Cutting tool

Abstract: A coated cemented carbide tool insert with a lengthened life is provided which comprises a substrate of a cemented carbide and a coating film provided thereon consisting of one or more layers 2 to 20 µm in thickness of a compound with a higher hardness than the substrate, in which at least the surface of the coating film, on and near the ridgeline of the cutting tip, of the tool is substantially composed of a smooth surface with a surface roughness Rmax of at most 0.2 µm to the standard length of 5 µm.

Description: This invention relates to coated hard metal tools suitable for use in cutting of metals, for example, turning, milling, threading, drilling, boring, etc. In the field of cutting of metals, working conditions have become severer with the years and there is a requirement to increase the hardness, wear resistance and heat resistance of cutting tools used to this end. Cemented carbide tools are capable of satisfying these requirements to some extent, but tools of cemented carbides with various hard layers have lately been used more widely. A typical form of the tools is as exemplified in Fig. 3 in which a cemented carbide insert 1 is fixed to a holder 2. This is generally called a throwaway insert which is to be thrown away to be exchanged for a new insert after using cutting edge corners (in the case of Fig. 3, 6 corners). In these coated cemented carbide tools, the surface of the cemented carbide insert 1 is ordinarily coated by a CVD method, PVD method or the like. Generally, cutting tools are required to be excellent in both of wear resistance and toughness. It is known from the prior art in the case of the coated cemented carbide inserts however, that if the thickness of the hard coating is increased so as to improve the wear resistance, the toughness is decreased. In order to solve the above described problems, various methods have been proposed, for example, wherein the coating film near the cutting edge is partly made thinner or removed (Japanese Patent Publication No. 37553/19 and Japanese Patent Laid-Open Publication Nos. 219122/1984, 24371/1985 and 447203/1985). US-A-4 239 819 discloses an improved hard metal coated alloy appropriate for manufacturing cutting tools. The improvement lies in the deposit on a substrate of fine grained crystals free of columnar orientation which yields a smooth surface layer of an approximate thickness of 25 µm. However, the requirements for the grade of a finished surface of a workpiece to be machined or cut have lately become severer. As shown in Fig. 4, for example, very localised damage 6 of a coating film has often been regarded as limiting the life of a tool although the life of a tool is generally judged by the microscopic average wear (flank wear 5, crater depth 4) or breakage of a cutting edge (ridgeline of cutting tip) because this is transferred to a machined surface of a workpiece. Thus, there are many cases wherein a cutting tool is exchanged for a new one when the flank wear width reaches about 0.1 µm which is much less than the standard for the life of a tool due to wearing, i.e. a flank wear width of 0.2 to 0.3 µm, as ordinarily allowed in the art. Under this situation, an even further reduction in localised damage is required when a higher quality is required on the finished surface of a workpiece. To this end, it has been considered to employ a thin coating film, but the wear resistance, as a whole, is lacking. Therefore, a structure in which the localised damage is minimised even if the film thickness is large is desirable. According to this invention there is provided a cutting tool having a ridgeline comprising a substrate of a sintered hard alloy and a coating film 2 to 20 µm in thickness provided thereon consisting of one or more layers of a compound with a higher hardness than the substrate, characterised in that at least the surface of the coating film on and near the ridgeline of the cutting tip of the tool is substantially composed of a smooth surface with a surface R max of at most 0.2 µm over a standard length of 5 µm. The sintered hard alloy substrate can be formed from a cemented carbide with WC as a predominant component and a binder metal such as an iron group metal or from cermets comprising TiC or TiN as a predominant component and a binder metal such as Ni. The or each layer of the coating film comprises at least one member selected from the group comprising carbides, carboxides, carbonitrides and carboxynitrides of Group IVa, Va and VIa metals of the Periodic Table and oxides and oxynitrides of Zr and Al and solid solutions thereof. The coating film comprises two layers, at least one of which is a layer comprising of an oxide or oxynitride of Al or Zr. The invention provides a tool with a long life. Further, the invention provides a tool having such a structure that the coating film near the cutting edge is resistant to damage. Examples of the invention will now be described with reference to the drawings, in which:- Figs. 1(a) and 1(b) are 8000-multiplied photographs of particle structures by SEM of a coated cemented carbide tool according to one embodiment of the invention, (a) being that of the surface part and (b) being that of a film cross section of the surface part thereof; Figs. 2(a) and 2(b) are similarly enlarged photographs of particle structures by SEM of a coated cemented carbide tool according to the prior art, (a) being that of the surface part and (b) being that of a film cross section of the surface part thereof; Fig. 3 is a perspective view of a throwaway insert, as one embodiment of the invention, clamped by a holder; and Fig. 4 and Fig. 5 are enlarged schematic views of the corner part of a tool insert to illustrate the merits of the present invention, Fig. 4 showing a worn and damaged state of the corner part and Fig. 5 showing a contacted and worn state of a workpiece and insert. The inventors have made various efforts to solve the above-described problems of the prior art and consequently, have found as a result of observing local damage on a coating layer of a tool, occurring prior to a large degree of wear, that impacts suffered through contact of the surface of the coating layer with a workpiece many times when the workpiece is subjected to cutting, damage of the tool. As a method for reducing the damage it is effective to reduce the surface roughness on the surface of the coating layer. Generally, the larger the thickness of the coating layer, the larger the crystal grains, i.e. the surface roughness of the coating layer. Therefore, the most simple method for obtaining both a relatively thick coating layer from the viewpoint of the wear resistance and a smooth surface of a coating layer form the view point of preventing the coating film from a localised damage consists in subjecting the surface of the coating layer of a tool, including at least a cutting edge, to mechanical polishing after coating and thus improving the surface roughness. The above described smooth surface can be obtained by barrel finishing, buff polishing, brush honing or lapping treatments using an elastic grinding wheel or diamond wheel. In addition to these mechanical treatments, the above described object can be achieved by chemical etching or improving the coating method itself to give the above specified surface roughness. The coating film should have a thickness of 2 to 20 µm, since if the thickness is less than 2 µm, the wear resistance by the coating is not sufficient and if more than 20 µm, the strength of the coating film itself is lowered and the toughness is also deteriorated, which are not suitable as a cutting tool. The relationship between the properties of the surface of a coating film and the performances of a coated cemented carbide insert will now be illustrated, for example, as to a coated cemented carbide insert having an Al 2 O 3 film as the outermost coating layer. Fig. 4 is an enlarged schematic view (perspective view) showing a worn state at the cutting corner of a throwaway insert and Fig. 5 is a schematic view showing a contacted and worn state of a workpiece 9 and a corner part of a tool insert consisting of a hard coating film 7 and cemented carbide substrate 8 during machining. Up to the present time, flank wear width 5 and crater depth 4 have generally been used as a standard measure of the life of a tool. Observation of wear in a coated cemented carbide insert in greater detail teaches that the wearing progress is relatively slow in the vicinity of a ridgeline 3 as shown in Fig. 4 and Fig. 5. The coating film 7 is locally peeled off and carried away by chips in a region where the coating film remains sufficiently thick, whereby causing the film-peeled damage 6, exposing the cemented carbide substrate 8, causing local wearing to progress deteriorating the grade of the finished surface of a workpiece and promoting the flank wear itself. According to our studies to prevent coated cemented carbide inserts from such damage of the cutting edge, it is found that the life of a coated cemented carbide insert, until the coating film is peeled, can largely be lengthened by making the surface of the coating film 7 on and near the ridgeline 3 smooth. The surface of an ordinary coated cemented carbide insert consists of a polycrystalline film with a cross-sectional roughness corresponding to the crystal size of the coating material as shown in Fig. 2(a) and (b). Figs. 2(a) and (b) are 8000-multiplied photographs of particle structures by SEM (scanning electron microscope), (a) being a plan view of the surface and (b) being a side view of a film cross-section of the surface part. Since the Al 2 O 3 film as the outer-most surface of the coating layers has a poor electroconductivity, a sample to be observed is subjected to ion sputtering as a pretreatment and thus coated with a thin film of Au so as to clarify the image. The coated Au thin film has such an adequately homogeneous thickness of at most 100 Å that there is little error in the microscopic surface roughness of the sample. In the example as shown in Fig. 2(b), the surface roughness R max over the standard length 5 µm is about 0.4 µm. The surface of an Al 2 O 3 coating film of a tool according to one embodiment of the invention is shown in Figs. 1(a) and (b), in which R max is about 0.15 µm. R max of 0.2 µm or less is effective for retarding occurrence of the film-peeled damage 6 at the ridgeline 3 as described above (Fig. 4). When the surface of the coating film on or near the ridgeline 3 has a surface roughness R max exceeding 0.2 µm over the standard length 5 µm, film peeling takes place in a region where the coating film remains sufficiently thick in the vicinity of the ridgeline of cutting tip before the life of the insert is exhausted by an ordinary flank wear or rake wear. Following which welding or surface napping or surface roughening of a workpiece to be cut starts, or the peeling further spreads to result in abnormal wearing. Therefore, a surface roughness of 0.2 µm or less is preferable. As shown in Fig. 5, wearing gradually proceeds from the initial stage of cutting on the rake face and flank face to give smooth surfaces, whereas wearing of the coating film does not so proceed and the surface roughness of the coating film is substantially maintained during cutting as it is at the ridgeline 3 that is present as "dead zone" during cutting. On the other hand, in general, a cutting insert is repeatedly exposed to intermittent thrusting in and releasing from a workpiece to be cut in machinings such as by not only milling but also turning. If the surface roughness is maintained on and near the ridgeline of cutting tip 3 during the same, welding of the workpiece occurs thereon or complicated forces are applied thereto, as a result of a number of impacts, thus resulting in a localized damage or breakage of the film. When the surface of a coating layer is made smooth however, the stress concentration during the impact cycles can be moderated to decrease the welding of a workpiece and lengthen the life of the tool. As is evident from the above described mechanism of machining, it is required that the surface is made smooth at least on and near the ridgeline of the cutting tip. Ordinarily, the surface roughness of a coating film is composed of an undulation or roughness substantially transferred from the surface of a cemented carbide substrate before coating and a roughness of a polycrystalline coating film, in combination. However, it is surprisingly found by the inventors that the above described phenomenon is mainly related with not the undulation but the roughness, so the surface roughness of a cemented carbide substrate can sufficiently be of the level of an ordinarily ground surface. That is, in order to exclude the effect of the undulation of a substrate surface, the standard length for measuring the surface roughness Rmax is defined to 5 µm. To obtain the advantages of the invention, 100 % of the surface on and near the ridgeline of the cutting tip need not be made smooth; if the surface with an area of at least 50 % has a surface roughness of at most 0.2 µm, these advantages can substantially be obtained. The surface roughness of a cemented carbide substrate can sufficiently be that of an ordinarily ground surface as described above, but in another embodiment of the present invention, of course, the surface roughness of a cutting insert can be controlled by previously reducing the surface roughness of a cemented carbide substrate, for example, by lapping, followed by usually coating. The following examples are given in order to illustrate the present invention. Example 1 A cutting insert having a Form No. of ISO and a shape of SNMG 120408 was made of a cemented carbide with a composition of 88 % WC-3 % TiC - 3 % NbTaC - 6 % Co (% : by weight) and the whole ridgeline of cutting tip was subjected to an edge treatment for curved-surface chamfering with a width of 0.05 mm viewed from the rake face side using a vibrating barrel, followed by measuring the surface roughness in the vicinity of the ridgeline. On the thus resulting substrates were formed hard coating films of various structures as shown in Table 1 by an ordinary CVD method (Chemical Vapor Deposition) to obtain various products. Some of them were used as samples as they were and the others were subjected to lapping by the use of diamond powder with an average grain size of 4 µm to make smooth the outermost surface of the coating films, thus obtaining other samples. Thus, the samples with two kinds of surface roughness were prepared and subjected to measurement of the surface roughness, in which the surface roughness to the standard length of 5 µm was examined by a SEM photograph of the cross section of the insert sample so as to distinguish it from undulations transferred from the substrate. Enlarged views of the surface particle structure of Insert B are shown in Fig. 2 (a) and (b) and those of Insert G are shown in Fig. 1 (a) and (b). Using these sample inserts for cutting, a cutting test was carried out under the following conditions to compare the number of cutting-feasible samples until the coating film at the cutting edge met with a localized peeling, thus obtaining results as shown in Table 1. Cutting conditions Workpiece: SCM 304 (H B 210), 200 mm diameter x 15 mm width Cutting Speed: 120 m/min Cutting Depth: 1.5 mm Feed: 0.15 mm/rev Cutting by Dry Process Example 2 Insert A of the prior art, used in Example 1, and Insert F of the present invention, used in Example 1, were subjected to a cutting test under the following conditions: Cutting Conditions Workpiece: SCM 415 (Hs 200) 200 mm diameter x 30 mm width Cutting Speed: 200 m/min Cutting Depth: 1.5 mm Feed: 0.3 mm/rev Water-soluble Cutting Oil In this test, Insert A of the prior art met with peeling of the coating film at the ridgeline when 90 workpieces were machined, while in the case of Insert F of the present invention, the coating film was partially peeled when 160 workpieces were machined. As can be seen from Table 1, the tool is capable of processing or machining more work-pieces until the coating film is partially peeled, than those of the prior art, thus teaching that the insert has a longer life. In the cutting tool damage of the coating film in the vicinity of the cutting edge or ridgeline of cutting tip is not readily caused and accordingly, there are given advantages that lowering of the grade of a finished surface of a workpiece through welding of the workpiece due to peeling of the coating film can be prevented, progressing and developing of the overall wearing due to peeling of the coating film can be suppressed and the effects of formation of the coating layers can sufficiently be brought to thus lengthen the life of the tool.

Claims: A cutting tool having a ridgeline comprising a substrate of a sintered hard alloy and a coating film 2 to 20 µm in thickness provided thereon consisting of one or more layers of a compound with a higher hardness than the substrate characterised in that at least the surface of the coating film on and near the ridgeline of the cutting tip of the tool is substantially composed of a smooth surface with a surface R max of at most 0.2 µm over a standard length of 5 µm, and in that the coating film comprises at least two layers, at least one of which is a layer comprising an oxide or oxynitride of Al or Zr. A cutting tool as claimed in Claim 1, characterised in that the sintered hard alloy is selected from the group consisting of cemented carbides comprising WC as a predominant component and a binder metal such as iron group metals and cermets comprising TiC or TiN as a predominant component and a binder metal such as Ni. A cutting tool as claimed in Claim 1 or Claim 2, characterised in that the or each other layer comprises at least one member selected from the group comprising carbides, carbonitrides and carboxynitrides of Group IVa, Va and VIa metals of the Periodic Table and solid solutions thereof. A cutting tool as claimed in any preceding claim, characterised in that the ridgeline of the cutting tip has a curved chamfered shape.

Inventors: CHUDO MASUO, NOMURA TOSHIO, CHUDO, MASUO, NOMURA, TOSHIO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1988
Main classification: C23C30
IPC codes: C23C30, B23B27, C23C16