Cutting forces should be kept constant during machining operations to prevent changes in spindle speeds or work position. Such changes would deteriorate the accuracy of the work's dimensions. A control system is proposed to control the cutting force. The plant is difficult to model, since the

FIGURE P$6\mathrm{.}16$ Cutting force control system $($Reprinted with$)$ permission of ASME.$)$

factors that affect cutting force are time varying and not easily predicted. However, assuming the simplified force control model shown in Figure P$6\mathrm{.}16,$ use the Routh$-$Hurwitz criterion to find the range of $K$ to keep the system stable $($Rober$,1997).$