METHOD FOR OBTAINING WELL-DEFINED EDGE RADII BY ELECTROPOLISHING

Abstract: The present invention relates to a method for edge rounding of cutting tool inserts, in combination with a high surface finish over the whole insert, of cemented carbide or titanium based carbonitride alloys. An electrolytic method is used with an electrolyte which provides an even removal of both binder phase and hard constituent phases such as perchloric (HC1O4) or sulphuric (H2SO4) acid, 15-50 vol %, in methanol or other organic liquid. The method is easier to control than conventional mechanical methods and is particularly useful for providing very small edge radii of about 10 mu m in combination with a high surface finish over the whole insert which can not be made by mechanical or other electrolytic methods.

Description: Method for obtaining well defined edσe radii by electropolishinα The present invention relates to a method for ob- taining well defined edge radii on cutting tool inserts in combination with a high surface finish over the whole insert by electropolishing technique. Inserts for chip forming machining made of cemented carbide or of titanium based carbonitride (cermet) con- sist of at least one main cutting edge and a connecting nose (corner) . Such inserts are produced by the powder metallurgical methods of milling of powders of the hard constituents and binder phase, pressing to bodies of de¬ sired shape and finally sintering the pressed bodies. The pressing is generally done as tool pressing between two opposing punches in a die. As a result of the pres¬ sing operation the inserts obtain rather sharp edges . In addition, because of the small gap, a few microns wide, that always exists between the punches and the die wall the insert edges also have burr. Such edges break too easily when used. Therefore, after sintering, the inserts are sub¬ jected to an edge rounding operation including methods such as lapping, tumbling, brushing or blasting. These operations, however, are difficult to control with any desirable accuracy. For this reason, the edge rounding values usually range between 30 and 75 μm on cemented carbide inserts for a majority of machining applica¬ tions. Smaller edge rounding values are generally not possible to obtain with mechanical methods. Also, the edges often obtain defects in the initial stage of the mechanical operation. These defects disappear during the continued treatment provided that the final edge round¬ ing obtained is larger than the defect size. A finer edge rounding, however, means lower cutting forces. The choice of edge rounding is a compromise bet¬ ween desired edge strength and acceptable cutting forces. For certain cutting operations such as threading and machining of heat resistant materials, aluminium or cast iron, lower cutting forces are desirable. However, the above mentioned methods for edge rounding are gene¬ rally not useful at least on a large, industrial scale. Electrolytic smoothing or deburring is a commonly employed technique. Two well-known processes are called electrochemical deburring and electropolishing. US 4,405,422 discloses methods for electrolytic deburring of copper or copper alloys and 4,411,751 of steel or aluminium alloys. However, when subjecting materials with phases of differing chemical properties such as ce¬ mented carbide to chemical treatments the binder phase is often dissolved first, resulting in a porous surface layer with reduced strength and often containing por¬ tions comprising several grains that have disappeared, so called pitting. It is therefore essential that an electrolyte is used which provides an even removal of material, essentially without depth effect. An example of this is US 5,380,408 which discloses a method for re¬ moving cobalt from the surface of cemented carbide using an electrolyte of sulphuric and phosphoric acids. This method, however, does not generate edge rounding. In US 5,591,230 a method for edge rounding of cut¬ ting inserts by electropolishing in an electrolyte con¬ taining 2-15 vol-% perchloric (HCIO4) or sulphuric (H2SO4) acid in methanol is presented. With this method for geometrical reasons the material removal is signifi¬ cantly larger along an edge than on an essentially flat surface resulting in an excellent fine edge rounding whereas the polishing effect is smaller on essentially flat sur aces. However, on such surfaces there often ap- pear spots in which, due to unstable conditions, Co is preferentially etched away. The surface finish after this treatment is also generally not good enough for the inserts to be coated directly by CVD- or PVD-methods or delivered to customer. Additional mechanical or chemical treatment is needed before that. This further treatment can lead to an enlargement of the desired fine edge ra¬ dius. It is an object of the present invention to provide a method for edge rounding of cutting tool inserts, which can be more carefully controlled, in combination with a high surface finish over the whole insert. It is a further object of the present invention to provide a method of manufacturing inserts with a small edge radius of the order of 10 μ in combination with a high surface finish over the whole insert. It has now surprisingly been found that by using a method similar to the one disclosed in US 5,591,230 but using an electrolyte comprising perchloric (HCIO4) or sulphuric (H2SO4) acid in higher concentrations, an even removal of the burr and rounding of the edge is obtain¬ ed, resulting in a smooth edge with an edge rounding which is essentially constant around the insert in com¬ bination with a high surface finish over the whole in- sert. The method is easier to control than conventional mechanical methods and is particularly useful for pro¬ viding very small edge radii of about 10 μm in combina¬ tion with a high surface finish over the whole insert which can not be made by mechanical methods . Fig. 1 shows in 5X the clearance face of an insert treated according to prior art. Fig. 2 shows in 5X the clearance face of an insert treated according to the invention. Fig. 3 shows in 500X an edge of an insert with burr. Fig. 4 shows in 500X an edge after rounding accor¬ ding to the invention. According to the present invention the inserts are thoroughly cleaned e.g. by ultrasonic cleaning in metha- nol so that dust, loose particles, grease stains etc. that may affect the polishing result are removed from the surfaces. The inserts are then submerged in the electrolytic bath and a voltage is applied between the inserts (anode) and a cathode. Strong agitation is car¬ ried out in order to obtain stable conditions with elec¬ trolyte flowing along all sides of the inserts. The cathode should be made of an acid resistant material, e.g. platinum or acid resistant stainless steel, and have a surface area comparable to or preferably larger than the total surface area of the inserts. The electrolyte shall contain >15 but <50 vol%, preferably 20-30 vol% perchloric (HCIO4) or sulphuric (H2SO4) acid, or a mixture thereof, in methanol. Metha¬ nol may be partly or fully substituted by more viscous fluids, e.g. another lower alcohol such as butanol, glycerol or ethyleneglycol-monobutylether, in order to decrease the polishing speed or as a means for obtaining more stable conditions. The temperature of the electrolyte may be varied between room temperature and -60 °C, mainly in order to change the viscosity of the electrolyte. The voltage shall be lower than 50 V but higher than 3 V, preferably 10-30 V. Generally a DC-voltage is used. But it is also possible to use pulsed or AC-voltage. The proper choice of voltage depends on the design of the equipment used, the degree of agitation obtained and the choice of electrolyte and temperature. The choice of electrolyte, temperature, applied voltage and polishing time should be adapted for each insert grade to obtain the best result. It is within the purview of the skilled artisan to determine these condi¬ tions . Immediately afterwards the inserts are rinsed, e.g. in methanol, in order to avoid corrosion caused by the electrolyte. With a correct choice of the different parameters described above a thin, highly viscous layer is formed at the interface between insert and electrolyte. Since the voltage drop occurs mainly across this layer the polishing speed will depend strongly on its thickness. Therefore, on a rough surface, protruding parts will be polished faster than grooves, leading to a continuously decreasing surface roughness. On the other hand, if the choice of parameters is too far from the optimum, the viscous layer will never be formed or will be unstable, leading to oxidation or even pitting of the surface. The method is suitable for mass production since large quantities of inserts can be polished simultane¬ ously with high polishing speed. The accuracy and repro- ducibility is extremely high. Edge defects due to pressing or grinding will de¬ crease in size or even vanish depending on the size re¬ lation between defect and final edge radius. For geometrical reasons, the material removal rate is substantially larger along the edges than on the flat surfaces of the insert. Thus, the method can be used also for gradient sintered grades, i.e. grades with a binder phase enriched surface layer, without risking that the gradient is removed. Example 1 A commercially available cemented carbide insert (SANDVIK H10F) with as-sintered sharp edges with remain¬ ing burr was electropolished for 6 minutes using an electrolyte consisting of 22 vol% sulphuric acid in methanol, cooled to -50 °C, and a DC-voltage of 20 volts. A 30 cm2 platinum sheet was used as cathode and the electrolyte was stirred strongly using a magnetic mixer. Smooth rounded edges were obtained with small edge radii about 20 μm and excellent surface finish. For comparison a similar insert was treated accord¬ ing to prior art, US 5,591,320. In Fig. 1 a clearance face is shown from the insert treated as disclosed in US 5,591,320 and in Fig. 2 treated according to the invention. Fig. 3 shows an edge with remaining burr and Fig. 4 after treatment according to the invention. Example 2 Example 1 was repeated but with a shorter polishing time of 4 minutes. Smooth rounded edges were obtained with small edge radii about 10 μm and excellent surface finish.

Claims: Claims 1. Method for edge rounding of cutting tool inserts in combination with a high surface finish over the whole insert of cemented carbide or titanium based carboni- tride alloys c h a r a c t e r i s e d in the following steps providing an electrolyte of 15-50 vol% perchloric (HC10 4 ) or sulphuric (H2SO4) acid, or a mixture thereof, in methanol or other organic liquid carrier submerging said inserts into the electrolyte providing an electrode of an acid resistant mate¬ rial, e.g. platinum or acid resistant stainless steel within the electrolyte applying an electrical potential between the inserts (anode) and the electrode (cathode) for a period of time sufficient to round the edges of said inserts to a de¬ sired degree while also providing a high surface finish over the whole inserts . 2. Method according to the previous claim c h a r a c t e r i s e d in that an edge rounding of about 10 μm in combination with a high surface finish over the whole inserts is obtained.

Inventors: JOHANSSON HANS, ROLANDER ULF, JOHANSSON, HANS, ROLANDER, ULF
Applicants: SANDVIK AB, SANDVIK AB (PUBL)
Application year: 1997
Main classification: C25F3
IPC codes: C25F3