The present invention relates to a toolholder for centrally apertured indexable inserts intended for chipforming machining of metallic workpieces.

Indexable inserts are usually provided with nose point angles below 60° and they are usually intended for copying machining. There exist a variety of toolholders for various types of indexable inserts with nose point angles of 55° such as rhombic, rhomboidic and triangular inserts with equal angles all around the insert. The material used for such inserts is normally cemented carbide material or ceramic material .

A commonly used locking system for such indexable inserts includes a pin in the holder extending upwardly into the insert-receiving pocket to engage a hole in the insert located in said pocket, and means for forcing said pin to be subjected to a pivoting movement inwardly towards said holder, said forcing means being arranged to actuate the lower portion of said pin, thus urging both the pin and the insert inwards towards an upwardly extending wall formed on said holder rearward- ly of said pocket. Such type of toolholder is disclosed in US-Patent 3,314,126. The drawback of such holder, however, is that the forward nose portion of the cutting insert has a tendency of coming apart from the shim plate located underneath said insert which sometimes can lead to insert failure.

In view thereof it is an object of the invention to provide a novel arrangement for clamping the insert in place in the pocket which will ensure that a clamp is acting on both the top surface and in the central hole wall of the insert in such a way that both downwardly

CONFIRMATION COPY

and laterally acting clamp forces will keep the insert firmly and safely in place in its location without any tendency of coming apart from its underneath supporting surface.

According to the invention, a toolholder is provided having a pocket for receiving an insert with a central hole therein, and a clamp is arranged having a forward end, a rear end and a clamping bolt downwardly extending therebetween whilst threadably engaged in the holder body, said forward end of the clamp being provided with two clamping surfaces one of which is arranged to engage with a portion of the hole wall of the insert and the other being arranged to abut with an upper surface portion of the insert located between said insert hole and a rearwardly located upstanding supporting wall of the insert-receiving pocket.

In accordance with a preferred embodiment of the invention the rear end portion of the clamp is adapted to come into abutment with a forwardly and upwardly inclined cam surface provided on the holder body.

The invention will now be described more in detail by reference to the enclosed drawings which illustrate a preferred embodiment of the invention.

Fig. 1 is a side view of a toolholder according to the invention.

Fig. 2 is a plan view of the toolholder shown in Fig. 1, and

Fig. 3 is a side view of an alternative embodiment of the invention.

In Figs. 1-2 there is shown a portion of a cutting tool for turning operations comprising a partially shown toolholder 10 for left hand turning the forward nose portion of which is provided with a pocket for the receipt of an indexable cutting insert 11 of polygonal shape. This insert has rhombic basic shape with nose point angles less than 60°. The insert has an upper flat surface 12 and a lower flat surface 13 parallel with said upper surface. The insert has four side surfaces extending perpendicularly towards said upper and lower flat surfaces 12, 13, two of which are arranged to come into surface abutment with two up¬ standing support surfaces 16 and 17 of the insert receiving pocket whereas said lower flat surface 13 is intended to rest against a bottom supporting surface 15 of said insert-receiving pocket. The central hole of the insert is designated 14. The bottom supporting surface 15 could either be a partial surface portion of the holder itself or be provided as a separate shim plate in the insert-receiving pocket of said holder.

There is a clamp 18 provided for the clamping of the insert into its pocket in the holder 10, said clamp being actuatable by means of a clamp screw 20 that extends entirely through said clamp and threadably engages a bore 19 in the holder 10, the central axis SI of said screw extending parallel with the central axis of the hole 14 in the insert. The forward end of said clamp is in the shape of a downward-inwards inclined first protrusion 21 intended to engage with the inwardly towards said holder directed hole wall of said hole 14 at a contact point 22 at a distance downwardly from the upper surface 12 of the insert . The opposite end of said clamp is in the shape of a second protru- sion 23 that is received in a recess 24 in the holder 10, the end surface 25 of said protrusion 25 being in

the form of a flat surface or preferably in the shape of two laterally spaced convex surfaces for surface abutment with a planar contact surface 26 in said recess 24 that is inclined at an angle in relation to the central axis SI of the clamp screw 20.

Fig. 3 shows an alternative embodiment of the inven¬ tion. In this embodiment the abutment surface 25 of the rearward end portion of the clamp 18 is provided preferably in the shape of two laterally spaced flat surfaces for surface abutment with a corresponding flat surface 26 in the holder, said flat surface 25 of the clamp being plane parallel with the upper surface 12 of the insert. In this case the clamp-actuating screw 20 has a conically shaped head 27, the central axis SI of which is laterally displaced in relation to the central axis S2 of the recess 28 in the clamp. More specifically, the central axis SI of said screw is excentrically displaced inwards towards the holder as compared with central axis S2 of said recess 28, thereby securing that the clamp 18 is urged inwards towards the holder upon tightening said screw and bringing it downwards into threaded engagement with the corresponding threaded bore 19 of said holder body 10.

The clamp 18 has a flat underneath surface 30 located between the first protrusion 21 and the position of the clamp screw 20 intended for flat surface abutment against a partial surface of top surface 12 that is located between the central hole 14 and the upstanding side support surface 17. In a preferred embodiment said underneath surface 30 is so dimensioned that it comes into flat surface abutment along a major portion of the surface area of said top surface 12 located between the hole 14 and support surface 17.

With the embodiment shown in Fig. 1-2 the forward end of the clamp 18 is made in the shape of a protrusion 21 that has an axis of symmetry Z that extends downwards- inwards at an angle β that is substantially equal to or somewhat smaller than the angle a . The toolholder in

Fig. 1-2 is furthermore provided with a helical spring 31 so that the clamp is pressed upwards upon loosening the screw 20. The spring 31 is partially received in a cylindrical counterbore 32 in the holder body and partially received in a similar cylindrical counterbore 33 in the underneath portion of the clamp.

Upon actuating the clamp 18 by threadably tightening the screw 20 the clamp 18 is pressed downwards, thus urging the surface or the laterally spaced surfaces 25 to slide downwards-inwards along contact surface 26 at the same time as the underneath clamp surface 29 is brought in planar surface abutment with said top surface 12 whilst said forward protrusion 21 engages the insert hole 14 at a downwardly located point 22 distantly from said top surface 12. Thanks to providing said inclined surface 26 at an angle a a favorable wedging action is achieved which ensures providing effective vertical and lateral clamping forces acting on said insert. The size of the angle should be selected in the range 15 to 50 degrees, preferably within the range 20 to 40 degrees.