Tool and cutting head for cutting machining

A tool for cutting machining includes a holder, a cutting head and a threaded fastener for fastening the cutting head to the holder. The cutting head is formed of injection molded cemented carbide and includes at least one cutting edge. The holder has a front surface and the cutting head has a support surface provided to releasably abut the front surface substantially in a radial plane. A central blind hole is provided in the support surface. An insert formed of a material softer than injection molded cemented carbide is secured in the blind hole. The insert includes a thread cooperating with a thread of the fastener for pulling the cutting head toward the holder in response to rotation of the fastener. By further tightening the fastener after the support surface abuts the front surface of the holder, the contact between the thread of the fastener and the thread of the insert forces the insert radially outwardly against a wall of the blind bore.

BACKGROUND OF THE INVENTION
 The present invention relates to a machining tool and a cutting head
 therefor.
 PRIOR ART
 Through U.S. Pat. No. 5,947,660 there is previously known a tool in which a
 cutting head is mounted on a rotatable holder. The cutting head has a
 blind hole formed in a rear surface thereof facing a front surface of the
 holder. A fastener screw extends axially through the holder and is
 threadedly connected within the blind hole to pull the rear surface and
 the front surface against one another. The rear surface of the cutting
 head and the front surface of the holder include ribs and grooves that
 form an interlocking relationship in response to rotation of the fastening
 screw. Such tools, when being of small dimensions have been found to lack
 sufficient strength or stability in the coupling between the cutting head
 and the tool body.
 OBJECTS OF THE INVENTION
 One object of the present invention is to provide a coupling for two tool
 parts, whereby a great load capacity can be transferred therebetween.
 Another object of the present invention is to provide a tool which is
 continuously stable.
 Still another object of the present invention is to provide a tool having
 good precision.
 These and other objects have been achieved by a tool for cutting machining,
 the tool comprising a holder, a cutting head, and a fastener. The holder
 defines a longitudinal axis of rotation and includes a front surface
 oriented radially with respect to the axis. The cutting head includes a
 carrier body and an insert. The carrier body is formed of injection molded
 cemented carbide. The carrier body carries a cutting edge at its front end
 and includes a rear surface oriented radially relative to the axis and
 abutting the front surface of the holder. The rear surface has a central
 blind hole formed therein. The insert is secured in the blind hole and is
 formed of a material which is softer than that of the carrier body. The
 fastener is disposed in the holder and engages the insert for pulling the
 insert and the carrier body toward the holder.
 The invention also pertains to the cutting head per se.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
 FIGS. 1A and 1B show a, preferably injection molded, hard metal (e.g.,
 cemented carbide) carrier portion 10 according to the present invention.
 The hard metal carrier portion 10 has a cylindrical basic shape and
 comprises a base or support portion 11, which connects to an intermediate
 shank portion 12, which in turn connects to a cutting edge portion 13
 provided at the free end of the carrier portion 10. The base 11 has a
 diameter D1, which is substantial greater than the diameter D2 of the
 intermediate part 12 and the diameter D3 of the cutting edge portion 13.
 The diameter D3 is somewhat greater than the diameter D2 to create space
 for chips during drilling or milling. The portion 13 has, in this example,
 the shape of a rotary file with a number of bent cutting edges but may
 alternatively have a drill geometry with only two edges extending towards
 the rotational axis CL. In case the portion 13 is a drill, it is an
 advantage if also the part 12 has chip flutes for conducting-away
 cuttings. The base 11 connects to the intermediate part 12 via a softly
 rounded shoulder portion 14. The axial distance X between the shoulder
 portion 14 and the cutting edge portion 13 defines the maximum machining
 depth.
 A blind hole 15 is provided symmetrically about the rotational axis CL and
 is located centrally in the base 11 so as to open in a direction away from
 the intermediate part 12. The hole 15 terminates in a support surface 16
 and has an axial depth d. The hole 15 has the cross-section of a dove
 tail, that is, the smallest diameter is located at the surface 16 such
 that a bottom 17 of hole 15 connects to a wall 18 of the hole at an acute
 angle. The blind hole and the cutting edges are integrated with the hard
 metal carrier portion 10.
 The support surface 16 has a ring-like basic shape and comprises two rows
 of groove parts orient at right angles. The groove parts occupy
 substantially the entire support surface 16. Each groove part comprises a
 number of mutually separate, identical flutes or grooves 19A or 19B. The
 grooves 19A and 19B extend in respective directions perpendicular to each
 other. Substantially all of the grooves 19A, 19B intersect the jacket
 surface 20 of the bases 11 at two places. Each groove 19A, 19B is
 elongated and is substantially V-shaped in cross-section. That is, each
 groove has two flanks that connect to a bottom, via a sharp or rounded
 transition. The flanks form an acute angle with each other. The angle lies
 within the interval of 40.degree. to 80.degree., preferably 55.degree. to
 60.degree.. The number of grooves in each groove part depends on how the
 front surface of the holder is shaped, and the number is chosen in the
 interval of 5 to 30 grooves. The design of the groove parts gives a
 considerably greater specific surface than if the surface were planar. The
 groove parts cover at least 80%, preferably 90-100%, of the accessible
 area of the support surface 16. The function of the support surface 16 has
 been more closely described in PCT/SE98/01146 which hereby is incorporated
 in the present description as regards cooperation between the support
 surface 16 and a front surface of a shank, described below, where the
 coupling comprises two cooperating surfaces and means for forcing the
 surfaces together. The surfaces are profiled with grooves in order to
 allow locking by shape against each other.
 A bushing or insert 21 of a material softer than injection molded cemented
 carbide is shown in FIG. 2. The bushing is preferably made of steel and
 has a cylindrical basic shape and has a certain dimension h in the axial
 direction. The jacket or outer peripheral surface of the bushing comprises
 a number of circumferentially extending grooves or segments 22. The
 bushing comprises first 23 and second 24 end surfaces. The end surfaces 23
 and 24 are preferably parallel with each other and perpendicular to the
 rotational axis CL. Each of the segments 22, except for a cylindrical
 segment located closest to the first end surface 23, is in the form of a
 truncated conical part, whose imaginary tip (vertex) is directed outwardly
 towards the end surface 23. Each segment 22 has a base which is of greater
 diameter than the diameter of the narrowest part of the segment, whereby a
 sharp circumferential flange is formed. The end surface 23 defines the
 greatest diameter of the bushing and is smaller than the smallest diameter
 of the hole 15. The bushing has a central through-going, finely threaded
 boring 25. By "finely threaded" is here meant that the pitch of the thread
 is smaller than a "coarse or normal" pitch according to ISO-standard. For
 example, the "normal" pitch under M5 is according to standard 0.8 mm while
 the "fine" pitch is 0.5 mm.
 The manufacture of a cutting head 26 according to the present invention
 from the hard metal carrier portion 10 and the bushing 21 is done as
 follows. The bushing 21 is inserted into the blind hole 15 such that the
 end surface 23 abuts against the bottom 17, whereafter melted brazing
 material M is inserted radially outside the bushing. Subsequently, the
 brazing material is cooled such that the bushing is fixed centrally in the
 blind hole 15, see FIGS. 3A and 4. It should be noted that since the
 height h of the bushing is shorter than the depth d of the hole, a gap
 will develop between the end surface 24 and the plane of the support
 surface 16, as is visible in FIG. 4.
 In FIGS. 4 and 5 is shown a milling tool 27 according to the present
 invention. The milling tool 27 comprises a substantially cylindrical
 holder 28, the multi-edged cutting head 26 and a screw or other clamping
 means 29.
 The holder 28 may be made of steel, cemented carbide or high speed steel.
 One free end of the holder 28 is intended to be fastened in a rotatable
 spindle (not shown) in a machine while the opposite other free end surface
 comprises a front surface 30 and has a non-threaded axial hole 31 therein.
 The front surface 30 has a circular basic shape and comprises two groove
 parts 32A, 32B. Each groove part covers a respective substantially half
 portion of the front surface 30 and comprises a number of mutually
 separate, identical flutes or grooves. The grooves of the groove parts
 have two main directions that are perpendicular to each other. The second
 groove part 32A is bordered by the first groove part 32B. Substantially
 each groove in the first groove part 32B intersects the jacket surface of
 the holder at two places while substantially each groove in the second
 groove part 32A intersects the jacket surface of the holder at one place.
 Each groove of the two groove parts is elongated and substantially
 V-shaped in cross-section.
 The finely threaded hole 25 of the bushing 21 is provided to receive the
 screw 29 which fits loosely in the hole 31. The screw is formed as an
 internal pull rod with a threaded free end. Thereby, a possibility to
 provide cutting edges towards the rotational axis CL for drilling and
 milling is obtained. In the shown embodiment the groove parts 19, 32 have
 been made by direct-pressing and sintering or by grinding.
 The tool 27 is mounted by pushing the support surface 16 of the cutting
 head 26 by hand against the front surface 30 of the holder 28 in one of
 four possible positions. Thereby, the grooves 19 of the cutting head are
 aligned with ribs formed between the grooves 32 of the holder. The screw
 29 is brought against the threaded hole 25. During rotation of the screw
 29 via a key (not shown) which is in engagement with a key grip of the
 screw (not shown) situated in an axially rearwards part of the holder
 (i.e., the lower part in FIG. 5), the cutting head will be drawn firmly
 against the front surface, i.e. the position according to FIG. 4 will be
 achieved wherein the ribs are locked in respective grooves. The cutting
 head 26 is now anchored against the holder 28 in a satisfactorily manner.
 The gap between the end surface 24 of the bushing and the front surface 30
 allows elastic movement of the bushing during additional tightening of the
 screw. Therefore, the bushing will be wedged harder in the dove-tail hole.
 The fine thread 25 in engagement with the fine thread of the screw 29
 allows relatively great traction in the joint.
 The reason why only one surface, i.e., the surface 16, has a waffle pattern
 over its entire area is that the tool obtains a more distinct locking
 effect than if both of the surfaces 16, 30 were waffle patterned.
 When the cutting head 26 must be replaced, the mounting process is
 reversed, whereafter the cutting head can be removed from the holder 28
 and replaced by another cutting head. Thereby also the geometry of the
 cutting head can be changed to other embodiments, which all fit in the
 same holder. For example there can be used a chip-dividing cylindrical end
 mill head or thread milling cutter; a cylindrical head without chip
 dividing for example for end mill cutting or reaming; a head for grooving
 and a truncated conical head for end mill cutting of dove tail slots.
 Common for all these tools is that they replace conventional end mills in
 high speed steel where smaller dimensions (2 to 15 mm in diameter) often
 are desirable.
 Although it is preferable that the surfaces 16 and 30 be formed with
 grooves to form the interlocking relationship, it is possible within the
 scope of the invention that those surfaces instead be planar (smooth).
 FIGS. 6A-8 show alternative embodiments of a hard metal carrier portion 10'
 for a cutting head, a bushing 21' to cooperate with the carrier portion
 10' as well as a cutting head 26' according to the present invention. The
 hard metal portion 10' is externally as described above. What differs
 relative to the earlier example is that the bushing 21' and the blind hole
 15' cooperate via an external thread 22' and an internal thread 18',
 respectively. This means that brazing material is inserted in the thread
 18' during the manufacture of the cutting head. The bushing is then
 threaded against the bottom of the blind hole 15', whereafter the brazing
 material is heated and cooled.
 FIGS. 9A-11 show alternative embodiments of a hard metal carrier portion
 10" for a cutting head, a bushing to cooperate with the portion as well as
 a cutting head 26" according to the present invention. Externally the hard
 metal carrier portion 10" is the same as described above. What differs
 relative to the earlier example is that the bushing 21" is wedged in the
 blind hole 15". This is done by providing the blind hole with a projecting
 central, conical projection 17", the tip of which is directed towards the
 mouth of the hole 15". The projection is brought to cooperate with a
 corresponding central, conical recess 23" of the bushing. Thereby the
 bushing is urged radially outwardly in the hole 15" and is plasticized
 against the wall 18". In this case it is possible to provide the hole with
 a dove-tail profile just as described above.
 The bushings 21-21" in the above-described embodiments are secured in the
 respective blind holes 15-15" such that the insert, by the effect of the
 screw 29, is forced against the bore wall 18-18". This is evident since
 the threads in the screw and the insert during axial strain will be
 forcing the bushing apart, which then is pushed even harder against the
 bore wall.
 Thus the present invention relates to a coupling for two tool parts such
 that a large traction can be transferred from the screw to the cutting
 head, a tool which is continuously rigid during use especially for smaller
 tool dimensions while maintaining good precision at mounting.
 Although the present invention has been described in connection with
 preferred embodiments thereof, it will be appreciated by those skilled in
 the art that additions, deletions, modifications, and substitutions not
 specifically described may be made without departing from the spirit and
 scope of the invention as defined in the appended claims.