END MILL TOOL WITH A COMPOUND MATERIAL CORE AND A HARD MATERIAL COATING

Abstract: An end mill which both drills and mills has main cutting edges (4) and end cutting edges (5) as well as reinforcing chamfers (6) between these. The end mill consists of a core of high speed steel or tool steel and an over-lying layer made of a hard material with 30-70 % by volume of sub-micron hard constituents in the form of carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a metallic matrix based on Fe, Co and/or Ni, whereby the external surface of the overlying layer at least partly is coated with a thin layer of TiN, Ti(C,N) and/or (Ti,Al)N. The tool's pitch angle is 40 +/- 5 DEG and its rake angle is 12 +/- 5 DEG .

Description: END MILL TOOL WITH A COMPOUND MATERIAL CORE AND A HARD MATERIAL COATING The present invention relates to an end mill tool which through material and cutting geometry attains an improved result. End mills according to the invention belong to an advanced group of tools which shall be able to both drill and mill. They shall manage the most changing working conditions at one and the same time. The most distinguishing is that it is required good cutting properties in the form of wear resistance and ability to resist high temperatures at the very periphery, i.e. at full nominal measure at the same time as the tool is capable of generating chips and function as a cutting tool in the center, where the cutting speed approaches zero. Between these a continuous change of the cutting speed is attained through all possible built up edge areas etc. End mills are often used in difficult operations, where the requirements are very high in terms of surface smoothness. This is for example the case in the aerospace industry when milling wing spars and the like. Here no unevenness and notches whatsoever are acceptable, since these later may give rise to failure. Another important area of application is the finecopying of moulding tools where the requirements of high surface smoothness and shape accuracy, with simultaneous high productivity and predictable long tool life, are specially severe. Further, it is a necessity that the tool does not have to be replaced during the continuous machining operation, something that results in inferior precision of the manufactured article. When machining stainless material it is important with sufficiently large chip spaces, chips which slip off the cutting edges, a positive cutting geometry which gives soft machining, as well as safety at machining so that tool damage is prevented. The end mills which exist today are frequently of HSS or cemented carbide. Although these function satisfactorily at several applications, they are impaired with many drawbacks. Hence, high speed steel often causes so called built up edge formation (chips stick to the cutting edge) and they have a relatively short tool life. Cemented carbide is, as is well known, a more brittle material and is therefore not so safe. In SE-B-392 482 a material is disclosed which contains 30-70 % by volume of sub-micron hard constituents in a metallic binder phase. This material has superior wear resistance compared to advanced high speed steel and can therefore be placed in the gorge between cemented carbide and high speed steel as to material properties. Further, in SE-B-440 753 is disclosed how superior compound tools have been made with, inter alia, the above material in the areas which are subjected to a high cutting speed, with high speed steel in the center for .drilling applications. The purpose with this later invention was on the one hand to obtain a tool with a better macro toughness with the aid of a tougher core, and on the other hand to achieve a better grinding economy since hard constituent rich material is experienced as considerably more difficult to grind than for example high speed steel. Moreover, inconveniences were experienced with the zero speed problem and built-up edge formation areas for the sub-micron hard material with 30- 70 % by volume of hard constituents. Both these SE documents are herewith incorporated by reference. In US-A-5 026 227 an end mill is disclosed which consists in its entirety of a solid so called cermet material, mainly consisting of a NbC-TiC-TiN-based composition. This tool is relatively brittle and is used in principle only in finishing applications. It is inappropriate for coarser end mill cutting where high demands are made on the tool's toughness. It has now been shown possible to produce end mills with a high productivity, very fine surfaces, high tool life and good machining economy in modern metal working machines. This has been achieved by manufacturing an end mill of a compound material and with a geometry according to claim 1. Besides, built-up edge formation is avoided over a very broad cutting speed range. This results in a low wear and a durable sharp cutting edge which generates good shape accuracy and very good surfaces on the work piece. This is true even for work piece materials which of tradition are regarded as very difficult to machine. The tools were used in coated condition and the so called PVD-method which is the most used coating method. The coating is made mainly with titanium based hard material, such as TiN, Ti(C,N), (Ti,Al)N etc. Particularly good properties have been obtained with 2-4 μm Ti(C,N) . Thus, according to the invention there now exist long, slender and solid end mill tools with a cover of a hard material with 30-70 % by volume of sub-micron hard constituents in the form of carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a metallic matrix based on Fe, Co and/or Ni. Preferably, the hard material consists of 30-70 % by volume of hard constituents of mainly TiN in a matrix of high speed steel type, where the concentrated hard constituents have a grain size of <1 μm, preferably <0.5 μm. By means of a well balanced combination between tool material and tool geometry, a unique tool with superior performance has been obtained. By its fine grain-size and good dispersion between hard constituents and binder phase, an extraordinarily good adhesiveness has been attained for pure hard constituent layers which have been applied by the PVD- ethod. Otherwise, this method often gives less good adhesion, compared with the more metallurgical bond which arises at the so called CVD-method. The reason is foremost that CVD takes place at higher temperatures. The applied layers are primarily titanium based and specially good properties have been obtained with Ti(C,N) but also (Ti,Al)N shows great advantages. Tools according to the invention function very well not only in normal steels but also in hard metal piece materials, in the range of 250-500 HB, such as tool steel, adhering materials such as stainless steel, aluminum alloys or titanium alloys. By the fact that the hard material is metallurgically bound to a tougher core material, one has obtained a very advantageous combination of wear resistance and toughness behaviour for end mills. The invention is described in the appending drawings, wherein: Fig 1 shows an end mill tool with three cutting edges in a side view, Fig 2 shows a cross-section of the tool along the line A-A in Fig 1, Fig 3 shows an end view of a tool with three cutting edges, Fig 4 shows an enlarged cross-section view of the tool along the line B-B in Fig 1, radially from the central axis , Fig 5 shows a cross-section along line C-C in Fig 4. The tool consists generally of an oblong cylindric body 1 which consists of a compound material according to SE- B-440 753 and which has been made in a way as described in this patent document. Consequently, the tool's core consists of high speed steel or tool steel and an external coating, whose thickness constitutes generally 15 % of the the tool's diameter (however, at least 0,5 mm) , consisting of an above more closely specified hard material. Helically along the tool's central axis run a number of protruding lands 2, usually two or three. The protruding lands 2 are delimited by flutes 3. According to the invention, it has been proved to be surprisingly advantageous to arrange these helically formed protruding lands and flutes with 40±5 degrees helix or pitch angle, suitably 40 ± 3°, and preferably 40 ± 2°. Each protruding land is along its whole fore edge, seen in the direction of rotation, equipped with a main cutting edge 4 with a positive rake angle β. According to the invention, it has turned out to be surprisingly advantageous to arrange this main cutting edge with 12 ± 5 degrees positive rake angle, more exactly 12 ± 3°, and preferably 12 ± 2°. The clearance angle a of the main cutting edge 4 varies depending upon the tool's diameter. The following values illustrate this variation: The tool's cutting edges have an edgerounding of 10-30 μm, preferably 10-20 μm. Since the tool also shall be able to drill in axial direction, a number of end cutting edges 5, 5' corresponding to the number of protruding lands 2 are provided at the tool's tip. These end cutting edges are formed obliquely inwards and downwards in an angle of about 1,5° in relation to a radial plane that is perpendicular to the central line. The end cutting edge 5 extends past the center while the remaining end cutting edges 5' have a shorter length which is delimited by a ground chip space 7. According to the present invention, a reinforcing chamfer 6 is arranged on the corner between each main cutting edge 4 and the adjacent end cutting edge 5, 5' . This reinforcing chamfer is there to diminish the strains that these corners are submitted to when they are in contact with the workpiece. They are suitably angled at an angle y of 20°±10° in relation with the central axis of the end mill, preferrbly 20°±5°. Generally, this pitch angle of the reinforcing chamfer amounts to roughly half the helix angle of the main cutting edge 4. Further, each reinforcing chamfer 6 is inclined by an angle δ in relation with the adjacent end cutting edge 5,5'. This angle can generally be 0°±10°, preferrably 0°±5°. The maximal axial width of the reinforcing chamfers can vary within a wide range and depends inter alia on the diameter of the tool. Generally it should be between 0,05 and 1,5 mm, suitably between 0,10 and 1,0 mm and preferrably between 0,15 and 0,50 mm. Owing to these reinforcing chamfers, a number of inconveniences have been overcome in a surprisingly simple manner, such as damages and chippings in these corner parts, as well as vibrations. In order to enable the end mill tools to hold the dimension tolerances even at requirements of long tool life and high cutting data, they are also covered with a thin layer of TiN, Ti(C,N) and/or Ti(Al)N. Preferably TiCN is used, in a thickness of between 2 and 4 μm; specially with regard to the very good adhesiveness which has been achieved with specifically this material. Coating of the hard material with a thin layer is effected with the so called PVD-procedure, which is well known for the man skilled in the art. In order to facilitate the tool's all functions (in particular lateral milling, grooving, drilling) , it has been provided with a substantial chip space, particularly with the purpose of avoiding possible chip jamming. This is demonstrated by the size of the flutes 3 in Fig 1 and 2. For illustrating but non-limiting purposes some examples are presented below, which further illustrate the excellent properties of the end mill tool according to of the invention. Example 1 The example below shows how it has been possible to combine the material with the new geometries to products with superior properties. A rod of compound material produced in accordance with SE-B-440 753 example 3, was extruded to measure 9,6 mm. After heat-treatment, the rod was ground to 8 mm's end mills comprising two or three cutting edges, with different geometries. Subsequently, a comparative lateral milling test in stainless austenitic steel SS 2343 was performed with tools according to the invention, other in-house made tools and conventional HSS tools. All test tools were covered with a thin Ti (C,N) -layer of PVD-type. Tool A: End mill with three cutting edges according to the invention, with pitch angle 40 degrees, chip angle + 12 degrees, as well as a reinforcing chamfer between main and end cutting edges. Tool B: Tool according to A but without reinforcing chamfer. Tool C: Tool according to A but with a pitch angle of 45°. Tool D: End mill with three cutting edges of a material according to the invention but with a pitch angle of 30 degrees. The chip angle is +12 degrees; a reinforcing chamfer between main and end cutting edges is foreseen. Tool E: An HSS end mill with three cutting edges and with a pitch angle of 35 degrees, a chip angle of +10 degrees, without any reinforcing chamfer. The tools were tested in medium coarse milling with an axial cutting depth of 8 mm and a radial cutting depth of 2 mm. The durability criterium was an average flank wear of 0,08 mm or chippings. Cutting data were choosen according to the manufacturer's recommendations. Cutting data and results ax 3 shown in the table below. For tools B, C and E a large life spread was obtained because of sudden chippings on the tool corners. The test showed that the test tools according to the invention clearly offered the best result in the sense of long tool life and good reliability. Example 2 End mills according to the invention with two cutting edges with a diameter 8 mm were produced in agreement with Example 1 and were tested in grooving in stainless austenitic steel SS 2343 in comparison with other coated conventional end mills of high speed steel or cemented carbide. Tool A: An end mill with two cutting edges according to the invention, with a pitch angle of 40 degrees, a chip angle of +12 degrees and a reinforcing chamfer between main and end cutting edges. Tool B: An end mill with two cutting edges of solid fine-grained cemented carbide, with a pitch angle of 40 degrees, a chip angle of +12 ' degrees, without any reinforcing chamfer. Tool C:. An end mill with two cutting edges in a solid, highly alloyed high speed steel with a pitch angle of 40 degrees, a chip angle of +13 degrees and without any reinforcing chamfer. The tools were tested in grooving at am axial cutting depth of 6 mm. The durability criterium was an average flank wear of 0,06 mm or an interrrupted test due to chippings, tool damage or unacceptable notches Cutting data according to producers' recommendations and results are shown in the table below. Tools B and C exhibited a considerably larger spread in tool life because of a large percentage of chippings on the tools' corners. Tool A according to the invention gave the clearly best results in the sense of high productivity, long tool life and safety.

Claims: Claims 1. An end mill tool comprising a plurality of main cutting edges and end cutting edges consisting of a compound material which forms the core and an over-lying layer of the tool, c h a r a c h t e r i z e d in that the core consists of high speed steel or tool steel and the over-lying layer consists of a hard material with 30- 70 % by volume of sub-micron hard constituents in the form of carbides, nitrides and/or carbonitrides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and/or W in a metallic matrix based on Fe, Co and/or Ni, whereby the external surface of the over-lying layer is at least partly covered with a thin layer of TiN, Ti(C,N) and/or (Ti,Al)N, that the tool's pitch angle is 40 ± 5° and its rake angle is 12 ± 5°, and that it is provided with a reinforcing chamfer (6) between the main cutting edge and the end cutting edge. 2. Tool according to claim 1, c h a r a c h t e r i z e d in that there are two or more main cutting edges, preferably two or three. 3. Tool according to claims 1 or 2, c h a r a c h t e r i z e d in that the reinforcing chamfer is angled at 20±10° in relation with the central axis of the end mill tool (angle £) and 0±10° in relation with the adjacent end cutting edge (5, 5', angle δ) . 4. Tool according to any of claims 1-3, c h a r a c h t e r i z e d in that the cutting edge has an edgerounding of 10-30 μm, preferably 10-20 μm. 5. Tool according to any of claims 1-4, c h a r a c h t e r i z e d in that the tool's cutting diameter is 3-50 mm, preferably 4-25 mm, whereby the clearance angle at the periphery is 17 ± 2° for a diameter of 4 mm and diminishes to 11 + 2° for a diameter of 25 mm. 6. Tool according to any of claims 1-5, c h a r a c h t e r i z e d in that the end cutting edge has a clearance of about 1,5° towards the tool's central axis and about 6° tangentially aback, against the direction of rotation. 7. Tool according to any of claims 1-6, c h a r a c h t e r i z e d in that the over-lying layer has a thickness of at least 0,5 mm. 8. Tool according to any of claims 1-7, c h a r a c h t e r i z e d in that the hard material consists of 30-70 % by volume of hard constituents of mainly TiN in a matrix of high speed steel type in which the enriched hard constituents have a grain size of < 1 μm, preferably < 0,5 μm. 9. Tool according to any of claims 1-8, c h a r a c h t e r i z e d in that it is coated with a 2-4 μm thin layer of TiN, Ti(C,N) or (Ti,Al)N, preferably Ti(C,N) . 10. Tool according to claims 1 - 3, c h a r a c t e r i z e d in that the angle of the reinforcing chamfer (6) to the central axis of the tool (β) amounts to about half the value of the helix angle of the main cutting edge.

Inventors: HAAKANSSON BJOERN, VON HOLST PEDER, HAAKANSSON, BJOERN, VON HOLST, PEDER
Applicants: HAAKANSSON BJOERN, HOLST PEDER VON, SANDVIK AB
Application year: 1993
Main classification: B23C5
IPC codes: B23C5, C22C1, C23C30