The present invention relates to a method for obtaining 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) have 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 desired 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 pressing 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 burrs. Such edges break too easily when used.
Therefore, after sintering, the inserts are subjected to an edge rounding operation including mechanical 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 .mu.m on cemented carbide inserts for a majority of machining applications. 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 mechanical operation. These defects disappear during the continued treatment provided that the final edge rounding obtained is larger than the defect size.
A finer edge rounding, however, means lower cutting forces. The choice of edge rounding is a compromise between desired edge strength and acceptable cutting forces. For certain cutting operations such as threading and machining of heat resistant materials, aluminum or cast iron, lower cutting forces are desirable. However, the above-mentioned methods for edge rounding are generally 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. U.S. Pat. No. 4,405,422 discloses methods for electrolytic deburring of copper or copper alloys and U.S. Pat. No. 4,411,751 of steel or aluminum alloys. However, when subjecting materials with phases of differing chemical properties such as cemented carbide to chemical treatments, the binder phase is often dissolved first, resulting in a porous surface layer with reduced strength and often containing portions 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 U.S. Pat. No. 5,380,408 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.
In U.S. Pat. No. 5,591,320, a method for edge rounding of cutting inserts by electropolishing in an electrolyte containing 2-15 volume % perchloric (HClO.sub.4) or sulphuric (H.sub.2 SO.sub.4) acid in methanol is presented. With this method, for geometrical reasons, the material removal is significantly 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 surfaces. However, on such surfaces there often appear 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 to be delivered to the customer. Additional mechanical or chemical treatment is needed. This further treatment can lead to an enlargement of the desired fine edge radius.