WO2016165929A1

WO2016165929A1 - Boring device having centering means

Info

Publication number: WO2016165929A1
Application number: PCT/EP2016/056595
Authority: WO
Inventors: Kevin Nedzlek
Original assignee: Walter Ag
Priority date: 2015-04-17
Filing date: 2016-03-24
Publication date: 2016-10-20

Abstract

The present invention is related to a boring device (100) having a cutting head (20) and a shaft (30) defining a longitudinal axis (50) and comprising a generally cylindrical guide section (10) including guiding means for guiding the shaft (30) in a guide hole during machining. In order to provide a bor- ing device as defined in the preamble of claim 1, which is further adapted to produce machined holes with small tolerances and which are properly centered with respect to a guide hole potentially having larger tolerances or even offset from an envisaged position of a new hole to be machined or an already prepared hole with smaller diameter to be reamed to a final diameter, the present inven- tion provides a plurality of radially moveable guide elements (4) arranged within the guide section of said shaft and comprising guiding surfaces exposed to the periphery of the guide section (10), and means providing a simultaneous radial movement of the guide elements (4) in the same radial direc- tion and by the same amount upon any forced radial movement of at least one of said guide ele- ments (4).

Description

Boring device having centering means

FIELD OF THE INVENTION

The present invention is related to a boring device having a cutting head and a shaft defining a longitudinal axis, the boring device further comprising a generally cylindrical guide section including guiding means for guiding the shaft in a guide hole during boring.

BACKGROUND OF THE INVENTION

Boring devices of the afore mentioned kind are sometimes required in situations where a hole to be machined needs to be aligned with respect to another hole already provided in a separate member arranged at some distance to the new hole to be machined. In such a case, the already existing hole in a first member is used as a guide hole for guiding the shaft of the boring device upon machining a properly aligned hole in a second member.

The extended length of the shaft of a boring device to be inserted through already existing holes causes problems with regard to the stability and accuracy of the d+bored or reamed holes, because the longer shaft renders the tool more susceptible to deflection due to the cutting forces acting on the cutting head at some distance from any chuck or receptacle for the shaft. Therefore, the use of intermediate holes as guide holes is almost inevitable when the succession of holes is to be properly aligned.

By means of such a boring device using guide holes for a proper alignment of the shaft and boring head with already existing guide holes, it is generally possible to drill a succession of aligned holes in a number of different members arranged one behind the other in a spaced relationship, provided that the shaft has a sufficient length and diameter to extend through a succession of aligned machined holes.

Principally, a boring device comprising a cylindrical shaft of a sufficient length and having an appropriate diameter adapted to the diameter of the guide hole would be suited for that purpose. However, the shaft diameter of the boring device would only fit to a single diameter of the guide hole. With some of the prior art boring devices, the diameter of the guide section is defined by a number of separate guiding pads distributed about the circumference of the guide section. This allows to adapt the diameter of the boring device to the diameter of the guide hole by using appropriate guiding pads, which might be exchanged if required. Of course, for the use of any guide holes the shaft diameter of the guide section must always be smaller than the diameter of the guide hole.

In addition, for an easier adaption to a variety of guide hole diameters, boring devices have been developed which provide one or several guiding pads that are radially moveable on said guide section and spring biased in the radial outward direction in order to adapt the diameter of the guide section to the diameter of the guide hole. However, if the boring device is under any radial load such spring biased guiding elements would allow the guide section to be arranged in the guide hole in an position offset from the central axis, while all guide pads may still be in contact with the wall of the guide hole. This may then cause that the center of the hole to be machined is radially offset from the center of the guide hole.

Moreover, once a guide hole has not been prepared with the desired accuracy, also the hole to be machined will suffer from the limited accuracy of the guide hole, because the shaft is rotated and guided within the corresponding inaccurate guide hole, thereby transferring or even enhancing the inaccuracy of the guide hole with the hole to be machined.

With the inherent tolerances occurring even when the guide hole is machined with the available accuracy, the tolerances and deviations from preceding holes may even add up to larger tolerances when a succession of holes is machined with each preceding hole being used as the guide hole for the next following hole.

Moreover, in some cases a particular accuracy may not be required for the guide hole but may be required for the next hole to be machined. With the known drilling or boring devices providing fixed or alternatively radially biased guiding means, any improvement for reducing the tolerances of the hole to be machined over the tolerances of the guide hole has not yet been possible.

SUMMARY OF THE INVENTION

Accordingly, it is the object of the present invention to provide a boring device as defined in the preamble of claim 1 , which is further adapted to produce holes with small tolerances and which are properly centered with respect to a guide hole potentially having larger tolerances or even offset from an envisaged position of a new hole to be machined or an already prepared hole with smaller diameter to be reamed to a final diameter.

This object is achieved by means of a boring device identified above which is further characterized by

a plurality of radially moveable guide elements arranged within said shaft and comprising guiding pads exposed on the periphery of the guide section,

means providing a simultaneous radial movement of the guide elements in the same radial direction and by the same amount upon any forced radial movement of at least one of said guide elements.

By providing a simultaneous radial movement of the guide elements in the same radial direction and by the same amount, there is ensured that the guide elements or the guiding pads thereof are always defining the same single diameter and radius with respect to a common center. If a particular guide hole has a larger diameter than the hole to be machined, the guide elements will be extended and still ensure that the shaft is centered within the respective hole and the center of the hole to be machined is properly aligned with the center of the guide hole. The same holds for any guide hole which may be narrower than the hole to be machined, provided that the guide section of the shaft still fits into the guide hole.

One option to provide a simultaneous radial outward movement of the guide elements by the same amount is the engagement of a centrally arranged piston with guide elements arranged around the periphery of the piston, wherein the piston and the guide elements are each comprising surfaces acting as cooperating wedge surfaces arranged to push the guide elements simultaneously radially outward once the piston is moved towards a first direction which will here be indicated as the forward direction.

By the term "wedge surfaces" there is meant that the surfaces of the piston and the guide element which are in abutting contact with each other extend at an angle with respect to the shaft axis, so that any axial movement of the piston when engaging a guide element is transferred into a radial movement of the guide element and vice versa, provided that the angle between the shaft axis and the wedge surfaces is sufficient to effect the wedge action both ways. Accordingly, once any of the guide elements engages the wall of a guide hole and is on its radial inner (wedge-) side in contact with the piston, it will stop the piston from any further axial forward movement and thereby also stop any radial outward movement of other guide elements. Further, once the guiding section enters a guide hole which is narrower than the diameter defined by the guiding elements in an actual state, the guide elements will be pushed radially inward by engaging the wall of the hole, whereupon due to the engagement between the wedge surfaces of the guide elements and the piston, the latter will be pushed back in the axially rearward direction. The guide elements may have proper ramping surfaces in order to be entered into a narrower hole.

Accordingly, in order to provide the means for causing a simultaneous identical movement of the guide elements, an axial bore may be arranged within said shaft receiving an axially moveable actuating piston, said axially moveable actuating piston being spring biased and engaging a radially inner surface of the radially moveable guide elements for urging the same radially outward upon axial movement of the piston in a first axial or forward direction.

Accordingly, once the axially moveable actuating piston is advanced due to the action of a spring, it engages a radially inner surface (slightly tilted with regard to the axis) of the guide elements and urges the same radially outwards into contact with the wall of the guide hole whereupon any further axial movement of the piston is blocked because the guide elements cannot move radially outward any further.

Accordingly, with a plurality of such guide elements distributed around the circumference of the periphery of the guide section and simultaneously engaging a piston arranged within the bore of the shaft, also the piston which is an engagement with the guiding elements and the bore receiving the piston in an axially moveable manner are properly centered with respect to the hole. Thereby also the shaft receiving the piston in the corresponding central bore therein is properly centered and is kept in this centered position upon rotation of the shaft, during which the guiding elements or corresponding guiding pads defining the outer surface of the guiding elements are kept in contact with the wall of the guide hole. The piston and thus the shaft are always centered with respect to the guide hole.

It goes without saying, that of course the piston and the bore receiving the piston are centrally disposed within said shaft at the guide section and the piston does not have any substantial radial play. Rather, in the preferred embodiment, the piston is in a tight slip fit engagement with the centrally arranged bore in the shaft receiving the piston.

Accordingly, by radially extending or retracting the guide elements due to the action of the piston, the diameter of the guide section may be varied and adapted to the diameter of the guide hole, but at the same time the radial outer surfaces of the guide elements defining the radial outer diameter of the guide section by will always be the same with respect to the same central axis. Therefore, once the guide elements are in contact with the wall of the guide hole, the axis of the boring device will always coincide with the axis of the guide hole.

Accordingly, even if the guide hole may have a slightly larger or smaller diameter due to tolerances than the hole to be machined, the shaft and thus the tool axis and the boring head arranged at the front end of the shaft are always kept centered with regard to the center of the guide hole which allows a very precise machining of the hole, because the shaft is continuously supported and prevented from being bent due to cutting forces acting on the cutting head.

The radial retraction or repelling of the guide elements will be effected either by the wall of the guide hole or by means of additional spring elements acting on the guide elements, which will be described later.

In order to have the boring device properly centered with respect to the guide hole, at least three guide elements are provided at substantially the same axial position and angularly offset with respect to each other about the axis or circumference of the shaft. In particular, it is preferred that the guide elements are distributed around the periphery of the shaft guide section at about equal angular spacing, which does not only hold for the provision of three guide elements but principally for any number of guide elements such as four or six. However, with any number of guide elements larger than three, it is generally sufficient if the angular spacing between any adjacent two guide elements is less than 100 degrees and no special care needs to be taken to provide an exactly equal angular spacing between all adjacent guide elements.

In one embodiment the axial bias of the piston is provided by means of a spring, in particular a helical compression spring received within a bore provided for the piston or adjoining such bore.

In one embodiment, the piston and the guide elements are defining pairs of mutually abutting actuating surfaces, each pair of abutting actuating surfaces extending in a plane which is perpendicular to a sectional plane including the shaft axis wherein the plane of the actuating surfaces is tilted with respect to the shaft axis by an angle a from between 10 and 70 degrees when measured in said sectional plane including the shaft axis.

Preferably, the angle a is in the range from between 15 and 60 degrees. Due to the angle a between the actuating surfaces and the shaft axis, the piston exerts a wedge- action on the guide elements when axially moved towards a first direction. Preferably, a radial outward movement of the guide elements due to the wedge-action of the central piston is not only limited by the walls of the guide hole but in addition spring members may be provided exerting a radially inward force, i.e. towards the axis of the shaft, wherein the spring elements urging the guide elements radially inward are designed such as to move the guide elements to a radially inward position in which none of the guide elements projects beyond the peripheral surface of the guide section, i. e. the shaft portion of the guide section. The latter may for instance have some axially extending slots for receiving plate like guide elements extending through said slots in a radially moveable manner, when acted upon by the piston and spring elements.

In particular, the piston may comprise a cylindrical outer surface with intermediate axially extending grooves, the bottom of said grooves defining an actuating surface tilted with respect to the shaft axis for abutting the cooperating actuating surface of the guide element. In operation, the abutting actuating surfaces of the piston and the guide elements are continuously in contact with each other wherein the axial position of the piston defines the outer diameter of the guide section which in turn is limited by the guide hole diameter determining the amount by which the radial outer surfaces of the guide elements may extend beyond the cylindrical outer surface of the shaft of the guide section.

Preferably, the piston is received in a tight slip fit engagement within the respective bore of the shaft which ensures that the centered position of the piston which is obtained due to the engagement of the guide elements with the wall of the hole and the engagement of the inner actuating surfaces of the guide elements with the corresponding actuating surfaces of the piston, is also transferred into a correspondingly centered position of the shaft. Nevertheless, the piston is still axially moveable in order to be able to adapt the diameter defined by the radial outer surface of the guide elements to any particular diameter of the guide hole.

Still, the radial movement of the guide elements, more precisely the maximum extension of the radially outer surface of the guide elements from the cylindrical peripheral surface of the guide section of the shaft should be limited to at most 10% and preferably at most 5% of the radius of the guide section of the shaft.

In one embodiment, the guide elements are plate-shaped elements which are partly received in axially extending slots in the peripheral wall of the guide section surrounding the bore and piston of the guide section. In addition, the radial inner edge surface of the plate-like guide element extends into the axially extending grooves in the peripheral wall of the piston for engaging the cooperating actuating surface at the bottom of said grooves.

In one embodiment, the boring device comprises a push rod received in the same or another adjoining bore of said shaft as said bore receiving the piston, wherein the push rod is adapted to engage and adjust a loading spring for biasing said pistons axially and, by means of the tilted actuating surfaces radially against the guide elements. The loading spring ensures some flexibility of the piston and guide elements upon entering a guide hole or upon rotation in a not perfectly round guide hole.

Further, the shaft may include a return spring received in a further bore of said shaft for urging the piston from the side opposite the loading spring and against the direction A when the thrust of the push rod on the loading spring is released.

Accordingly, upon retracting the push rod, the loading spring will be released and the piston will be forced back under the action of the return spring, wherein the additional spring elements for the guide elements will retract the guide elements radially inward and out of engagement with the wall of a guide hole so that the radial outer surfaces of the guide elements, which may be provided with a particular pad material, are retracted into the slots in the peripheral wall of the guide section out of contact with the wall of the guide hole and preferably do no longer extend radially beyond the periphery of the guide section.

In such inactive state, the boring device may be retracted from the guide hole or further advanced into the next guide hole for boring or reaming another hole, wherein the previously machined hole may now act as a guide hole for the next boring operation.

Of course, the return spring is designed for overcoming the biasing force of the loading spring only if the push rod is sufficiently retracted towards an inactive position because during operation the loading spring is intended to keep the guide elements or radially outer pads thereof continuously in contact with the wall of the guide hole.

In one embodiment, the diameter of the guide section of the shaft is between 90% and 99% of the cutting diameter of the cutting head, which in turn means that any gap between the peripheral wall of the guide section and the guide hole wall is to be bridged by the guide elements projecting radially outward through slots provided in the peripheral wall of the guide section. As already mentioned, this in turn requires that the guiding pads or guide elements may have an extension from the peripheral surface of the guide section of the shaft which is between 0.5% and 5% of the guide section diameter.

For a proper function, it is of course necessary that the loading springs and the biasing elements for urging the guide elements towards the shaft axis are designed such that upon pushing the push rod into an active position towards the piston in the forward direction, the force exerted by the piston on the guide elements urging them radially outward exceeds the radially inward force from the spring elements urging the guide elements radially inward.

While the boring device of the present invention may of course be designed as an ordinary drill having helically extending chip flutes, the cutting head of a preferred embodiment comprises at least one recess for receiving a cutting insert. Such a design of course limits the diameter of the boring device to a minimum of at least 12 mm because it is otherwise difficult to provide cutting inserts with maximum dimensions of less than 8 mm.

DESCRIPTION OF THE DRAWINGS

Further features and possible applications of the present invention will become apparent on basis of the following description of preferred embodiments and the associated figures.

Figure 1 shows side views and a front view of a boring device according to the present invention.

Figure 2 shows a cross-section through a shaft and a cutting head including the axis of the tool.

Figure 3 shows a perspective view on the cutting head and guide section of the tool according to the invention.

Figure 4 shows a perspective view of the inner elements of the shaft and guide section, namely the piston, the guide elements, the push rod, the loading spring and a return spring.

Figure 5 shows the piston and associated members of another embodiment of a boring device.

Figure 6 shows a shaft and guide section including the piston and guide elements shown in figure 5. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Figure 1 a is a front view from the cutting head side on a boring device 100, the side view of which is shown in figures 1 (b) and 1 (c), respectively.

The complete tool comprises a cutting head 20, a shaft 30 including a guide section 10 and a flange 60 on the rear end of the shaft 30 to be mounted within an adapter 70 which may be coupled to the spindle of a machine tool.

Guide elements 4 are visible in the guide section 10, wherein the shaft portion of the guide section 10 comprises axially and radially extending slots 16.

The wall of a guide hole is indicated by 23 and the wall of a hole to be reamed is indicated by 24.

Details of the guide section 10 are better visible in figure 2 which is a cross-section through a boring device according to the present invention taken along a plane including the axis 50 of the tool. In the present embodiment, the shaft 30 and the guide section 10 have substantially the same outer diameter d, which is slightly smaller than the diameter D, which is defined by twice the radius of the cutting edge of a cutting insert 22 received in a corresponding recess 21 of the cutting head 20. The cutting insert 22 or the cutting edge thereof define a maximum outer diameter of the hole to be machined. The diameter of the guide hole may be about the same as the diameter of the hole to be machined. In the present case, no cutting elements are provided in the center of the cutting head, which means, that the present tool is designed for reaming the wall of an already existing central hole to the desired diameter.

The guide section is formed by a portion of the shaft adjacent the cutting head 20. The guide section is comprised of a shaft portion 17 which may be integral with the major part of the cutting head 20 wherein the shaft portion 17 comprises a central inner bore 2 having adjacent (central) bore sections 2' and 2" of a smaller diameter than the bore 2. The bore 2 receives a substantially cylindrical piston 3 having three longitudinal grooves 18 extending along and within the outer cylindrical surface of the piston and which are clearly visible in the embodiment of figure 5. The cylindrical wall of piston 3 is in tight slip fit engagement with the wall of bore 2.

Plate-shaped guiding elements 4 are received within said grooves 18, wherein the section of figure 2 is taken along a plane including the axis 50 and intersecting the bottom 7 of one of the grooves 18 as well as the radial inner edge surface 6 of the guide elements 4. The radial outer edge face of the guide elements 4 is cladded with a pad 5 having a desired wear resistance and designed for low friction with the wall of a guide hole.

The bore 2 provides a sufficient axial stroke space for the piston 3 and a loading spring 1 1 is provided in another bore 2' having a narrower diameter than the bore 2 and adjoining the upper end of the bore 2. On the opposite side another bore 2" of a narrower diameter is provided for receiving a return spring 12.

The shaft portion 17 of the guide section 10 may be connected to the upper shaft portion 30 by means of a thread 19.

As better visible in the embodiment of figure 6, the guide portion 10 also comprises a number of axi- ally and radially extending slots 16 for receiving the plate-like guide elements 4. The sectional plane of figure 2 is intersecting a corresponding slot 16 receiving the guide element 4.

A push rod 9, the lower head of which is an engagement with a loading spring 11 in the bore 2', extends up through another narrower central bore of the shaft and may be actuated via the spindle driving the respective tool, in particular by means of hydraulic or pneumatic pressure.

Upon finishing a hole by machining or reaming and by arranging the guide section in a corresponding guide hole during machining, the push rod 9 will be retracted and thus the loading spring 1 1 is released and the return spring 12 thereby pushes the piston 3 upward towards its uppermost position.

A retention pin 13 having a lateral (radial) spring action (cf. figures 5 and 6) is placed and designed such as to exert a force on the guiding elements 4 towards the center of the tool, i.e. towards the axis 50 as long as the surface of the pads 5 project over the peripheral surface of the peripheral surface of the shaft portion 17 of the guide section.

Once the guide elements 4 are sufficiently retracted into the slots 16 such as to be flush with the circumferential surface of the shaft section 17 or even slightly retracted radially inside of the surface, the shaft may retracted from the guide hole or be further advanced there through. In particular, the cutting head may be inserted through another guide hole, which may even be the just machined hole until the guide section and in particular the guide elements 4 thereof are axially positioned within the new guide hole (not shown), while the cutting insert 22 may be in engagement with another member to be machined. Thereupon, the push rod 9 is pushed downward for biasing the loading spring 1 1 , wherein the loading spring 1 1 and the return spring 12 are designed such that the force of the return spring 12 may easily be overcome when pushing the rod 9 downward in order to compress the loading spring 1 1 . Therefore, the loading spring 1 1 pushes the actuating piston 3 downward, wherein, by engagement of the wedge surfaces 6, 7 of the guide elements 4 and the bottom 7 of the piston grooves, the guide elements 4 are pushed radially outward until all pads engage the wall of the guide hole.

In the embodiments shown, there are either three or four guide elements distributed about the circumference of the guide section 10 at equal angular distances. All guide elements 4 have the same shape and dimensions and the same axial position so that each of the guide elements 4 is engaged by the piston or better to say the bottom surface 7 of the groove 18 thereof, so that each of the guide elements 4 is moved radially outward by the same amount, until the pads abut the wall of the guide hole.

The retention pins will be slightly compressed thereby. However - at least when using such retention pins as biasing means - the radial movement of the guide elements 4 is rather limited, i.e. there is only a little radial play between the peripheral surface of the guide section 10 and the wall of the corresponding guide hole which is to be bridged by the guide elements. This means, that the guide elements 4 and pads 5 thereof need to be pushed out of the slots 16 only by a rather small amount in the order of for instance 1 mm or less before engaging the wall of the guide hole.

Even if the shaft 30 may originally be slightly off center, the simultaneous extension of the guide elements 4 by the same amount will ensure, that very quickly all pads 5 engage the wall of the guide hole at the shaft 30 and the axis 50 thereof is at least properly centered with regard to the axis of the guide hole once the push rod 9 is actuated.

It goes without saying that with a narrower guide hole, the actual movement of the piston 3 is more restricted because the guide elements 4 and pads 5 thereof may be pushed out of the slots by an even smaller amount.

The axial ends of the pads 5 are chamfered, so that even in a situation where the guide section enters a guide hole which may be slightly narrower than the diameter defined by twice the radius of the pads 5 while the pads are not completely retracted with the slots of the shaft portion, the pads may still be inserted with the guide hole due to their chamfered ends and in the course of inserting the guide portion 10 into the guide hole, the pads 5 and guide elements 4 will be pushed inwardly and in turn push the piston 3 upward and rearward against the loading spring 1 1.

Of course, the proper function of the device in such a situation will also depend on the slope of the wedge surfaces 6 and 7, respectively, which should be sufficiently large in order to allow the piston 3 to be pushed back against the loading spring 11 once the pads and guide elements 4 are pressed radially inward due to the engagement of the pads 5 with a narrow guide hole.

Figure 3 is just an enlarged perspective view of the cutting head and adjacent guide portion 1 1 of the embodiment shown in figures 1 and 2, respectively. Wherein the guide elements 4 including the pads 5 thereof are received in slots 16 extended through the wall of the shaft portion 17 of the guide section 10, which are better visible in figure 6. The cutting head comprises a recess 21 for receiving a cutting insert 22 and leaving also some space in front of the cutting face of the cutting insert 22 for receiving chips generated by the cutting insert 22 during a machining action, i. e. during chip forming boring or reaming.

Figure 4 shows again the elements inside the guide section 10. The guide elements 4 are in this case designed somewhat different from the plate-like guide elements 4 disclosed in figure 5 and 6, respectively. Nevertheless, the principal structure of the guide elements 4 is the same in both cases in that they are received and guided within slots 16, 18 and will be pushed radially outward through the slots 16 by the mutually engaging wedge surfaces 6, 7 once the loading spring 1 1 is compressed. The piston 3 has a substantially cylindrical peripheral surface with axially extending grooves 18 for receiving the radial inner ends of the guide elements 4 and for engaging the inner edge surfaces 6 thereof by the corresponding bottom surface 7 of the grooves 18. The cooperating faces of the piston and guide element are thus the same and act in the same way as in the embodiment of figures 5 and 6. In the embodiment of figures 1 through 4, the guide pads and pad holders which may be screwed onto a slide which in turn is received in a groove 18.

The principal structure of the embodiment shown in figures 5 and 6 is the same as that of the embodiment of figures 1 through 4, wherein the piston 3 and the embodiment of figure 5 comprise bearing balls received in small cages in the periphery of the piston 3 in order to allow a tight fit within the bore 2 of the guide section shaft with a very low friction and easy axial movement.

For the purpose of original disclosure it is to be noted that any features which may be gathered by a skilled person from the present description, the drawings and the claims, even if only described in connection with particular further features, may be combined individually as well as in arbitrary com- binations with any other of the features or groups of features disclosed herein, unless this is explicitly excluded or technical conditions would render such combinations impossible or senseless. The comprehensive, explicit discussion of any combinations of features which might be thought of is dispensed with just for the sake of brevity and legibility of the description and claims.

Claims

C l a i m s

1 . Boring device (100) having a cutting head (20) and a shaft (30) defining a longitudinal axis (50) and comprising a generally cylindrical guide section (10) including guiding means for guiding the shaft (30) in a guide hole during machining, characterized by

a plurality of radially moveable guide elements (4) arranged within the guide section of said shaft and comprising guiding surfaces exposed to the periphery of the guide section (10),

means providing a simultaneous radial movement of the guide elements (4) in the same radial direction and by the same amount upon any forced radial movement of at least one of said guide elements (4).

2. Boring device according to claim 1 , characterized in that the shaft (30) comprises a central axial bore (2) within said guide section and an axially moveable actuating piston (3) tightly and slidably received within said bore, said axially moveable actuating piston (3) being biased for engaging radially inner surfaces (6) of the guide elements (4) for urging the same radially outwards upon axial movement of the piston (3) in a first axial direction (A).

3. Boring device according to any of the preceding claims, characterized in that the piston (3) and the guide elements (4) are defining pairs of mutually abutting actuating surfaces (6, 7), each pair of abutting actuating surfaces (6, 7) extending in a plane which is perpendicular to a plane including the shaft axis (50) and tilted with respect to the shaft axis (50) by an angle (a) from between 10 and 70 degrees when measured in said plane including the shaft axis.

4. Boring device according to claim 3, characterized in that said angle (a) is in the range from between 15 and 60 degrees.

5. Boring device according to any of the preceding claims, characterized in that the guide elements (4) are individually biased towards the shaft axis (50).

6. Boring device according to any of the preceding claims, characterized in that the piston (3) comprises a cylindrical outer surface (8) with intermediate grooves, the bottom of said grooves defining an actuating surface (6) for abutting actuating surfaces (7) of the guide elements (4).

7. Boring device according to any of the preceding claims, characterized in that the guide elements are plate shaped elements partly received in and radially extending through axially and radially extending slots (16) in the peripheral wall of the guide section surrounding the bore and piston of the guide section.

8. Boring device according to any of the preceding claims, characterized in that at least three guide elements (4) are provided at substantially the same axial position and angularly offset with respect to each other about the axis (50) of the shaft (30).

9. Boring device according to any of the preceding claims, characterized in that the guide elements (4) are distributed around the periphery of the shaft guide section (10) at about equal angular spacings.

10. Boring device according to any of the preceding claims, characterized by a push rod (9) received in another bore (2') of said shaft (30) and adapted to engage and adjust a loading spring (1 1 ) for biasing said piston (3) against said guide elements (4).

1 1 . Boring device according to any of the preceding claims, characterized in that the shaft (30) includes a return spring (12) received in a further bore (2") of said shaft for urging the piston (3) from the side opposite the loading spring (11 ) towards and against the loading spring (11 ).

12. Boring device according claim 1 1 , characterized in that the return spring (12) is designed for overcoming the biasing force of the loading spring (11 ) only if the push rod (9) is retracted to an inactive position

13. Boring device according to any of the preceding claims, characterized in that the diameter of the guide section (10) of said shaft (30) is between 90 and 99% of the cutting diameter of the cutting head (20).

14. Boring device according to any of the preceding claims, characterized in that guiding pads 5 defining the radial outer periphery of the guide elements have a maximum extension from the peripheral surface of the shaft guide section (10) in the range of 0,5 % to 10 % of the guide section diameter (d).

15. Boring device according to any of the preceding claims, characterized in that the loading spring (1 1 ) and the spring elements (13) for urging the guide elements (4) towards the shaft axis (50) are designed such that upon pushing the push rod (9) into an active position towards the piston in the first direction, the force exerted by the piston on the guide elements urging them radially outward exceeds the radially inward force from the spring elements (13) urging the guide elements radially inwards.

16. Boring device according to any of the preceding claims, characterized in that the cutting head (20) comprises at least one recess (21 ) for receiving a cutting insert (22).