A slot is the hardest simple feature in machining. A hole has a drill. A face has a facemill. A slot has a cutter buried on both flanks at once, and most of the rules of good milling collapse the moment full radial engagement begins. Run an endmill the same width as the slot and it is engaged across 180 degrees of arc. Several things go wrong together: - One wall is climb milled and the other conventional, so deflection flips direction mid-cut - Chips have nowhere to escape and are recut, which is the fastest way to chip a carbide edge - Heat stays in the cut instead of leaving with the swarf - Radial load bends the tool, producing tapered walls and a slot that measures narrow at the bottom The workaround is to stop slotting. Rough with a cutter around 70 per cent of the finished width using trochoidal or peel passes, keeping radial engagement low and arc of contact short, then take a light finishing pass on each flank to correct for deflection. Enter by helix or ramp, never by plun...