / or functional purposes. The radius of the sphere is labelled 94 and its diameter is labeled 96. The radius 94 is substantially constant and is determined by the relative positional setting of the cutting tool 20 and base 22 of the power tool attachment 10 when the sphere is created. FIG. 10B depicts a workpiece 14 that has been worked on by the power tool attachment 10 such that it has an internal concave surface 12 that is hemispherical. The resulting object that isproduced from the workpiece 14 is a bowl that may be used for ornamental and / or functional purposes.

[0053] In the example depicted, the power tool attachment 10 includes a single base 22 that is attachable to the sleeve 30 at different positions to adjust the radius of curvature of a spherical or partially spherical surface that is formed. In other examples, the power tool attachment 10 may comprise a kit of parts that includes a set of different bases that are each attachable to the sleeve 30. Each base will have a different height such that the respective annular guide surfaces 25, 28 of each base are positioned at unique heights relative to the cutting tool 20 when the base is attached to the sleeve 30. In such examples, the user of the power tool attachment 10 can select a base corresponding to the required radius of curvature and attach it to the sleeve 30.

[0054] In the example depicted, the cutting tool 20 comprises a pair of blade members 70, 72 for cutting directly into the workpiece 14. However, it will be appreciated that the cutting tool 20 may use a different number and / or type of cutting devices. Furthermore, in other examples the cutting tool 20 may comprise an abrasive disc, or another type of abrasive tool, for wearing away the workpiece 14. In the example depicted, the base 22 of the power tool attachment 10 comprises both an outward facing annular guide surface 25 and an inward facing annular guide surface 28 for forming concave and convex surfaces respectively. In other examples, the base 22 may comprise only the outward facing annular guide surface 25 or only the inward facing annular guide surface 28. In examples where the power tool attachment 10 includes a set of different bases attachable to the sleeve 30, one or more of the bases may comprise only an outward facing annular guide surface and one or more of the bases may comprise only an inward facing annular guide surface. The user can, therefore, select one of the bases and attach it to the sleeve 30 to form a concave or convex surface as may be required.

[0055] In the example depicted, the base 22 is attachable to the sleeve 30 and remains static while the cutting tool 20 axially rotates in use. In other examples, the base 22 may be attachable to the bit 16 such that the base 22 rotates with the bit 16 in use. The base 22 may engage the workpiece 14 directly when the base 22 is rotating or it may comprise a slipping ring that engages the workpiece 14. In the example depicted, thepower tool attachment 10 is a standalone device that is releasably connectable to a power tool 18. In other examples, the power tool attachment 10 may be an integral part of a power tool that is specifically configured to create spherical or partially spherical surfaces in workpieces. In either case, the power tool may be provided with a dust extraction tube (not shown). A vacuum inside the tube operates to pull air and dust particles away from the part of the workpiece 14 that is being worked on by the cutting tool 20 when the surface 12 is being formed.

[0056] Now that example embodiments of the power tool attachment have been described, it will be apparent that it provides a number of advantages over the prior art, including the following:(i) The power tool attachment can be attached to a range of different power tools to form spherical or partially spherical surfaces including angle grinders and routers;(ii) The cutting tool of the power tool attachment is guided by the base in a smooth spherical or partially spherical trajectory to form a spherical or partially spherical surface that has a substantially constant radius of curvature;(iii) The position of the base relative to the cutting tool is adjustable. This allows the user to switch between concave and convex modes of operation and to adjust the radius of curvature of the surface that is formed.

[0057] Embodiments of the present invention provide power tools, and related power tool attachments, that are useful for forming concave and convex spherical or partially spherical surfaces in workpieces made of wood, stone and other materials.

[0058] For the purpose of this specification, the word “comprising” means “including but not limited to”, and the word "comprises" has a corresponding meaning. It is to be understood that, if any prior art is referred to herein, such reference does not constitute an admission that the prior art forms a part of the common general knowledge in the art, in Australia or any other country.

[0059] The above embodiments have been described by way of example only and modifications are possible within the scope of the claims that follow.

Claims

Claims1. A power tool attachment for forming a spherical or partially spherical surface on a workpiece, the power tool attachment comprising: a bit that is connectable to a rotatable drive head of a power tool, wherein an end of the bit comprises a cutting tool; and a base engageable with the power tool for bearing against the workpiece, wherein the base comprises at least one annular guide surface, wherein the annular guide surface is dimensioned such that, when the base is engaged with the power tool, the annular guide surface extends around the bit and faces toward or away from a longitudinal axis of the bit for guiding the cutting tool relative to the workpiece to form the spherical or partially spherical surface on the workpiece.

2. The power tool attachment according to claim 1 , wherein the annular guide surface is rounded such that the workpiece contacts the annular guide surface tangentially when the cutting tool is being guided by the annular guide surface.

3. The power tool attachment according to claim 1 or 2, wherein the base comprises a pair of annular guide surfaces dimensioned to face toward and away from the longitudinal axis respectively for forming convex and concave surfaces on the workpiece respectively.

4. The power tool attachment according to claim 3, wherein the pair of annular guide surfaces are together partially toroidal in shape.

5. The power tool attachment according to any one of the preceding claims, wherein a position of the base relative to the cutting tool is adjustable for adjusting a radius of curvature of the spherical or partially spherical surface to be formed on the workpiece.

6. The power tool attachment according to claim 5, wherein the power tool attachment further comprises a sleeve engageable with the power tool, and wherein thebase is attachable to the sleeve at a plurality of positions for adjusting the radius of curvature.

7. The power tool attachment according to claim 6, wherein the base is moveable into each of the positions relative to the sleeve by a sliding action.

8. The power tool attachment according to claim 7, wherein the base and sleeve comprise one or more ribs received into cooperating channels for governing the sliding action.

9. The power tool attachment according to claim 8, wherein the ribs are provided on an internal surface of the base and the channels are provided on an external surface of the sleeve.

10. The power tool attachment according to any one of claims 7 to 9, wherein the base comprises a rotatable collar comprising an internal screw thread that is engageable with a cooperating external screw thread on the sleeve for moving the base into each of the positions.

11. The power tool attachment according to any one of claims 7 to 10, wherein the base comprises a clamp assembly for fastening the base into each of the positions.

12. The power tool attachment according to claim 11, wherein the clamp assembly comprises a band extending about a circumference of the base and a restraint mechanism for tightening the band around the sleeve.

13. The power tool attachment according to claim 12, wherein the band extends at least partially around the circumference and comprises a pair of end portions, and wherein the restraint mechanism is configured to pull the end portions together to tighten the band.

14. The power tool attachment according to claim 13, wherein the restraint mechanism comprises a pin, wherein the pin extends through a pair of aperturesprovided in the end portions respectively, and a fastening mechanism for causing the pin to pull the end portions together.

15. The power tool attachment according to claim 14, wherein the fastening mechanism comprises a lever pivotably connected to the pin by a cam, wherein the cam causes the pin to pull the end portions together when the lever is pivoted.

16. The power tool attachment according to any one of the preceding claims, wherein the power tool attachment comprises an arm configured to bear against the power tool to prevent the power tool attachment from rotating about the longitudinal axis relative to the power tool in an uncontrolled manner in use.

17. The power tool attachment according to any one of the preceding claims, wherein the cutting tool comprises one or more blade members for cutting into the workpiece.

18. The power tool attachment according to claim 17, wherein the cutting tool comprises a pair of the blade members that are in an annular configuration.

19. The power tool attachment according to any one of claims 1 to 16, wherein the cutting tool comprises an abrasive disc for wearing away the workpiece.

20. A power tool for forming a spherical or partially spherical surface on a workpiece, the power tool comprising: a rotatable drive head with a bit connectable thereto or provided thereon, wherein an end of the bit comprises a cutting tool; and a base for bearing against the workpiece, wherein the base comprises at least one annular guide surface, wherein the annular guide surface is dimensioned such it extends around the bit and faces toward or away from a longitudinal axis of the bit for guiding the cutting tool relative to the workpiece to form the spherical or partially spherical surface on the workpiece.

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