In orthogonal cutting when the chip flows along the orthogonal plane, πO, the cutting force (resultant) and its components PZ and PXY remain in the orthogonal plane. Fig is schematically showing the forces acting on a piece of continuous chip coming out from the shear zone at a constant speed. That chip is apparently in a state of equilibrium.
The circle(s) drawn taking R or R1 as diameter is called Merchant’s circle which contains all the force components concerned as intercepts. The two circles with their forces are combined into one circle having all the forces contained in that as shown by the diagram called Merchant’s Circle Diagram (MCD)
The significance of the forces displayed in the Merchant’s Circle Diagram are :
PS – the shear force essentially required to produce or separate the chip from the parent body by shear
Pn – inherently exists along with PS
F – friction force at the chip tool interface
N – force acting normal to the rake surface
PZ – main force or power component acting in the direction of cutting velocity