<p>Plunge milling is suitable for machining deep groove parts and increasingly applied in integral impeller processing. When significant variations in impeller runner width, using tools with varying diameters improves efficiency, but replacing large diameter tools poses a risk of cutting force overload, leading to premature tool failure. Adjusting the tool path post-overload is time-consuming and often relies on operator experience. To address this, a transitional tool path planning method is proposed for impeller plunge milling during large diameter tool replacement. Experiments revealed that the key factor affecting cutting force during replacement is the instantaneous radial cutting width. To prevent overload quickly, the maximum cutting force was used as a constraint, with a prediction model based on the instantaneous radial cutting width. Additionally, an instantaneous radial cutting width calculation model for five-axis plunge milling was developed using the micro-element discretization method. This enables fast maximum cutting force prediction from tool path parameters, allowing automatic tool path adjustments to prevent overload during large tool replacement. Simulations and experiments validate the method, achieving automatic tool path correction in 2.62&#xa0;s, with transition paths adding only 6% to total machining time.</p>