WO2018031671A1

WO2018031671A1 - End mill for machining ceramics

Info

Publication number: WO2018031671A1
Application number: PCT/US2017/046119
Authority: WO
Inventors: Christian Travert; Jeremi RAKES
Original assignee: Tratech Corp.
Priority date: 2016-08-09
Filing date: 2017-08-09
Publication date: 2018-02-15

Abstract

A tool bit (20) for use as an end mill for machining a dense ceramic workpiece has a cutting portion (22) with a cutting surface that is coated with a metal matrix within which are embedded diamond particulates. Shank (24) has a central passageway (30) to which coolant is provided. The passageway is connected to a multiplicity of transverse passageways (28). Each transverse passageway (28) terminates at a port which is located within a preferably elliptical shape concavity (26) on the cutting surface of the end mill. The cutting surface is substantially cylindrical and is continuous around the exterior surface of the cutting portion of the end mill, except there are concavities.

Description

END MILL FOR MACHINING CERAMICS

TECHNICAL FIELD

The present invention relates to tool bits, in particular to end mills that are particularly suited for machining ceramics.

BACKGROUND

Milling cutters used in machine tools engage the surface of a workpiece to remove material, often by passing back and forth over the surface multiple times. An end mill, which is a species of milling machine cutter, is a generally cylindrical tool bit that removes workpiece material when rotating about the end mill length axis by laterally translating the end mill while in contact with a workpiece.

End mills coated with a metal matrix in which are embedded diamonds are well known for use in machining difficult materials like structural ceramics such as aluminum oxide, silicon carbide and the like. Such kinds of end mills are commercially available from, for example, G. G. Shultz Tool Inc., Taveres, Florida, which in U.S. Patent Publication 2016/0306685, dated October 29, 2015 describes a particular "diamond plated" rotary grinding tool (end mill), how coolant flows within and about the end mill, and details of the material removal process.

Fig. 1 show a prior art end mill 10 which has helical land portions (also called grinding elements) spaced apart by flutes 14, which alternatively may be straight as shown in the Schultz publication.

During use of end mill 10, coolant is delivered to the shank end 18 of the end mill and flows within an interior lengthwise passageway that leads to port 16 at the distal end of the end mill, where there are three radial grooves on the end surface. Coolant also flows within the end mill from the central passageway through interconnected radial conduits that terminate at ports 13 which are located within the channels of the flutes on the end mill exterior. While end mills of the foregoing type work well, when cutting hard ceramics they suffer degradation of the cutting surface over time, as do all cutting tools. In particular, diamond particles tend to become rounded due to wear and that can generate excess heat from friction. Particles can be pulled out of the typical nickel base matrix coating that holds the diamond particles on the tool cutting surface.

The coolant which flows within the tool and is discharged from the ports along the length of the tool is intended both to carry away heat and workpiece debris generated by the cutting action. If workpiece debris is not carried away the effectiveness of the diamond particle cutting action can be diminished. If there is excessive heating, that can cause premature degradation of the diamond particulate coating cutting surface or can damage the workpiece. As a result, the user has to be cautious with respect to the workpiece rotary speed and other machining parameters when using a subject end mill. When end mills have flutes like those shown in Fig. 1, it would appear that fluid discharged from the ports 13 can easily flow lengthwise along the channel of the flutes and thus away from intimacy with the region where workpiece and end mill are engaged with each other, that is, where a lot of heat is being generated.

There is a continuing need for better diamond coated end mills that cut faster or enable greater material removal rates, while being long-lasting.

SUMMARY

An object of the invention is to provide an improved diamond coated end mill that has an increased life diamond coating cutting surface, and to provide an end mill that enables higher cutting speeds and material removal rates. A further object is improve the flow of coolant for such an end mill. A still further object is to enable such an end mill to cut while rotating in opposing directions.

In accord with the present invention a tool bit, in particular an end mill having an electroplated hard particle cutting surface has internal passageways for flowing coolant along the length of the tool bit and for discharging the coolant from ports along the generally cylindrical exterior of the cutting surface of the tool bit. In an embodiment of the invention, there is a multiplicity of transverse passageways, each passageway terminating at a port on the tool exterior; and each port has an associated concavity or depressed region. Preferably, each concavity is many times bigger in dimension that the port opening and has an elliptical shape edge, with the major ellipse axis thereof aligned with the length axis of the tool. And preferably the cutting surface coating has continuity, except for the concavities, e.g., in comparison to a tool bit having flutes.

Preferably, the concavities are spaced apart lengthwise along spaced apart imaginary axial lines running along the cylindrical exterior of the tool bit; and the concavities staggered lengthwise relative to each other from one line to the next. And, at any circumference along the tool length, at least one concavity will be intersected by an imaginary circumferential line, so that for any full rotation of the workpiece at least one concavity will come to be in intimate contact with the workpiece which is being machined.

Preferably the area of each concavity is at least 10 times greater than the area of the port which is within the concavity; and the total area of all the concavities is between 30 and 70 percent of an imaginary cylindrical area that envelopes the cutting portion of the tool bit.

When a tool bit embodiment of the invention is in cutting contact with a workpiece, coolant is intimately and copiously delivered to the interface between the cutting surface and the workpiece. When used to cut hot pressed ceramic material, a tool bit of the present invention provides higher material removal rates and/or improved tool life compared to prior art tool bits having the same kind of electroplated diamond surface.

The foregoing and other objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a perspective view of a prior art tool bit.

Fig. 2 is a perspective view of a tool bit of the present invention.

Fig. 3 is a partial cut-away of the tool bit shown in Fig. 2.

Fig. 4 is a detail view of a portion of the exterior surface of the tool bit shown in Fig. 2. Fig. 4A is a lengthwise cross section through the portion of tool bit shown in Fig. 4.

Fig. 5 is a partial lengthwise cross section of a tool bit showing a transverse passageway with a tapered port opening.

Fig. 6 is a partial lengthwise cross section of a tool bit showing a transverse passageway with a stepped diameter port opening.

Fig. 7 is a side view showing the spindle of a milling machine holding a tool bit in contact with a workpiece.

DESCRIPTION

Fig. 1 shows a diamond coated tool bit 10 especially suited for machining dense ceramic materials. It is like that which is taught by U.S. Patent Publication 2016/0306685 of Rakes et al. now Pat. No. 9,555,485.

While the present invention is described in terms of machining ceramic materials, an end mill of the present invention may be useful for fabricating workpieces made of other materials such as plastics, composites, non-ferrous metals (and with the right choice of particulate) ferrous metals.

Fig. 2 shows an embodiment of the present invention, namely tool bit mill 20, which also may be referred to as an end mill or tool or rotary grinding tool. Tool bit 20, which has a central lengthwise axis C, comprises a body that has shank portion 24 integral with cutting portion 22. Cutting portion 22, which has a substantially cylindrical shape (i.e., it has concavities on the surface), may also be referred to as the grinding portion or the working portion or the distal end portion of the tool bit. The shank portion may also be referred to as the proximal end portion of the tool bit. The cutting portion of a tool bit herein is the portion which removes material from a workpiece by shearing off microscopic size chips; and the "cutting" nomenclature ought not to be construed as meaning a workpiece is divided into sub-workpieces by the tool bit. Cutting portion 22 preferably mostly has a cutting surface which is comprised of a coating of electroplated metal within which are embedded particulates. Preferably, the particulates are diamond material, for example particulates having an average U.S. Sieve Size between 600 and 36 mesh. The electroplating may be about one to several mils thick, or as otherwise known in the art. In shorthand parlance, such coating might be referred to as a diamond coating; probably better it is referred to as an electroplated diamond coating. The coating is indicated in the drawings by speckling. The small diamond particulates have sharp edges to effect removal of material when the tool bit is contacted with a work piece. Preferably, the particulates are natural or synthetic diamonds and the metal matrix is a nickel base. Other hard particulate substances may be used in substitution of diamond, for instance cubic boron nitride. Other metal plating matrices than nickel base metal may be used. The tool bit preferably has a substrate made of hardened M2 tool steel; alternatively of another tool steel, or tungsten carbide plus cobalt, or some other stiff, high strength material.

Embodiments of tool bits of the present invention have an array of internal passageways for flow of coolant. Coolant may be pumped at 250 psig or another pressure from a reservoir (not shown) and made to flow through a hollow milling machine spindle to the proximal end of the shank of tool bit 20, so the coolant will then flow within central passageway 30 which is shown in the partial cutaway of Fig. 3. Passageway 30 runs from an open-end feed point at the proximal end portion 24, to vicinity of the distal end of the tool bit. From central passageway 30, smaller passageways 28 run transversely/laterally, preferably radially, with respect to the tool bit central length axis C.

Preferably each passageway 28 terminates at a port which is within a depression or concavity 26 in the cutting surface, as discussed below.

Optionally, a plurality of smaller lengthwise passageways may be substituted for a single lengthwise passageway 30. Passageway 30 may optionally be stepped and or tapered along its length, for example with the diameter decreasing along the length toward the distal end of the tool bit; and preferably in such an embodiment the cross sectional area (and thus the flow capacity) of passageway 30 at any given point along the length of the tool bit is either equal to or greater than an area which is the summation of the cross sectional flow areas of the passageways 28 which are downstream of the given point. The distal end of tool bit 20 comprises a circular end surface 32 which preferably has a lightly dished central portion 37. At the distal end of the tool bit there is a center port 33 and two or more surrounding spaced apart ports 34 within the dished portion, that are in flow communication with the end of passageway 30 by means of small passageways about the same size as passageways 28.

In preferred embodiments, each transverse passageway 28 terminates at a port which is located within a shallow concavity 26 on the generally cylindrical cutting surface of end mill 20. A port of a passageway is preferably centered within a concavity; alternatively the port may be off-center.

Concavities of the kind which are described herein are believed to cause the coolant to spread locally in a desirable way under particular conditions when the tool is engaged with a workpiece for cutting. For example, depending on the depth of cut and the size of the tool bit, at times the whole periphery of a particular concavity will be in intimate contact with the curved workpiece surface with which the rotating tool bit is engaged. When that happens, it is believed that the coolant fills the particular concavity and is forced to escape by flowing into the thin region where the tool bit cutting surface which is adjacent the concavity is cutting the workpiece, to provide both better cooling and debris removal. Regardless of the physics, the result is that some of all of the benefits are higher tool speeds, higher feed rates, and improved tool life.

Compared to any tool bit having a cylindrical cutting surface where there are only coolant ports (such as may exist) and no concavities, a tool bit of the present invention provides substantially increased fluid contact with the work piece for reasons described above. Compared to a tool bit which has ports within flutes, coolant which flows out of such ports would be less effective because it can more easily escape from vicinity of where cutting is taking place because it is not forced into the small gap between the tool bit and the workpiece. Notwithstanding, a less preferred

embodiment of the tool bit of the present invention may have some spiraling flutes or linear flutes; and a lesser advantage might be expected.

A preferred concavity is a shallow dished depression, preferably having a gently curved shape in cross section as shown in Fig. 4A. The tool bit surface which is within the concavity may have other contours, for example, it may have the shape of a simple or complex cone segment. Other configuration concavities are discussed below. As shown in the close-up details of Fig. 4 and Fig. 4A, the periphery of a preferred concavity is elliptically shaped. And the elliptical outer edge of such concavity has a major axis Dl that is parallel to the axis C and the edge has a minor axis D2 that runs in a plane which is transverse to the length axis C. The edge of any concavity 26 is of course bent in shape, owing to the curved cylindrical surface on which it lies. Thus the shape of the edge will be nominally elliptical (or as applies, nominally circular; see just below). Having such a particularly oriented elliptical shape edge, compared to some other orientation or compared to having a circular edge, means there is greater likelihood that the concavity-trapping effect described just above will occur when a small fraction of the circumference of the tool bit is in intimate contact with the workpiece.

Generally, in a tool bit it is desirable to maximize the amount of cutting surface because that will maximize tool life and material removal rate. Thus, there is a tradeoff in providing concavities for coolant ports, because it means the diamond plated cutting surface that contacts the workpiece is reduced. An elliptical shape concavity aids in providing good cooling and debris removal, helping to make the tradeoff effective. Nonetheless, in alternative embodiments of the invention, tool bits may have some or all of the concavities with circular edges. In still other alternative embodiments, concavities may have some other shape, such as a teardrop shape.

Concavities 26 are circumferentially spaced apart evenly from each other as well as preferably spaced apart from each other along spaced apart imaginary axial surface lines 40 that run parallel to the length axis C. The concavities lying along one length line 40 are preferably staggered lengthwise from the concavities that lie along an adjacent length line 40, as illustrated in Fig. 2 and Fig. 3. In the generality of the invention, concavities may be disposed on the cutting surface without being aligned along lengthwise-running lines 40.

Preferably, the spacing of concavities in the lengthwise direction is such that, at any length location along the cutting surface, a given imaginary circumferential line P (as shown in Fig. 3) which runs around the exterior of the cutting surface will intersect at least one concavity. When that is done, during any one rotation of the tool bit the workpiece will be directly cooled by contact with coolant flowing from a port associated with a concavity. The foregoing characteristic is applicable to the preponderance of the cutting surface rather than the entirety of the cutting surface since there may be "end effects". That is, there may be mechanical constraints and space limitations that require small portions of the cutting surface to lack the desired configuration, near either the tip of the distal end or where the cutting surface ends in the direction of the shank portion.

In the generality of the invention, a tool bit having concavities associated with ports may also have transverse passageways which terminate at ports on the exterior surface at locations where there is no associated concavity.

Fig. 7 is a side view of the spindle 42 of a vertical milling machine (not shown) that is holding and rotating tool bit / end mill 20. In the drawing, the side of the tool bit 20 is about to contact the vertical edge of workpiece 44 which is held on work table 46 of the milling machine by a vise, clamps, or other known devices, not shown. In typical use, spindle 42, and thus end mill 20, translates horizontally in well-known x and y axes of the machine.

An exemplary tool of the present invention has a nominal 0.25 inch outside diameter and a length of about 2.25 inch. Other tool bits can be made which are smaller or larger in both diameter and length. The coating on the coated/cutting surface of portion 22 of the tool bit is about 0.500 inch long. The exemplary tool has a lengthwise passageway 30 of somewhat more than 0.1 inch diameter, and transverse passageways 28 that run radially are about 0.024 inch diameter. The port opening of a passageway 28 at the exterior surface of the tool, within a concavity, may be about 0.024 inch diameter. A preferred elliptical concavity may have a major axis of about 0.1 inch, measured in the C axis direction of the tool bit, and a minor axis dimension of about 0.08 inch, measured transverse to the C axis.

The depths of concavities in exemplary tool bits can be characterized in terms of the depth measured from an imaginary cylinder that sleeve-like runs around the exterior surface of the tool bit in intimate contact with the cutting surface coating. The depth of a preferred concavity relative to such imaginary cylinder is about 0.025 inch, measured very near the center of a symmetrical concavity. For an exemplary tool bit of about 0.25 inch diameter, depth is about 10 percent of the tool bit diameter The opening of any concavity measured were it intersects the cutting surface has a nominal area based on a projection of the concavity peripheral edge onto a flat plane that is tangent to the surface of the workpiece at the concavity location. As an example, an ellipse having axes of 0.1 inch and 0.08 inch has an area of is about 0.0064 square inches. The nominal area of the port, i.e., the opening of a passageway 28 at the bottom of the concavity, is about 0.00046 square inches. Thus the area of an exemplary concavity is about 14 times the area of the port which feeds coolant to the concavity. In other embodiments, the tool may have concavities with areas that are between 10 and 25 times the area of the port opening which feeds coolant to the concavity.

An exemplary tool having a 0.25 inch diameter and a 0.55 inch long diamond coated cutting surface has a total area of about 0.027 square inches - which is the area of an imaginary cylinder surrounding the portion and includes the areas occupied by the concavities. The total area of a multiplicity of preferred concavities is about 30-70 percent, more preferably about 40-60 percent, of the total cutting surface area.

In embodiments of the invention, a diamond-nickel coating may or may not be present within the concavities 26. There may be a preference for allowing the coating to be present, since that can avoid the cost of masking the concavities. But since any coating within a concavity does not contact a workpiece during useful tool life, that portion of the coating would not be considered to be part of the cutting surface that defines the exterior of the cutting portion / distal end of the tool bit.

During use, a preferred embodiment tool bit may be rotated clockwise or counterclockwise since the cutting surface lies on an imaginary substantially cylindrical surface; as a corollary, at any axial location the cutting surface has a substantially circular shape. When, during use, rotation of the tool bit is in a first clockwise direction with the result that the grinding particles become worn and rounded, the tool may then be rotated in the opposite clockwise direction to make use of the unworn "backsides" of the particles. The rotation of the tool may be changed in connection with different steps of a machining process. That feature of reversibility of direction enables longer tool bit life than is provided by a tool designed for rotation in only one direction, for instance, where a cutting surface has a relief. In alternate embodiments of the invention, a tool bit may have other shape concavity cross sections. Fig. 5 is a partial lengthwise cross section of a tool bit (like that of Fig. 4A) showing a tool bit cutting portion 222 that has a transverse passageway 228 with a frusto-conical port opening 226. Fig. 6 is a cross section of a tool bit showing a cutting portion 322 of a tool that has a transverse passageway 328 with a stepped diameter port opening 326. In Fig. 5 and Fig. 6 embodiments, the edge of each opening will preferably be elliptical, optionally circular or some other shape.

Preferably, the cross sectional area of each opening 226, 326 at the substantially cylindrical exterior surface of the tool bit is at least four times the area of the passageway port within the concavity of the opening 226, 326; preferably it is between 10 to 25 times, as aforesaid. Other configurations of concavities would be within the scope of the invention. For example, the circular bottom of stepped depression could be curved, or the sides that define the step could be tilted outwardly. One kind of depression may be combined with another kind.

While the Figures show a tool with a constant diameter along the axial length, in the generality of the invention a tool bit may have a shank portion that has a different diameter from that of the cutting portion. Also, the cutting portion may have a non-constant diameter. For example, the tool bit cutting portion may have a circumferential step, bulge or groove.

A tool bit of the present invention may be used, as suggested, in a vertical milling machine. With reference again to Fig. 7, the tool bit is mounted in the spindle of a machine. Coolant is flowed into the interior channel of the tool bit by a pump so it flows through the port openings during engagement of the lengthwise side of the cutting surface of the tool bit with a workpiece, to remove material therefrom.

While the invention has been described in terms of an end mill, the principles of the invention can be applied to other types of milling cutters that heretofore have had electroplated abrasive particulates adhered to the surface thereof. For instance, the invention may be used in connection with face mills (where the axial end surface of the tool bit is the cutting surface that engages the workpiece and translates relative to such) and with side mills (where a nominally cylindrical body - which may be contoured - is mounted on a shaft running between two supports so the side of the cylindrical body engages the workpiece which translates in a direction perpendicular to the mounting shaft). The features and use of those other milling cutters are well known. The channeling of coolant to concavities in the cutting surface within other milling cutters would be carried out as taught by the description of the end mill herein.

The invention, with explicit and implicit variations and advantages, has been described and illustrated with respect to several embodiments. Those embodiments should be considered illustrative and not restrictive. Any uses of words which relate to the orientation of an article pictured in space are for facilitating comprehension and should not be limiting should an article be oriented differently. Any use of words such as "preferred" and variations thereof suggest a feature or combination which is desirable but which is not necessarily mandatory. Thus embodiments lacking any such preferred feature or combination may be within the scope of the claims which follow. Persons skilled in the art may make various changes in form and detail of the invention embodiments which are described, without departing from the spirit and scope of the claimed invention.

Claims

Claims:

1. A tool bit having an axial length and a lengthwise axis, for mounting in a machine tool to remove material from a workpiece, comprising: a proximal end shaped for being held in the machine tool, a distal end shaped for contacting and for removing material from a workpiece while rotating; and, at least one central passageway within the tool bit running from the proximal end to proximity of the distal end, for lengthwise flow of coolant provided to the tool bit at said the proximal end; wherein the distal end has a cutting portion comprising

a cutting surface having a coating comprised of a multiplicity of hard particles embedded within a metal matrix;

a plurality of concavities spaced apart lengthwise and circumferentially on said cutting surface, each concavity having within the concavity a port for flow of coolant into the concavity; and,

a plurality of passageways, each passageway running from said at least one central passageway to one of said ports.

2. The tool bit of claim 1 wherein said cutting surface is substantially cylindrical and continuous except where there are concavities.

3. The tool bit of claim 1 wherein the concavities are staggered along spaced apart imaginary lengthwise lines running parallel to the length of the cutting surface portion of the tool bit, so that in the preponderance of the cutting surface at least one concavity lies along any imaginary

circumferential line running around said cutting surface.

4. The tool bit of claim 1 wherein the edge of each concavity has nominally the shape of an ellipse, and wherein the major axis of the ellipse shape is aligned with the lengthwise axis of the tool bit.

5. The tool bit of claim 1 wherein the area of each concavity is at least 10 times greater than the area of the port which is within the concavity.

6. The tool bit of claim 1 wherein the total area of the concavities is between 30 and 70 percent of an imaginary cylindrical area that envelopes the cutting portion of the tool bit.

7. The tool bit of claim 1 wherein each port is centered within the concavity.

8. The tool bit of claim 1 wherein each port has nominally the shape of a circle.

9. The tool bit of claim 1 wherein the particulates are diamonds and the metal matrix is comprised predominately of nickel.

10. The tool bit of claim 1 wherein the each concavity is defined at least in part by a frusto-conical shape.

11. The tool bit of claim 1 wherein the each concavity is defined at least in part by a stepped depression.

12. The tool bit of claim 1 further comprising one or more ports on a circular surface at the distal end, each port in flow communication with the central passageway.

13. A milling cutter for mounting in a machine tool to remove material from a workpiece, comprising: a proximal end shaped for being held in the machine tool, a distal end shaped for contacting and for removing material from a workpiece while rotating; and, at least one central passageway within the tool bit running from the proximal end to proximity of the distal end, for lengthwise flow of coolant provided to the milling cutter at said the proximal end; wherein the distal end has a cutting portion comprising

a plurality of passageways, each passageway running from said at least one central passageway to one of said ports; wherein said cutting surface is substantially cylindrical and continuous except where there are concavities; wherein the edge of each concavity has nominally the shape of an ellipse, the ellipse having a major axis aligned with the lengthwise axis of the tool bit.

14. The milling cutter of claim 13 wherein the concavities are staggered along spaced apart imaginary lengthwise lines running parallel to the length of the cutting surface portion of the milling cutter, and wherein, for the preponderance of the cutting surface, at least one concavity lies along any imaginary circumferential line running around said cutting surface.

15. A method of machining ceramic workpiece material which comprises providing a tool bit in accord with claim 1 ; mounting the tool bit in the rotatable spindle of a milling machine; flowing coolant to the proximal end of the tool bit which rotating the tool bit, so coolant flows out of said ports; and, simultaneously contacting the cutting surface of the tool bit with a workpiece to cut away a portion of the workpiece.

16. A method of machining ceramic workpiece material which comprises providing a tool bit in accord with claim 13; mounting the tool bit in the rotatable spindle of a milling machine; flowing coolant to the proximal end of the tool bit which rotating the tool bit, so coolant flows out of said ports; and, simultaneously contacting the cutting surface of the tool bit with a workpiece to cut away a portion of the workpiece.