There is nothing subtle about the first cut into a rusty forging. The skin is the enemy. A forged billet leaves the furnace wearing a layer of oxide scale — wustite, magnetite and haematite stacked in that order from the steel outwards — and it is harder than the metal beneath it. Magnetite sits around 5.5 on the Mohs scale, and embedded sand or refractory debris picked up in the forging operation can be harder still. Run a cutting edge along that surface and you are not machining steel, you are lapping tungsten carbide with abrasive grit. The fix is depth, not caution: - Set the first pass deeper than the scale layer so the edge engages clean parent metal and the oxide rides up the rake face - Drop surface speed and raise feed; abrasive wear scales with sliding distance, not chip thickness - Choose a tough substrate with a thick CVD alumina coating, because Al2O3 is the layer that survives the temperatures roughing generates - Hone or T-land the edge, since a sharp edge in a forgin...