METHOD FOR OBTAINING WELL-DEFINED EDGE RADII ON CUTTING TOOL INSERTS BY ELECTROPOLISHING TECHNIQUE

Abstract: There is disclosed a method for edge rounding of cutting tool inserts 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. The electrolyte comprises perchloric (HClO4) or sulphuric (H2SO4) acid, 2-15 vol %, or mixtures thereof in methanol or other suitable 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 which cannot be made by mechanical methods.

Description: Method for obtaining well-defined edσe radii on cut¬ ting tool inserts bv electropolishinσ technique. The present invention relates to a method for ob- taining well defined edge radii on cutting tool inserts by electropolishing technique. Inserts for chip forming machining made of cemented carbide or of titanium based carbonitride (cermets) have at least one main cutting edge and a connecting nose (corner) . Such inserts are produced by the powder metal¬ lurgical methods of milling of powders of the hard con¬ stituents and binder phase, pressing to form bodies of a desired shape and finally sintering the pressed bodies. The pressing is generally done by tool pressing between two opposing punches in a die. As a result of the press¬ ing operation the inserts have 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 burrs. Such edges break too easily when used. Therefore, after sintering, the inserts are subject¬ ed to an edge rounding operation including mechanical methods such as lapping, tumbling, brushing or blasting. These operations, however, are difficult to control with 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 get defects in the initial stage of the me¬ chanical 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 the desired edge strength and acceptable cutting forces. For certain cutting operations such as threading and machining of heat resistant materials, aluminum or cast iron, low 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 Pa- tent No. 4,405,422 discloses methods for electrolyte de¬ burring of copper or copper alloys and U.S. Patent No. 4,411,751 of steel or aluminium alloys. However, when subjecting materials with phases of differing chemical properties such as cemented carbide to chemical treat- ments, the metallic binder phase is often dissolved first, resulting in a porous surface layer with reduced strength and often containing portions comprising seve¬ ral 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 Swedish pa¬ tent application SE 9101469-6, which discloses a method for removing cobalt from the surface of cemented carbide using an electrolyte of sulphuric and phosphoric acids. This method, however, does not generate edge rounding, since it only removes cobalt, leaving the carbide or carbonitride grains intact. It is an object of the this invention to avoid or alleviate the problems of the prior art. A primary object of the invention is to provide a method for edge rounding of cutting tool inserts which can be more carefully controlled. A second object of the present invention is to pro¬ vide a method of manufacturing inserts with a small edge radius of the order of 10 μin. The invention provides a method for edge rounding of cutting tool inserts of cemented carbide or titanium based carbonitride alloys comprising an electrolyte selected from the group consisting of 2-15 vol% per- chloric (HC104) , sulphuric (H2SO4) acid and mixtures thereof, in an organic liquid carrier; submerging said inserts into the electrolyte; providing an electrode of an acid resistant material within the electrolyte; applying an electrical potential between the inserts and the electrode for a period of time sufficient to round the edges of said inserts to a desired degree. FIG.l is a SEM-image in 600x magnification of the edge of a cemented carbide cutting tool insert treated according to a prior art electrolyte method disclosed in U.S. Patent No. 4,411,751. FIG.2 is a corresponding image in 1500x magnifica¬ tion of a cemented carbide cutting tool insert edge rounded according to the present invention. FIG.3 is a corresponding image to FIG.2 of a cermet cutting tool insert. It has now surprisingly been found that by using a method similar to the one disclosed in U.S. Patent No. 4,405,422 and 4,411,751 but using an electrolyte com- prising perchloric (HCIO4) or sulphuric (H2SO4) acid or mixtures thereof, an even removal of the burr and round¬ ing of the edge is obtained, resulting in a smooth edge with an edge rounding which is essentially constant around the insert. The method is easier to control than conventional mechanical methods and is particularly use¬ ful for providing very small edge radii of about 10 μm which can not be made by mechanical methods. According to the presently claimed invention, the inserts are thoroughly cleaned, e.g., by ultrasonic cleaning in methanol, so that dust, loose particles, grease stains, etc., that may affect the polishing re¬ sult are removed from the surf ces. The inserts are then submerged in the electrolytic bath and a DC-voltage is applied between the inserts (anode) and a cathode. Strong agitation is carried out in order to obtain stab¬ le conditions with electrolyte 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 be 2-15 vol% perchloric (HCIO4) or 2-15 vol% sulphuric (H2SO4) acid, or a mix¬ ture thereof, in methanol. Methanol may be partly or fully substituted by more viscous fluids, e.g., another lower alcohol such as butanol or glycerol or ethylene- glycol-monobutylether, in order to decrease the polish¬ ing speed or to obtain 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 between +10 and +40 volts. 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. Polishing time is generally from about 5 seconds to about 5 minutes . 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 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 after electropolishing, the inserts are rinsed, e.g. in methanol, in order to avoid corrosion caused by the electrolyte. The method is suitable for mass production since large quantities of inserts can be polished simulta¬ neously with high polishing speed. The accuracy and re- producibility 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 risk that the gradient is removed. Example 1 A commercially available cemented carbide insert (SANDVIK HIOF) with as-sintered sharp edges was electro- polished for 15 seconds using an electrolyte consisting of 5 vol% sulphuric acid in methanol, cooled to -20 °C, and a DC-voltage of 20 volts. A 30 cm 2 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 10 μm and con¬ siderably improved surface finish as shown in Fig 2. Example 2 A commercially available cermet insert ( SANDVIK CT530) with sharp edges (after grinding of the flat sur- faces) was electropolished under identical conditions as above. Smooth rounded edges were obtained with small edge radii about 10 μ and considerably improved surface finish as shown in Fig 3. Example 3 A commercially available cermet insert (S-AΝDVTK CT530) with sharp edges (also after grinding) was elec¬ tropolished using an electrolyte consisting of 5 vol% perchloric (HC10 ) acid and 35 vol% n-butanol in metha- nol, cooled to -30 °C, and a DC-voltage of 22.5 volts. The other conditions were identical as above. Smooth rounded edges were obtained with small edge radii of about 10 μm and considerably improved surface finish es¬ sentially similar to Fig 3. The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification. The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims: laims 1. A method for edge rounding of cutting tool in¬ serts of cemented carbide or titanium based carbonitride alloys c h a r a c t e r i z e d in the following steps providing an electrolyte selected from the group consisting of 2-15 vol% perchloric (HC10 ) , sulphuric (H2SO4) acid and mixtures thereof, in an organic liquid carrier; submerging said inserts into the electrolyte; providing an electrode of an acid resistant material within the elctrolyte; applying an electrical potential between the inserts and the electrode for a period of time sufficient to round the edges of said inserts to a desired degree. 2. The method of claim l c h a r a c t e r i z e d in that an edge rounding of about 10 μ is obtained. 3. The method of claim l c h a r a c t e r i z e d in that the organic liquid carrier is a lower alcohol. 4. The method of claim l c h a r a c t e r i z e d in that the organic liquid carrier is methanol. 5. The method of claim l c h a r a c t e r i z e d in that the electrode is made of platinum. 6. The method of claim l c h a r a c t e r i z e d in that the electrical potential is applied at a voltage of 10 to 40 volts.

Inventors: ROLANDER ULF, ROLANDER, ULF
Applicants: SANDVIK AB
Application year: 1995
Main classification: C25F3
IPC codes: B23B27, B24B53, C25F3