Inconel 718 is a challenging alloy to machine, commonly employed in the aeronautic and energy industries. There is a continual need to enhance our understanding of its cutting processes to improve its machining applications. This research presents an in situ analysis of the kinematic and thermal fields behaviour of a serrated chip, during orthogonal cutting in the primary shear zone. This study involves a specific self-designed optical system enabling the simultaneous acquisition of both a visible high-speed CCD camera and an infrared camera via a single × 25 magnification reflective objective. A particular attention is brought to evaluate the accuracy of the whole optical system to measure the thermomechanical fields in the unfavourable peculiar cutting conditions among such are high strain rates, texture evolution, strong thermal gradients, and very narrow observation window. The description of the digital image correlation technic within the visible range and the infrared images post-processing are both affronted, and their limitations exposed. To conclude, a deeper characterisation of the primary shear zone shape is done from the kinematic and the thermal point of views showing that, at least under the employed cutting conditions and for Inconel 718 alloy, the primary shear zone width exhibits no thickness despite the literature.