The present invention relates to a method for the manufacturing of cutting tools, such as milling cutter bodies with seats for indexable cutting inserts, and drills with seats for several cutting inserts of a hard material.
When milling, the tool is given a rotating main motion at the same time as the workpiece or the tool performs a rectilinear or a rotating feeding motion. A feeding along a curve may also take place in copying milling machines and in numerically operated milling machines. For instance, the milling cutter body may consist of a face milling cutter, a corner milling cutter or a cylindrical milling cutter. These have in common that they usually carry a plurality of cutting inserts of a hard material, such as cemented carbide. These cutting inserts are normally of the indexable type, i.e. they may be detached and turned or "indexed" when a cutting edge has been worn out and the operator wishes to turn a new cutting edge into the operative position. To obtain a fine and smooth surface on the workpiece, it is necessary that the operative abutting edges of the cutting inserts describe substantially identical rotation orbits. For instance in a face cutting miller or a corner cutting miller the inserts should have substantially the same axial and radial positions in the milling cutter body. This implies extraordinarily high precision requirements when manufacturing the insert seats in the milling cutter body. If for instance the axial positioning is unsatisfactory, a so-called axial throw arises, which causes a deteriorated surface smoothness.
In several different face cutting miller applications, very high demands are set on the surface smoothness and R.sub.a values of maximally 1.5 .mu.m, R.sub.z values of between 10 and 15 .mu.m, R.sub.max of 10 .mu.m and WT values of between 5 and 8 .mu.m often occur. In order to achieve these surface criteria, it is necessary to adjust the cutting edges with a very high precision, both absolutely and relative to the other cutting edges in the milling cutter body. Thus, the axial height difference between two cutting edges should not exceed a few .mu.m. Suitably, one should be able to position cutting edges within an axial tolerance range of 5 .mu.m, and preferably even less. This has proved to be impossible because of the internal differences that occur between the positions in the milling cutter body of the insert seats, and thereby, also the cutting inserts.
According to one known technique, a milling cutter body is manufactured by starting off from a piece of bar material of tool steel, such as, e.g., one with the code designation 42CrMo4H in toughened condition (hardness 270-310 HB). Shape and form are established by cutting machining, such as turning of the basic shape and milling out of the seats for the cutting inserts. Further, the bottom support surfaces of the insert seats are provided with a central, threaded hole for receiving the locking screw of the cutting insert, or a tubular shim screw for the fastening of a shim to the cutting insert. When the shaping of the milling cutter body is terminated, it is hardened. The purpose of hardening is to give the steel (in this case, normally the tool steel) a suitable hardness, either throughout its entire thickness or only to a certain depth under the surface. By heating to the austenitic range, followed by a fast cooling in water, oil or air (depending on the composition) martensite is obtained. The hardness of the martensite depends on the carbon content. Milling cutter bodies are either locally hardened in the regions around the insert seats, or they are hardened throughout, normally to a hardness degree of between 40 and 52 H.sub.R C, preferably between 43 and 47 H.sub.R C. During this hardening and the subsequent oil cooling, it is unavoidable that yieldings and distortions in the material bring about minor shifts of the positions of the insert seats. Thus, differences of 50 to 60 .mu.m may for instance occur between the axial positions of two insert seats. Thereby, the positions of the cutting inserts will of course deviate to the same extent. In many cases, this may turn out to be unacceptable, and therefore, in some applications, a possibility must exist to finely adjust the cutting edges axially, for instance by fastening the cutting inserts in movable cassettes; see for instance Swedish Patent 501 915 (Hessman et al.).
Thus, a primary object of the present invention is to form cutting positions in a milling cutter body with an increased position precision.
A second object of the present invention is to reduce the axial throw between two insert seats in a milling cutter body to less than 20 .mu.m, preferably 10 .mu.m.
Still another object of the present invention is to avoid complicated constructions with a plurality of different machine parts.