Prior art Polycrystalline cubic boron nitride, PCBN, is next to diamond the hardest material. PCBN is, contrary to diamond, stable at high temperatures, which means that turning of hardened steel, 45-65 HRC, becomes possible also with economically advantageous cutting data. This is utilized within today's modern machining industry by turning the hardened components that earlier used to be ground. Efficient CNC lathes enable quick and flexible machining to an equivalent quality as with grinding in the sense of surface roughness and tolerances but to a lower cost and with shorter machining times.

Martensitic transformations, so called white layers, just as residual strains in the machined surface can occur during both grinding and hard turning. These phenomena depend on the generation of heat and the mechanical strain in the cutting process and they are influenced by cutting data, tool shape as well as the choice of tool paths.

The hard turning methods currently used for machining try to have as little contact between the cutting insert and work piece surface as possible in order to meet the quality requirements for the finished component. This means that machining is made with small cutting depth and feed and relatively pointed cutting corners. The cutting corner, which is in little engagement, is brought from the beginning of the machined surface to its end, by using typical cutting data in the order of ; cutting depth ap=0.1 mm, feed f=0. 1 mm/r and cutting speed 150 m/min.

Objects of the invention One object of the present invention is to provide a method for hard turning, which comprises the advantages of prior art.

Another object of the present invention is to provide a method for hard turning in order to drastically reduce the machining time at fine turning of hardened components.

Another object of the present invention is to provide a method for hard turning in order to improve the surface quality at fine turning of hardened components.

Another object of the present invention is to provide a method for hard turning in order to minimize the existence of white layers and residual strains in the machined surface.

These and other objects have been achieved by a method for hard turning such as it is defined in the subsequent claims with reference to the drawings.

Description of the drawings Figs. 1 and 1A show a cogwheel in a cross-section before engagement with a tool. Figs. 2 and 2A show a ball-bearing ring in cross-section before engagement with a tool.

Detailed description of the invention In Figs. 1 and 1 A a turning tool 10 is shown comprising a cutting insert holder 11 and a cutting insert 12 mounted therein. The cutting insert is made of a material harder than cemented carbide that is harder than 1500 HV1 (Vickers hardness with the load 1 kg). With"cemented carbide"is here meant WC, TiC, TaC, NbC, etc. in combination with a binder metal such as for example Co or Ni. The cutting insert in Figs.

1 and 1 A has a triangular basic shape with three cutting edges 14. Each cutting edge 14 is located between two adjacent cutting corners of the insert and has a cutting edge length LI, which is defined by the maximum available insert edge length. By shaping the insert of PCBN to the same profile as the guiding surface 15 of the finished component 13 the machining occurs much faster since all of the cutting edge profile comes into engagement simultaneously and the feed occurs in a direction opposite to prior art feed. The cutting insert is consequently fed in only one direction F a distance that is shorter than the cutting edge length Ll. The finished surface 15 of the component 13 has a length L2. The cutting edge length LI is always greater than the length L2 of the zone of contact such that the ends of the active cutting edge 14 during turning are outside the zone of contact

with the work piece 13. Thereby, a surface roughness Ra is obtained which is maximum 0.2 u,m.

Tests have shown that with the method according to the present invention the time for machining can be reduced to a tenth of the time when using the conventional hard turning method. Completely surprising it has been shown that the existence of white layers is reduced and that it seems as the level of tension also becomes less.

Example 1. Machining of an external guiding surface 15 of a cogwheel 13, with hardness 59 HRC, for a gearbox according to Figs. 1 and 1A.

Conventional method: a. Rough turning with a cutting insert of a neutral rhombic type with vc=130m/min, f=0.16 mm/r, ap=0.1 mm, there vc is the cutting speed, f is the feed speed and ap is the cutting depth. b. Fine turning according to above but with f=0.06 mm/r, ap=0.05mm gave the surface finish Ra 0.3 um with a surface quality typical for conventional turning, that is a fully visible helical ridge on the generated surface with the partition 0.06 mm.

The method according to the present invention: Machining with a triangular cutting insert 12 with 80% of the cutting insert edge in engagement and vc=200m/min, F=0.05 mm/r, where the parameter"cutting depth"is missing, gave the surface finish Ra 0.2 pm during feed in only the direction F perpendicularly to the rotational axis CL of the workpiece. By "missing"is here meant that cutting depth is not a parameter in the method of the present invention. Savings in time compared to the conventional turning became about 60% and the turning gave a completely bright generated surface.

An alternative embodiment of a turning tool 10'is shown in Figs. 2 and 2A, comprising a cutting insert holder 11'and a profiled insert 12'mounted therein. The hardness of the cutting insert 12'relative to the hardness of the work piece 13'is the same as has been discussed above in connection with Figs. 1 and 1 A. The cutting insert 12'has a triangular basic shape but with at least one convex cutting edge. The cutting edge length Ll', which is defined by the maximum available insert edge 14'length is always greater than the length L2'of zone of contact, such that the ends of the active cutting edge 14'during turning are outside the zone of contact with the work piece 13'.

Example 2. Machining of an internal bearing race 15'of a ball bearing ring 13', with the hardness 58 HRC, according to Figs. 2 and 2A.

Conventional method: a. Rough turning with a cutting insert of a positive triangular type with vc= 150m/min, f=0. 1 mm/r, ap=0.15 mm. b. Fine turning according to above but with vc= 180m/min, ap=0.05mm gave the surface finish Ra 0.3 llm with a surface quality typical for conventional turning, that is a fully visible screwshaped ridge on the generated surface with the partition of 0.1 mm.

The method according to the present invention: The generation of finished surface with the profiled insert 12'with vc=400m/min, f=0.02 mm/r, where the parameter"cutting depth"is missing, gave the surface finish Ra 0.2 am at feed in direction F only perpendicularly to the rotational axis CL of the workpiece. Time savings compared to conventional turning became 50- 90% depending on the size of the component. The internal bearing race 15'of the ball bearing ring 13'obtained a wholly bright generated surface with the method according to the present invention, which improves the fatigue strength for the bearing race and which reduces resistance against rolling in the bearing.

With the method according to the present invention subsequent honing and polishing operations can be avoided in most cases.

The present invention consequently relates to a method for turning of a rotating metallic work piece where cutting depth is missing, wherein the machining time at fine turning of hardened components is drastically reduced and the existence of white layers and tensions in the finished surface is reduced.