Counterbalanced boring tool

The invention concerns a boring tool comprising a main tool body and a tool holder which is adjustable relative thereto, as well as a counterweight coupled to the tool holder in such a way that upon a movement of the tool holder in one direction a movement of the counterweight occurs substantially in the opposite direction, wherein both the tool holder and also the counterweight are respectively arranged at least in part in a bore in the main tool body. To provide a boring tool of the above-mentioned kind which is suitable in particular for high rotary speeds, which is advantageous in terms of manufacture and which in addition provides a reliable precise balancing function, it is proposed in accordance with the invention that a tool holder and a counterweight are arranged in the same bore in the main tool body.

The present invention concerns a boring tool with a main tool body, at the end of which there is provided at least one tool holder which is adjustable relative to the main tool body, wherein the tool holder is movable in the radial direction and can be held fast.

Such boring tools are frequently operated at very high rotary speeds so that it is necessary for the boring tool to be balanced prior to operation. As any radial displacement of the tool holder provides for an unbalance, the tool has to be re-balanced after each such displacement. Boring tools are therefore already known having a counterweight which is coupled to the tool holder in such a way that, upon a movement of the tool holder in one direction, there is a movement of the counterweight substantially in the opposite direction, wherein both the tool holder and also the counterweight is respectively arranged at least in part in a bore in the main tool body.

Such a boring tool is known for example from EP 0 804 984. That boring tool has a clamping member which is intended to come into contact simultaneously with the tool holder and the counterweight. That construction is very complicated and expensive to manufacture. In addition in the course of use it can happen that the clamping member no longer comes exactly simultaneously into contact with the counterweight on the one hand and the tool holder on the other hand so that there is possibly a slight unbalance after fine adjustment has been effected. In addition the counterweight and the tool holder are spaced from each other in the axial direction so that any displacement of the tool holder alters the moments which occur in the tool.

Therefore, taking that state of the art as the basic starting point, the object of the invention is to provide a boring tool of the above-specified kind, which is suitable in particular for high rotary speeds, which is advantageous in terms of manufacture and which in addition provides a reliable precise balancing function.

That object is attained in that the tool holder and the counterweight are arranged in such a way that the movement of the tool holder and the substantially opposite movement of the counterweight are effected on the same axis.

In other words the tool holder and the counterweight are arranged in the same bore in the main tool body.

That arrangement ensures that the center of gravity does not change even in the plane of the tool holder so that the forces within the tool are reduced. The total of the radial forces is preferably zero.

In a preferred embodiment the main tool body has a clamping device, by means of which the main tool body can be elastically deformed so that at least the tool holder and possibly also the counterweight are respectively clamped fast in the bore.

The elastic deformation of the main tool body means that the counterweight and the tool holder are braced in the bore so that the two parts are clamped fast in embracing relationship.

To permit such elastic deformation the main tool body may for example have a slot so that the slot becomes narrower due to the elastic deformation by means of the clamping device.

The clamping device can comprise for example a clamping screw which engages into a screwthreaded bore in the main tool body.

In a particularly preferred feature the clamping device does not come into direct contact either with the tool holder or with the counterweight. That ensures that the tool holder and the counterweight are braced by the bracing action of the main tool body and not by coming into contact with the clamping device, in the main tool body.

In a further particularly preferred embodiment there is provided a guide means for guiding the counterweight in the bore. The guide means serves to ensure the exact movement of the counterweight in the bore. The guide means can comprise for example an opening and a projection, preferably a guide groove and a guide pin. Desirably the guide opening is provided on the counterweight and the guide projection on the main tool body. The projection engages into the opening and thus prevents rotation of the counterweight about its axis within the bore.

In a further particularly preferred embodiment, there is provided a screw element having two screwthreads (double screwthread), for the movement of the tool holder in the main tool body, wherein the one screwthread is in engagement with the tool holder and the other screwthread with the counterweight.

Accordingly, rotation of the screw element produces an oppositely directed movement of the counterweight on the one hand and the tool holder on the other hand. As the counterweight and the tool holder are possibly of different masses, the two screwthreads can also involve a different pitch so that, with a slight movement of the heavier element, the lighter element moves to a correspondingly greater degree in the opposite direction.

Furthermore a particularly preferred embodiment provides that the tool holder and the counterweight comprise materials of different density, wherein preferably the material of the counterweight is of higher density than the material of the tool holder.

Advantageously at least one screwthread and preferably the screwthread coming into engagement with the tool holder is in the form of a fine screwthread. That permits highly precise balancing.

FIG. 1shows a longitudinal section through an embodiment by way of example of the boring tool1according to the invention. The boring tool1comprises a main tool body2which in operation rotates about the tool axis3. The boring tool1also has a tool holder4to which a cutting bit holder18with a cutting bit seat is mounted by means of the screw19. A cutting bit21is accommodated in the cutting bit seat. Disposed in the main tool body2is a bore8into which both the tool holder4and also the counterweight6are fitted on a screw element15.

The tool holder can be moved in a radial direction, that is to say towards the right or the left inFIG. 1, by means of the screw element15which engages with a screwthread flight17into a screwthreaded bore in the tool holder4. That measure makes it possible to precisely adjust the machining radius of a cutting bit accommodated on the cutting bit seat5.

The screw element15has two opposite screwthreads so that upon rotation of the screw element15that entails for example a displacement of the tool holder4together with the cutting bit holder18which is fixed to the tool holder4by means of the screw19and in which the cutting bit21is fitted, in a radial direction outwardly, that is to say towards the right inFIG. 1, while the counterweight6is moved towards the left inFIG. 1, so that the boring tool is balanced at any time.

Arranging the counterweight6and the tool holder4on an axis affords various advantages. On the one hand, that means that only one corresponding bore has to be provided in the main tool body2, which leads to higher stability for the boring tool1. On the other hand the boring tool1can be of a markedly more compact structure.

In addition arranging the counterweight6and the tool holder4on an axis ensures that the moments of force in the tool are reduced or in the ideal case the forces cancel each other out so that no moments of force occur.

In the illustrated embodiment the counterweight6is made from a material which is of higher density than the material from which the tool holder4is made. As generally the tool holder4together with the cutting bit holder18and the cutting bit21is of comparatively large dimensions, the choice of a material of higher density for the counterweight6means that it can be of smaller dimensions, which also leads to the boring tool1being of a more compact structure.

FIG. 1shows a clamping element9. As can be seen fromFIGS. 2 and 3the main tool body2is provided with a slot10so that when the clamping screw9is tightened the main tool body is elastically deformed and the tool holder4and possibly also the counterweight6are braced in the main tool body2. That ensures that during the machining operation of the boring tool, no unintentional movement of the counterweight6and/or the tool holder4occurs.

In that case the clamping screw9does not come into engagement directly with the counterweight6or the tool holder4.

In addition there is basically the possibility of making various tool holders4and/or cutting bit holders18available, which are admittedly each of the same weight but are of a different configuration in terms of arrangement so that the possible adjustable radius of the cutting bit21is increased.

The screw element15or the cover22can also have a scale.

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