This study investigates the cutting characteristics of direct milling of cemented tungsten carbides performed using a diamond-coated carbide end mill. The diamond-coated carbide end mills have cutting edges that are both treated and untreated, and the sharp cutting edge can be developed at the ridgeline of the diamond coating on the flank face via treatment of the cutting edge. Two types of cemented tungsten carbide were used as workpiece materials, i.e., TAS VM-40 and VC-70. The influence of the cutting length on the cutting characteristics was also studied. In the case of cutting of the VM-40, the cutting force of the treated tool was significantly lower than that of the untreated tool. The cutting forces of both tools were observed to be similar after the diamond coating on the rake face of the untreated tool was flaked. The cutting edge at the ridgeline of the diamond coating on the flank face of both tools was retreated during the cutting progress, and the thickness of the diamond coating of the untreated tool was also decreased. The finished surface integrity was drastically altered owing to the flaking of the diamond coating on the rake face of the untreated tool and the irregularity of the cutting edge of both tools. The accuracy of the machined shape obtained by using the treated tool was better than that obtained using the untreated tool, and the tendency was significantly observed as the cutting progressed. In the case of cutting of VC-70, flaking of the diamond coating of untreated tool did not occur, and the cutting force of the treated tool was significantly lower than that of the untreated one. The cutting edge of the treated tool was maintained sharp up to a groove length of 500 mm, although the workpiece material was clearly observed to be adhering to the round corner of the cutting edge of the untreated tool. Moreover, the accuracy of the shape of the machined groove obtained using the treated tool was better than that obtained using the untreated tool.

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