WO2016114702A1

WO2016114702A1 - Self-propelled cutting tool and method for turning a workpiece

Info

Publication number: WO2016114702A1
Application number: PCT/SE2015/051331
Authority: WO
Inventors: Jacek Kaminski
Original assignee: Aktiebolaget Skf
Priority date: 2015-01-12
Filing date: 2015-12-14
Publication date: 2016-07-21
Also published as: SE1550017A1; SE538368C2

Abstract

A self-propelled cutting tool for use in turning which comprises a housing having a first part, a second part and at least a side wall interconnecting the first and second parts, the housing comprises a receiving space formed between the first part, second part and side wall. The receiving space is adapted to accommodate a cutting insert having a first and a second face and a cutting edge. Furthermore, the cutting tool comprises an arrangement for conducting fluid under pressure to the receiving space to form a hydrostatic bearing in the form of a fluid film surrounding the cutting insert when accommodated in the receiving space, and thereby allowing the cutting insert to rotate in the receiving space. When the cutting insert is accommodated in the receiving space the first face of the cutting insert is facing the first part and the second face of the cutting insert is facing the second part and a part of the cutting edge is protruding out of said receiving space forming a cutting zone. So, that during rotation of the cutting insert the cutting edge is rotated within the receiving space.

Description

SELF-PROPELLED CUTTING TOOL AND METHOD FOR TURNING A WORKPIECE

TECHNICAL FIELD

The present invention relates generally to a self-propelled cutting tool for use in turning, a self-propelled cutting tool and insert assembly and a method for turning a workpiece.

BACKGROUND OF THE INVENTION

Various high performance and high quality metal, steel, or ceramic components, such as bearing components, rods, axles, shafts, couplings, engine members, etc., may be manufactured from a workpiece using a turning process. The turning process involves a machining operation which physically removes material from a machining surface of the workpiece with a cutting tool, often comprised of a replaceable tool insert or tip, such as a carbide, cubic boron nitride (CBN), or ceramic insert. Material removal during the machining operation is achieved by providing a relative movement between the workpiece and the cutting tool, contacting a machining surface of the workpiece with a tip of the cutting tool, and moving the cutting tool in relation to the workpiece such that a desirable amount of material is removed during each revolution of the turning process. Especially in hard turning applications the tool life is a limiting factor. High cutting temperatures are generated during hard turning, i.e. where turning is carried out on materials with Rockwell C hardness greater than 45. The generated temperatures cause thermal softening of the tool in the cutting zone leading to wearing out of the tool. The generated tool wear affects the integrity of the generated surface and therefore controlling it is a major challenge.

A rotary tool is a cutting tool with a cutting edge that rotates about its own axis. Rotary cutting tools can be classified as either driven or self-propelled. The former is provided rotational motion by an external source while the latter may be rotated by the chip flow over the rake face of the tool and forces generated in turning.

Rotary cutting tools are known to reduce the heat generated during turning operations and can result in the rotary cutting tool itself enjoying a longer life span compared to static cutters. During use of a static tool the tool wear is concentrated on a small part of the cutting edge and higher heat is thus generated. The life of the cutting tool may be limited to about three to seven kilometers spiral cutting length (SCL) depending on the cutting conditions, which, for larger workpieces, means that the tool is worn out and that the cutting insert needs to be replaced before the cutting for this workpiece is finalized.

In known rotary cutting tools, the cutting insert is mounted on an external spindle and a shaft that rotates about its axis between suitable bearings. The involvement of an external spindle and shaft limits access to internal surfaces, such as for examples the inner surface on a ring. Other disadvantages are limited accuracy regarding out-of-roundness of the rotating cutting insert due to inaccuracy of the external spindle, straightness and stiffness of the shaft and roundness of the insert, further disadvantages of an external spindle and shaft are limited static and dynamic properties of the rotating cutting insert, low static stiffness, low dynamic stiffness and low damping.

US 5,014,581 discloses a further rotary cutting tool wherein a spindle/cutter floats on a high pressure fluid, the fluid acting as a hydrostatic thrust bearing separating the spindle from the walls of the surrounding housing. SUMMARY OF THE INVENTION

It is thus an object of the present invention to provide a more efficient and accurate self- rotating cutting tool for use in turning which enables easy access of the cutting tool to internal surfaces, which are otherwise difficult to access with good static and dynamic properties.

In view of the above, there is provided a self-propelled cutting tool for use in turning which comprises a housing having a first part, a second part and a side wall extending between the first and second parts. The housing furthermore comprises a receiving space formed between the first part, the second part and the side wall. The receiving space is adapted to accommodate a cutting insert having a first and a second face and a cutting edge. Furthermore, the cutting tool comprises an arrangement for conducting fluid under pressure to the receiving space to thereby form a hydrostatic bearing in the form of a fluid film surrounding the cutting insert when accommodated in the receiving space, and thereby allowing the cutting insert to rotate in the receiving space. When the cutting insert is accommodated in the receiving space the upper face of the cutting insert is facing the first part and the second face of the cutting insert is facing the second part and a part of the cutting edge is protruding out of said receiving space forming a cutting zone, i.e. an area in which the cutting edge of the cutting insert comes into direct contact with a workpiece. So, that during use of the self-propelled cutting tool the cutting insert may be brought to rotate by the movement of a workpiece, whereby it will rotate within the receiving space containing the liquid.

The self-propelled cutting tool may also be provided as a self-propelled cutting tool and insert assembly comprising a housing having a first part, a second part and a side wall extending between the first and second parts. The housing furthermore comprises a receiving space formed between the first part, the second part and the side wall. The self- propelled cutting tool and insert assembly also comprises a cutting insert, having a first and a second face and a cutting edge, accommodated in the receiving space.

Furthermore, the cutting tool comprises an arrangement for conducting fluid under pressure to the receiving space to thereby form a hydrostatic bearing in the form of a fluid film surrounding the cutting insert in the receiving space, and thereby allowing the cutting insert to rotate in the receiving space. The cutting insert is accommodated in the receiving space so that the upper face of the cutting insert is facing the first part and the second face of the cutting insert is facing the second part and a part of the cutting edge is protruding out of said receiving space forming a cutting zone. So, that during rotation of the cutting insert the cutting edge is rotated within the receiving space containing the liquid.

There is also provided a method of turning a workpiece comprising the steps of:

- providing a housing having a first part, a second part and a sidewall extending between the first and second parts, the housing comprising a receiving space formed between the first part, second part and side wall;

inserting into the receiving space a cutting insert having a first and a second face and a cutting edge with the first face of the cutting insert facing the first part, with the second face of the cutting insert facing the second part and with part of the cutting edge protruding out of the receiving space forming a cutting zone;

conducting liquid to the receiving space via an arrangement for conducting liquid and thereby forming a hydrostatic bearing in the form of a liquid film between the cutting insert and the housing; inclining the cutting insert axis with respect to the surface of a workpiece that is to be machined at a cutting edge inclination angle of > 0° and≤ 45° OR at a cutting edge inclination angle of < 0 ° and≥ -45°;

providing a relative movement between the workpiece and the cutting tool and thus a rotational movement to the cutting insert.

The cutting insert may be a disc-shaped cutting insert having a first flat face and a second flat face, a circumferential face and a circumferential cutting edge. The continuous rotation of the cutting insert provides cooling since a new portion of the cutting edge continuously is brought into contact with the workpiece at the cutting zone. An advantage of the present invention is that the problem of heat generation during cutting is even further reduced since the cutting edge after being in contact with the workpiece rotates within the receiving space comprising the hydrostatic bearing in the form of a fluid film. The temperature decrease will be even higher when rotating the cutting edge in a temperature controlled fluid due to the fact that fluid cooling is more efficient than air cooling, thus leading to a significantly prolonged spiral cutting length. An advantage with self-propelled cutting tools according to the present invention comprising a disc-shaped cutting insert and/or a cutting insert comprising a

circumferential cutting edge is that the spiral cutting length of the cutting insert is prolonged, compared to for example a "manually" rotated static disc-shaped insert. This is due to an increase of the effective edge length (EL) since the effective edge length for these types of disc-shaped cutting insert or a cutting insert comprising a circumferential cutting edge used in a self-propelled cutting tool according to the present invention is:

EL = 2 x π x (insert radius)

During finish turning of larger objects, for example a large ring for use in a bearing, the spiral cutting length of the cutting insert/tool is crucial. If the cutting insert would need to be exchanged before the cutting of the ring is finalized, the surface finish of the workpiece would be negatively impacted since the cutting insert/tool exchange would result in an indent or cut on the surface of the workpiece when applying the new cutting insert/tool against the workpiece. Non-limiting examples on fluids suitable for use in the present invention are water-in-oil emulsions and straight oils. Straight oils are petroleum or vegetable based oils that do not contain water.

5 A cutting tool according to the present invention also has the advantage compared to conventional cutting tools that no external spindle and shaft is needed. This allows for the cutting tool to access surfaces which otherwise may be difficult to access, such surfaces may for example be internal surfaces on bearings, like a bore on the inner ring or raceway on the outer ring. Hence, with a cutting tool according to the present invention both o internal and external turning is facilitated.

Another problem with conventional rotating cutting tools is limited accuracy relating to the concentricity or out-of-roundness of the cutting insert, meaning when a round or discshaped cutting tool/insert has some irregularities. When using conventional bearings there5 is no or poor compensation for such out-of-roundness, however, with a hydrostatic

bearing according to the invention the out-of-roundness will, at least partly, be

compensated for and the accuracy be increased.

The accuracy may also be improved due to that the shape of second part, at least the0 inner surface of the second part facing the receiving space and the cutting insert, may correspond to the shape and surface area of the cutting insert, thus completely supporting the cutting insert during cutting. The shape of the first part, or at least the inner surface of the first part facing the receiving space and the cutting insert, also corresponds to the shape and surface area of the cutting insert but comprises a cut-of section or the like5 allowing the cutting insert to protrude out from the receiving space at the intended cutting zone. The side wall surrounds the cutting insert so that it encloses more than 50% of the cutting insert in order to retain the cutting insert in the receiving space during rotation. It may also enclose more than 55% of the cutting insert. If the cutting insert is a disc-shaped cutting insert and the part of the cutting edge protruding out from the receiving space0 constitutes a sector of a circle, the angle measured between the side wall ends may be < 180 °, this angle may also be <170 °.

The term "self-propelled" in this context means that a rotary drive system is not used to turn the cutting insert. The freely rotating cutting insert is rotated by the movement of the workpiece being machined and to initiate the self-propelling motion an inclination angle of the cutting edge is needed.

Typically, cutting insert material hardness of at least three times harder than the work material is recommended. Cutting materials such as cubic boron nitride (CBN), Ceramic 5 or Carbide are recommended for turning hardened steel because of their ability to sustain the high temperature generated during the metal removal process. Suitable cutting inserts are disc-shaped cutting inserts well known in the art and commercially available. For example such disc-shaped cutting inserts may be available from SECO Tools,

SUMITOMO or Sandvik Coromant. Suitable dimensions for the cutting insert are a o diameter of between about 6 μπΊ and about 12 μηι.

The first and second parts may, on their inner surfaces facing the receiving space and the cutting insert, comprise at least one pocket each. The pockets are connected to the arrangement for conducting fluid. 5 The self-propelled cutting insert may during the cutting process rotate at fairly high speeds and both concentricity and accuracy of the cutting insert are critical. The presence of at least two hydrostatic pockets facing both the first and second faces of the cutting insert gives a possibility to control and maintain the axial and radial position of the insert, which leads to a very high accuracy and a better damping giving even lower vibration during0 turning.

Moreover, the housing side wall may comprise at least one pocket on its inner surface facing the receiving space and the cutting insert. The first part, second part and sidewall may each comprise at least two of said pockets on their respective inner surfaces facing5 the receiving space and the cutting insert. Furthermore the first part, second part and sidewall may each comprise at least four of said pockets on their respective inner surfaces facing the receiving space and the cutting insert. The pockets may either be evenly or unevenly distributed over the inner surface of the top part, bottom part and sidewall.

0

The "pockets" herein represent cavities in the inner surface of the first part, second part and/or sidewall which are facing the receiving space and thus the cutting insert. The pockets may for example have one or several of the following shapes; round, triangular, square, rectangular, circle sector or a truncated circle sector. The hydrostatic pockets are designed to withstand forces acting on the cutting insert. Integration of the pressure distribution over the area at which the pressure distribution is acting defines geometric and pressure requirements of the hydrostatic pockets. The arrangement for conducting fluid under pressure to the receiving space, either via the pockets or directly into the receiving space, may comprise channels adapted to connect to a fluid source outputting a stream of pressurized fluid to the pockets or the receiving space via the channels. In order for the hydrostatic bearing to secure that the cutting insert maintains an accurate axial and radial positioning within the receiving space, the pressure of the fluid needs to balance the cutting force exactly. Therefore, the fluid source may output a stream of the fluid at a pressure of about 5 to about 100 MPa, which allows said fluid film to form and maintain a thickness of about 10 μηι to about 30 μηι during use of the cutting tool.

The rotation of the cutting insert is achieved by the interaction between the cutting insert and the chip resulting from the cutting. The cutting insert axis should be inclined with respect to the workpiece at a cutting edge inclination angle since the peripheral velocity of the cutting insert is determined by the cutting speed and the inclination angle. The inclination angle of the cutting insert cutting edge may therefore for example be between - 45° and 45°. However in order to provide rotation to the cutting insert, the inclination angle should not be 0°.This inclination angle may be achieved by inclination of the housing. In order to balance the pressure from the hydrostatic pockets in relation to the cutting force exactly and to equalize the pressure distribution within the hydrostatic bearing and between the hydrostatic pockets pressurized fluid may be supplied to the hydrostatic pockets via restrictors or independently controlled pressure regulators automatically compensating for forces acting on the cutting insert.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is an exploded view of a self-propelled cutting tool according to the invention Fig. 2 is a top view of a self-propelled cutting tool according to figure 1 . Fig. 3 is a cross sectional view through the cutting tool according to the line Ill-Ill in figure 2

5 Fig. 4 is a perspective of a self-propelled cutting tool attached to a tool holder.

Fig. 5 is a front view of the self-propelled cutting tool according to figure 4.

It should be noted that the drawings have not necessarily been drawn to scale and that o the dimensions of certain features may have been exaggerated for the sake of clarity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 shows an exploded view of a self-propelled rotary cutting tool 1 according to this invention. The rotary cutting tool 1 according to figure 1 includes a housing 3 having a first 5 part 4, a second part 5 and a side wall 6 interconnecting the first part 4 and the second part 5. The housing 3 comprises an annular receiving space 7 which is adapted for accommodating and allowing a disc-shaped cutting insert 8 to rotate therein. The inner surface of the second part 5a facing the receiving space 7 and the cutting insert 8 corresponds to the shape and surface area of the cutting insert 8, thus completely

0 supporting the cutting insert during cutting. Also the shape of the inner surface of the first part 4a corresponds mainly to the shape and surface area of the cutting insert 8 but comprises a cut-of section allowing the cutting insert 8 to protrude out from the receiving space. The side wall 6 encloses the cutting insert 8 around. The disc-shaped cutting insert 8 has an upper face 9, a lower face 10 and a circumference face 1 1 . At an intersection5 between the upper flat face 9 and the circumference face 1 1 a circumferential cutting

edge 12 is formed. The side wall 6 surrounds the cutting insert 8 so that it covers more than 50% of the circumference face 1 1 and the cutting edge 12 in order to keep the cutting insert in place during rotation. 0 As can be seen in this figure the first part 4 is preferably a removable part of the housing 3 or at least possible to open up, either upwards or sideways for example, to allow for the cutting insert 8 to be easily removable from the housing 3 when a change of the cutting insert 8 is required. The first part 4 may for example be fixed to the housing 3 by screwing it to the housing 3. According to fig. 1 the first part 4, second part 5 and side wall 6 each comprises four pockets 16 at their inner surfaces 4a, 5a, 6a facing the receiving space 7 and the cutting insert 8. The pockets 16 each represent cavities formed in the inner surfaces of the first and second parts 4a,5a and sidewall 6a, and the pockets in the first and second parts 4a,5a each have the form of a truncated circle sector and the pockets 16 in the inner surface of the side wall 6a are of rectangular shape. Each of the pockets 16 comprises inlet openings 18 for allowing the pressurized fluid to enter the pockets and thus the receiving space.

At least one part of any of the following components may be subjected to a turning process using a self-propelled rotary cutting tool according to an embodiment of the invention: a bearing, such as a ball bearing, a roller bearing, a needle bearing, a tapered roller bearing, a spherical roller bearing, a toroidal roller bearing, a ball thrust bearing, a roller thrust bearing, a tapered roller thrust bearing, a wheel bearing, a hub bearing unit or a slewing bearing, a ball screw, a gear tooth, a cam, a shaft, a fastener, a pin, an automotive clutch plate, a tool, a die.

Fig. 2 is a top view of the self-rotating cutting tool 1 shown in figure 1 and illustrates that the first part 4 does not completely extend over the entire upper face 9 of the cutting insert 8 so that the circumferential cutting edge 12 partly protrudes out from the receiving space 7 forming a cutting zone 13.

Fig. 3 is a cross sectional view through the rotary cutting tool 1 according to the line Ill-Ill in Fig. 2 and serves, amongst other things, to illustrate how the pressurized fluid is supplied to the pockets 16 by a fluid source 14a which is a discharge pump that provides the hydraulic channels 14b with a pressurized stream of fluid with a pressure of about 5 - 100 MPa or higher, if necessary. The pockets 16 are supplied with the pressurized fluid via restrictors or controlled pressure regulation may also be used so that equilibrium yields. The fluid entering the pockets 16 fills the pockets 16 and also leaks out from the pockets and surrounds the cutting insert 8 at an interface 19 between the housing 3 and cutting insert 8 and forms a fluid film 15 having a thickness of about 10 μηι to about 30 μηι which supports the cutting insert 8. In this way, the cutting insert 8 floats on the fluid in the housing 3 and the fluid acts as a hydrostatic bearing since the cutting insert 8 and the housing 3 are separated by the film 15 which prevents friction between the cutting insert 8 and the housing 3. The interface 19 between the housing 3 and cutting insert 8 is sized so as to minimize leakage while still allowing rotation of the cutting insert 8 with a slight cushion of fluid.

The rotary cutting tool 1 according to figures 4 and 5 includes a housing 3, which is adapted to be secured in a tool holder 2a via an intermediate part 2b. The intermediate part 2b to which the housing 3 is connected, is rotatably connected to the fixed tool holder 2a and may be rotated and fixed into different predetermined positions in relation to the tool holder 2a. This provides the possibility to adjust the inclination of the housing 3 and thus the inclination angle of the cutting insert 8 with respect to the workpiece in order to provide rotation to the cutting insert 8. The inclination angle of the cutting insert cutting edge 12 may for example be between - 45° and 45°. However in order to provide rotation to the cutting insert, the inclination angle should not be 0 °.

While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention as understandable by one skilled in the art on the basis of the present disclosure.

For example, the first part 4, second part 5 and side wall 6 need not necessarily be separate components but may be parts of the same components. A side wall 6 may for example be integrated with a second part 5, and a first part 4 may be removably connected to the second part 5. Any of the first part 4, second part 5 or sidewall 6 may also be non-removably connected to each other, by means of a hinge for example, as long as a cutting insert 8 can be inserted into the receiving space 7.

Claims

A self-propelled cutting tool (1 ) for use in turning which comprises:

a housing (3) having a first part (4), a second part (5) and a side wall (6) extending between said first and second parts (4,5), said housing (3) comprising a receiving space (7) formed between said first part (4), second part (5) and said side wall (6), said receiving space (7) being adapted to accommodate a cutting insert (8) having a first and a second face (9,10) and a cutting edge (12), said cutting tool (1 ) further comprises an arrangement for conducting fluid (14) to said receiving space (7) to thereby form a hydrostatic bearing in the form of a fluid film (15) between said cutting insert (8) and said housing

(3) allowing said cutting insert (8) to rotate within said receiving space (7),

characterized in that said receiving space (7) is adapted to accommodate said cutting insert (8) with said first face (9) of said cutting insert (8) facing said first part (4), said second face (10) of said cutting insert (8) facing said second part (5), with part of said cutting edge (12) protruding out of said receiving space (7) forming a cutting zone (13), so that during rotation of said cutting insert (8) said cutting edge (12) is rotated within said receiving space (7).

The self-propelled cutting tool (1 ) according to claim 1 characterized in that the first and second parts (4,5) comprises at least one pocket (16) each, said pockets (16) being in the form of a cavity to which said arrangement for conducting fluid (14) is connected.

The self-propelled cutting tool (1 ) according to claim 1 or 2 characterized in that the sidewall (6) comprises at least one pocket (16) in the form of a cavity, which is facing the cutting insert (8) and to which said arrangement for conducting fluid (14) is connected.

The self-propelled cutting tool (1 ) according to claims 2 or 3 characterized in that the first part

(4) , second part (5) and sidewall (6) each comprises at least two of said pockets (16).

5. The self-propelled cutting tool (1 ) according to any of claims 2 - 4 characterized in thatVne first part (4), second part (5) and sidewall (6) each comprises at least four of said pockets (16).

6. The self-propelled cutting tool (1 ) according to any of claims 2 to 5 characterized in that said pockets (16) have one of the following shapes; round, triangular, square, rectangular, a circle sector or a truncated circle sector.

5

7. The self-propelled cutting tool (1 ) according to any of claim 2-6 characterized in that said arrangement for conducting fluid (14) comprises channels (14b) adapted to connect a fluid source (14a) with each of said pockets (16). o

8. The self-propelled cutting tool (1 ) according to claim 7 characterized in that said channels (14b) are provided with restrictors or independently controlled pressure regulators.

9. A self-propelled cutting tool and insert assembly for use in turning which

comprises: a housing (3) having a first part (4), a second part (5) and a sidewall (6) extending between said first and second parts (4,5), said housing (3) comprising a receiving space (7) formed between said first part (4), said second part (5) and said side wall (6),

a cutting insert (8) having a first and a second face (9,10) and a cutting edge (12),0 said cutting tool (1 ) further comprises an arrangement for conducting liquid (14) to said receiving space (7) and to thereby form a hydrostatic bearing in the form of a liquid film (15) between said cutting insert (8) and said housing (3) allowing said cutting insert (8) to rotate within said receiving space, characterized in that said receiving space (7) is adapted to accommodate said cutting insert (8) with said5 first face (9) of said cutting insert (8) facing said first part (4), said second face (10) of said cutting insert (8) facing said second part (5), with part of said cutting edge (12) protruding out of said receiving space (7) forming a cutting zone (13), so that during rotation of said cutting insert (8) said cutting edge (12) is rotated within said receiving space (7).

10. A self-propelled cutting tool assembly according to claim 9 characterized in that W comprises a self-propelled cutting tool (1 ) according to any of claims 1 -8.

1 1 . A self-propelled cutting tool assembly according to claim 9 or 10 characterized in5 that said cutting insert (8) is a disc-shaped cutting insert having a first and second flat face (9,10), a circumferential face (1 1 ) and a circumferential cutting edge (12).

12. A method for turning a workpiece, which method comprises the steps of:

providing a housing (3) having a first part (4), a second part (5) and a sidewall (6) extending between said first and second parts (4,5), said housing (3) comprising a receiving space (7) formed between said first part (4), second part (5) and side wall (6);

inserting into said receiving space a cutting insert (8) having a first and a second face (9, 10) and a cutting edge (12);

conducting liquid to said receiving space (7) via an arrangement for conducting liquid (14) and thereby forming a hydrostatic bearing in the form of a liquid film (15) between said cutting insert (8) and said housing;

inclining the cutting insert axis with respect to the workpiece at a cutting edge inclination angle of >0° and <45° OR at a cutting edge inclination angle of <0° and≥-45°;

providing a relative movement between the workpiece and the cutting tool and and thus a rotational movement to the cutting insert;

characterized in that it comprises

inserting said cutting insert with said first face (9) of said cutting insert (8) facing said first part (4), with said second face (10) of said cutting insert (8) facing said second part (5) and with part of said cutting edge (12) protruding out of said receiving space (7) forming a cutting zone (13), so that during rotation of said cutting insert (8) said cutting edge (12) is rotated within said receiving space.

13. A method for turning a workpiece according to claim 13, wherein the first and second parts (4,5) comprise at least one pocket (16) each, said pockets (16) being in the form of a cavity to which said arrangement for conducting fluid (14) is connected.