A polycrystalline diamond (PCD) ball end mill is effective for processing molds made of hard and brittle materials, such as cemented carbide and ceramics and having fine and complex shapes with a high aspect ratio. The PCD ball end mill has a distinctive feature of its cutting edge, which it has a simple spherical shape without sharp cutting edge found on conventional cemented carbide and cubic boron nitride (cBN) ball end mills. The rake angle varies depending on the depth of cut. In cutting using the PCD ball end mills, flow type chips are observed and mirror finish surface without micro fracture can be realized by adopting the ductile-cutting mode. However, the mechanism of chip generation in this cutting with the PCD ball end mills has not been fully clarified. In this study, to simulate the cutting with the PCD ball end mill, an orthogonal cutting experiment was conducted using a commercial cutting insert tool having a fine and sharp cutting edge made of PCD. In addition, we designed an experimental setup with a spring constant force system for applying simulated thrust force of the PCD ball end mill. As a result, the chips formed at all rake angles down to −75° and the relationship between the negative rake angles and the chip generation mechanisms were clarified.