This invention relates to rotary tool holder assemblies, and tool holder shaft assemblies and tool holder spindles comprising such rotary tool holder

assemblies. Such rotary tool holder assemblies may typically be used to hold drill bits for drilling holes.

There are various considerations to take into account when designing rotary tool holder assemblies. On the one hand the rotary tool holder assembly needs to be able to properly grip a tool when under load and operating at a maximum

operational speed but at the same time the tool also needs to be gripped at start up and when being operated at a lower operational speed. Further the rotary tool holder assembly needs to form part of a shaft and spindle arrangement as a whole which gives good stability characteristics to ensure accurate drilling and/or machining, and a convenient tool change operation needs to be accommodated.

WO00/44519 describes a rotary tool holder assembly which makes use of centrifugal forces and sprag forces for gripping a carried tool.

A sprag effect force is a force due to an offset pressure in a direction opposed to that of rotation. Sprag effect forces were originally used on railway wagons in the nineteenth century where a sprag, or piece of timber, was placed against a wagon wheel on the railway line at an orientation directed slightly above the centreline of the wheel to act as a brake. In such as case as the wheel tries to move towards the sprag the pressure increases due to the offset force of the sprag with respect to the axle of the wheel. Clutches have been devised based on the sprag effect where a wedging effect increases against the direction of rotation to provide for a clutching action. Similarly, the tool holder in WO00/44519 makes use of sprag effect forces. A hinged gripping arm is provided where a leading edge of a jaw for gripping the tool is disposed such that a line between the pivot axis of the hinge for the gripping arm and the leading edge of the jaw does not pass through the axis of rotation of the tool holder and hence nor the axis of rotation of a carried tool. The sprag effect force opposes the relative rotation of the tool to the tool holder. In the tool holder of WO00/44519 centrifugal effects are relied upon to initiate the gripping force by suitably moving the jaw portion so that the sprag effect force can begin to take hold as the tool is used at operational speed. However, a problem with a tool holder such as that described in WO00/44519 is that there may be insufficient grip on a tool where it is used at a lower speed. This can become particularly inconvenient where larger diameter tools, for example larger diameter drill bits, are used where the operational speed will tend to be lower.

The present invention is aimed at providing a rotary tool holder assembly which makes use of sprag effect forces and is aimed at addressing problems of the prior art.

According to one aspect of the present invention there is provided a rotary tool holder assembly having a tool receiving bore and comprising tool gripping means for gripping a tool received in the tool receiving bore, the tool gripping means comprising at least one tool gripping arm which is mounted on an outer rim support and is moveable relative to the rim between a tool gripping position in which off centre sprag effect forces can act on a carried tool and a tool release position, the tool gripping means further comprising at least one wedge portion arranged to act on the tool gripping arm and arranged for axial movement relative to the outer rim support wherein axial movement of the wedge portion in a first direction moves the tool gripping arm towards the tool gripping position and the tool gripping means comprises biasing means for biasing the wedge portion towards the first direction.

This can help to initiate and/or enhance gripping of a carried tool as well as enable a simple and effective design and facilitate a straightforward tool change operation. The tool gripping arm may comprise a jaw portion for engaging with a carried tool.

The tool gripping arm may comprise a moveable mass portion which is arranged to move, during rotation of the tool holder, away from a main axis of the tool holder under action of centrifugal effects and is arranged in so doing to move the tool gripping arm towards the tool gripping position. This can allow centrifugal effects to help initiate and/or enhance gripping of a carried tool. The action of the wedge portion and biasing means may help gripping a tool at relatively low speed of rotation whilst the centrifugal effects may help as rotation speeds increase.

At least in some cases the sprag effect forces will provide the main gripping action on a tool once it is up to speed and in use, however, the wedge based grip and/or centrifugal based grip can be important when there is no load, when at lower speeds, and/or at start up.

The tool gripping arm may be hingedly mounted to the outer rim support. The tool gripping arm may be mounted to the outer rim support via a solid deformable hinge. The tool gripping arm and outer rim support may be integrally formed of a single piece of material.

Where there is a hinged mounting, the tool holder assembly will be arranged so that a line between a pivot axis of the hinge and a leading edge of the jaw portion does not pass through the axis of rotation of the tool holder assembly thus facilitating the spragging action.

The biasing means may comprise spring means.

The biasing means may comprise a spring pack. In an alternative a gas spring or hydraulic arrangement might be provided.

The jaw portion may be invariant in cross section along its axial length.

The tool gripping arm may be invariant in cross section along its axial length.

The outer rim support may comprise a wedge receiving portion which has a taper which is complementary with a taper of the wedge portion. A first wall of the wedge receiving portion may be defined by the tool gripping arm. A second wall of the wedge receiving portion facing the first wall may be inclined relative to the first wall to provide said complementary taper. The wedge may be inserted axially into the outer rim support from an end which is opposite to the end in which a tool will be inserted into the tool holder assembly. Thus the wedge may be inserted from an internal end of the outer rim support.

The wedge may be arranged to project axially beyond an external end of the outer rim support.

This can facilitate application of force to the end of the wedge against the action of the biasing means for aid in the removal of a carried tool. A separate pusher may be used to apply such force in tool release.

Where there are a plurality of tool gripping arms there may be a respective plurality of wedges with each wedge arranged to act on a respective one of the arms. A plate may be provided between the or each wedge and the biasing means.

In some embodiments the or each wedge may be mounted on the plate. In other embodiments the plate may be separate and unconnected to the or each wedge. Where the plate is separate it leads to more separate components but can ease insertion of the wedge(s) and/or allow for greater manufacturing tolerances.

In a further alternative, the plate may be flexible. This can ease insertion of the wedges where mounted on the plate and/or allow for greater manufacturing tolerances.

According to a second aspect of the present invention there is provided a tool holder shaft assembly comprising a rotary tool holder assembly as defined above housed in a hollow shaft member. According to another aspect of the present invention there is provided a tool holder spindle comprising a spindle body comprising at least one radial bearing in which is journalled a shaft assembly according to the second aspect of the invention. The tool holder assembly, shaft assembly and spindle may be a drill holder assembly, a drilling shaft assembly and a drilling spindle respectively. These may be for PCB drilling.

According to another aspect of the invention there is provided a method of changing a tool held in rotary tool holder assembly as defined above, comprising the step of pushing on the wedge portion against the action of biasing means to release grip on a carried tool.

The method may comprise the step of using a Z-axis motor of a machine in which the tool holder assembly is mounted to drive the wedge portion against the action of the biasing means. The method may comprise pressing an end of the wedge portion against a fixed face in a tool change station, or the region of a tool change station. The method may comprise the step of using retraction of a pressure foot of a machine in which the tool holder assembly is mounted to drive the wedge portion against the action of the biasing means. The method may comprise pressing an actuating portion of the pressure foot against an end of the wedge portion. Each of the optional features following each of the aspects of the invention above is equally applicable as an optional feature in respect of each of the other aspects of the invention and could be written after each aspect with any necessary changes in wording. The optional features are not written after each aspect merely in the interests of brevity.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 shows, in highly schematic form, a drilling machine comprising a tool holder spindle; Figure 2 is a sectional view of part of a shaft assembly of the tool holder spindle shown in Figure 1 ; Figure 3 shows part of a rotary tool holder assembly of the shaft assembly shown in Figure 2 whilst holding a carried tool;

Figure 4 shows a side view of the part of the tool holder assembly shown in Figure 3;

Figure 5 shows an end view of the part of the tool holder assembly shown in Figure 4;

Figure 6 shows a wedge assembly of the tool holder assembly shown in Figures 3 to 5;

Figure 7 is a section on line VII-VII of the part of the tool holder assembly shown in Figure 5; and Figure 8 shows the part of the tool holder assembly shown in Figures 3 to 5 with the wedge assembly moved in a first direction so as to move a tool gripping arm of the tool holder assembly to a tool gripping position.

Figure 1 shows in highly schematic form, a PCB drilling machine which comprises a tool holder spindle 1 , comprising a spindle body 1 a comprising at least one radial aerostatic air bearing (not shown) within which is journaled a shaft assembly 1 b. The shaft assembly 1 b comprises a rotary tool holder assembly 2 which is arranged for carrying a tool D, in this case a drill bit. The drilling machine and tool holder spindle will include conventional means for driving the shaft assembly 1 b for rotation and so on. However such details are not particularly pertinent to the present invention and therefore description of them is omitted. What is of interest is the construction and operation of the rotary tool holder assembly and this is described in further detail below with references to Figures 2 to 8. Figure 2 shows a part of the shaft assembly 1 b in section and more detail. The shaft assembly 1 b comprises a hollow, tube like, shaft member 1 c within which is housed the rotary tool holder assembly 2.

The rotary tool holder assembly 2 comprises a body portion 3, a wedge assembly 4 received in the body portion 3, a spring pack 5 for acting on the wedge assembly 4 and a threaded stop member 6 against which the spring pack 5 can react.

Thus in the present arrangement the tool holder assembly 2 is housed within the hollow shaft member 1 c and in particular the wedge assembly 4 as well as the spring pack 5 are housed within the shaft.

Figure 3 shows the body portion 3 and wedge assembly 4 in isolation and holding a drill bit D. These components, i.e. the body portion 3 and wedge assembly 4, are shown again in isolation in Figure 4 as a side view and as an end view in Figure 5. Figure 6 shows the wedge assembly 4 in isolation and Figure 7 is a sectional view showing the wedge assembly 4 inserted in the body portion 3.

As best seen by considering Figures 5 and 7 the body portion 3 comprises an outer rim support 31 which supports a plurality of (in this case three) tool gripping arms 32. Each of the tool gripping arms 32 has a main support portion 32a and comprises a jaw portion 33 which faces a tool receiving bore 34.

The outer rim support 31 and tool gripping arms 32 are formed of a single piece of material with each tool gripping arms 32 being hingedly mounted on the outer rim support via a solid hinge 35 which is a deformable part of the material of the body portion 3 as a whole.

Each of the tool gripping arms 32 comprises a bob-weight portion 36 which is arranged to move outwards under centrifugal effects as the shaft assembly 1 b and hence the rotary tool holder assembly 2 is rotated. As the bob-weight portion 36 moves outwards under centrifugal effects this will tend to cause the tool gripping arm 32 to move around the solid hinge 35 so bringing the jaw portion 33 and in particular a leading edge 33a of the jaw portion into contact with a carried tool. Thus as the bob-weight portion moves radially outwards, the leading edge 33a of the jaw portion moves radially inwards. However this action requires the shaft assembly and hence rotary tool holder assembly 2 to be rotating above a certain predetermined speed of rotation.

The wedge assembly 4 is provided for enhancing grip and helping initiate the sprag gripping force at speeds where the centrifugal effects will not yet have taken hold.

As most clearly seen in Figure 6, the wedge assembly 4 comprises a plurality of (in this embodiment 3) wedges 41 mounted to a plate 42. Note however in alternatives the wedges may be separate from the plate 42.

As best seen in Figures 5 and 7 the outer rim support 31 comprises a respective plurality (in this embodiment 3) of wedge receiving portions 37. Each wedge receiving portion 37 and the respective wedge 41 have complimentary tapers. The tapers are such that axial movement of the wedge assembly 4 in a direction such as to further insert the wedge assembly 4 into the body portion 3 causes each wedge portion 41 to act on a respective one of the tool gripping arms 32 so driving the jaw portion 33, and in particular the leading edge 33a, into engagement with a carried tool for gripping the carried tool and enabling sprag effect forces to take hold. Thus again the action of the wedges is to drive the leading edge of the jaw portion 33a radially inwards.

By considering Figure 7 it can be seen that the taper in each wedge receiving portion 37 is provided by an inclination (relative to the axis of the body portion 3) in the wall of the wedge receiving portion 36 which is opposite to/faces the respective gripping arm 32. As such the cross section (perpendicular to the axis of the body portion 3) of the gripping arm 32 along the axial length of the body portion 3 is invariant. This helps ensure uniform gripping of the tool shank along the length of the tool holder assembly. Figure 8 shows the situation with the wedge assembly 4 inserted into the body portion 3 to such a degree as to deform the tool gripping arms 32 into a gripping position. The rest position is illustrated by the solid lines, with the leading edge indicated as 33a. The deformed, gripping, position is shown in solid block form with the leading edge indicated as 33a'. This degree of deformation/movement of the arms is exaggerated in Figure 8 for the sake of illustration.

In an alternative, rather than wedges 41 having a polygonal cross section, the wedges may have the form of pins with a part circular surface and a tapered wedge surface. Of course in such a case the wedge receiving portions in the body portion 3 will be correspondingly shaped.

By considering Figure 2 it will be noted that in operation, the action of the spring pack 5 tends to drive the wedge assembly 4 into engagement with the body portion 3 which in turn will bias the wedge assembly 4 and hence gripping arms 32 towards a gripping position. To put this another way, the spring pack 5 provides a biasing force in a first direction on the wedges 41 to bias the arms 32 towards the gripping position. This active loading or spring loading of the tool holder assembly is advantageous insofar as it provides a dynamic gripping force and also facilitates a tool change operation without complex components internal to the shaft assembly 1 b.

By considering Figures 2, 3 and 7 it will be seen that the distal ends of each wedge portion 41 projects axially beyond the end of the body portion 3. This facilitates a tool change operation. An appropriate form of pusher may be provided for pushing on the end of the wedges 41 to force the wedges 41 back into the shaft assembly 1 b against the action of the spring pack 5. It will be appreciated that, as this occurs, the biasing force on the arms 32 will be removed such that they can return to a tool release position. At this stage, a tool change can be effected.

As will be appreciated a machining or drilling spindle of the current type will generally be provided in a machine having at least a Z-axis motor for driving the spindle perpendicular relative to a work piece for machining operations. This same Z-axis motor may be used for actuating a tool change operation by virtue of providing the necessary force for acting on the ends of the wedge portions 41 to release a tool. Of course such a use of a Z-axis motor for a tool release operation would be possible even were the wedges 41 not to project beyond the end of the body portion 3, but the operation and/or tool change station may need to be more complex in such a case.

Thus in one particular implementation, during actuation, whilst the Z-axis motor is driving the spindle 1 downwards to change tools, the end of the wedges 41 (or at least one of the wedges 41 ) will contact a fixed face in the tool change station, or the region of the tool change station causing the wedges 41 to be pushed back into the shaft assembly 1 b to release the carried tool. When the spindle 1 is moved upwards away from the fixed face, the spring pack 5 acting on the wedges will cause re-gripping of a carried tool.

In another particular implementation, the machine of which the spindle 1 is part may comprise a pressure foot provided for acting on a work piece and holding it against a table of the machine during machining. The pressure foot may be mounted on a carriage which also supports the spindle 1 . The machine may be provided with a mechanism for retracting the pressure foot from the work piece between machining operations. Typically such a mechanism causes relative movement between the tool carrying spindle 1 and the pressure foot. Thus in such an implementation, retraction of the pressure foot may be used to release a carried tool. In particular the pressure foot may comprise an actuating portion arranged to act against the ends of the wedges 41 (or at least one of the wedges 41 ) as the foot is retracted.

Note also that in the present embodiment the biasing of the tool gripping arms 32 towards the gripping position means that the need for some other mechanism such an O ring for gripping the tool initially before spinning the shaft up to speed may be avoidable. This in turn may lead to more reliable operation.