A crankshaft converts combustion force into rotation, and every rod journal on it sits deliberately off axis. That eccentricity is the whole machining problem. Turning an offset journal means either swinging the entire forging around the pin centre in a counterweighted chuck, or accepting cutting loads that bend a long slender part like a spring. Milling solves it differently. In internal milling the crankshaft stays on its main axis while an annular cutter ring, toothed on its inside diameter, encircles the journal. The machine interpolates that ring around the eccentric path as the crank slowly rotates. External milling inverts the geometry, using a large disc cutter carrying inserts on its periphery. Both replace one continuous cut with hundreds of short interrupted ones. That distinction matters: - Radial force stays low, so the crank deflects less and roundness holds - Each insert engages briefly, so heat leaves with the chip instead of soaking into the part - Chips arrive shor...