Speeds, Feeds, and What the Chip Is Telling You

Stop and look at what is coming off. The chip is the only direct report you get from the cutting edge, and it is unambiguous once you know the vocabulary. SHORT TIGHT CURLS, warm and silver-grey: correct. LONG STRINGY RIBBONS that wrap the work: feed too light or no chipbreaker, and dangerous — they are razor-sharp and they will grab. DARK BLUE OR STRAW chips: the cut is running hot, speed too high for the tool, and on HSS you are minutes from a blunt edge. DUST rather than chips on steel: the tool is already blunt and rubbing. A chip that changes colour part way through a job is telling you the tool is wearing, not that the metal changed. Listen too: a steady hiss is a cut, a rhythmic chatter is the tool or work springing and needs less stick-out, a slower speed or a smaller nose radius.

Flood steel generously with cutting oil. It cools the edge, floats the chip out of the cut and stops material welding to the tool, and it buys a large multiple of tool life on stainless in particular. Cast iron is cut DRY. Its graphite lubricates the cut by itself, and fluid turns the fine dusty swarf into an abrasive paste that gets into the slideways and wears the machine. The one rule that matters with carbide: apply fluid steadily or not at all. An intermittent squirt onto a hot carbide tip thermally shocks it and cracks the edge, and a cracked tip fails suddenly rather than gradually. Aluminium wants something to stop it welding to the tool — paraffin or a dedicated aluminium cut fluid works where general cutting oil is indifferent.

Keep a page per material: tool, diameter, spindle speed, feed, depth of cut, and one word about the chip and the finish. It takes fifteen seconds at the end of a job. Published cutting-speed tables are a STARTING POINT computed for industrial machines with rigid setups and plenty of power. A small lathe on a bench will often chatter at the book figure and be perfectly happy at two thirds of it. What you actually want is a table for YOUR machine. After a dozen jobs you will stop calculating for the common cases, which is the point: the formula is how you handle the unfamiliar material, and the notebook is how you stop re-deriving the familiar ones.